Design and Analysis of Self Centering Steady Rest for Supercut-6 CNC Turning Machine Using CAD & FEA

Design and Analysis of Self Centering Steady Rest for Supercut-6 CNC Turning Machine Using CAD & FEA 1 Satish G. Bahaley, 2 Rajendra L. Bharambe Prof. Ram Meghe Institute of Technology & Research, Badnera, Amaravati. Abstract : Turning is a machining process in which a cutting tool, typically a non-rotary tool bit, describes a helical tool path by moving more or less linearly while the workpiece rotates. This time is termed as Set up time while time required to remove material is known as machining time. Total machining time is summation of both the setup time and machining time. So if the set up time will increase, eventually total machining time will also increase. To make turning operation fast and accurate there are many accessories used. To reduce the set up time and increase accuracy one of the accessories is generally used which is called as Steady Rest Steady rests are supplementary intermediate, supports, used in turning long slender work to prevent it from being bent by the action of the cutting forces. When the length and stiffness of a work piece make it difficult to machine without distorting or deflecting the part, many manufacturers turn to the steady rest as a work piece support device. This is especially true for long axles, shafts and similar parts used in automotive or heavy equipment applications, and in oil drilling components. Steady rest can be mounted and used to good advantage on most types of lathes for lengths, face and internal turning, centering, boring, plunge cutting and parting off etc. In this project the effort is being made to design a Steady Rest then model and simulate it by using solid works and validate the design by using Ansys for a client who wants to reduce the lead time in machining process and achieve his target in specified given time limit. Keywords steady rest, outer arm, work holder, cutting tool, bending by action of cutting forces, lead time. I. INTRODUCTION Steady rests have been used for many years in connection with machining operations. They typically employ three hydraulically or pneumatically operated fingers which are adapted to concentrically support a shaft-like workpiece about its outer periphery to provide evenly distributed support for it. The steady rest serves to resist the tendency of the machine tool to throw the workpiece off center during machining by turning, grinding or the like. In the past, known steady rests have been designed only for use in contacting the outer periphery of the workpiece. Consequently, different mechanisms must be employed to support inner surfaces of the workpiece at its free end. When performing machining operations on solid bar stock it is possible to use the point of conventional dead centering devices to engage indentations in the end of the workpiece and hold it in place. With tubular work pieces, however, other techniques must be employed. The need to support the inner periphery of a tubular workpiece is especially important when a lathe is used to make a circumferential cut near the end of the workpiece. One commonly used support technique for tubular work pieces is to employ a plug to fill the open end of the tube. Unfortunately, these techniques take additional time thereby decreasing productivity, not to mention the expenditures required for procuring the different support devices. The conventional steady rest assembly employs a hydraulic or pneumatic cylinder connected to the fingers. The cylinder must exert heavy pressure on the fingers during machining operations to prevent the part from going off center. This requirement for a constant and large amount of cylinder pressure is difficult to achieve in an economical manner [1]. Steady rests are supplementary intermediate, supports, used in turning long slender work to prevent it from being bent by the action of the cutting forces.in various types of machine tools, a chuck is utilized to grip, hold and rotate one end of a workpiece upon which the machine tool performs one or more operations. However, because of the length of some workpiece, it is frequently desirable to support the free end of the workpiece. Generally, such devices supporting the free end of the workpiece, commonly known as steady rests, are manually adjustable and require a considerable amount of time to set up in order to hold the workpiece coaxially aligned with the axis of the chuck. When the length and stiffness of a work piece make it difficult to machine without distorting or deflecting the part, many manufacturers turn to the steady rest as a work piece support device. This is especially true for long axles, shafts and similar parts used in automotive or heavy equipment applications, and in oil drilling components. Steady rest can be mounted and used to good advantage on most types of lathes for lengths, face and internal 14

