Production Engineering Textbook PDF

 A TEXTBOOK OF PRODUCTION ENGINEERING P.C. SHARMA B.Sc. Engg. (Mech.) (Hons.) M.Sc. Eng...

 A TEXTBOOK OF PRODUCTION ENGINEERING P.C. SHARMA B.Sc. Engg. (Mech.) (Hons.) M.Sc. Engg. (Mech.) (Distinction) Ph.D., I.I.T. (Delhi), LMISME, MISTE Visiting Professor SUS College of Engineering and Technology Tangori (Mohali), Punjab Formerly of Punjab Engineering College CHANDIGARH S. CHAND & COMPANY LTD. (An ISO 9001 : 2000 Company) RAM NAGAR, NEW DELHI - 110 055 Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  S. CHAND & COMPANY LTD. (An ISO 9001 : 2000 Company) Head Office: 7361, RAM NAGAR, NEW DELHI - 110 055 Phone: 23672080-81-82, 9899107446, 9911310888 Fax: 91-11-23677446 Shop at: schandgroup.com; e-mail: [email protected] Branches : AHMEDABAD : 1st Floor, Heritage, Near Gujarat Vidhyapeeth, Ashram Road, Ahmedabad - 380 014, Ph: 27541965, 27542369, [email protected] BANGALORE : No. 6, Ahuja Chambers, 1st Cross, Kumara Krupa Road, Bangalore - 560 001, Ph: 22268048, 22354008, [email protected] BHOPAL : 238-A, M.P. 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[email protected] CHANDIGARH : S.C.O. 2419-20, First Floor, Sector - 22-C (Near Aroma Hotel), Chandigarh -160 022, Ph: 2725443, 2725446, [email protected] CHENNAI : 152, Anna Salai, Chennai - 600 002, Ph: 28460026, [email protected] COIMBATORE : Plot No. 5, Rajalakshmi Nagar, Peelamedu, Coimbatore -641 004, (M) 09444228242, [email protected] CUTTACK : 1st Floor, Bhartia Tower, Badambadi, Cuttack - 753 009, Ph: 2332580; 2332581, [email protected] DEHRADUN : 1st Floor, 20, New Road, Near Dwarka Store, Dehradun - 248 001, Ph: 2740889, 2740861, [email protected] GUWAHATI : Pan Bazar, Guwahati - 781 001, Ph: 2738811, 2735640 [email protected] HYDERABAD : Sultan Bazar, Hyderabad - 500 195, Ph: 24651135, 24744815, [email protected] JAIPUR : A-14, Janta Store Shopping Complex, University Marg, Bapu Nagar, Jaipur - 302 015, Ph: 2719126, [email protected] JALANDHAR : Mai Hiran Gate, Jalandhar - 144 008, Ph: 2401630, 5000630, [email protected] JAMMU : 67/B, B-Block, Gandhi Nagar, Jammu - 180 004, (M) 09878651464 KOCHI : Kachapilly Square, Mullassery Canal Road, Ernakulam, Kochi - 682 011, Ph: 2378207, [email protected] KOLKATA : 285/J, Bipin Bihari Ganguli Street, Kolkata - 700 012, Ph: 22367459, 22373914, [email protected] LUCKNOW : Mahabeer Market, 25 Gwynne Road, Aminabad, Lucknow - 226 018, Ph: 2626801, 2284815, [email protected] MUMBAI : Blackie House, 103/5, Walchand Hirachand Marg, Opp. G.P.O., Mumbai - 400 001, Ph: 22690881, 22610885, [email protected] NAGPUR : Karnal Bag, Model Mill Chowk, Umrer Road, Nagpur - 440 032, Ph: 2723901, 2777666 [email protected] PATNA : 104, Citicentre Ashok, Govind Mitra Road, Patna - 800 004, Ph: 2300489, 2302100, [email protected] PUNE : 291/1, Ganesh Gayatri Complex, 1st Floor, Somwarpeth, Near Jain Mandir, Pune - 411 011, Ph: 64017298, [email protected] RAIPUR : Kailash Residency, Plot No. 4B, Bottle House Road, Shankar Nagar, Raipur - 492 007, Ph: 09981200834, [email protected] RANCHI : Flat No. 104, Sri Draupadi Smriti Apartments, East of Jaipal Singh Stadium, Neel Ratan Street, Upper Bazar, Ranchi - 834 001, Ph: 2208761, [email protected] VISAKHAPATNAM : Plot No. 7, 1st Floor, Allipuram Extension, Opp. Radhakrishna Towers, Seethammadhara North Extn., Visakhapatnam - 530 013, (M) 09347580841, [email protected] © 1982, P.C. Sharma All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the Publishers. F irst E d it io n 1 9 8 2 S u b s e q u e n t E d itio n s a n d R e p rin ts 1 9 8 6 , 8 8 , 8 9 , 9 0 , 9 2 ( T w ic e ) , 9 4 , 9 5 , 9 6 , 9 7 ( T w ic e ) , 9 9 , 2 0 0 0 , 2 0 0 2 , 2 0 0 3 , 2 0 0 5 , 2 0 0 6 ( T w ic e ) , 2 0 0 8 R e p rin t w ith C o rre c t io n s 2 0 0 9 IS B N : 8 1 -2 1 9 -0 1 1 1 -1 C o d e :10A 038 printed in india By Rajendra Ravindra Printers Pvt. Ltd., 7361, Ram Nagar, New Delhi -110 055 and published by S. Chand & Company Ltd., 7361, Ram Nagar, New Delhi -110 055. Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  PREFACE TO THE ELEVENTH EDITION The author feels very happy to present the revised edition of the book to the readers. This standard treatise on ‘‘Production Engineering’’ was first publishing in 1982. During the last about 26 years, it has kept a close rapport with the readers. The publication of the new edition has given an opportunity to me to incorporate the latest developments in the field in the text. Additional material has been included in chapters : 1, 2, 4, 5, 9, 10, 11, 14, 15, 20, 21 and 24. While every attempt has been made to ensure that no errors (printing or otherwise) enter the text, the possibility of these creeping into the text is always these. I shall be grateful to the readers to bring these errors to my notice so that these may be rectified in the subsequent editions. AUTHOR PREFACE TO THE TENTH EDITION The author is grateful to the readers for the tremendous response to the Ninth edition of the book. The author has done his best to remove all the errors in the text. The author shall feel grateful if the readers point out the errors in the text, which might have been overlooked. In the present revised edition of the book, about 200 problems from various competitive ex- aminations (GATE, IES, IAS) have been included. The author does hope that with this, the utility of the book will be further enhanced. AUTHOR (iii) Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/ Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  CONTENTS Chapters Pages 1. JIGS AND FIXTURES 1 – 68 1.1 General (1); 1.2 Locating and clamping (2); 1.3 Design principles common to jigs and fixtures (17); 1.4 Drilling jigs (20); 1.5 Milling fixtures (31); 1.6 Lathe fixtures (34); 1.7 Grinding fixtures (35); 1.8 Broaching fixtures (36) 1.9 Assembly fixtures (37); 1.10 Inspection fixtures (39); 1.11 Boring fixtures (40); 1.12 Planing and Shaping fixtures (41); 1.13 Indexing jigs and fixtures (41); 1.14 Automated jigs and fixtures (43); 1.15 Fundamentals of jig and fixture design (47) 1.16 Jig and fixture construction (49); 1.17 Materials for Jigs and Fixtures (50); 1.18 Tolerance and Error analysis (53); 1.19 Analysis of Clamping Forces (61); PROBLEMS (66). 2. PRESS TOOL DESIGN 69 – 146 2.1 General (69); 2.2 Press operations (69); 2.3 Press working equipment (72); 2.4 Press selection (76); 2.5 Press working terminology (76); 2.6 Types of dies (78); 2.7 Principle of metal cutting (80); 2.8 Clearance (81); 2.9 Cutting forces (83); 2.10 Methods of reducing cutting forces (84); 2.11 Minimum diameter of piercing (87); 2.12 Blanking die design (88); 2.13 Piercing die design (108); 2.14 Pilots (110); 2.15 Drawing dies (111); 2.16 Bending dies (116); 2.17 Design procedure for progressive dies (127); 2.18 Dimen- sions of Back-post die-sets (128); 2.19 Materials and manufacture of sheet metal working dies (129); 2.20 Solved examples (132); PROBLEMS (144). 