Design principles common to jig and fixture

Design principles Common to jigs and fixtures:
There are some principles which are useful to design jigs and fixtures.
1: Rigidity:
Jigs and fixtures should be sufficiently stiff to secure the preset accuracy of machining.
2: Fool proofing:
It can be defined as “ the incorporation of design feature in the jig or fixture that will make it possible to lead the work into jig and fixture, in an improper position , but will not interfere with loading and unloading the work piece.” There are many fool proofing devices , such as fooling pegs, blocks or pins which clears correctly position parts but prevent incorrectly loaded parts from entering the jig and fixture body.
3: Clearance:
Clearance is provided in the jig or fixture body
(A)   to allow for any variation in component sizes specially casting and forging.
(B)   To allow for hand movements so that the work piece can easily placed in the jig or fixture and removal after machining .
4: Burr Grooves:
A burr raised on the work piece at the start of the cut is termed a minor burr and at the end of a cut is called a major burr. Jigs should be designed so that the removal of the work piece is not obstructed by these burr for this suitable clearance grooves or slots should be provided.

5: Ejectors:
The use of ejection devices to force the work piece out from the jig or fixture is important in two positions.
(A)   the work piece is heavy
(B)   machining pressure forces the work piece to the slides or based on the jig or fixture and the pressure and oil or coolant fill will cause the work to strick and difficult to remove on small jigs and fixtures , a pin located under the work will remove the part radially.

6: Inserts:
To avoid any damage to fragile and soft work piece and also to the finished surfaces of the work piece while clamping. Inserts of some soft material such as copper, lead , fiber , leather , hard rubber and  plastic should be fitted to the faces of the clamps.
7: Design for Safety:
Jigs and fixtures must be safe and convenient in use, following are the some
Factors for the safety of worker working on jigs and fixtures.
(A): Sharp corner on the body of jig and fixture should be avoided.
(B): Sighting surfaces should be cleared.
(C): Bolt and nut should be inside the body of jig or fixture and not protrude on the surface.
8: Sighting Surface:
Machining on the work piece must be clearly visible to the worker. He should not be required to bend is neck for seeing the work piece or work surfaces.
9: Simplicity in design:
Design of the jig and fixture should be a simple one. A completed design require a large maintenance. They should be easily to set , cheap in manufacture.
10: Economical:
Jig and fixtures should be simple in construction, give high accuracy , be sufficiently rigid and lightly weight. To satisfy these conditions an economical balance has to be made.

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