PRODUCT DESIGN AND PRODUCTION TOOLING

Transcription

1 PRODUCT DESIGN AND PRODUCTION TOOLING By Prof.(Dr) MANOJ KUMAR PRADHAN B.E, (University College of Engineering, Burla, Odisha) M.Tech, (National Institute of Technology, Rourkela) Ph.D, UCE, Sambalpur University, Odisha Professor and Head Department of Mechanical Engineering Gandhi Institute for Technological Advancement (GITA) Bhubaneswar, Odisha

2 Module I PRODUCT DESIGN & PRODUCTION TOOLING Product design considerations, product planning, product development, value analysis, product specification. Role of computer in product design. Product design for sand casting: design of gating system and risering. Module II Forging design: allowances, die design for drop forging, design of flash and gutter, upset forging die design. Sheet metal working: Design consideration for shearing, blanking piercing, deep drawing operation, Die design for sheet metal operations, progressive and compound die, strippers, stops, strip layout. Module III Design of jigs and fixtures, principle of location and clamping, clamping methods, locating methods, Drill Jig bushing, Indexing type drilling Jig. Design of single point cutting tool, broach and form tool. Design of limit gauges. Process Planning selection of processes, machines and tools. Design of sequence of operations, Time & cost estimation, Tooling design for turret lathe and automats. Text Books: 1. Fundamentals of Tool Engineering design, S.K. Basu, S.N. Mukherjee, R. Mishra, Oxford & IBH Publishing co. 2. Manufacturing Technology, P.N. Rao, Tata McGraw Hill 3. A Textbook of Production Engineering, P.C. Sharma, S. Chand & Co Reference Books: 1. Product Design & Manufacturing, A K Chitale, R C Gupta, Eastern Economy Edition, PHI. 2. Product Design & Development, Karl T Ulrich, Steven D Eppinger, Anita Goyal, Mc Graw Hill 3. Technology of Machine Tools, Krar, Gill, Smid, Tata Mc Graw Hill 4. Production Technology, HMT

3 MODULE-I

4 Introduction to Product Design Definition of Product Design. Product design deals with conversion of ideas into reality which aims at fulfilling human needs. A designer produces the prototype which issued as a sample for reproducing the particular goods or, services as many times as required. In the course of production, on error made by the producer in manufacturing an item may lead to its rejection, but an error in design, which will be repeated in all products, may lead to an economic misadventure of enormous, proportions. The designer s responsibility is therefore serious. Design by Evolution Product Designers are usually asked to develop an existing design, rather than to design a new product from scratch. Updating successful or even unsuccessful designs is a regular occurrence in the working life of a Product Designer. Take for example the humble and tried and tested pen knife. The existing design, as seen with the Swiss Army Knife, has evolved slowly over many years. Pen knifes and multifunction knives have been used for well over a century. Even the Romans developed multifunctional tools. Manufacturers know from experience, that they must continue to develop new designs in order to remain successful and to continue to sell their products.

5 Design by Innovation Following a scientific discovery, a new body of technical knowledge develops rapidly; the proper use of this discovery may result in an almost complete deviation from past practice. Every skill, which the designer or, the design team can muster in analysis and synthesis, is instrumental in a totally novel design. e.g- Implementation of laser beam. Product design considerations 1. Need A design must be in response to individual or, social need, which can be satisfied by the technological status of the times where the design is be prepared. 2. Physical realizabilaty A design should be convertible into material goods or, service, is it must be physically realizable. 3. Economic worth wholeness T he goods or services, described by a design, must have a utility to the consumer which equals or, exceeds the sum of the total costs of making it available to horn. 4. Financial feasibility The operation of designed producing and distribution the goods must be financially supportable.

6 5. Optimality The choice of a design concept must be optimal amongst the available attentive the selection of the choice design concept must be optimal among all possible design proposal. 6. Design criterion Optimality must be established relative to a design criterion which represents the designer s compromise among possibly conflicting value judgments which include those of the consumer, the producer, the distributer and his own. 7. Morphology Design is progression from the abstract to the concrete. This gives a chronologically horizontal structure to a design project. Primitive need Phase-I Feasibility Study Phase-II Preliminary Design Phase-III Detailed Design Phase-IV Planning for Production Phase-V Planning for Distribution Phase-VI Planning for Consumption Phase-VII Planning for Retirement

7 8. Design Process Design is an iterative problem solving process. 9. Sub problems During the process of solution of a design problem, a sub-layer of Sub- Problems appears; the solution of the original problem is dependent on the solution of the Sub- problems. 10. Reduction of uncertainty Design is derived after processing of information that results in a transition from uncertainty, about the success or, failure of a design towards certainty. 11. Economy worth of evidence Information gathering and processing have a cost that must be balanced by the worth of the evidence, which affects the success or, failure of the design. 12. Bases for decision A design project is terminated when it is obvious that its failure calls for its abandonment. It is continued when confidence in an available design solution is high enough to indicate the commitment of resources necessary for the next phase. 13. Minimum commitment- In the solution of a design problem at any stage of the process, commitments which will fix future design decisions must not be made beyond what is necessary to execute the immediate solution. 14. Communication A design is a description of an object and prescription for its production; it will exist to the extent of is expressed in the available modes of

8 communication. The best way to communicate a design is through drawings, which is the universal language of designers. The present day impact of computer added modeling and drafting has resulted in very effective communication between the designer and sponsor. Product Planning- The product planning process takes place before a product development project is formally approved, before substantial resources are applied and before the large development team is formed. Product planning is an activity that considers the components (portfolio) of projects that and organization might pursue and determines what subset of these projects will be pursued over what time period. The product planning actuate ensures that product development projects support the broader business strategy of the company and addresses these questions: 1. What product development projects will be undertaken? 2. What mix of fundamentally new products, platforms, and derivative products should be pursued? 3. How do the various projects relate to each other as a portfolio? 4. What will be the timing and sequence of the projects? The product plan identifies the portfolio of products to be developed by the organization and the timing of their introduction to the market. The planning

9 process considers product development opportunities indentified by many sources, including suggestions from marketing, research, customers, current product development teams and benchmarking of competitors. From among these opportunities, a portfolio of projects is chosen, timing of projects is outlined and resources are allocated The product plan is regularly updated to reflect changes in the competitive environment, changes in technology, and information on the success of existing products. Product plans are developed with the company s goals, capabilities, constraints, and competitive environment in mind. Product planning dews ions generally involve the senior management of the organization and may take place only annually or a few times each year. Product Development Each of the selected projects is then completed by a product development team. The team needs to know its mission before beginning development. The answers to these critical questions are included in a mission statement for the team. : What market segments should be considered in designing the product and developing its features? 1. What new technologies (if any) should be incorporated into the new product?

10 2. What are the manufacturing and service goals and constraints? 3. What are the financial targets for the project? 4. What are the budget and time frame for the project? Four type of product Development Projects A. New product platforms This type of project involves a major development effort to create a new family of products based on a new, common platform. The new product family would address familiar markets and product categories. B. Derivatives of existing product platforms These projects extend an existing product platform to better address familiar markets with one or, more new products. C. Incremental improvements to existing products These projects may only involve adding or modifying some features of existing products in order to keep the product line current and competitive. Product life cycle (PLC) is one conceptual tool which helps to analyze the requirement, growth, maturity and decline. Many a time, the third stage of PLC that is maturity stage of product life cycle, primarily gives clue for making incremental improvements in existing products to remain competitive and to keep the product in the market.

11 D. Fundamentally new products These projects involve radically different product or, production technologies and may help to address new and infamiliar markets. Such projects inherently involve more risk; however, the long term success of the enterprise may depend on what in learned through these important projects. Value Analysis Value analysis is defined as an organized creative approach, which has for its purpose the efficient identification of unnecessary cost i.e, cost which provides neither nether quality nor use, life, appearance or, customer features. A Product or, service is generally considered to have good value if that product or, service has appropriate performance and cost. 1. Value is always increased by decreasing cost, (While of course, maintaining performance) 2. Value is increased by increasing performance, if the customer has needs and wants, and is willing to pay for more performance. Nature and Measurement of Value Value can be perceived as the ratio of the sum of the positive and negative aspects of an object.