3. FORGING DIE DESIGN 147 – 193 3.1 General (147); 3.2 Forging equipment (149); 3.3 Design of a forging (156); 3.4 Die- design for Drop forging and Press forging (163); 3.5 Die-Design for Machine forging (171); 3.6 Determination of stock size (174); 3.7 Selection of forging equipment (176); 3.8 Selection of sizes of forging equipment (177); 3.9 Die-inserts (179); 3.10 Solved examples (180); 3.11 Tools for flash trimming and hole piercing (181); 3.12 Materials and manufacture of forging dies (183); 3.13 Die-Manufacture (183); 3.14 Electro-re- moval processes (189); 3.15 Cast dies (190); 3.16 Resinking of dies (190); 3.17 Size of Die Blocks (192); 3.18 I.S. Code (192); PROBLEMS (192). 4. COST ESTIMATION 194 – 235 4.1 Definition (194); 4.2 Cost accounting or costing (194); 4.3 Elements of cost (195); 4.4 Estimation of cost elements (198); 4.5 Methods of cost estimating (204); 4.6 Data requirement for cost estimating (205); 4.7 Steps in making a cost estimate (205); 4.8 Chief factors in cost estimating (206); 4.9 Numerical examples (206); 4.10 Calcula- tion of machining Times (211); 4.11 Estimation of total unit time (231) PROBLEMS (234). 5 ECONOMICS OF TOOLING 236 – 297 5.1 Introduction (236); 5.2 Machine tool replacement (244); 5.3 Mathematical analysis (265); 5.4 Economics of small tool selection (269); 5.5 Break even point analysis (273); 5.6 Economic lot size (286); PROBLEMS (292). 6. PROCESS PLANNING 298 – 320 6.1 General (298); 6.2 Contents of a process plan (299); 6.3 Process operations (299); 6.4 Steps of process planning (301); 6.5 How process plans are expressed (307); 6.6 Planning and tooling for low cost processing (308); 6.7 Solved examples (314); Problems (319). (v) Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  (vi) 7. TOOL LAYOUT FOR CAPSTANS AND TURRETS 321 – 339 7.1 General (321); 7.2 Types of turret lathes (322); 7.3 Main parts (323); 7.4 Work- holding equipment (326); 7.5 Standard equipment and tools (328); 7.6 Machine opera- tions (330); 7.7 Advantages of turret lathe (331); 7.8 Tool layout (331); 7.9 Bar stock feeding mechanism (332); 7.10 Solved examples (334); 7.11 Programme controlled turret lathes (338); PROBLEMS (338). 8. TOOLLAYOUT FORAUTOMATICS 340 – 358 8.1 Automatic lathes (340); 8.2 Classification of Automatic machines (340); 8.3 Classification of semi-automatics (348); 8.4 Setting up of automatics and semi- automatics (348); 8.5 Tooling layout and operation sheet (348); 8.6 Cam design (350); 8.7 Tool layout for automatic screw machine (351); 8.8 Programmed automatic lathes (356); 8.9 Bar stock feeding (356); PROBLEMS (357). 9. LIMITS, TOLERANCES AND FITS 359 – 385 9.1 General (359); 9.2 Terminology for limits and fits (359); 9.3 Meaning of limits (364); 9.4 General limits of tolerance (364); 9.5 Limit systems (368); 9.6 Selective assembly (376); 9.7 Solved examples (376); PROBLEMS (383). 10. GAUGESAND GAUGE DESIGN 386 – 413 10.1 Introduction (386); 10.2 Plain gauges (386); 10.3 Design of limit gauges (390); 10.4 Manufacture of limit gauges (398); 10.5 Choice of limit gauges (399) 10.6 Thread or screw gauges (401); 10.7 Advantages of limit gauges (402); 10.8 Limits of limit gauges (402); 10.9 Care of gauges (403); 10.10 Other types of gauges (404); 10.11 Solved examples (407); PROBLEMS (411). 11. SURFACE FINISH 414 – 439 11.1 Introduction (414); 11.2 Elements of surface roughness (415); 11.3 Evaluation of surface roughness (417); 11.4 Representation of surface roughness (422); 11.5 Rela- tionship of surface roughness to production methods (423); 11.6 Effect of surface roughness on the performance of machine parts (425); 11.7 Measurement of surface roughness (426); 11.8 Surface finishing processes (428); 11.9 Superfinishing (435); 11.10 Polishing (435); 11.11 Numerical Examples (436); PROBLEMS (437). 12. MEASUREMENT 440 – 476 12.1 General (440); 12.2 Calipers (441); 12.3 Vernier calipers (442); 12.4 Vernier height gauge (444); 12.5 Vernier depth gauge (445); 12.6 Micro-meter caliper (445); 12.7 Slip gauges (448); 12.8 Checking straightness and flatness (449); 12.9 Checking squareness (453); 12.10 Dial gauge Indicator (453); 12.11 Surface gauge (455); 12.12 Checking of parallelism (455 ); 12.13 Measurement of angles (456); 12.14 Telescopic gauge (462); 12.15 Small hole gauges (462); 12.16 Feeler gauges (462); 12.17 Radius gauges (463); 12.18 Comparators (463); 12.19 Optical measuring instruments (470); PROBLEMS (475). 13. ANALYSIS OF METAL FORMING PROCESSES 477 – 528 13.1 Theoretical basis for metal forming (477); 13.2 Classification of metal forming processes (483); 13.3 Effect of variables on metal forming processes (486); 13.4 Meth- ods of analysis of manufacturing processes (487); 13.5 Open die forging (488); Closed die forging; 13.6 Rolling (496); 13.7 Drawing (Wire, Rod, Tube) (505); 13.8 Extrusion (512); 13.9 Solved examples (515); PROBLEMS (527). 14. THEORY OF METALCUTTING 529 – 585 14.1 Introduction (529); 14.2 The mechanics of chip formation (529); 14.3 Single point cutting tool (530); 14.4 Methods of machining (537); 14.5 Types of chips (537); 14.6 Determination of shear angle (539); 14.7 Determination of undeformed chip thickness Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  (vii) (541); 14.8 Force relations (543); 14.9 Energy considerations is metal cutting (548); 14.10 Oblique cutting (549); 14.11 Tool wear and tool life (550); 14.12 Economics of metal cutting (556); 14.13 Machineability (559); 14.14 Surface roughness (561); 14.15 Solved examples (563); PROBLEMS (577). 15 DESIGNAND MANUFACTURE OF CUTTINGTOOLS 586 – 633 15.1 Types of cutting tools (586); 15.2 General problems of cutting tool design (589); 15.3 Single point cutting tools (589); 15.4 Milling cutters (592); 15.5 Broach design (598); 15.6 Drills (604); 15.7 Reamers (611); 15.8 Form tools (618); 15.9 Combination tools (620); 15.10 Manufacture of cutting tools (622); PROBLEMS (631). 16 GEAR MANUFACTURING 634 – 655 16.1 Introduction (634); 16.2 Materials (634); 16.3 Methods of manufacture (635); 16.4 Gear cutting by milling (636); 16.5 Gear cutting by single point formed tool on shaper/planer (638); 16.6 Broaching (638); 16.7 Shear speed process (638); 16.8 Gear planing (638); 16.9 Gear shapers (640); 16.10 Gear hobbing (641); 16.11 Bevel gear generating (643); 16.12 Miscellaneous gear manufacturing methods (645); 16.13 Gear finishing operations (645); 16.14 Gear inspection (646); 16.15 Design of gear hob (651); PROBLEMS (654). 17 THREAD MANUFACTURING 656 – 672 17.1 Introduction (656); 17.2 Casting (656); 17.3 Thread chasing (656); 17.4 Thread rolling (658); 17.5 Die-threading and tapping (660); 17.6 Thread milling (666); 17.7 Thread grinding (668); 17.8 Thread measurement and inspection (668); PROB- LEMS (672). 18 DESIGN OFMACHINE TOOLELEMENTSAND MACHINE TOOL TESTING 673 – 692 18.1 Design of machine tool elements (673); 18.2 Machine tool testing (683); PROBLEMS (691). 19 MACHINE TOOLINSTALLATION AND MAINTENANCE 693 – 709 19.1 Machine tool installation (693); 19.2 Machine tool maintenance (698); 19.3 Ma- chine tool lubrication (701); 19.4 Reconditioning of machine tools (703); 19.5 Safety in machine tools (703); PROBLEMS (708). 20 DESIGN OF PRODUCT FOR ECONOMICAL PRODUCTION 710 – 727 20.1 Introduction (710); 20.2 Suggestions for designing for production (710); 20.3 Design for Manufacturability (725); PROBLEMS (727). 