1) 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. 2) Following the purpose and advantages of jigs and fixtures :
(a) It reduces or sometimes eliminates the efforts of marking, measuring and setting of workpiece on a machine and maintains the accuracy of performance. (b) Easy, quick and consistently accurate locating, supporting and clamping the blank in alignment of the cutting tool. So it reduces product cycle time. (c) jigs and fixtures maintain a consistent quality and reduce measurement and its cost (d) Due to low variability in dimension assembly operation becomes easy, low rejection due to les defective production is observed. (e) It reduces the production cycle time so increases production capacity. (f) The operating conditions like speed, feed rate and depth of cut can be set to higher values due to rigidity of clamping of workpiece by jigs and fixtures. (g) Operators working becomes comfortable as his efforts in setting the workpiece can be eliminated. (h) Semi-skilled operators can be assigned the work so it saves the cost of manpower also. (i) There is no need to examine the quality of produce provided that quality of employed jigs and fixtures is ensured.
3) Factors essentially considered during design, fabrication and assembly of jigs and fixtures 1. Easy, quick and consistently accurate locating of the blank in the jig or fixture in reference to the cutting tool 2. Providing strong, rigid and stable support to the blank
3. Quick, strong and rigid clamping of the blank in the jig or fixture without interrupting any other operations. 4. Tool guidance for slender cutting tools like drills and reamers. 5. Easy and quick loading and unloading the job to and from the jig or fixture. 6. Use of minimum number of parts for making the jig or fixture. 7. Use of standard parts as much as possible.
8. Reasonable amount of flexibility or adjustability, if feasible, to accommodate slight variation in the job - dimensions. 9. Prevention of jamming of chips, i.e. wide chips-space and easy chip disposal 10. Easy, quick and accurate indexing system if required.
11. Easy and safe handling and moving the jig or fixture on the machine table, i.e., their shape, size, weight and sharp edges and corners 12. Easy and quick removal and replacement of small parts
13. Manufacturability i.e. ease of manufacture
14. Durability and maintainability
15. Service life and overall expenses
4) Locating – principles and methods
• Principles or rules of locating in jigs and fixtures
For accurate machining, the workpiece is to be placed and held in correct position and orientation in the fixture (or jig) which is again appropriately located and fixed with respect to the cutting tool and the machine tool. It has to be assured that the blank, once fixed or clamped, does not move at all. Any solid body may have maximum twelve degrees of freedom as indicated in Fig. 8.1.4. By properly locating, supporting and clamping the blank its all degrees of freedom are to be arrested as typically shown in Fig. 8.1.5.
Some basic principles or rules need to be followed while planning for locating blanks in fixtures, such as; ο One or more surfaces (preferably machined) and / or drilled / bored hole(s) are to be taken for reference ο The reference surfaces should be significant and important feature(s) based on which most of the dimensions are laid down ο Locating should be easy, quick and accurate
ο In case of locating by pin, the pins and their mounting and contact points should be strong, rigid and hard ο A minimum of three point must be used to locate a...
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