12 Value = (+) ( ) In reality, this equation, is more complex, since we are dealing with many valuables of different magnitudes. A more descriptive eq n is ( mb1 mb2... mbn ) Value ( ma mc... mc ) 2 n Where, m = the magnitude of a given factor or, criterion b = a specific benefit c = a specific cost Max m. Value is probably never achieved. The degree of value of any product depends on the effectiveness with which every usable idea, process material, and approach to the problem have been identified, studied and utilized. In a free enterprise system, with competition at full play, success in business over the long term hinges on continually offering the customer the best value for the price. The best value determined by two considerations : performance and cost. The value Analysis Job Plan In the job plan, the problems are recognized and faced, with the functions to be accomplished clearly in mind. It is a five step process.

13 (i) Information step Record of all the relevant information pertaining to the problem is done by individuals or by groups of any number of persons. (ii) Analysis step In the analysis step, extensive essential function thinking is developed. Functions are evaluated and problem setting is made precise; functions are separated for single study and then they are grouped as needed for best solutions. (iii) Creativity step When there is a problem to be solved: Creativity is more important than knowledge. Having acquired understanding and information, we have laid the foundation for the application of various techniques, to generate every possible solution to the overall problem involved, to the part of problem, and to the individual problems. To meet real life situations, the strategy of value engineering must be to 1. provide logic, 2. communicate emotionally in credible terms, 3. Identify new types of knowledge needs. 4. Provide research techniques that will find that knowledge efficiently, and 5. Cause creativity that will usefully combine the knowledge from diverse sources.

14 (iv) Use preliminary judgment - Select the approaches that show so much promise than it is believed. They should be thoroughly studied, extended and judged. (v) Evaluation This phase is a feasibility and cost analysis phase. The alternative ideas suggested during the creative phase were refined and analyzed with a view to ascertain, whether they could achieve the desired functions. This was carried out in two stages: In the first stage, all suggestions were studied and those which could not be adopted because of quality, reliability or, other basic reasons were eliminated, and the others were shortlisted. In the second stage, the ideas short listed after first evaluation were critically studied and discusses with the concerned personnel, for feasibility and practicability of production. Value Analysis Tests Each product or, component is subjected to the following tests : 1. Does its use contribute value? 2. Is its cost, proportionate to its usefulness? 3. Does it need all its features? 4. Is there anything better for the intended use? 5. Can a usable part be made by a lower cost method?

15 6. Can a standard product be found, which will be usable? 7. Is it made on proper tooling, considering the quantities used? 8. Do material reasonable labouur, overhed and profit total its cost? 9. Will another dependable supplier provide it for less? 10. Is anyone buying it for less? Product Specifications. Customer needs are generally expressed in the language of the customer. The term product specifications mean the precise description of what the product has to do. A specification (singular) consists of a metric and a value. For example, average time to assemble is a metric. Less than 75 seconds is the value of this metric. The value may take on several forms, including a particular number, a range or, an in equality values are always labeled with the appropriate units. (e.g., seconds, kilograms, joules) Together, the metric and value form a specification. The product specifications (plural) are simply the set of the individual specifications. The specifications must reflect the customer needs, differentiate the product from the competitive products and be technically and economically realizable.

16 Specifications are typically established at least twice. Immediately after identifying the customer needs, the team sets target specifications. After concept selection and testing the team develops final specifications. Target specifications represent the hopes and aspirations of the team, but they are established before the team knows the constraints the product technology will place on what can be achieved. The teams Efforts may fail to meet some of these specifications and may exceed others, depending on the details of the product concept the team eventually selects. The process of establishing the target specifications entails four steps. 1. Prepare the list of metrics. 2. Collect competitive benchmarking information. 3. Set ideal and marginally acceptable target values. 4. Reflect on the results and the process. Final specifications are developed by assessing the actual technological constraints and the expected production costs using analytical and physical models. During this refinement phase the team must make difficult trade-offs among various desirable characteristics of the product. The five-step process for refining the specifications is : 1. Develop technical models of the product. 2. Develop a cost model of the product.

17 3. Refine the specifications, making trade offs where necessary. 4. Flow down the specifications as appropriate. 5. Reflect on the results and the process. Role of Computer in product Design The design related tasks which are performaed by a modern CAD system can be grouped into four functional areas : 1. Geometric modeling. 2. Engineering analysis. 3. Design review and evaluation. 4. Automated drafting. 1. Geometric modeling. In CAD, geometric modeling involves computer complatible mathematical description of the geometry of an object. In geometric modeling, the designer constructs the image of the object on the CRT screen of the interactive computer graphies system, by inputting three types of command to the computer. 1. The first type of command generates basic geometric elements such as prints, lines and corcle. 2. The second type of command is meant to accomplish translation scaling (size change), rotation or, other transformations of the elements, 3. The third type of command joins the various elements to give the desired object.

18 During the above process, the computer converts the commands into a mathematical model, stores it in the computer data files and displays it as an image on the CRT screen. 2. Engineering Analysis In the formulation of any design project, some sort of analysis is required. The analysis may be stress stralin calculations, heat transfer computations or, the use of differential equations to described the dynamic behavior of the system being designed. The computer can be used to assist in this work. CAD/CAM systems can be interfaced to engineering analysis software to test a given product design. Probably the most powerful analysis feature of a CAD system is the finite element Method (FEM). 3. Design Review and Evaluation Checking the accuracy of design can be accornplished conveniently on the graphics terminal. Semi- Automatic dimensioning and tolerancing routines which, assign size specifications to surfaces indicated by the user help in reducing the possibility of dimensioning errors. The designer can zoom in on any details and closely sorutinize the magnified image. Animation helps in checking kinematic performance of like mechanisms without resorting to pinboard experiments. Gear simulations can be carried out and tooth contact analysis can be done. Interference checking of shaft hole assemblies and the link can be done.

19 4. Automated Drafting This procedure results in saving a lot of time and labour. Computer aided drafting is known as the design workstation. The CAD work station is the system interface with the outside world. A good CAD workstation must accomplish five functions. It must 1. Interface with the central processing unit of the computer. 2. Generate a steady graphic image for the user; 3. Provide digital description of the graphic image. 4. Translate computer commands into operating function ; and 5. Be user friendly.

20 Product Design for Sand Casting Gating systems refer t o all those elements, Which are connected with the flow of another metal from the ladle to the mould cavity.

21 The various elements that are connected with a gating system are _ (i) Pouring basin, (ii) (iii) (iv) Sprue, Sprue base well, runner (v) runner extension, (vi) in gate. (vii) riser. Pouring Bason D- sprue entrance diameter The main function of a pauring basin is to reduce the momentum of the liquid flowing into the mould by setting first in to it in order that the natal enters into the sprue without an y turbulence, it is necessary that the pousing basin be deep enough, and also the entrance into the sprue be a smooth radous of at least 258mm. The pouring basin depth of 2.5 tomes the sprue entrance diameter is enough for smooth metal flow and to prevent vortex formation.. Also,the pouring basin is kept full to avoid vortex formating Constant conttains of flow through the sprue are established by using strainer Core cevarnee from fulter

22 Sprue Sprue is the channel through which the molten metal is brought in to the parting plane where it enters the runners and gates to ultimately reach the mould cavity. The sprue tapering is obtained by equation of continuity. T demites top secton c demotes choke section At Vt = AcVc or, A t Vc Ac V t From, Bernoulli s equation ( h Cons tan t) p w v 2 2 g A t A c hc ht The square root suggests that the profile of the sprue should be parabolic if exactly done as per equation. But making a parabolic sprue as too inconvenient in practice and therefore a straight taper is preferable. A straight tapered sprue is able to effectively reduce the air aspiration as well as increase the flow rate compared to a cylindrical sprue. The dimensions of the sprue at the top and subsequent top per depends on head of the metal in the pouring basin. Metal at the entry of the sprue would be moving with a velocity of 2 gh

23 Hence, At AC ht h Where, H = Actual sprue height & ht = h + H Theoretical ratios of At Ac based on pouring basin depth. Sprue height (mm) Depth of pouring basin (mm) Sprue Base well - This is a reservoir for metal the bottom of the bottom of the sprue to reduce the momentum of the mother metal and there by mould erosion as reduced.

