Machining. Module 1: Introduction to Machining. Curriculum Development Unit PREPARED BY. August 2013

Transcription

1 Machining Module 1: Introduction to Machining PREPARED BY Curriculum Development Unit August 2013 Applied Technology High Schools, 2013

2 Module 1: Introduction to Machining Module Objectives After the completion of this module, the student will be able to Recognize the function and importance of machine tools. Differentiate between the basic categories of machine tools. Read and understand the main technical drawings symbols. Take precise measurement using Venier Caliper and Micrometer. Module Contents Topic Page No. 1 Introduction 3 2 What is a machine tool? 3 3 Basic machine tools 3 4 Non-chip producing machine tools 4 5 Conventional chip producing machine tools 4 6 New generation of machine tools 6 7 Skills required to use a machine tool 7 8 Engineering drawing 7 9 measurement Prepare the blank part References 16 2 Module 1: Introduction to Machining

3 Introduction How much a nation produces determines how well its people live. Today, every known Product from a paper clip to a space vehicle is a product of machine tools. If machine tools are not used directly in the manufacture of the product itself, machine tools are required to produce the machinery and the equipment necessary for its processing. Without machine tools modern civilization, could not exist. Think of what would the world be without the automobile, electric power generators, aircrafts etc. Fig Fig. 1.1: Aircraft is an example of a product of machine tools 1. What is a Machine Tool? A machine tool is a power-driven machine not portable by hand, used to shape or form metals or materials by cutting, impacting, forming, eroding, or a combination of these processes. Example of machine tools is shown in Fig Basic Machine Tools There are three main categories of machine tools: 1. Non-chip producing machine tools. 2. Conventional chip producing machine tools. 3. New Generation of machine tools Fig. 1.2: CNC milling is one of the machine tools Module 1: Introduction to Machining 3

4 2.1 Non-chip producing machine tools. This type of machines shapes metals by shearing, pressing, and drawing to a desired shape. Fig. 1.3 Fig. 1.3: Non-chip producing machine tool 2.2 Conventional chip producing machine tools. Machines of this type shape metal to a size and contour by cutting away the unwanted portions in form of metal chips. Fig. 1.4 The collection of material-working processes in which conventional machine tools are used is called conventional machining. This is the type of machine tools that we are going to cover in this course. Fig. 1.4: Process of Chip removal Fig. 1.5 shows many parts of different shape that made by this type of machine tools. These parts could be used in the production of engines, machines or any industrial product. 4 Module 1: Introduction to Machining Fig. 1.5: Machine tools products

5 With conventional machine tools, the operator uses machine handwheels to manually control the table or spindle movements to produce the part. Fig. 1.6 The accuracy of the part produced depends upon the skills of the operator or machinist. Fig. 1.6: Hand wheels of conventional machine tool Lathe and Milling machines are examples of this type of machine tools. Fig 1.7 (a) shows lathe machine. Fig. 1.7 (b) shows milling machine. Fig. 1.7 (a): Conventional lathe machine Fig 1.7 (b): Conventional milling machine Module 1: Introduction to Machining 5

6 2.3 New Generation of machine tools To increase the rate of production as well as preciseness of machined parts, automatic programming has also been added to conventional machines and called NC or CNC (Computer Numerical Control) machines. With Computerized Numerical Control machine tools (example, CNC lathe and CNC milling), the programmer programs the machine control unit (MCU), through the use of symbols, letters, and numbers (coded instructions) which automatically control the machine tool movements to produce the desired part. Fig 1.8 Fig. 1.8 : Computerized Numerical Control machine tool The cutting action of CNC machine tools is similar to conventional machining. Fig 1.9 (a) and 1.9 (b) show a CNC milling machine and CNC lathe machine respectively. Fig. 1.9 (a) : CNC milling machine Electric discharge machines (EDM) and electrochemical machines are also examples of new generation machine tools but are completely different in construction and in the way they shape metals so they will not be part of this course. Fig. 1.9 (b) : CNC lathe machine 6 Module 1: Introduction to Machining