21 STATISTICALQUALITY CONTROL 728 – 765 21.1 Introduction (728); 21.2 Statistical tools for quality control (729); 21.3 Frequency distribution (729); 21.4 Control charts (732); 21.5 Sampling inspection (739); 21.6 Average outgoing quality limit (AOQL) (748); 21.7 Total Quality Management (TQM) (754); PROBLEMS (763). 22 KINEMATICS OF MACHINE TOOLS 766 – 804 22.1 Introduction (766); 22.2 Drives in machine tools (767); 22.3 Stepless mechanical drives (795); 22.4 Hydraulic Drives (797); 22.5 Electrical drives (801); PROBLEMS (803). 23 PRODUCTION PLANNINGAND CONTROL 805 – 847 23.1 Introduction (805); 23.2 Types of Productio (808); 23.3 Sales forecasting (810); 23.4 Economical batch quantity (815); 23.5 Production planning and control functions (818); 23.6 Production planning and control for different types of production (828); 23.7 Inventory control (830); 23.8 Network techniques (835); PROBLEMS (844). 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To remove this line, buy a license at: http://www.software602.com/  (viii) 24 MANUFACTURING SYSTEMS AND AUTOMATION 848 – 867 24.1 Introduction (848); 24.2 Material movement (848); 24.3 Classification of manufac- turing systems (855); 24.4 Characteristics of manufacturing systems (863); 24.5 Pro- duction systems (863); 24.6 Automation (864); PROBLEMS (866). 25 COMPUTER INTEGRATED MANUFACTURING 868 – 888 25.1 Introduction (868); 25.2 Computer-Aided Design (CAD) (868); 25.3 Computer- Aided Manufacturing (CAM) (873); 25.4 Computer-Integrated manufacturing (CIM) (879); 25.5 Computerized Information System (880); 25.6 The Automatic factory (882); 25.7 Simultaneous engineering (883); 25.8 Rapid prototyping technique (883); 25.9 Enterprise Resource Planning (886); PROBLEMS (887). 26 PLANT LAYOUT 889 – 909 26.1 Introduction (889); 26.2 Definition (889); 26.3 Necessity of plant layout (889); 26.4 Importance of plant layout planning (890); 26.5 Objectives of plant layout (890); 26.6 Advantages of Good Plant Layout (891) 26.7 Factors influencing plant layout (892); 26.8 Types of plant layout (893); 26.9 Types of flow patterns (890); 26.10 Prin- ciples of plant layout (898); 26.11 Steps in plant layout planning (898); 26.12 Visualisation aids of the layout engineer (906); 26.13 Single storey versus Multi-storey buidling (906) 26.14 Computerized techniques for planning a plant layout (907) 26.15 Effect of “Push” or “Pull” type of Repetitive manufacturing system on plant layout (908); PROBLEMS (908). 27 PRODUCTIONAND PRODUCTIVITY 910 – 918 27.1 Definition (910); 27.2 Difference between production and productivity (911); 27.3 Importance of productivity (911); 27.4 Measurement of productivity (911); 27.5 Factors affecting productivity (913) 27.6 Techniques for improving productivity (914); PROBLEMS (918). APPENDIX-I 919 – 923 APPENDIX-II : Process Planning Sheets for Some Mechanical Components 924 – 932 APPENDIX-III : Cutting Force Measurement : Dynamometery 933 – 938 APPENDIX-IV : Gear Manufacturing 939 – 944 APPENDIX-V : Problems from Competitive Examinations (GATE, IES, IAS) 945 – 979 INDEX 980 – 983 Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  1 JIGS AND FIXTURES 1.1. GENERAL Jigs and fixtures are special purpose tools which are used to facilitate production (machining, assembling and inspection operations) when workpieces are to be produced on a mass scale. The mass production of workpieces is based on the concept of interchangeability according to which every part will be produced within an established tolerance. Jigs and fixtures provide a means of manufacturing interchangeable parts since they establish a relation, with predetermined tolerances, between the work and the cutting tool. They eliminate the necessity of a special set up for each individual part. Once a jig or fixture is properly set up, any number of duplicate parts may be readily produced without additional set up. Hence jigs and fixtures are used : 1. To reduce the cost of production, as their use eliminates the laying out 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 partially automates the machine tool. 11. Their use improves the safety at work, thereby lowering the rate of accidents. A jig may be defined as a device which holds and positions the work, locates or guides the cutting tool relative to the workpiece and usually is not fixed to the machine table. It is usually lighter in construction. 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. Jigs are used on drilling, reaming, tapping and counterboring operations, while fixtures are used in connection with turning, milling, grinding, shaping, planning and boring operations. Jigs and fixtures, because of their functions and advantages are also called “Production Devices”. To facilitate interchangeability, we need “Inspection Devices, i.e., the different types of Gauges (Refer to Chapter 9 and 10)”. 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. 1 Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  2 A Textbook of Production Engineering 2. Locating elements. 3. Clamping 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. Locating pins are stops or pins which are inserted in the body of jig or fixture, against which the workpiece is pushed to establish the desired relationship between the workpiece and the jig or fixture. To assure interchangeability, the locating elements are made from hardened steel. The purpose of clamping elements is to exert a force to press a workpiece against the locating elements and hold it there in opposition to the action of the cutting forces. In the case of a jig, a hardened bushing is fastened on one or more sides of the jig, to guide the tool to its proper location in the work. However, in the case of a fixture, a target or set block is used to set the location of the tool with respect to the workpiece within the fixture. Most jigs use standard parts such as drill bushings, screws, jig bodies and many other parts. Fixutres are made from grey cast iron or steel by welding or bolting. Fixtures are usually massive bodies because they have to withstand large dynamic forces. Because the fixtures are in between the machine and the workpiece, their rigidity and the rigidity of their fastening to the machine table are most important. Jigs are positioned or supported on the machine table with the help of feet which slide or rest on the machine table. If the drill size is quite large, either stops are provided or the jig is clamped to the machine table to withstand the high drilling torque. Fixtures are clamped or bolted to the machine table. A simple jig and a fixture are shown in Fig. 1.1. Drill Bush Cutter Work Table Work Locating pin Key T-slot (a) Jig (b) Fixture Fig. 1.1. A Simple Jig and a Fixture. According to the degree of mechanization and automation, jigs and fixtures are classified as : (a) hand operated (b) power (c) semi-automatic (d) automatic. 