24 The sprue base area should be five times that of the sprue choke area and the well depth shold be approximately equal to that of the runner. For a narrow and deep runner, the well diameter should be 2.5 times the width of the runner in a two-runner system, and twice its width in a one runner system. Runner It is generally located in the horizontal plane (parting plane), which connects the sprue to its in gates, thus allowing the metal enter the mould cavity. The runners are normally made trapezoidal in cross section. It is a general practice for ferrous metals to cut the runners in the cope and the ingates in the drag, there by slag are entrapped. For effective trapping of the slag, runners should flow full. Figure Runner Extension The runner is extended a little further after it encounters the in-gate. This extension is provided to trap the slag in the molten metal Gates or in gates

25 Depending on the application, various types of gates are used in the casting design. Top Gate IN this type of gating, the molten metal enters the mould cavity from the top. This is sudtable only for simple casting shapes of famous days. Bottom Gate When molten metal enters the mould cavity slowly from bottom, it would not cause any mould erosion. Bottom gate is generally used for very deep moulds. Parting Gate This is the most widely used gate in sand castings. The metal enters the mould at the parting plane when a part of the casting is in the cope and a part in the drag. Step Gate Such, gates are used for heavy and large castings. The molten, metal enters mould cavity through a number of in gates, which are arranged in vertical steps. The size of in gates are normally increased from top to bottom such that the metal enters the mould cavity from the bottom most gate and then progressively moves to the higher gates. Riser Most of the foundry alloys shrink during solidification. Hence a reserved of molten metal is to be maintained from which the metal can flow readily into the casting when the need arises. These reservoirs are called risers.

26 Material Shrinkage (%) Medium carbon steel High carbon steel No Morel Al Cu Brass Bearing bronze Grey cast iron Mg Zn 2.5% to 3.5% 4% to negative The metal in the riser should solidify in the end. The riser volume should be sufficient for compensating the shrinkage in the casting. The risers are normally of the following types top risers which are open to the atmosphere, blind risers which are completely concealed inside the mould cavity itself and internal risers which are enclosed on all sides by the casting. W CastingYield x100 %

27 When, W actual casting mass W mass of metal poured into the mould. Gating System Design Pouring Time There is an optimum pouring time for any given casting. 1. Grey cast Iron, mass less than 450 kg T Pouring time, t K 1.41 W sec. Fluidityof ironininches where, K 40 T = average section thickness, mm. W = mass of the casting, Kg 2. Grey cast iron, mass than 450 kg T Pouring time, t K W sec Steel castings, logW sec. Pouring time, t W 4. Shell moulded ductile iron (Vertical pouring) Pouring time, t K1 W sec. Where, K 1 = for thinner sections

28 = for sections 10 to 25mm thick. =2.970 for heavier sections. 5. Copper alloy casting Pouring time, t K 2 W sec. 3 K 2 is constant given by Top gating = 1.3 Bottom gating = 1.8 Brass = 1.9 Tin Bronze = Intricately shaped thin walled casting of mass upto 450 kg 3 ' Pouring time, t K 3 W sec. W 1 = mass of the casting with gates & risers, Kg K 3 = a constant. 7. For castings above 450 kg and upto 1000 kg. Pouring time, t K 3 ' ' 4 W T sec. Q- Calculate the optimum pouring time for a casting whose mass is 20 kg and having an average section thickness of 15mm. The materials of the casting are

29 grey cast iron and steel. Take the fluidity of iron as 28 inches. Calculate for grey cast Iron and steel. Solution Grey cast Iron T Pouring time, t K W sec sec. Steel, Pouring time, t ( logW ) W sec. ( log 20) sec ond 20 Q. Calculate the optimum pouring time for a casting whose mass is 100 kg and a thickness of 25mm. Fluidity of iron is 32 inches. Calculate both for cast Iron Steel Solution Grey cast Iron Pouring time, t Steel, sec ond. Pouring time, t ( log100) sec ond Choke Area The choke area can be calculated using Bernoulli s equation as

30 W A dtc 2gH Where, A = choke area, mm 2 W = Casting mas, Kg. T = pouring time, s D = mass density of the molten metal, kg/ mm 3 G = acceleration due to gravity, mm/s 2 H = effective metal head (sprue height), mm C = efficiency factor which is a function of the gating system used. The effective sprue heads can be calculated using the following relations. Top gate, H = h Bottom gate, H = h - c 2 Parting gate, H = h - P2 Where, h = height of sprue P = height of mould cavity in cope c = total height of mould cavity. 2 c

31 Q Figure For the casting sloon, which is to be made in cast iron, calculate the choke area. Sol Volume of the casting = 500 x 250 x 50 = 6.25 x 10 6 mm 3 Weight of the casting = 7.86 x 10-6 x 6.25x 10 6 kg = kg Assuning a composition factor of 4.0 and a pouring temperature of C and fluidity is 22 inches., Pouring time t sec Calculate effective sprue height, Assuring a top gating system with 100 mm cope height, Effective sprue height = 100 mm. Selecting effodency factor C = 0.73 Assumed the density of the liquid metal = 6.9 x 10-6 kg/mm Chokearea, A mm 6 6.9x10 x19x0.73 2x9800x100 2 In a pressurized gating system, the choke is located in gates, with four ingates, the ingate area of each is 90 mm 2, each = (15.x6) mm Gating Ratios

32 The gating ratio refers to Sprue ara : runner area : in gate area. Depending on the choke area, there can be two types of gating systems : Non Pressurized Pressurized. A non pressurized gating system having choke at the bottom of the sprue base, having total runner area and in-gate areas higher than the sprue area. In this system there is no pressure existing in the metal flow system and thus it helps to reduce turbulence. E.g, sprue : runner : in-gate : : 1: 4:4 In the case of a pressurized gating system, normally the in-gate area is the smallest, thus madntaining a back pressure throughout the gating system. Because of this bnack pressure in the gating system, the metal is more turbulent and generally flows full and thereby, can minimize the air aspiration. e.g Sprue : runner : in-gate : : 1 : 2: 1 Al 1:2:1 or 1:3:3 or 1:4:4 Al bronze 1:2.88:4.8 Brass 1:1:1, OR 1:1:3 OR 1.6:1.3:1 Cu 2:8:1 OR 3:9:1

33 Steels 1:1:7 OR 1:2:1 OR 1:2:1.5 In gate Design The in-gates are generally made wider compared to the depth, upto a ratio of 4. Sometimes of is proffered to reduce the actual connection between the in-gate and the casting by means of a neck down, wash burn or, dry sand core so that the removal of the gating is simplified. In-gate should not be located near a protruding part of the mould to avoid the striking of vertical mould walls by the molten metal stream. In-gates should preferably be placed along the longitudinal axis of the mould wall. In-gates should not be placed near a core point or a chill. In-gate cross-sectional area should preferably be smaller than the smallest thickness of the casting so that the in-gates solidify, forest and isolate the castings from the gating system. Slag Trap systems, Runner Extension Whirl Gate

34 Risering Design Caine s Method Chvorinov has shown that the solidification time of a casting is proportional to the square of the ratio of volume to surface area of the casting. The constant of proportionality called mould constant depends on the pouring temperature, casting and mould thermal characteristics. t s V K SA 2 Where, ts = solidification time, S V = Volume of the casting SA = Surface area K = mould constant. The freezing ratio, X, of a mould is defined as the ratio of cooling characteristics of casting to the riser. SAcosting / V casting X SAriser / V riser In order to be able to feed the costing, the riser should solidify last and hence its freezing ratio should be greater than unity. For that sphere has the lowest surface

35 area to volume ratio and hence should be used as a riser. But in a sphere, the hottest metal being at the centre, it is difficult to use it for feeding the casting. The next best is the cylindrical type which is most commonly used. Based on the Chvorinov s rule, Caine developed a relationship empirically for the freezing ratio as a X c Y b Where, Y = Riser volume / Casting volume. A, b & c are constants whose values are different for different materials. Riser Most of the foundry alloys shrink during solidification. As a result of this volumetric shrinkage during solidification, voids are likely to form in the castings unless additional molten metal is fed into these places which are termed as hot spots. Hence a reservoir of molten metal is to be maintained from which the metal can flow reading into the casting when the need arises. These reservoirs are called risers. Risering requirements, vary from material to material. Risers should be designed keeping the following in mind 1. The metal in the riser should solidify in the end.