7 3. Skills required to use Conventional machine tools: In order to machine any part using the chip removal machine tools you need to acquire certain skills, e.g for the part shown in Fig. 1:10, you need to have the skills of drafting, read drawings and taking precise measurements. Fig. 1.10: machined part 3.1 Engineering drawing: Engineering drawing is a common language by which drafts persons, tool designers, and engineers indicate to the machinist and toolmaker the physical requirements of a part. Drawings are made up of a variety of lines, which represent surfaces, edges, and contours of a workpiece. By adding symbols, dimension lines and sizes, and word notes, the draftsperson can indicate the exact specifications of each individual part. Fig Fig. 1.11: Technical drawing Orthographic Projection: Orthographic Projection is a way of drawing a 3D object from different directions. Usually a front, side and top view are drawn so that a person looking at the drawing can see all the important sides. Orthographic drawings are useful especially when a design has been developed to a stage whereby it is almost ready to manufacture. Fig Fig. 1.12: three standard views in orthographic drawings Module 1: Introduction to Machining 7

8 3.1.2 Sectional Views: Sectional views are used to clarify interior or hidden details on a multi-view drawing of an object. Fig Sectional views are located by creating a Cutting Plane Line in one view. "Section Lining" or "Hatching" is added to the Section view mainly to distinguish the solid portions from the hollow areas of an object. Fig. 1.13: Sectional Views Isometric drawing: Isometric drawings consist of two-dimensional drawings that are tilted at some angle to expose other views and give the viewer the feeling that what he or she is viewing is a three-dimensional drawing. Fig Tolerance It is the permissible variation of specified size of a part. The basic dimensions plus or minus the variation allowed is given on a drawing. Example: From Fig.1.15 the following can be calculated: The largest permissible dimension = 70.1 mm ( = 70.1 mm) The smallest permissible dimension = 69.7 mm ( = 69.7 mm) The tolerance = 0.4 mm ( = 0.4 mm) 8 Module 1: Introduction to Machining Fig. 1.14: Isometric drawing Fig. 1.15: The basic dimension of the shaft plus or minus the

9 3.1.5 Allowance: Allowance (Fig. 1.16) is the intentional difference in the sizes of mating parts, such as the diameter of a shaft and the size of the hole. On a shop drawing, both the shaft and the hole would be indicated with maximum and minimum sizes to produce the best fit. Fit is the range of tightness between two mating parts. There are two general classes of fits: 1. Clearance fits, whereby a part may revolve or move in relation to a mating part 2. Interference fits, whereby two parts are forced together to act as a single piece variation allowed Fig. 1.16: Largest and smallest dimensions of two mating parts Common Symbols and abbreviations: R: Radius of a circle. Fig Ø: Dia. = Diameter. Fig TYP: Typical dimensions. Fig P: Pitch of the thread Fig mm: the unit of measurement is millimeter M: Metric Thread Fig. 1.17: R: means radius Fig. 1.18: The diameter of the two similar holes is 5.5 mm Example: M10 X 1.5: M = metric thread (Screw) 10 = diameter, 1.5 = Thread pitch Module 1: Introduction to Machining 9

10 3.2 Measurement: There are two measuring systems: 1. International system of measurements (Abbreviated SI from Systeme Internationale, the French version of the name). Meter (m) is the basic unit of length measurement. 1m=100 cm=1000 mm 1cm=10 mm Note: In machine shop, most dimensions will be given in millimeters (mm). Large dimensions will be given in meters (m) and millimeters (mm). Fig 1.20 shows a steel rule with mm scale at the top and inch scale on the bottom. Fig. 1.19: Pitch of a thread Fig Steel rule with two measuring systems 2. Imperial system (Old system using Yard as basic unit of length) Note: 1 yard = 36 inch I inch = 25.4 mm Care must be taken when using measuring tools. Most of tools have very sharp edges that may result in severe injuries. (Fig. 1.21), Tools could be damaged easily if bended, twisted or screwed with extra force. When using the graduated measuring tools, you must look at 90 degrees for accurate measurement. Fig Sharp edges of tools may result in severe injuries Steel Rule Metric steel rules, are usually graduated in millimeters and half-millimeters, and used for making linear metric measurements that do not require great accuracy. Fig Fig Metric steel rule A: 12 mm B:31.5 mm 10 Module 1: Introduction to Machining