turning, centering, boring, plunge cutting and parting off etc [10]. The most common application is to support a work piece during turning or milling and, increasingly, during secondary operations such as ID drilling, boring and producing end face bolt-hole patterns. In conventional steady rest some limitations are found such as 1) High clamping times because of manual adjustments. 2) Low degree of centering accuracy and repeatability. 3) Difficult to integrate with the CNC machine control. 4) Requires more space for mounting on machine. 5) Can be used only for fixed operation. Figure No. 1 Conventional Steady Rest Figure No. 1 shows the conventional type of steady rest. It is clamped in the required position on the bed by base clamp 1 and bolt 2. Then the work is centered by means of adjustable jaws 3. The jaws are locked by screws. Therefore there is a need of the new type of clamping device which will overcome above problems with improved production efficiency. In this project a self centering steady rest for providing additional support to a work piece. The self centering steady rest comprising a cylinder mounted on the self centering steady rest, the cylinder is driven either by hydraulic or pneumatic means. A piston is inside the cylinder, and the cylinder enables movement of the piston. A central cam lever is connected to piston at one end enabling forward movement of the central cam lever when piston moves forward and backward movement when the piston rod moves back. A couple of arms are configured to move up the slope of a cam when the central cam lever moves forward to provide support to the work piece, and the arms move back down the slope of the cam when the central cam lever moves backward releasing the work piece. A. Purpose of Designing the Steady Rest A task for designing a steady rest such that it could machine 10,000 components per month what studied. Table 1 : Operation time in minutes by conventional method Steps Current Machining Time taken Operation Process (Minute) 1 Rod loading to chuck 1 2 Clamping with 5 conventional steady rest 3 1 st machining cut 1 4 Removal of steady rest 1 5 Clamping with steady rest 5 to another location 6 2 nd machining cut 1 7 Removal of job from lathe 1 Total 15 Total time required for machining 10,000 components is 167 days. So to complete machining of 10000 components Batch one machine will require 167 days, so to complete the task within one month i.e. 26 working days, we will require 167/26= 6.4 ~ 7 machines and because of it total manufacturing cost become Rs. 92022 which is more. So to overcome the above draw backs it should be, Design the self-centering steady rest for supercut-6 CNC machine. Show that this design is safe for manufacturing by using CAD & FEA. Reduce the setup time and complete the job in specified time limit and only on two supercut-6 CNC turning machine. Basic Steps involving in the project 1) Mechanism layout of steady rest. 2) Force calculations. 3) Design calculations for each part 4) Drive selections Hydraulic, Pneumatic or other. 5) Lubrication. 6) CAD Modeling in Solid works. 7) Mechanism Simulation. 8) FEA Analysis to validate the Design in Ansys. II. GENERAL CONSIDERATIONS IN WORKHOLDER DESIGN AND SELECTION Work holder design and selection start in general with an analysis of the workpiece and the manufacturing operation to be performed. The shape, material, and tolerances of the workpiece for the manufacturing operation are specified by the product engineer. Certain functional and appearance requirements must be met. The manufacturing engineer specifies the most economical steps in the manufacturing process. 15

A. Physical Characteristics The first consideration is the condition of the work piece to be held by the proposed work holder for the next operation to be performed. This include the physical characteristics of the workpiece, i,e. whether it is round, irregular, large, heavy, weak or strong sections and also operations to be performed. This will indicate whether the workpiece has to remain stationary or be moved along a definite path relative to the cutting tool. Different machine tools have means to provide the needed motions for the workpiece and the cutting tools. Most of these motions are straight-line or rotary, or combinations of both. There are operations that to all appearance seem to be producing and requiring an irregular path of the cutting tool on the workpiece. This seemingly irregular path is often the resultant of the combination of several straight-line and rotary motions. Except for speed consideration and the degree of simplicity obtainable, only the relative motion of the cutting tool to the workpiece is of importance [3]. For instance, the turning of flanges on a valve body or a flanged T pipe fitting could be performed with either the work-piece or the tool revolving (Figure No.2). Figure No2. Machining a valve body either by workpiece rotation or tool rotation Whether the workpiece or the cutting tool moves in a straight line, revolves or moves in some combination of both, design requires careful coordination of work holder to the workpiece, and the work holder to the machine tool. Operations in which the workpiece revolves require great care in the attachment of the work holder to the machine tool and the means of actuation of the work