36 2. The riser volume should be sufficient for compensating the shrinkage in the casting. The risers are normally of the following types. (a) Top risers open to the atmosphere. (b) Blind, risers completely concealed inside the mould cavity. (c) Internal risers which are enclosed on all sides by the casting. Out of the above three, the best is the internal riser which is surrounded on all sides by the casting such that heat Iron the casting keeps the metal in the riser hot for a longer time. These are normally used for castings which are cylindrically shaped or have a hollow cylindrical portion. a b c Steel Al Cost Iron brass Grey cast Iron Al bronze So - bronze

37 Q Calculate the size of a cylindrical riser (height and diameter equal) necessary to feed a steel slab costing of dimensions 25 x 25 x 5 cm with a side riser, casing poured horizontally into the mould. Son n Volume of the casting = 25x25x5 =3125 cm 3 Surface area of the casting = 2x25x25 + 4x25x5 = 1750 cm 2 Volume of the riser = π 4 D3 Surface area of the riser = πd 2 + π 4 D2 = 1.25πD 2 When D Riser diameter and height 1750/ 3125 Freezing ratio, X D / 0.25 D 3 = 0.112D Y Riser volume casting volume 0.25 D D 3 Sub situating in Caine s eqation, X= a Y b + C D D 0.03 On simplification, D D D = 2490 By trial & error, D = cm 12cm

38 Modulus Method If the module of the riser exceeds the modules of the casting by a factor of 1.2, the feeding during solidification would be satisfactory. The modulus is the inverse of the cooling characteristic (surface area/ volume) as defined earlier. In steel castings, it is generally preferable to choose a riser with a height to diameter natoo of 1. Volume 3 4 D The bottom and of the riser is in contact with the casting and thus does not contribute to the calculation of surface area. Surfacearea D 4 2 D 2 5x D 4 2 The modules of such a cylindrical riser, MR = 0.2 D Since MR = 1.2 Mc = 0.2 D = 1.2 Mc = D = 6Mc Where, Mc = Modulus of the casting

39 Thus in this method, the calculation of the riser size is simplified to the calculation of the modules of the casting. Though this takes into account the cooling effect of the riser, it does not consider exactly the amount of feeding metal required to compensate for the shrinkage of the costing. If allowance is made for the volume of metal to be fed to counteract the contraction of the costing the equation would be D McD Vc = 0 Where, Vc = Volume of the casting. Module of simple Geometric shapes. Figure Q Calculate the size of a cylindrical riser (height & dia. Equal), using Modulus Method, necessary to feed a steel slab costing of dimensions 25x25x5 cm with a side riser, casting poured horizontally into the mould. Solution Since it is a slab of dimensions 25x25x5cm, it can be considered as a long bar with cross section of 25x5cm ab 25x5 Modulus, Mc cm 2( a b) 2(25 5)

40 The riser diameter, D = 6Mc = 6x = 12.5 cm Naval Research Laboratory Method In NRL Method a shape factor is used in place of freezing ratio The shape factor is defined as Length Width Thickness The corresponding riser volume to casting volume is obtained from the graph Figure Ex Calculate the size of a cylindrical riser (H& Di) using NRL method necessary to feed a steel slab casting of dimensions 25x255 cm with a side riser, casting poured horizontally in to the mould. Solution Shape factor 10 5 Riser volume Corresponding, 0.47( from graph) Casting volume = Riser Volume = 0.47 x (25x25x5) = cm 3

41 2 3 D xd cm 4 D 3 4x cm The same can also be directly obtained from graph Figure For calculation of shape faction for 1. Circular plates, the length and width are same as that of the diameter. 2. Cylinders, the width and thickness are same as the diameter. Chills Chjills are provided in the mould so as to increase the heat extraction capability of the sand mould. A chill normally provides as steaper temper. Gradient so that directional solidification as required in a casting can be obtained. The chills are metallic objects having a higher heat absorbing capability than the sand mould. Depending upon the place of applications, chills can be of two types external, and internal. External chills when placed in the mould should be clean and dry. The chills should not be kept for long to avoid moisture condensation resulting in below holes.

42 Internal chills are placed inside the mould cavity where an external chill can not be provided. The material of the chill should be approximate to the composition of the pouring metal for proper fusing. Internal chills should be troughly cleaned before use. However, because of serious draw backs with use of internal chills, they should be sparingly used. Grouping of castings Feeding Aids To increase the efficiency of a riser, it is necessary to keep the metal in the riser in liquied form for as long a period as required so that it would feed the casting till it solidifies. The aids used for thos purpose are called feeding aids. The feeding aids can be either exothermic materials or, insulators. The exothermic materials that can be used are graphite or, charcoal powder, rice husks and thermit mixtures. For steel costing, an insultating shield on the top of an open riser is very effective since it reduces heat loss by radiation. Rice husk is used as top insulation. For non ferrous materials, plaster of pairs is generally used as insulator.

43 MODULE-II

44 FORGING DESIGN Drawing out : Upsetting Forging Types.- Smith Forging (Open dies) Drop Forging (Closed impression dies) Press Forging (closed up dies Hydraulic Press) Machine forging (Upsetting) Allowance (i)shrinkage Allowance The forgings are generally made at a temperature of C to 1300 o C. At this temperature the material gets expanded and when it is cooled to the atmospheric temperature its dimensions would be reduced It is very difficult to control the temperature, at which the forging process would be complete. A Therefore to precisely control the dimensions, a shrinkage allowance is added on all the linear dimensions. (ii) Die Wear Allowance The die wear allowance is added to account for the gradual wear of the die which takes place with the use of the die. (iii) Finish Allowance Machining allowance is to be provided on the various forged surfaces, which need to be further machined. The amount of allowance to be provided should account for besides the accuracy, the depth of the decarburized layer. Also, the scale pits that are likely to form on the component should also be removed during machining.

45 FORGING - No loss of Material - Blacksmith operation - Plastic Deformation Process. Classification 1 Hot Forging & Cold Forging 2- Closed die Forging - carried out for complex shapes - For small size, mass pad suitable. - Close tolerance - Rough forsing Blokege pre Finishing due Open die Forging - Carried out for simple shapes - For large size - Production volume less - Wide tolerance - carried out in one stage 3- Impact foring - Pressure is applied suddenly - Definition is limited to surface - Hagh velosity of material flow. - Repeated rapid inpact brows.

