Purposes of jigs and fixtures and their Design principles - Jigs and Fixtures for Machine shops Mechanical Engineering Notes | EduRev

Mechanical Engineering : Purposes of jigs and fixtures and their Design principles - Jigs and Fixtures for Machine shops Mechanical Engineering Notes | EduRev

 Page 1


 
 
 
 
 
 
 
 
 
Module 
8 
Jigs and Fixtures for 
Machine shops 
 
Version 2 ME, IIT Kharagpur 
Page 2


 
 
 
 
 
 
 
 
 
Module 
8 
Jigs and Fixtures for 
Machine shops 
 
Version 2 ME, IIT Kharagpur 
 
 
 
 
 
 
 
 
 
Lesson 
33 
Purposes of jigs and 
fixtures and their 
Design principles 
Version 2 ME, IIT Kharagpur 
Page 3


 
 
 
 
 
 
 
 
 
Module 
8 
Jigs and Fixtures for 
Machine shops 
 
Version 2 ME, IIT Kharagpur 
 
 
 
 
 
 
 
 
 
Lesson 
33 
Purposes of jigs and 
fixtures and their 
Design principles 
Version 2 ME, IIT Kharagpur 
 
 Instructional objectives  
 
This lesson will enable the students 
 
(i)  Define Fixture and Jig that aid machining 
(ii)  Conceive the purposes of use of jigs and fixtures 
(iii) State the design considerations for jigs and fixtures 
(iv) Illustrate the methods of  
(a) locating 
(b) supporting and 
(c) clamping of blanks 
(d) guidance of cutting tools 
      in fixtures and jigs to facilitate machining 
 
 
(i)  Definition Of Fixture And Jig 
 
Fixtures, being used in machine shop, are strong and rigid mechanical 
devices which enable easy, quick and consistently accurate locating, 
supporting and clamping, blanks against cutting tool(s) and result faster and 
accurate machining with consistent quality, functional ability and 
interchangeability. 
Jig is a fixture with an additional feature of tool guidance. 
 
(ii)  Purpose Of Using Fixtures And Jigs 
 
For a machining work, like drilling a through hole of given diameter 
eccentrically in a premachined mild steel disk as shown in Fig. 8.1.1 in a  
 
 
hole to be made 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 8.1.1  A through hole has to be drilled in a pre-machined mild steel disc. 
 
 
Version 2 ME, IIT Kharagpur 
Page 4


 
 
 
 
 
 
 
 
 
Module 
8 
Jigs and Fixtures for 
Machine shops 
 
Version 2 ME, IIT Kharagpur 
 
 
 
 
 
 
 
 
 
Lesson 
33 
Purposes of jigs and 
fixtures and their 
Design principles 
Version 2 ME, IIT Kharagpur 
 
 Instructional objectives  
 
This lesson will enable the students 
 
(i)  Define Fixture and Jig that aid machining 
(ii)  Conceive the purposes of use of jigs and fixtures 
(iii) State the design considerations for jigs and fixtures 
(iv) Illustrate the methods of  
(a) locating 
(b) supporting and 
(c) clamping of blanks 
(d) guidance of cutting tools 
      in fixtures and jigs to facilitate machining 
 
 
(i)  Definition Of Fixture And Jig 
 
Fixtures, being used in machine shop, are strong and rigid mechanical 
devices which enable easy, quick and consistently accurate locating, 
supporting and clamping, blanks against cutting tool(s) and result faster and 
accurate machining with consistent quality, functional ability and 
interchangeability. 
Jig is a fixture with an additional feature of tool guidance. 
 
(ii)  Purpose Of Using Fixtures And Jigs 
 
For a machining work, like drilling a through hole of given diameter 
eccentrically in a premachined mild steel disk as shown in Fig. 8.1.1 in a  
 
 
hole to be made 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 8.1.1  A through hole has to be drilled in a pre-machined mild steel disc. 
 
 
Version 2 ME, IIT Kharagpur 
conventional drilling machine without using any fixture or jig, the following 
elementary steps are to be sequentially followed 
  
? cleaning and deburring the blank (disc) 
? marking on the blank showing the location of the hole and its axis 
on the blank 
? punch the centre at the desired location and prick punch the 
periphery of the hole to be made in the disc 
? mount the blank in a drilling vice using parallel block, a small Vee 
block etc. to provide support and clamp the blank firmly 
? position the vice along with the marked blank to bring the hole axis 
in alignment with the drill axis by 
? either adjusting the vise position w.r.t. the fixed drill axis 
? or moving the drilling machine table and then locking the 
table position 
? or moving the radial arm and the drilling head, if it is a 
radial drilling machine 
? after fixing the blank, vise and the table, alignment is checked again 
? if error, like eccentricity, is found to occur then readjustment of 
location of the hole – axis is to be done before and even after 
starting drilling 
? drilling is accomplished. 
 