1.2. LOCATING AND CLAMPING The question of properly locating, supporting, and clamping the work is important since the overall accuracy is dependent primarily on the accuracy with which the workpiece is consistently located within the jig or fixture. There must be no movement of the work during machining. Locating refers to the establishment of a proper relationship between the workpiece and the jig or fixture. The function of clamping is to exert a force to press the workpiece against the locating surfaces and hold it there against the action of cutting forces. 1.2.1. Principle of Location. In order to study the complete location of a workpiece within a jig or fixture, let us consider a workpiece in space (Fig. 1.2). The workpiece is assumed to have true and flat faces. In a state of freedom, it may move in either of the two opposed directions Fig. 1.2. Workpiece in Space. along three mutually perpendicular axes, XX, YY and Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  Jigs and Fixtures 3 ZZ. These 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 in space. To confine the workpiece accurately and positively in another fixed body (jig or fixture), the movement of the workpiece in any of the twelve degrees of freedom must be restricted. For this, let us refer to Fig. 1.3. X D F Y Y E X Z Z E F Y Y X X A C B Z Z Fig. 1.3. Workpiece Located in a Fixed Body. (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. In this way, the five degrees of freedom 1, 2, 3, 4 and 5 have been arrested. (b) Two more pins D and E are inserted in the fixed body, in a 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 and 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 and 12 are still free. To restrict these, three more pins, one for each of these degrees of freedom are needed. But this will completely enclose the workpiece making its loading and unloading into the jig or fixture impossible. Due to this, these remaining three degrees of freedom may be arrested by means of a clamping device. The above method of locating a workpiece in a jig or a fixture is called the “3-2-1” principle or “six point location” principle. 1.2.2. Principles of pin location. 1. The principle of minimum locating points. According to this principle, only the minimum locating points should be used to secure location of the workpiece in any one plane. Considering the “3-2-1” principle, there pins are used in the base of the fixed body. Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  4 A Textbook of Production Engineering This is due to the reason that this is the minimum number of locating points through which a plane can be drawn on which the workpiece will seat. The workpiece may rock and get strained if more than three locating points are provided. Now considering the second plane, it is clear that if one locating point is provided, the workpiece will swivel about this point, but not if there are two such location points. These two locating points establish a line parallel to the first plane. With the workpiece located against a plane and a line, it has only one direction of movement in third plane. Therefore, one locating point is sufficient in the third plane to prevent this movement. 2. The principle of mutually perpendicular planes. The “3-2-1” principle can also be put as : a workpiece may be fully located by supporting it against three points in one plane, two points in second plane and one point in a third plane. These three planes are not parallel and are preferably perpendicular to one another. If the locating surfaces are not perpendicular to one another, the following two difficulties will arise : (i) The workpiece will tend to lift due to the wedging action between the two locating surfaces. (ii) A large error in the movement of the workpiece introduced due to the displacement of a locating point or a particle (chip or dirt) adhering to it, as is clear from Fig. 1.4. The difference of resulting error and introduced error, that is, the projection factor is zero Work piece when the locating surfaces are normal and increases as the angle between them becomes more acute. 3. The principle of extreme posi- Chip or particle of thickness 't' tions. The locating points should be adhering to a locating point placed as far away from one another as causing mislocation of work piece possible, to achieve the greatest accu- racy in location. 1.2.3. Locating devices. Pins of Resulting error various designs and made of hardened Introduced error 't' steel are the most common locating Projection error devices used to locate a workpiece in a Jig or fixture jig or fixture. The shank of the pin is press fitted or driven into the body of Fig. 1.4. Principle of Mutually Perpendicular Planes. the jig or fixture. The locating diameter of the pin is made larger than the shank to prevent it from being forced into the jig or fixture body due to the weight of the workpiece or the cutting forces. Depending upon the mutual relation between the workpiece and pin, the pins may be classified as : 1. Locating pins 2. Support pins 3. Jack pins 1. Locating pins. When reamed or finelly finished holes are available in the workpiece, these can be used for locating purposes in the manner shown in Fig. 1.5. Depending upon their form, the locating pins are classified as : (i) Conical locating pins. These pins are used to locate a workpiece which is cylindrical and with or without a hole as shown in Fig. 1.5 (a) and (b). Any variation in the hole size will be easily accommodated due to the conical shape of the pin. (ii) Cylindrical locating pins, 1.5 (c), (d) and (e) : In these pins, the locating diameter of the pin is made a push fit with the hole in the workpiece, with which it has to engage. The top portion of these pins is given a sufficient lead either by chamfering [Fig. 1.5 (c) and 1.5 (d)] or by means of radius [Fig. 1.5 (e)] to facilitate the loading of the workpiece. 2. Support pins, Fig. 1.6. With these pins (also known as rest pins, buttons or pads), workpieces with flat surfaces can be supported at convenient points. In the fixed type of support Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  Jigs and Fixtures 5 pins, the locating surface is either ground flat [Fig. 1.6 (a)] or is curved [Fig. 1.6 (b)]. Support pins with flat head are usually Work Work employed to provide location and support to machined surfaces, because more con- tact area is available during location. It would ensure accurate and stable location and would not indent the work. The spheri- cal head or rounded-head rest buttons are conventionally used for supporting rough surfaces (unmachined and cast surfaces), (a) Conical (b) Conical recess because they provide a point support which Work may be stable under these circumstances. Adjustable type support pins [Fig. 1.6 (c) and Fig. 1.6 (d)] are used for workpieces whose dimensions can vary, e.g., sand castings, forging or unmachined faces. If the component is to be located in the jig/fixture body, without the aid of these support pins, then the surface of the jig/ fixture body where the component will be supported, will have to be machined. This (d) Cylindrical flanged (c) Cylindrical will involve unnecessary machining time. The use of support pins saves machining Locating pin time as only seats for the pins can be Work machined instead of the entire body of a large fixture. For small workpieces, how- (e) Cylindrical (Bullet nosed) ever, no support pins are necessary. The fixture body itself can be machined suitably to provide the locating surfaces. An ample Fig. 1.5. Locating Pins. Component recess should be provided in corners so that burr on the workpiece corners or dirt and swarf do not obstruct proper location through positive contact of the workpiece with the locat- ing surface. Support pins in large (a) Fixed type (b) fixtures automatically provide similar recesses. 