46 - Pressure is max,at begriming &gradually bows Press Forging - Pressure is applied gradually. - Law velocity of material flow. - Single step pressure application. - Pressure is from zero to max. - Pressure increases as the metal is deformed & is maximum at the moment the pressure is released - Deeper Penetration of metal deformation. Drop-forging Die Design The first step in the design of a drop forging due is the decision regarding what impressions (or stages) are necessary to achieve the necessary fiber flow direction to obtain the requisite strength. A blocking impression becomes a necessity only when the component is to be accurately made or the component has deep pockets or thin ribs, which are difficult to be obtained in a single finishing impression. A bending impression is required when the part is of bent nature and the growth direction is to be along the bend lone. In such a case, the bending impression is to be obtained before the blocking impression or finishing impression when no blocking is used. Similarly, a

47 flattening impression is used when the component is thin and perpendicular to one plane. Flash The excess metal added to the stock to ensure complete falling of the due cast in the finishing impression is called flash. Gutter Proportions. Flash Gutter Stock size Width Thickness(mm) Width (mm) Thickness (cm) (cm) (mm) Upto to to to to Forging load is greatly influenced by the flash thickness and width. The forging load can be decreased by increasing the flash thickness. However, this increases the scrap losses. In addition to the flash, provision should be made in the die for additional space so that any excess metal can flow and help in the complete closing of the die. This is called gutter. Without a gutter, flash may became excessively thick, not allowing the dies to close completely.

48 Stock As a rule drop forgings do not get upset and therefore the stock size to be chooser depends on the largest cross sectional area of the component. To get the stock size, the necessary flesh allowance is to be provided over and above the stock volume. The stock to be used is either round, rectangular or any other section depending on the nature of the component. Knowing the section of the stock, the length of the stock can be found out. In addition to stock length, about 50 to 60mm tong hold is provided for effective handling and movement of the stock in the die. As the metal is being processed at high temperature, the iron of the forging combines with atmospheric oxygen to form an oxide which is adhering over the forging as scale. The loss of metal in this way is about 6% of net weight of forging. Lengthof the stock Gross weight Density of metal x greatest crosssec tion area Impressions in a Multiple Impression Die Six types of impressions may be incorporated in a drop forging die for shaping the material progressively from bar form to the finished forging. They are 1. Fuller This impression is required to reduce the cross section of a portion of the forging stock between the ends of the stock. 2. Edger or, roller This impression distributes the stock so that it will fill the next impression without excessive waste.

49 3. Bender impression Bender impression is included in the die block when curves or, angles in the forging make it necessary to bend the stock before it will fit properly in the finishing impression. Bending is a very important operation to keep the flow lines continuous in a job like crane hook. 4. Block impression Block impression gives the forging its general shape and allows the proper gradual flow of metal necessary to prevent laps and cold shuts. Blocking impression is of same contour as the finished parts but the radio and fillots are lange to permit the easiest flow of metal. A block impression is without a flash or, gutter. 5. The finishing impression This impression bring the forging to its final size. The finishing impression has both a gutter and flash impression cut round it to provide space for the excess metal. Both blocker and finisher have a necking or, running impression so that the forging is still held with tongs. 6. Cut off It is also part of the die block. It cuts the forging from the bar by cutting off the tong hold. The flash and gutter are however removed in separate trimming dies, under punch, presses. Die block Dimensions The dimensions of the die-block depend upon the length of the finish forging impression, depth of the impression and the number of impressions in the die block.

50 For a single impression die, the length of the die block may be taken as, L = l + 3 h (minimum) B = c x b Where, l Total length of forging impression, h = maximum depth of the impression, b = Maximum width of the impression c = Constant = 3 for b upto 5cm = 2.5 for b upto 25 cm = 2 for b above 25 cm The height of the die block determines the maximum impression depth, since adequate die material must be there between the bottom, of ompression and bottom face of the die block to provide strength in the die. From the strength point of view of the dies and the die wear, the ratio of h and b is Material h/b l = b l 2b Al, Mg Steel, Titanium For multi impression dies, a gap of at least 25mm should be left between two adjacent impressions. a 1 - the inter impression distance,

51 a the distance of impression from the edge of the die block H the height of the die block Dimension in mm h a a 1 H h = the maximum depth of impression. In terms of the maximum depth of impression, corresponding other values given. Upset Forging Die Design (Machine Forging) In upset forgings, as a rule, no reduction in cross section occurs. Depending on the shape of the upsetting to be done, the number of passes or blows in the die are to be designed. The amount of upsetting to be done in a single stage is limited. To arrive at the safe amount of upsetting in a given pass, the following rules are to be satisfied, to achieve defect free upset forgings. Rule 1 The maximum length of the up supported stock that can be gathered or, upset in a single pass is not more than three times the diameter of the bar. Beyond this length, the material is likely to buckle under, the axial upsetting load. In practice, it is better that the length of the unsupported stock is within 2.5 times the bar diameter.

52 Rule 2 Length of stock more than 3 times bar diameter that is within the limits of the stroke of machine can be successfully upset made is not more than 1.5 times bar diameter. If this is kept more than 1.5d, the buckling will be excessive and the stock will fold in. In practices it is advisable not to exceed 1.3 time bar diameter. Rule 3 In an upset requiring more than 3 rd in length, when the diameter of the upset is 1.5d, the amount of unsupported stock beyond the face of the die must not exceed one diameter of bar. However if the diameter of the hole in the die is reduced below 1.5d, then the length of unsupported stock beyond the face of the die can be correspondingly increased. Rule 4- Avoid using head diameter greater than four times the stock diameter. The ratings of Upset forging machines as per Metals Handbook Rated Size, Nominal Rated Average strokes per cms capacity kn min

53 Selection of correct size machine necessary to forge a part should be governed by the following factors. 1. Volume of stock required in the finished forging, 2. Size of stock used. 3. Maximum dimension of the finished forging. 4. Number of blows necessary to complete the forging. The following steps are formulated in sequences for the purpose of eliminating guess work. (i) Calculate the volume of metal in the part to be forged. (ii) Determine the proper cross section of metal and shape of the metal to be used to make the forging. (iii) With the shape and area of the cross section as well as the volume of the upset, calculate the length of the metal necessary.

54 (iv) Calculate the number of blows necessary to complete the forging, using the general rules that eliminate folding or, buckling. (v) Make a die layout to determine the size of the die blocks and heading tolls necessary to accommodate the required number of blows and cavity dimensions. (vi) Determine the size of forging machine to be used, bearing in mind the size of the bar to be used, size of the forging to be made, size of the die-blocks necessary length of header slide, length of stroke, length of gather, length of die opening etc. (vii) Use hot dimensions on all cavities. (viii) Provide for clearance between heading tools and their mating dies when these tolls enter the dies (ix) Provision should be made for proper grip of the stock. The length of this grip should not be less than 3d. Also, the cavity diameter in this area should measure approximately 0.30 to 0.50mm smaller then the diameter of the bar to be foged. (x) With the dies and tools designed and the impression machined in the die blocks, it is advisable to place the tools in the tool holders and make a preliminary set up of the dies and tools in a face placet for final checking before placing them in the forging machine. (xi) Setting of the dies and tools in the forging machine requires a check of parallelism for the die seats and also the travel of the header slide.

55 (xii) A small stream of coolant should be directed on the dies and tools to dissipate the heat and keep the dies free from scale, that may gather in the cavities. The best is solution of slable oil & water. Upset forging gives flowing added advantages 1. A high degree of accuracy in dimensional tolerances. 2. Die life is increased by minimusing the contact time between the dies and the hot metal. 3. Reduction in the man power required as compard to drop forging. 4. Die setting time is less than drop forging for similar jobs. 5. Die manufacturing cost is less as insert technology is very suitable for upsetter dies. 6. Preparation of raw material perform design ) is not needed as the bar stock is directly used in dies. Design suitable tooling for upset forging of the component shown. The material is mild steel. Solution For the design of the die, all the dimensions will be taken on M.S. contraction scale. Volume to be upset. Since l is less than 2.5, the forging can be upset in one blow. Applying rule 1- d

56 Total length required for the component = ( ) = cm Size of the M/c = Since the bar size is 3.8 cm, so the size of M/c of nominal rated capacity 3000 KN will be suitable. Size of the die block = For 3.8cm size of the machine, the die block sizes.- Length of bar to be gripped = 3d = 11.4cm. Parting line of the job is taken aling the diameter Half the impression is in the die block and the other half in the punch. The length of the conical portion is within two thirds of the maximum working length. The unsupported stock beyond the die face is 101mm which is 2.89 times the stock diameter and is acceptable since it is around 2.5 The average stock diameter after pass 1 is = 40mm Length of stock = 324 Diameter 40 = 8.1 This is still high and therefore one more conical gathering pass is desired. Length of conical portion = 12x π( x = 205.5mm The unsupported length is 118.5mm which is 2.96 times the stock diameter and therefore can be acceptable. Average stock diameter after pass 2 = = 50mm Length of stock Diamter = =4.11 2

57 This is still more than 3 but not too high. Therefore, we may check for the validity of Rule 2, since it has already violated Rule 1. Rule 2 is violated since the diameter of 135 mm of the cavity should be =2.7x stock diameter. Hence, one more cone gathering is desirable. Length of conical portion= 12x π ( x ) = mm After the third pass, the length to diameter ratio is now which therefore can be gathered in a single pass.