holder. Unbalanced masses in the work holder and the workpiece must be minimized by proper balancing. This is particularly true in high speed applications such as turning with tungsten carbide, diamond, and ceramic cutting tools. Cutting Forces: On all operations, the magnitude and the direction of the forces produced by the material-removing operation determine the necessary holding forces. Cutting forces must be held within limits; so that the part itself cannot be distorted to an amount that would affect the part itself cannot be distorted to an amount that would affect the required accuracy. Rigidity and strength of the workpiece limit the applicable holding forces and the speed and amount of metal removal per unit of time[2]. A thin-walled part may not be able to sustain heavy cutting and holding forces without distortion or damage. Machine Selection: Workpiece weight and size influence what type and size of machine tool can or should be used for a particular operation. The combined weight of the work holder and the part must be carefully matched to the capacity of the ways, bed, table, and spindles of the machine tools. Excessive weight may cause distortion in the machine tool and produce inaccurate work. Mounting of the Work holder to the Machine Tool: The mounting or attachment of the work holder or the workpiece to the machine tool should be so arranged that the forces produced in the material-removing operation are absorbed by the strongest and most rigid parts of the machine tool. The cutting forces should tend to hold the work holder down against the bed of the machine rather than lift it away. The Projection between the point at which the cutting forces that act parallel to the bed, table and face plate should be applied as close to it as possible. Cutting forces should never be permitted the advantages of a large lever arm which could increase the tendency to loosen or pry away the work piece and the work holder from their attachment. This is in contrast to the holding forces, where the effect of a large lever arm or mechanical advantages is always desirable. Physical Characteristics of the Workpiece: Cross-section: - Round Clamping range: - 6 to 55 mm diameter of the work piece. Turning length between centre (L) = 375 mm (Assumed) Volume, V = ( π d 2 4 max ) x L Taking maximum work diameter, d max = 58 mm 16 V = 990779.78 mm³ Considering the material of higher strength i.e. steel Specific weight of steel (w) = 7.85 x 10-6 kgf/ mm³ (CMTI Handbook Table 294)[7]. Maximum weight of workpiece (W) = w x V = 7.85 x 10-6 x 990779.78 = 7.78 kgf = 76.32 N Mounting of the Steady Rest on the Machine Tool: Steady rest, can be mounted in any angular position square to the axis to lathe. The most practical method is for the customer himself to make a suitable mounting bracket to suit both the steady and the machine [6]. Since for Supercut-6 CNC Machine the slant angle of the bed to the horizontal is 35 degree the recommended

position for the steady rest is inclined 35 degree to the horizontal axis. Steady rest can be screwed to the bracket which is fixed to the tailstock guide ways. Figure No. 3 shows the mounting of the steady rest on the supercut-6 CNC machine. Figure No.3. Mounting of Steady Rest B. Drive Layout: There should be some suitable drive to hold the steady rest against the work piece. We have following choices:- 1. Hydraulic 2. Pneumatic Because of the following reasons, we selected the hydraulic drive. 1. Hydraulic connection requires smaller cylinder sizes. 2. It provides large forces for actuation. 3. Accessories for hydraulic connection are easily available. Considering all above factors it is decided that hydraulic drive is more economical. C. Mechanism Layout: We have to design a mechanism for efficient operation of steady rest for which we have to consider the following requirements of the design. 1. Steady rest is the clamping device hence requires frequent closing and opening. 2. Assembly should be compact in size. 3. There should be rapid and automatic self centering over the full clamping range. 4. Manufacturing processes required to fabricate the parts should be available in the factory. Therefore a mechanism consisting of a linear cam with roller follower is selected. Components of the Mechanism: 1. Cam: - It is linear type of the cam. Outer arms are displaced in relation to the movement of the centre arm by cam. It is mounted at the base of the central arm. 2. Cam Roller: - It is the roller follower which traces the path on the cam profile. It is located to the upper end of the outer arm. 3. Central Arm: - It provides support for the cam. At its upper end, there is piston and at lower end there is roller it moves linearly according to the force applied by the piston. 4. Outer Arm :- These are two pivoted arms when central arm moves downwards, they moves towards the centre of the workpiece and when central arms moves upwards they moves away from the centre of the workpiece. 5. Rollers: - There are three rollers which rigidly support the workpiece at points on the periphery approximately 120 degree apart. Each roller moves with exactly the same tangential stroke towards the axis over the full clamping range in this way the workpiece is accurately centered and supported. 