11 3.2.2 Metric Vernier There are three basic types of Vernier caliper: standard, Fig a Dial, Fig b Digital, Fig c Fig a Fig b Fig c The Vernier caliper is used to measure outside, inside and depth measurement as shown in Fig below: Fig (a) Outside measurement Fig (b) Inside measurement Fig (c) Depth measurement The accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's actual (true) value. The Vernier caliper with 10 divisions in Vernier scale (Vernier scale is the bottom scale on the Vernier) is accurate to (1/10) ±0.1 mm. Fig (a) Fig (a): reading = 15.8 mm Module 1: Introduction to Machining 11

12 The Vernier caliper with 20 divisions in Vernier scale is accurate to (1/20) ±0.05 mm. Fig (b). Fig (b): reading = mm The Vernier caliper with 50 divisions in Vernier scale is accurate to (1/50) ±0.02 mm. Fig (c). Fig (c): Reading = mm 12 Module 1: Introduction to Machining

13 3.2.3 Metric Micrometer A Micrometer is a device incorporating a calibrated screw used widely for precise measurement of small distances in mechanical engineering and machining as well as most mechanical trades. There are three basic types of micrometers: Fig (a) outside micrometer Outside micrometer. For external measurement. Fig (a) Inside micrometer, used to measure the diameter of holes. Fig (b) Depth micrometer, measures depths of slots and steps. Fig (c) Fig (b) inside micrometer Reading the micrometer Each division on the upper scale on the sleeve equal 1 mm. (Fig. 1.27) Each line appears after the upper scale reading is equal to 0.5 mm. The micrometer screw gauge also uses an auxiliary scale (measuring hundredths of a millimeter) which is marked on a rotary thimble. (Fig.1.27). The micrometers in our laboratory have a pitch of 0.50 mm i.e. the thimble must be rotated through two revolutions to open the jaws by 1 mm. The rotating thimble is subdivided into 50 equal divisions. This means each division on the thimble = 0.01 mm. Fig (c) depth micrometer Fig sleeve and thimble of the micrometer Module 1: Introduction to Machining 13

14 Example 1: Fig (a) shows example 1 of a micrometer reading. Steps of solution: mm Fig (a): Micrometer reading = 5.78 mm Example 2: Fig (b), shows example 2 of a micrometer reading. Steps of solution: mm Fig (b): Micrometer reading = 5.28 mm Digital micrometers Digital micrometers are also available to give direct reading. Fig 1.29 Fig. 1.29: Digital Micrometer 14 Module 1: Introduction to Machining

15 4.Prepare the blank part: Power hacksaw (Fig. 1.30) is used to cut the blank part from a long bar (stock bar). Cutting fluids must be used when cutting hard materials (steel). (For practical tasks, the teacher will provide each group of students with a ready blank part). Fig. 1.30: Power Hacksaw For further reading, you can use the following links: References 1. Technology of Machine Tools. Seventh Edition, McGraw-Hill Companies, 2. Machine shop operations and setups, 4 th edition, Lascoe nelson Porter. 3. Machine tool and Manufacturing technology, Steve F. Krar, Mario Rapisarda, Albert F. Check., Delmar Publishers. 4. en.wikipedia.org/wiki/machining Module 1: Introduction to Machining 15

16 Student s notes 16 Module 1: Introduction to Machining

17 Worksheet 1. What is a machine tool? 2. Mention two main differences between conventional chip removal tools and CNC machine tools? Why sectional views are required in some cases to draw the part? 4. Define the term allowance? Module 1: Introduction to Machining 17

18 5. Use the drawing below and answer the following questions? a) What is the greatest permissible diameter?... b) What is the minimum permissible diameter? Mention two types of measuring system? Mention two measuring tools used in the machine shop? Module 1: Introduction to Machining

19 8. Use the drawing below to answer the following questions? a) What is the smallest diameter shown on the drawing?... b) What is the largest diameter shown on the drawing?... c) Explain: M20 X Module 1: Introduction to Machining 19

20 9. Find out the following readings of the measuring devices shown below? A) mm B) mm C) mm D) mm d) The reading is mm e) The reading is mm 20 Module 1: Introduction to Machining

21 f) The reading is mm g) The reading is mm Module 1: Introduction to Machining 21