58 SHEET METAL WORKING Shearing Action The metal is brought to the plastic stage by pressing the sheet between two shearing blades so that fracture is initiated at the cutting points. The metal under the upper shear is subjected to both compressive and tensile stresses. Clearances - The clearance between two shears is one of the principal factors controlling a shearing process. This clearance depends essentially on the material and thickness of the sheet metal. This clearance is given per side as. C = x t x τ Where t = sheet thickness, mm & = material shear stress, MPa Shearing operations In die shearing operations, the shears take the form of the component to be made. The upper shear is called the punch, and the lower shear is called the die. Blanking It is a process in which the punch removes a portion of material from the stock which is a strip of sheet metal. The removed portion is called a blank. Piercing It is also called punching. Piercing is making holes in a sheet. In the shearing operation, first the material is elastically deformed and then plastically and finally removed from the stock strip. After the final breaking, the

59 slug will spring back due to the release of stored elastic energy. This will make the blank cling to the die face unless the die opening is enlarged. This enlargement is normally referred as angular clearance or draft. The normal value is from 0.25 to 0.75 deg per side the die opening increase after every sharpening of the die because of the provision of angular clearance. So, to maintain the die size as per the design, the angular clearance is provided in the die opening along with a straight portion called die land or cutting land. The length of the cutting land is around same as the material thickness. Stripper = Due to the release of the stored elastic energy in the stock left on the die, the stock tends to grip the punch as the punch moves upward. This necessitates the use of a stripper to separate the punch from the stock. The stripping force varies from 2.5 % to 20% of the punch force. The stripping force is given by, Ps = KLt Where, Ps = Stripping force, kn L = Perimeter of cut, mm t= Stock thickness, mm K = Stripping constant. = for low-carbon steels thinner than 1.5mm with the cut at the edge or, near a preceding cut.

60 = for low-carbon steels thinner than 1.5mm for other cuts. = for low-carbon steels thinner than 1.5mm above 1.5mm thick. = for harder materials. In blanking the die size is same as the component size whereas in piercing the punch size is same as the actual hole size to be obtained. Punching Force The force required to be exerted by the punch in order to shear out the blank from the stock can be estimated from the actual shear area and the shear strength of the material. It is given by = P = Lt z Wher, P = Punching force, N z = shear strength, MPa t = Stock thickness, mm The punching force for holes which are smaller than the stock thickness is estimated as = Where, d = diameter of the punch, mm S = tensile strength of the stock, MPa Shear The maximum shear force when shear is applied to the punch or the die, is given as Where, p = Penetration of punch as a fraction

61 t 1 = shear on the punch or, die, mm The provision of the shear on the punch will change the slug where as shear provided on the die would make the stock left on the die to bend. Hence, the shear is provided on the die for blanking and on the punch for piercing. Q = Determine the die and punch sizes for blanking a circular disc of 20mm, diameter from a C20 steel sheet whose thickness is 1.5mm Take shear strength of C20 steel as 294 Mpa. Solution = The clearance to be provide is given as C = x t x τ = C = x 1.5 x 294 = mm 0.10mm Since it is a blanking operation, Die size = blank size = 20mm. Punch size = blank size 2C = 20 2x0.10 =19.8 mm If it were a piercing operation, Punch size = blank size = 20mm Die size = blank size +2C = x 0.1 = 20.2 mm Blanking Piercing Punch size, mm

62 Die size, mm Punching force (P) = Lt z =πd x t x z = π x 20x1.5x294 = kN. Stripping force (Ps) = KLt = x (πx20) x 1.5 = kn Q = A 100mm diameter hole is to be punched in a 6mm thick steel plate. The material is cold rolled C40 steel for which the maximum shear strength can be taken as 550 MPa. With normal clearance on the tools, cutting is complete at 40% penetration of the punch. Give suitable diameters for the punch and die, and shear angle on the punch in order to bring the work within the capacity of a 200 kn press available in the shop. Solution DEEP DRAWING - Drawing is the process of making cups, shells, and similar articles from metal blanks. Here, the setup is similar to that used in blanking except that the punch and die are provided with the necessary rounding at the corners to allow for the smooth flow of metal during drawing. Shallow drawing is defined as that where the cup height is less than half the diameter.

63 For drawing deeper cups it is necessary to make specific provisions to confine the metal in order to prevent excess wrinkling of the edges. For this purpose, a blank holder is normally provided on all deep-drawing dies. DRAW DIE DESIGN Corner Radius on the Punch The corner radius on the punch varies from four to ten times the blank thickness. Ideally, the punch radius should be the same as the corner radius of the required cup, because it takes its form. Draw Radius on Die Drew radius = 4t normal = (6 to 8 ) t when the blank holder is used. = 0.8 π (D-d) t Where, t = blank thickness Clearances = Ideally, the clearance between the punch and die should be same as the blank thickness. But the blank gets thickened towards the edge because of the metal flow and hence, the actual clearance provided is slightly higher to account for this thickening. It varies from 7% to 20% of the blank thickness. Blank size Where, r = corner radius on the punch, mm. h = height of the shell, mm

64 d = outer diameter of the shell, mm. D = blank diameter, mm. Trim Allowance These are only the theoretical blank sizes, based on the surface area of the shell and blank. Additional trimming allowance is added for trimming of uneven and irregular rim of the deep-drawn cup. The trim allowance is 3mm for the first 25mm cup diameter and additional 3mm for each of the additional 25mm of cup diameters. Drawing Force Where, P = drawing force N. t = thickness of the blank material, mm s = yield strength of the metal, Mpa. C= constant to cover friction and bending. It varies between 0.6 to 0.7 Blank Holding Force = The maximum limit of blank holding force is generally one third of the drawing force. However it is obtained more by trial and error depending on the wrinkling tendency. Ironing force = The objective of Ironing force is to reduce the wall thickness of the cup. Neglecting the friction and shape of the die, the ironing force = F= Where F = Ironing force, N

65 d 1 = mean diameter of the shell after ironing. t 1 = thickness of the shell after ironing. t o = thickness of the shell before ironing. S av = average of tensile strength before and after ironing. Percent Reduction = The drawing operation relies on the ductility of the blank material. The ductility is affected by the amount of strain a material takes. But there is a limit to which it can be strained. The amount of straining or, the drawability is represented by the % reduction which is expressed in terms of the diameter of the blank and the shell The percent reduction P is given by, Where d= Shell diameter (OD) P D = Blank diameter d D Theoretically, it is possible to get a percentage reduction upto 50, but it is practicllay limited to 40 Because of strain hardening, the percentage reducation gets reduced in the subsequent draws. Reduction in drawing with cup height (% Reduction) Ht. to dia. No. of First draw Second Third drew Fourth drew ratio draws draw

66 Up to to to to Air vent An air vent is normally provided in the punch to reduce the possibility of formation of vacuum in the cup, when it is stripped from the punch. For cylindrical shells, one vent located centrally would be enough, but for other shapes two vents are provided. The size of the air vent depends on the punch diameter. Punch dia. (mm) Air- Vent dia. (mm) Up to to to Over Drawing speed The speed with which the punch moves through the blank during drawing is termed the drawing speed. Suggested drawing speeds are = Drawing Speeds Material Drawing speed (m/s) Aluminum 0.9 Brass 1 Copper 0.75