6. Spring: - It is the helical extension type of spring. It keeps the cam and cam roller, always in contact. Figure No.4. General layout of mechanism D. Lubrication Arrangements Two methods of lubrication are suggested for proper working of the steady rest. 1. Manual lubrication. 2. Centralized lubrication. 1. Manual Lubrication: - This is the simple and low cost method of lubrication. The lubrication points and rollers are supplied with lubrication grease via the grease nipples and the grease gun. Lubrication intervals depending on the working conditions normally every 4 to 8 operating hours. 2. Centralized Lubrication: - In this type of lubrication the steady rest is provided with a lubricating connection at the area side to supply the lubrication points and rollers with lubrication oil via integral dosing elements. Minimum operating pressure of the lubrication pump is 12 bars and the maximum operating pressure is 30 bars. Suggested lubrication oil is HLP 46 DIN 51502. Figure No.5 shows the centralized lubrication for steady rest. 17

Figure No5. Centralized lubrication III. GEOMETRICAL CONSTRUCTION OF THE CAM PROFILE Design of the cam profile is important for obtaining required clamping range of the steady rest. We used the geometrical approach for this purpose [8]. 1. Draw the cam and follower mechanism to the scale. 2. Draw the circle of maximum work piece diameter 58 mm tangents to the circles of three rollers. 3. Draw another circle having centre at the pivot of the outer arm and radius = PQ. The roller traces this circle during the motion of the outer arm. 4. Draw the line RS inclined 30 degree to the horizontal. The intersection of this line with outer diameter of cam roller circle is the first point on the cam profile. 5. Draw another circle of diameter say 56 mm with centre 0. 6. Now the outer arm and central arm with cam is moved in such a way that the roller circles are again tangents to the new circle. 7. For this new position of the outer arm, a line is drawn from previous point tangent to the cam roller circle. The tangent point of this line on the cam roller circle is the second point on the cam profile. 8. This procedure is repeated up to the circle of minimum work piece diameter 6 mm. Finally we get the full curve on the cam which is the required cam profile. Maximum bending moment, M b = 645 x 27 = 17415 kgf.mm = 170667 N.mm We know that, f b = M b x y / I ------------------------------(1) For rectangular cross section [8] I= bh³ / 12, y = h/2 Where b = width of outer arm h = thickness of outer arm. Taking h = 31 mm Putting in equation (1) we get b = 5.43 mm (Taking b = 30 mm for safer side.) 2. Central arm:- Material - En8 Maximum load on central arm = 7704.8 N Allowable compressive stress f t = 147 N/mm² f b = load/area [9]. 147 = 7704.8/ h x b Where h = thickness of central arm b = width of central arm Taking h = 31 mm b = 1.69 mm Taking b = 25 mm for safer side. Figure No.6. Geometrical construction of the CAM Profile 1. Outer arm:- IV. DESIGN CALCULATIONS Material - 16 MnCr5 Allowable bending stress f b = 196.2 N/mm² Figure No.7-3-D Model Of Steady Rest V. ANALYSIS IN ANSYS Table 3 : Model > Static Structural > Solution > Results Object Name State Scope Geometry Definition Equivalent Solved All Bodies Maximum Shear Total Deformation 18 Type Equivalent (von- Maximum Shear Total Deformation

Display Time Results Minimum Maximum Mises) End Time 1.7207 204.73 0.98552 106.16 90473 mm 9.4546e+005 mm Figure No. 8-- Equivalent (von-mises) VI. CONCLUSION Finally, the conventional steady rest has been studied and due to its various limitations there is a need for design of new type of steady rest for Supercut-6 CNC turning machine the slant angle of the bed to the horizontal is 35 degree the recommended position for the steady rest is inclined 35 degree to the horizontal axis. It is clear from the results that the outer arm is working same under given operating conditions. The maximum bending stress value is 204 which is below the yield strength. REFERENCES [1] Bharat Joshi, John George, Brian Rose, Yin Chen, "Failure Analysis and Design Optimization Of the Steady Rest Hanger Rod Pipe Assembly" ASM International 2013. [2] Johannes Schmalz and Gunther Reinhart, Automated Selection and Dimensioning of Gripper Systems, Procedia CIRP 23 ( 2014 ),pp 212-216 [3] N. Acherkan, Machine Tool Design, Vol.1. MIR Publishers Moscow, pp 49-50. [4] American society of Tool and Manufacturing Engineers, Fundamentals of Tool Design, ASTME Publications,pp 137-146. [5] Paul J. Owsen, Multi-purpose Steady Rest, US Patent, oct 1985, P No-4546681. [6] Company Manual Supercut 6 CNC Lathe. [7] Central Machine Tool Institute, Machine Tool Design Handbook, Tata-McGraw Hill Publications, pp 805-830. [8] V.B. Bhandari, Design of Machine Elements, Tata-McGraw Hill Publications, pp 323-335 [9] R.S. Khurmi, G.K. Gupta, Machine Design, Eurasia Publishing House (Pvt) Ltd, pp 67-74, 283-295, 648-649 [10] Instruction Manual Steady Rest, KEL. 19