Definition of Jig
Jig is a device that firmly holds and locates work piece and also guides cutting tool during machining.
Elements of Jig
This is also called base. It supports work piece by locators and clamps. The base is fitted with four feet which rest on the machine table.
Jig has four feet at the bottom. Jig rests on machine table only with these feet. This increases accuracy. Jig feet are welded or cast with jig body. Sometimes, feet are press-fitted. They are usually hardened and ground to flat surface. In some cases jig feet is not present.
Drill bush or latch
Drill bush guides tools like drills and reamers during machining. Bushes are fixed in jig plates or bush plates. These bushes are made in hardened steel. When the bush wears out, it can be replaced with a new one. There will be one bush for each hole to be drilled. The size of bush depends on the size of the hole to be drilled.
Bush plate or Jig plate
Bushes are fitted to the bush plate. Bush plate may be a rigid one or may be of hinged type. Hinged type plate is called leaf or latch. This can be swung open to load as well as to un-load work piece. It is swung again for closing.
Locators help the work piece, to rest in proper position, with reference to tools. Therefore holes are drilled accurately in required position. Locators are fitted in these holes. Depending upon the work piece shape, various locators are used. Usually locators are detachable type. They are fixed to the jig body or frame. Worn-out locators can be replaced by new ones.
Clamps or knob
Clamps are used for holding the work piece rigidly in its position; they also keep work piece firmly in contact with locating pins. Clamps should be thick enough so that they do not bend, while clamping,. Generally, surface of clamps are case hardened.
Fool proof element
This element prevents loading of work piece in the wrong position. That is, this element makes sure that work piece is correctly loaded and positioned. Fouling pegs cross pieces or pins are used as fool proof element.
Advantages of Jigs
· Reduce manufacturing time-Marking out; measuring and setting the work piece are not required. This saves time.
· Increase dimensional accuracy: Work piece is located automatically; Tool is properly guided in machining.
· Identical parts are produced easily: Parts of same size and shape are interchangeable. Therefore assembly operations can be done without any difficulty.
· Increase rate of production:
Many work pieces are machined in single setup.
Many tools cut work piece at the same time.
· Work handling time is reduced-This is due to quick setting and locating of work piece.
· Metal removal rate is increased.
Work piece is rigidly clamped by jigs. Therefore speed, feed and depth of cut can be increased for machining. That is, more metal is removed in lesser time.
· Operator fatigue is reduced.
Work handling operations such as marking, centering, clamping are very much reduced. Hence operator’s tiredness reduced.
· Semi-skilled operator is adequate, because setting of tool and work are simpler. Salary to semi-skilled labour is lesser than skilled labour.
· Quality control cost is less.
· Overall cost of production is reduced.