67 Steel 0.28 Zinc 0.75 Q. A symmetrical cup of circular cross section with a 40 mm diameter and 60mm height having a corner radius of 2mm is to be obtained in C20 steel of 0.6mm thickness. Make the necessary design calculations, for preparing the die for the above cup. Q. The symmetrical cup work piece as shown is to be made from cold rolled steel 0.8 mm thick. Make the necessary design for the drawing die of this part. Progressive Dies In practice, components are produced by combinations of blanking, piercing, bending or, drawing operations in a certain order. Hence, practically dies have to do more than one operation for making a finished component. The progressive dies perform two or, more operations simultaneously in a single stroke, of a punch press, so that a complete component is obtained for each stroke. The place where each of the operations is carried out are called stations. The stock strip moves from station to station undergoing the particular operation. The distance moved by the strip from station one to two so that it is properly registered under the stations is called advance distance. The feed distance is the amount of stock fed under the punch when the ram comes for the next stroke. The feed distance may or, may not be the same as the advance

68 distance. This is because sometimes the sheet is overfed against a stop. The strip is therefore positioned correctly under the punch by pulling it backwards with the use of pilots. Progressive dies contain a large number of stations. It is generally preferred to have a piercing operation first in the sequence and a blanking or, cut off operation in the end to get the final component. Any of the pierced holes may be used as a pilot hole. The choice of progressive dies is made only when the production is of large numbers so that the handling costs are saved; stock material is not very thin, so that movement of the strip by pilots is convenient; stock material is not too thick so as to avoid the problems of stock straightening and the overall size of the die or, the press capacity are large. Compound Die- In a compound die, all the necessary operations are carried out at a single station, in a single stroke of the ram. To do more than one set of operations, a compound die consists of the necessary sets of punches and dies. Compound dies are somewhat slower than the corresponding progressive dies in operation. But higher tolerances can be achieved in them than in the progressive dies. This is mainly because the part located in one position under goes all the operations. Also in compound dies, small strips can be advantageously used, whereas in progressive dies very long strips are required to cover all the stations.

69 MODULE-III

70 Design of Jigs and Fixtures A jig may be defined as a device which holds and positions the work, locates or, guides the cutting tool relative to the work piece and usually is not fixed to the machine table. It is usually lighter in construction. Jigs are used on drilling, reaming, tapping and counter boring operations. A fixture is a work holding device which only holds and positions the work but does not in itself guide locate or, position the cutting tool. The setting of the tool is done by machine adjustment and a setting block or, by using slip gauges. A fixture is bolted or, clamped to the machine table. It is usually heavy in construction. Fixtures are used in connection with turning, milling, grinding, shapping, planning and boring operations. To fulfil their basic functions, both jigs and fixtures should possess the following components or, elements: 1. A sufficiently rigid body (plate, box or frame structure) into which the workpieces are loaded. 2. Locating elements. 3. Clemping elements. 4. Tool guiding elements (for jigs) or, tool setting elements (for fixtures). 5. Elements for positioning or, fastening the jig or, fixture on the machine on which it is used.

71 Jigs and Fixtures are used 1. To reduce the cost of production, as their use eliminates the layingout of work and setting up of tools. 2. To increase the production. 3. To assure high accuracy of the parts. 4. To provide for interchangeability. 5. To enable heavy and complex shaped parts to be machined by being held rigidly to a machine. 6. Reduced quality control expenses. 7. Increased versatility of machine tool. 8. Less skilled labour 9. Saving labour. 10. Their use partically automates the m/c torl. 11. Their use improves the safety at work, thereby lowering the rate of accidents. Locating and Clamping The overall accuracy is dependnt primarily on the accuracy with which the workpiece is consistently located within the jig on fixture. There must be no movement of the work during maching.

72 Locating refers to the establishment of a proper relationship between the workpiece and the jig or fixture. Clamping is to exert a force to press the workpiece agadust the locating surfaces and hold it there against the action of cutting forces. Figures In a state of freedom, it may move in either of the two opposed directions along three mutually perpendicular axes, xx, yy and zz. There six movements are called movements of translation. Also, the workpiece can rotate in either of two opposed directions around each axis, clockwise and anticlockwise. These six movements are called rotational movements. The sum of these two types of movements, gives the twelve degrees of freedom of a workpiece inspace. To confine the workpiece accurately and positively in another fixed body (jig or, fixture). The movements, of the workpiece in any of the twelve degrees of freedom must be restricted. (a) The workpiece is resting on three pins A, B and C which are inserted in the base of the fixed body. The workpiece cannot rotate about the axes XX and YY and also it cannot move downward. This wayt, the five degree of freedom 1,2,3,4, & 5 have been arrested.

73 (b) Two more pins D and E are inserted in the fixed body, ina plane perpendicular to the plane containing the pins A,B and C. Now the workpiece cannot rotate about the Z axis and also it cannot move towards the left. Hence, the addition of pins D & E restrict three more degrees of freedom, namely 6, 7 and 8. (c) Another pin F in the second vertical face of the fixed body, arrests degree of freedom 9. Thus, six locating pins, three in the base of the fixed body, two in a vertical plane and one in another vertical plane, the three planes being perpendicular to one another, restrict nine degrees of freedom. Three degrees of freedom, namely, 10, 11, 12 are still free. To restrict these, three move pins are needed. Buyt this will completely enclose the work piece making its loading and unloading into the jig or fixture impossible. Hence, these remaining three (10, 11, 12) degrees of freedom may be arrested by means of a clamping device. This method of locating a work piece in a jig or a fixture is called the principle or Six point location principle. Locating devices Pins of various designs and made of hardened steel are the most common locating devices used to locate a work piece in a jig or, fixture. The shank of the pin is press fitted or, driven into the body of the jig or, fixture. The locating diameter of the pin

74 is made larger than the shank to prevent it from being forced into the jig or, fixture body due to the weight of the work piece or, the cutting forces. Depending upon the mutual relationship between the work piece and pin, the pins may be classified as: 1. Locating pins 2. Support pins 3. Jack pins. Principles for location purposes 1. At least one datum or, reference surface should be established at the first opportunity, from which subsequent machining will be measured. 2. For ease of cleaning, locating, surfaces should be as small as possible consistent with adequate wearing qualities. Also, the location must be done from the machined surface. 3. The locating surfaces should not hold swarf and thereby misalign the workpiece. For this, proper relief should be provided where burr or, swarf will get collected. 4. Locating surfaces should be raised above surrounding surfaces of the jig or, fixture, so that chips fall or, can be swept off readily. 5. Sharp corners in the locating surfaces must be avoided. 6. Adjustable type of locators should be used for the location on rough surfaces. 7. Locating pins should be easily accessible and visible to the operator.

75 Clamping If the work piece cannot be restrained by the locating elements, it becomes necessary to clamp the work piece in jig or, fixture body. The purpose of clamping is to exert a pressure to press a work piece against the locating surfaces and hold it there in opposition to the cutting forces i.e, to secure a reliable (positive) contact of the work with locating elements and prevent the work in the fixture from displacement & vibration in machining. Principles for Clamping Purposes Since the proper and adequate clamping of a work piece is very important, the following design and operational factors should be taken care of 1. The clamping pressures applied against the work piece must counteract the tool forces. 2. The clamping pressures should not be directed towards the cutting operation. Whenever possible, it should be directed parallel to it. 3. The clamping pressure must only hold the workpiece and should never be great enough so as to damage, deform or change any dimensions of the workpiece. 4. The clamping and cutting forces should be directed towards the locating pins; otherwise the workpiece may get bent or forced away from the locating pins during machining. 5. Clamping should be simple, quick and foolproof.