3. Jack pins. Jack pins or Component spring pins are also used to locate the workpieces whose di- mensions are subject to varia- tion, Fig. 1.7. The pin is allowed to come up under spring pres- sure or conversely is pressed down by the workpiece. When the location of the workpiece is secured, the pin is locked in this position by means of the locking (c) Adjustable type (d) screw. Fig. 1.6. Support Pins. Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  6 A Textbook of Production Engineering Work Fig. 1.7. Jack Pin. 1.2.4. Radial or Angular Location. Workpieces such as connecting rod or lever, which have two previously machined and finished holes at the two ends, may be located with the help of two pins projecting from the base surface of a jig or fixture, which will fit into the two holes in the workpiece, Fig. 1.8. Assuming that the workpiece is effectively located on pin A, the only movement the workpiece can have is that of rotation about the pin A. Now, neither the workpiece nor the jig or fixture can be made to the exact dimensions. It means the centre distances between pins A and B and between holes A and B are subject to variation. Let the tolerance for the centre distance between the holes A and B be ‘x’ and that for the centre distance between the pins, A and B by ‘y’. Then if the workpiece is effectively located on pin A and if the pin B is a complete cylinder, the allowance between pin B and hole B will be x plus y. When the centre distance dimensions for the pins and holes are at maximum and minimum conditions, a large allowance will result between the hole and pin at B in the Y direction. Due to this, the workpiece will have undesirable rotation about the pin A and the pin B becomes useless. Therefore, to locate the component completely, location faces opposed to this rotational movement should be provided at the hole B. This is achieved by relieving the pin B Y Work piece dimension Y with tolerence x A B X X Y Y Pin A Work piece Pin B Work holder dimensions Jig or fixture body with tolerence y Fig. 1.8. Radial Location. Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  Jigs and Fixtures 7 B Y A Y Flattened locator X X Y Y Fig. 1.9. Flattened Pin Locator. on two sides perpendicular to the X-axis. This will allow for variations in the X-direction but will provide cylindrical locating surfaces in the Y-direction. This will result in a flattened or diamond pin locator as shown in Fig. 1.9 and Fig. 1.10 respectively. Y The important and accurate hole of the two holes should Pin Hole be used for principal cylindrical location with a full cylindri- cal pin. The diamond pin is used to constrain the pivoting of the workpiece around the principal location. The principal locator should be longer than the diamond pin so that the workpiece can be located and pivoted around it before engaging with the diamond pin. This simplifies and speeds X up locating of the workpiece. A workpiece with only one hole can also be fully located as shown in Fig. 1.11. The principal location is secured from pin A. The radial movement in both the directions of Y-axis is restricted by providing two pins B Diamond pin locator confining the periphery of the workpiece. The basic principle for radial locations so as to minimize deviations from true Fig. 1.10. Diamond Pin Locator. locations is to position the radial locators as far from the axis of rotation as possible. This is clear in Fig. 1.12. B Y A B Y Pin A Work piece Pin B Jig or fixture body Fig. 1.11. Location of Workpiece with only one Hole. A displacement ‘d’ at a distance ‘a’ from the axis O results in an angular error of AOA. The same displacement ‘d’ at a greater distance‘b’ gives an angular error of BOB which is smaller. Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  8 A Textbook of Production Engineering Axis A B d 0 A' B' a b Fig. 1.12 1.2.5. V-location. In V-location, workpieces having circular or semicircular profile are located by means of a Vee block. The V-block should be used correctly so that the variations Drill bushing Variation in work piece Displacement diameter (Error) No displacement (a) (b) Fig. 1.13. V-location. in the work piece size are not detrimental to location (Fig. 1.13). Vees can be used both for locating and clamping a workpiece. For this two Vees are employed, one fixed and the other sliding one. The fixed V acts to Straight vee Straight vee locate and the movable or sliding V acts to clamp and hold the workpiece at one end and forces it against the fixed V at the other end. To secure double clamping effect, the Vees may be made with in- clined locating surfaces, instead of these being perpendicular to the direction of location of clamping. With inclined faces Component of the Vees a vertical downward compo- nent of the clamping force is obtained in addition to its horizontal component, Fig. Fixed vee Sliding vee 1.14. The vertical force component presses the workpiece on the base of the jig or fixture. The usual inclination of the face is 3°. The fixed V is secured to the jig or fixture body by means of caphead screws or dowel pins. The sliding V block may be actuated by means of a hand operated Inclined vee Inclined vee screw (Fig. 1.15) or a cam. Fig. 1.14. Fixed and Sliding Vees. Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  Jigs and Fixtures 9 Hand screw knob Ways Movable jaw Work Stationary jaw Fig. 1.15. V-block. 1.2.6. Bush location. Shaft type workpieces can be easily located in hardened steel bushes. Small and medium sized bushes are usually press fitted into the jig or fixture body whilst the large bushes are push fitted in the body and located by means of screws. The bushes can be plain or flanged type. A flange strengthens the bush and also prevents it from being driven into the jig body if it is left unlocked. In all the bushes, the entrance of the bush is chamfered, coned or bell mouthed to facilitate loading of the workpiece. A typical bush location is shown in Fig. 1.16. Drill Bush Work Fig. 1.16. Bush Location. 1.2.7. Design principles for location purposes. In addition to the principles discussed under Art. 1.2.2, the following principles should be followed while locating surfaces : 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, as explained in Fig. 1.17. Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  10 A Textbook of Production Engineering Work Work (a) Incorrect Swarf (b) Correct Jig Locating Work pin Work Burr Burr and chip relief Corner relief (c) (d) Fig. 1.17. Provision of Relief. 