Therefore it appears that so many operations are needed to be carried out 
carefully and skilfully by the machinist or operator for such a simple job. Even 
after that there may be inaccuracies in machining. Such tedious and time 
consuming manual work are eliminated or drastically reduced in mass 
production by automatic or special purpose machine tools. But such machine 
tools are quite expensive and hence are economically justified for only huge 
or mass production and not viable for small lot or batch production. For batch 
production proper design and use of simple but effective jigs and fixtures are 
appropriate and economically justified. This is schematically illustrated in Fig. 
8.1.2. 
The basic purposes of developing and using suitable jigs and fixtures for 
batch production in machine shops are : 
• to eliminate marking, punching, positioning, alignments etc. 
• easy, quick and consistently accurate locating, supporting and 
clamping the blank in alignment of the cutting tool 
• guidance to the cutting tool like drill, reamer etc. 
• increase in productivity and maintain product quality consistently 
• to reduce operator’s labour and skill – requirement 
• to reduce measurement and its cost 
• enhancing technological capacity of the machine tools 
• reduction of overall machining cost and also increase in 
interchangeability. 
 
 
 
 
Version 2 ME, IIT Kharagpur 
Page 5


 
 
 
 
 
 
 
 
 
Module 
8 
Jigs and Fixtures for 
Machine shops 
 
Version 2 ME, IIT Kharagpur 
 
 
 
 
 
 
 
 
 
Lesson 
33 
Purposes of jigs and 
fixtures and their 
Design principles 
Version 2 ME, IIT Kharagpur 
 
 Instructional objectives  
 
This lesson will enable the students 
 
(i)  Define Fixture and Jig that aid machining 
(ii)  Conceive the purposes of use of jigs and fixtures 
(iii) State the design considerations for jigs and fixtures 
(iv) Illustrate the methods of  
(a) locating 
(b) supporting and 
(c) clamping of blanks 
(d) guidance of cutting tools 
      in fixtures and jigs to facilitate machining 
 
 
(i)  Definition Of Fixture And Jig 
 
Fixtures, being used in machine shop, are strong and rigid mechanical 
devices which enable easy, quick and consistently accurate locating, 
supporting and clamping, blanks against cutting tool(s) and result faster and 
accurate machining with consistent quality, functional ability and 
interchangeability. 
Jig is a fixture with an additional feature of tool guidance. 
 
(ii)  Purpose Of Using Fixtures And Jigs 
 
For a machining work, like drilling a through hole of given diameter 
eccentrically in a premachined mild steel disk as shown in Fig. 8.1.1 in a  
 
 
hole to be made 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 8.1.1  A through hole has to be drilled in a pre-machined mild steel disc. 
 
 
Version 2 ME, IIT Kharagpur 
conventional drilling machine without using any fixture or jig, the following 
elementary steps are to be sequentially followed 
  
? cleaning and deburring the blank (disc) 
? marking on the blank showing the location of the hole and its axis 
on the blank 
? punch the centre at the desired location and prick punch the 
periphery of the hole to be made in the disc 
? mount the blank in a drilling vice using parallel block, a small Vee 
block etc. to provide support and clamp the blank firmly 
? position the vice along with the marked blank to bring the hole axis 
in alignment with the drill axis by 
? either adjusting the vise position w.r.t. the fixed drill axis 
? or moving the drilling machine table and then locking the 
table position 
? or moving the radial arm and the drilling head, if it is a 
radial drilling machine 
? after fixing the blank, vise and the table, alignment is checked again 
? if error, like eccentricity, is found to occur then readjustment of 
location of the hole – axis is to be done before and even after 
starting drilling 
? drilling is accomplished. 
 
Therefore it appears that so many operations are needed to be carried out 
carefully and skilfully by the machinist or operator for such a simple job. Even 
after that there may be inaccuracies in machining. Such tedious and time 
consuming manual work are eliminated or drastically reduced in mass 
production by automatic or special purpose machine tools. But such machine 
tools are quite expensive and hence are economically justified for only huge 
or mass production and not viable for small lot or batch production. For batch 
production proper design and use of simple but effective jigs and fixtures are 
appropriate and economically justified. This is schematically illustrated in Fig. 
8.1.2. 
The basic purposes of developing and using suitable jigs and fixtures for 
batch production in machine shops are : 
• to eliminate marking, punching, positioning, alignments etc. 
• easy, quick and consistently accurate locating, supporting and 
clamping the blank in alignment of the cutting tool 
• guidance to the cutting tool like drill, reamer etc. 
• increase in productivity and maintain product quality consistently 
• to reduce operator’s labour and skill – requirement 
• to reduce measurement and its cost 
• enhancing technological capacity of the machine tools 
• reduction of overall machining cost and also increase in 
interchangeability. 
 
 
 
 
Version 2 ME, IIT Kharagpur 
 
 
 
W 
F 
A 
M B 
P
Machining cost, C
T 
Volume of production, Q 
breakeven points 
W – without using jig & fixture 
F – using jig and fixture 
A – automatic (special purpose) machine 
 P – piece production 
 B – batch production 
 M – mass production  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 8.1.2   Role of Jigs and Fixtures on machining cost 
 
 
 
 
(iii)   Design Considerations For Jigs And Fixtures 
 
Jigs and fixtures are manually or partially power operated devices. To fulfil 
their basic purposes, jigs and fixtures are comprised of several elements (as 
indicated in Fig. 8.1.3) : 
• base and body or frame with clamping features 
• locating elements for proper positioning and orientation of the blank 
• supporting surfaces and base 
• clamping elements 
• tool guiding frame and bushes (for jig) 
• indexing plates or systems, if necessary 
• auxiliary elements 
• fastening parts 
 
 
 
 
 
Version 2 ME, IIT Kharagpur 
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