76 6. The movement of a clamp should be strictly limited. 7. Whenever possible, the lifting of the clamp by hand should be avoided if it can be done by means of spring fitted to it. 8. Clamps should never be relied upon for holding the workpiece against the cutting force. The cutting force should be arranged against a fixed stop or a substantial part of the fixture body. 9. The clamps should always be arranged directly above the points supporting the work, otherwise the distortion of the work can occur. 10. Fibre pads should be riveted to the clamp faces, oterhwise soft and fragile workpiece can get damaged. 11. A clamp should be designed to deliver the required clamping force when operated by the smallest force expected. 12. A clamp should be strong enough to with stand the reaction imposed upon it when the largest expected operating force is applied. 13. Clamping pressure should be directed towards the points of support, otherwise work will tend to rise from its support. Clamping Devices The commonly used clamping devices are 1. Clamping screws 2. Hook blot clamp. 3. Lever type clamps 4. Quick acting clamps.

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79 DESIGN PRINCIPLES COMMON TO JIGS & FIXTURES 1. Since the total machining time for a work piece includes work handling time, the methods of location and clamping should be such that the idle time is minimum. 2. The design of jig and fixture should allow easy and quick loading and unloading of the workpiece. This wil also help in reducing the idle tiem to minimuse. 3. The Jig and fixture should be as open as possible to minimize chip or, burr accumulation and tto enable the operator to remove the chips easily with a brush or, an air jet. 4. The design features in the jig or fixture will be such that it becomes impossible to load the work into the jig or fixture in an improper position.

80 5. Clearance Clearance is provided in the jig or, fixture body for two main reasons : (i) To allow for any variation in component sizes, especially castings and forgings. (ii) To allow for hand, movements so that the workpiece can easily be placed in the jig or, fixture and removed after maching. 6. Rigidity Jigs and fixtures should be sufficiently stiff to secure the preset accuracy of machining. 7. To simplify the handling of heavy jigs or, fixtures, the following means can be adopted (i) Eye- bolts, lifting lugs can be provided for the lifting of the jig or, fixture. (ii) If the workpiece is heavy, then the jig design should allow for side loading and unloading by sliding the workpiece on the m/c table. 8. The use of ejection devices to force the workpiece out from the jig or, fixture. 9. Inserts To avoid any damage to fragily and soft workpieces and also to the fonishied surfaces of a workpiece clamping, inserts of some soft material such as copper lead fibre, leather, hard rubber or plastic should be fitted to the faces of the clamps. 10. Design for safety Jigs/ fixtures must be safe and convenient in use. For that (i) Sharp corners on the body of the jig/fixture should be avoided.

81 (ii) Bolts and nuts should be inside the body of the jig/fixture and protrude on the suface. 11. Sighting Surfaces Machining on a workpiece must be clearly visible to the worker. 12. Simplicity in Design. 13. Economical. 14. They should be easy to set in the machine tool. DRILLING JIGS Drilling jigs are used to machine holes in mechanical products. To obtain positional accuracy of the holes, hardended drill bushes or, jog bushes are used to locate and guide drills, reamers etc. in relation to the workpiece. The portin of the jig into which the hardened bushes are fitted is called bush plate. Drilling jigs are either clamped to the workpiece in which holes are to be drilled or, the workpiece is housed and clamped in the jig body. If more than one hole is to be drilled, the drill jig is made to slide on the table of the drilling machine. Drilling jigs make feasible the drilling of holes at higher speed, with greater accuracy and with less skilled workers than is possible when the holes are laid out and drilled by hand. Design Principles

82 1. A drilling jig should of light in construction consistent with adequate rigidity to facilitate its handling because it has to be handed frequently during the operation. 2. A drilling jig which is not normally clamped to the machine table should be provided with jour feet so that it will rock if it is not resting equeare on the machine table and so warm the operator. 3. The stability of a drilling jig should be as good as possible since it is not usual to clamp it to the machine table and to ensure this, the feet or, base of the jig should extend well outside the holes to be drilled. 4. Drill bushing should be fitted in fixed portoon of the jig. Drill Bushes Jig bushings eliminate the elastic spring back in machining and easily locate the tool relative to the wor. Drill bushings are classified as (1) Press Fit bushings. (ii) Renewable bushes. (iii) Linear bushes. (i) These bushings are used when little importance is put on accuracy or, finish. These bushing are installed directly in the jig body and are used mainly for short production runs out requiring bush replacement.

83 (ii) When the guide bushes require periodic replacement (due to the wear of the inside dia. of the bush, in the case of continuous or, large batch production), the replacement is simplified by using a renewable bush. These are of the flanged type and are sloding fit into the liner bush, which is installed (press fitted in the jig plate). The liner bush provides hardened wear resistant mating surface to the renewable bush. The renewable bushes must be prevented from rotating or, lifting with the drill. (iii) Liner Bushings also known as master bushings are permanently fixed into the jig body. These act as guides for renewable type bushings. These bushings can be with or, without heads. A liner bush is always used in conjunction with a renewable bush. Design Principles for Drill Bushings 1. To facilitate easy entry of drills, the entrances to drill bushes should be extremely smooth and well chamfered or, rounded. 2. There should not be any sharp corners on the body of the bush 3. Loose or, screwed in solid bushes should not be used where accuracy is important. 4. The effective length of the drill bushing should be sufficient to guide and support the drill.

84 5. Adequate provisions must be made for the chips that are produced and for their easy removal. 6. The hole of the drill bushing should be from to cm larger than the drill size. Jig bushings can be used to drill from 10,000 to 15,000 holes. Indexing jigs and Fixtures Indexing jigs and fixtures are used when holes or, slots are to be machined to some specific relationship, in a workpiece. Indexing Devices Many indexing jigs and fixtures employ a simple indexing plate for their operation. Suppose six holes are to be drilled in a flange. The flange can be mounted on an index plate which has six equispaced slots. The workpiece is revolved under the drill and each hole is drilled in turn. For this an index plunger is used which fots by turn into each slot in the index plate. To index the workpiece, the plunger is pulled out of the slot. The index place and thereby the workpiece is rotated ---- the next slpt comes in line with the index plunger into which it gets pushed due to spring action.

85 Design of Single Point Cutting Tools The work of a tool, designer consists of following stops (i) Determining the forces action on the cutting surfaces of the tool and determining the optimum tool geometry. (ii) Finding the most producible shapes of the cutting tool and determining the tolerances on the dimension of the cutting and mounting elements of the tools. (iii) Calculating the rigidily of the cutting and mounting elements of the tool. (iv) Making a working drawing of the tool and computing the manufacturing dimensions.

86 Rigidity considerations for a single point turning tool - The shank of a single point tool may be rectangular, square, round in section. The recteangular cross section is the most popular. Square shank tools are used for boring, turret and automatic lathes. Round shank tools are used for boring & thread cutting. The permissible size of the shank cross section is determined on the strength basis. For this purpose the actual bending moment (M b ) acting on the tool is equated to 1 the moment of resistance of the tool shank (M b1 ). M b = M b From Fig, Mb = Fc x L. The tool overhand (L) = (1-1.5) H. The maximum deflection which the tool undergoes durig the maching operation should also be limited. The maximum deflection of the tool would occur at the cutting point and could be found by assuming the tool shank to be a cantilever loaded at the free end. For a tool having rectangular cross section, deflection is BROACH DESIGN Broaching is a process of machining a surface with a pecial multipoint tool called a broach, whose teeth remove the while maching allowance in a single stroke. Broaching is widely used in the manufacture of special gears, Bushings and sleeves, compressor wheels, Rotors, Chain Sprocket teeth, turbine blades etc.

87 A broach is a multi point cutting tool consisting of a bar having a surface containing a series of cutting teeth or, edges which gradually increase in size from the starting or, entering end to the rear end. Broaches are used for machining either internal or, external surfaces (sizing of hole, straight or, helical splines, gun riflin and key ways). Each tooth of the tool takes a thin slice from the surface. Broaching of inside surface is called Internal or, hole broaching and outside surfaces as Surface broaching.