4. Locating surfaces should be raised above surrounding surfaces of the jig or fixture, so that chips fall or can be swept off readily, Fig. 1.18. Fig. 1.18 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. 8. To avoid distortion of the work, it should be supported as shown in Fig. 1.19. Drill Drill Work Work (a) Bad (b) Good Fig. 1.19. Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  Jigs and Fixtures 11 1.2.8. Clamping. If the workpiece cannot be restrained by the locating elements, it becomes necessary to clamp the workpiece in jig or fixture body. As already noted, the purpose of clamping is to exert a pressure to press a workpiece 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 and vibration in machining. The most common example of a clamp is the bench vise, where the movable jaw of the vise exerts force on the workpiece thereby holding it in the correct position of location in the fixed jaw of the vise. Principles for clamping purposes. Since the proper and adequate clamping of a workpiece is very important, the following design and operational factors should be taken care of : 1. The clamping pressures applied against the workpiece 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, Fig. 1.20. Fig. 1.20 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. Complicated clamps lose their effectiveness as they wear. 6. The movement of a clamp should be strictly limited and if possible it should be positively guided. 7. Whenever possible, the lifting of the clamp by hand should be avoided if it can be done by means of a 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, as illustrated in Fig. 1.21. 10. Fibre pads should be riveted to the clamp faces, otherwise 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. Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  12 A Textbook of Production Engineering Work Work Work Work (a) (b) (c) Fig. 1.21. Position of Clamp. 12. A clamp should be strong enough to withstand 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, Fig. 1.22. (a) Bad (a) Good Fig. 1.22 1.2.9. Clamping Devices. The commonly used clamping devices are discussed below. 1. Clamping screws. Clamping screws are used for light clamping and typical examples are shown in Figs. 1.17 and 1.21. 2. Hook bolt clamp. This is very simple clamping device and is only suitable for light work and where the usual type of clamp is inconvenient. A typical hook bolt clamp is shown in Fig. 1.23. Work Fig. 1.23. Hook Bolt Clamp. 3. Lever type clamps. The various designs in the lever type clamp used with jigs and fixtures are discussed below. Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  Jigs and Fixtures 13 (i) Bridge clamp. It is very simple and reliable clamping device. The clamping force is applied by the spring loaded nut Fig. 1.24 (a). x y Work piece Jig or fixture body (a) Simple bridge clamp (b) Slotted strap Fig. 1.24. (a, b) The relative positions of the nut, the point of contact of the clamp with the work and with outer support should be carefully considered, since the compressive force of the nut is shared between the workpiece and the clamp support inversely as the ratio of their distances from the nut. The distance ‘x’ is less than or equal to but never greater than the distance ‘y’. The spring is fitted with the clamp for its automatic lifting when the nut is loosened to remove the workpiece from the jig or fixture. To avoid the complete removal of the nut every time a workpiece is changed the clamp may be slotted to draw it back as shown in Fig. 1.24 (b). A two way clamping can be obtained by the bridge clamp as shown in Fig. 1.24 (c). (ii) Heel clamps. The various types of heel Jig Or clamps are shown in Fig. 1.25. These consist of a Fixture Body robust plate or strap, centre stud and a heel. The strap (c) Two way clamp should be strengthened at the point where the hole for Fig. 1.24. Bridge Clamps. the stud is cut out, by increasing the thickness around the hole. The design [Fig. 1.25 (a)] differs from the simple bridge clamp [Fig. 1.24 (a)] in that a heel is provided at the outer end of the clamp to guide its sliding motion for loading and unloading the workpiece. In design [Fig. 1.25 (b)], the heel is solid and one piece with the clamp. The workpiece is loaded into the jig or fixture or removed from these, by rotating the clamp. In design [Fig. 1.25 (c)], the clamp is guided by the loose heel which Heel clamp Component Slide Off Position Heel Work Stop pin (a) Dog (b) Solid heel clamp Fig. 1.25 (a, b) Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  14 A Textbook of Production Engineering is driven into the jig or fixture body. A short stem is turned on the end of the heel which fits loosely into a keyway in the clamp strap. The loading and unloading of the workpiece is obtained by reciprocating the clamp by hand. The design [Fig. 1.25 (d)] is similar to that in Fig. 1.25 (c) but, here the stem is provided at the end of the heel which forms part of the jig or fixture body casting. Clamp Loose heel Component Work (d) (c) Loose guided heel clamp Fig. 1.25. (c, d) Heel Clamps. (iii) Swinging strap (latch) clamp. This is a special type of clamp which provides a means of entry for loading and unloading the workpiece. For this, the strap (latch or lid) can be swung out or in. Two designs of swinging latch clamps are shown in Fig. 1.26. Work Work (b) (a) Fig. 1.26. Swinging Latch Clamp. (iv) Hinged clamps. This clamp is similar to swinging latch clamp in which the latch is hinged to enable the workpiece to be loaded and unloaded. The clamp can be made integral with the latch. Fig. 1.27 (a) shows a hinged clamp which is locked by means of a bolt. Fig. 1.27 (b) Handle Work (a) Hinged Clamp (b) Cam-operated Clamp Fig. 1.27. Hinged Clamps. Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  Jigs and Fixtures 15 shows a hinged clamp provided with a hook cam. This clamp is much quicker than the bolt type and is suitable for workpieces which maintain dimensional accuracy. The hooked end of the operating lever acts as a cam and engages a pin. Fig. 1.28 shows some other designs of the lids or straps which may be used for swinging latch or hinged clamps. Fig. 1.28. Straps. 4. Quick acting clamps. There are many mechanical clamping devices (pneumatic and hydraulic devices will be discussed later), which can be termed as quick acting. These devices are costlier than the other types but ultimately prove economical since these help in reducing the total operating time. Some of the quick acting clamping devices are discussed below : (i) C-clamps. The two types of C-clamps, free and captive are shown in Fig. 1.29. To unload the workpiece, the locking nut is unscrewed by giving it about one turn and this releases the C- clamp. When the clamp is removed or swung away, the workpiece can freely pass over the nut. 'C' clamp Work piece (a) Free 'C' clamp Locking nut (b) Captive 'C' clamp Fig 1.29. C - clamps. Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  16 A Textbook of Production Engineering The reverse procedure is adopted for loading the workpiece. The free C-clamp may be fastened to jig or fixture body to prevent it from being lost. (ii) Quick acting nut. This nut is shown in Fig. 1.30. The threads of the nut are not continuous but are D interrupted. The length of the nut is d0  about 2 to 3 times the thread diam- eter. The diameter of the clearance ‘D’ is slightly bigger than the outside diameter of the thread and the axis of the hole is inclined at angle (3° to Fig. 1.30. Quick Acting Nut. 7°) to the axis of nut. The use of the quick acting nut is explained in Fig. 1.31. When the nut is assembled over the male thread, it is inclined to the axis of the clearance hole. When the nut engages the male thread, it is dropped on to the screw threads and is then tightly locked by giving it about half a turn. Work holder Fixture Fixture Work Work piece piece Fig. 1.31. Use of Quick Acting Nut. (iii) Cam-operated clamp. These clamps find broad application and are fast and positive in action. These should not be used where vibrations are present or where the dimensions of the workpiece vary, e.g., sand castings. A cam operated clamp is shown in Fig. 1.32. Note. No clamping devices are used if a very heavy stable job is to be machined, whose weight is Handle very great compared to the forces developed in the cutting process, if these forces are in a direction that Spherical washer cannot disturb the setting of the job (as, for example, in drilling holes in a heavy baseplate). Clamping devices are also unnecessary if the job is deprived of Cam all of its degrees of freedom when it is loaded into a fixture (as. for example, turning a job between centres on a centre lathe and milling the two end faces of a connecting rod located on two pins from its two end holes, Fig. 1.9). 1.2.10. Materials for Locating and Clamping Elements. The locating and clamping elements are Fig. 1.32. Cam-operated Clamp generally made from steel. The elements which come in contact with the workpieces or are subjected to wear, should be hardened and wherever necessary, the working face should be ground. For dowel pins and handles, silver steel is generally used. For complicated shapes and when exceptional wear is liable to occur, good quality case-hardened steel, tool steel or a high tensile steel may be used. 1.2.11. Lever type clamps and spherical washers. In the lever type clamps discussed above, it is seen that the clamping face of the lever is curved. This makes the clamp operatable Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  Jigs and Fixtures 17 even if there is variation in the workpiece. At the other end of the strap (pressure pad), the top of the bridge or the heel should also be in the shape of raised and rounded toes to permit some tilting of clamp. This design provides more effective clamping than a design having flat strap ends. This design will also take care of small variations in workpiece height. Poor clamping conditions can result if there is a considerable variation in the workpiece or there is difference in workpiece and fulcrum block height. The misalignment between clamp surface and clamping nut due to tilting of clamp can be taken care of by interposing a pair (male- female) of spherical washers between the nut and the strap (instead of a plain washer), Fig. 1.33. The spherical bearing surfaces of the washers will allow the inclination of the strap caused by the difference in heights of the filcrum block and workpiece. The male washer (upper one) remains square with the nut while the female washer (lower one) tilts with the clamp; since the spherical bearing surfaces allow the pair of spherical washers to tilt with respect to each other. The angle of inclination of the strap that can be tolerated is limited by the clearance between the stud and the inside diameter of the washers. Spherical washers are thus commonly used for equalising clamping forces. Inclination Nut Spherical Curved washers clamping Male point Famale Strap Workpiece Spring Curved heel pin Fig. 1.33. Use of Spherical Washers. 1.3. DESIGN PRINCIPLES COMMON TO JIGS AND FIXTURES 1. Since the total machining time for a workpiece includes work-handling time, the methods of location and clamping should be such that the idle time is minimum. The various design principles regarding location and clamping have already been discussed. 2. The design of jig and fixture should allow easy and quick loading and unloading of the workpiece. This will also help in reducing the idle time to minimise. 3. The jig and fixture should be as open as possible to minimize chip or burr accumulation and to enable the operator to remove the chips easily with a brush or an air jet. 4. Fool proofing. It can be defined as the incorporation of design features in the jig or fixture, that will make it impossible to load the work into the jig or fixture in an improper position but will not interfere with proper loading and locating the workpiece. There are many foolproofing devices such as fouling pegs, blocks or pins which clear correctly positioned parts but prevent incorrectly loaded parts from entering the jig or fixture body. Figure 1.34 explains this principle. Three holes are to be drilled in the component shown. The operator locates the component on the bottom plate of the drilling jig with the projection C on the component fitting the locating hole in the jig. Now if the component is being located incorrectly so that the end A of the component is oriented towards the left of the locating hole, the fouling peg in the body of the jig will obstruct the component. Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  18 A Textbook of Production Engineering For correct locations, the end A of the component is to be towards the right and the curved end B of the component is to be towards the left of the locating hole. Fig. 1.34. Fool Proofing. 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 machining. 6. Rigidity. Jigs and fixtures should be sufficiently stiff to secure the preset accuracy of machining. 7. Trunnions. To simplify the handling of heavy jigs or fixtures, the following means can be adopted : (i) Eye-bolts, rings or lifting lugs can be provided for the lifting of the jig or fixture. (ii) If the workpiece is also heavy, then the jig design should allow for side loading and unloading by sliding the workpiece on the machine table. Fig. 1.35. Burr Grooves. Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  Jigs and Fixtures 19 8. Burr grooves. A burr raised on the work at the start of a cut is termed a ‘minor burr’ and that at the end of a cut a ‘major burr’. Jigs should be designed so that the removal of the workpiece is not obstructed by these burrs. For this, suitable clearance grooves or slots should be provided as shown in Fig. 1.35. 9. Ejectors. The use of ejection devices to force the workpiece out from the jig or fixture is important in two situations : (i) the workpiece is heavy. (ii) machining pressure forces the workpiece to the sides or base of the jig or fixture and the pressure and oil or coolant film will cause the work to stick and be difficult to remove. On small jigs or fixtures, a pin located under the work will remove the part readily [Fig. 1.36 (a), (b)]. Hinged ejectors [Fig. 1.36 (c)] are also very useful and can be easily mounted. Work Ejector Ejector Push Push (a) (b) Work Push (c) Fig. 1.36. Ejecting Devices. 10. Inserts. To avoid any damage to fragile and soft workpieces and also to the finished surfaces of a workpiece while clamping, inserts of some soft material such as copper, lead, fibre, leather, hard rubber, plastic or felt should be fitted to the faces of the clamps. 11. Design for safety. Jigs/fixtures must be safe and convenient in use. Following are some of the factors for the safety of the worker working with a jig/fixture : (i) Sharp corners on the body of the jig/fixture should be avoided. (ii) Sighting surfaces should be clear. (iii) Bolts and nuts should be inside the body of the jig/fixture and not protrude on the surface. 12. Sighting Surfaces. Machining on a workpiece must be clearly visible to the worker. He should not be required to bend his neck for seeing the work surface. 13. Simplicity in Design. Design of the jig/fixtures should be a simple one. A complicated design requires a large maintenance. They should be cheap in manufacture and should lend themselves readily to maintenance and replacement of worn-out parts. Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  20 A Textbook of Production Engineering 14. Economical. Jig/fixture should be simple in construction, give high accuracy, be sufficiently rigid and light in weight. To satisfy all these conditions, an economical balance has to be made. 15. They should be easy to set in the machine tool, which is so important in quatity production where jigs/fixtures are replaced at intervals. 1.4. DRILLING JIGS Drilling jigs are used to machine holes in mechanical products. To obtain positional accuracy of the holes, hardened drill bushes or jig bushes are used to locate and guide drills, reamers etc., in relation to the workpiece. These guide bushes are not essential but these prove to be economical and technically desirable as will be discussed ahead. The portion 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. Such a drill jig is moved by hand into position under the drill so that the drill readily enters the bush. During the drilling operation, the jig is held by hand. If the drill size is large enough to produce a high torque, either stops should be provided or the drill jig clamped to the table of the drilling machine. A drill jig is provided with feet which rest or slide on the table of drilling machine. These feet should be outside the cutting forces, thus providing solid support. 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”. Also, they produce interchangeable Minor burr parts, because each part drilled in a drilling jig should have the same hole pattern as every other part. It is clear that during the drilling operation, burrs will be produced. The burr produced at the start of a hole is smaller than that produced at the end of the hole. The first type is called ‘minor burr’and the second type ‘Major burr’ Major burr (when the drill breaks through the material, Fig. 1.37). When designing a drilling jig, these two types of burrs should be taken into consideration since they may cause Fig. 1.37. Major and Minor Burrs. difficulty in unloading the workpiece from the jig after a hole has been drilled. 1.4.1. Design Principles for Drilling Jigs. 1. A drilling jig should be of light construction consistent with adequate rigidity to facilitate its handling because it has to be handled frequently during the operation. 2. A drilling jig which is not normally clamped to the machine table should be provided with four feet so that it will rock if it is not resting square on the machine table and so warn 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 bushings should be fitted in fixed portion of the jig and not in clamps except for a few special cases (for example, leaf type jig). 1.4.2. Drill Bushes. Sometimes the stiffness of the cutting tool may be insufficient to perform certain machining operations. To eliminate the elastic spring back in machining and to locate the tool relative to the work, use is made of guiding parts, such as, jig bushings and templates. These must be precise, wear resistant and changeable. Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  Jigs and Fixtures 21 Jig bushings are used in drilling and boring jigs. Their use permits giving up the marking out, reduces drill run-off and hole expansion (ovalization). The diametric accuracy of holes in jig drilling is 50 per cent higher on the average compared to that of holes drilled conventionally. Drill bushings can be classified as : Press fit, Renewable and Liner bushings. (i) Press Fit bushings. These bushings are used when little importance is put on accuracy or finish, and the tool used is a twist drill. These bushings are installed directly in the jig body and are used mainly for short pro- duction runs not requiring bush re- placement. These are also employed where the centre distances of holes are too close to permit the fitting of liners and renewable bushes. There are two designs of press fit bushings : (a) Plain or headless (b) Flanged or headed. A flanged or headed bush (a) Headless or plain bush (b) Headed or flanged bush has a flange or head, Fig. 1.38. (b). It Fig. 1.38. Press Fit Bushings. is employed when the jig plate into which it is installed is thin, the flanged or headed portion increasing the length of the bush which provides longer guiding portion to the bush than would otherwise be available. The flange or head also acts as a stop for the tool. (ii) Renewable bushes. Figure 1.39 (a). When the guide bushes require periodic replacement (due to the wear of the inside diameter 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 sliding 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. Direction of rotation of tool Retaining screw Retaining screw Lip for drift Liner Liner Renewable bush Slip bush bush bush (a) Fixed renewable bush (b) Slip renewable bush Fig. 1.39. Renewable Bushes. The renewable bushes must be prevented from rotating or lifting with the drill. One common method is to use a retaining screw as shown in the Fig. 1.39 (a). The flange of the bush is provided with a flat. When the renewable bush is put into the liner bush and the retaining screw is tightened, the collar of the screw will press against the flat on the bush flange and prevent rotation of the bush and also its lifting up (when the drill is clearing the swarf or is being withdrawn at the end of the cut). When the renewable bush wears out, the retaining screw Created with Print2PDF. To remove this line, buy a license at: http://www.software602.com/  22 A Textbook of Production Engineering is removed and the worn bush is taken out. A new bush is then easily substituted. A normal press fit bush can be taken out only by driving it out of the jig plate. This usually damages the bore of the jig plate which has to be rebored for an oversize bush to be fitted. (iii) Slip Bushes. Fig. 1.39 (b). Slip bushes are used when more than one bushings are to be interchanged in a given size of the liner, that is, where two or more operations