3-2-1 principle used for jig and Fixture Locating Method

Principle Of Location in Jig And Fixture :

The principle of location is being discussed here with the help of a most popular example which is available in any of the book covering jigs and fixtures. It is important that one should understand the problem first.
Any rectangular body many have three axis along x-axis, y-axis and z-axis. It can more along any of these axes or any of its movement can be released to these three axes. At the same time the body can also rotate about these axes too. So total degree of freedom of the body along which it can move is six. For processing the body it is required to restrain all the degree of freedom (DOF) by arranging suitable locating points and then clamping it in a fixed and required position. The basic principle used to locate the points is desirable below.

See also: Introduction and Difference Between Jig And Fixture

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.  By properly locating, supporting and clamping the blank its all degrees of freedom are to be arrested as typically shown in

3 2 1 principle used in jig fixture design
3 2 1 principle used in jig fixture design

The three adjacent locating surfaces of the blank (workpiece) are resting against 3, 2 and 1 pins respectively, which prevent 9 degrees of freedom. The rest three degrees of freedom are arrested by three external forces usually provided directly by clamping. Some of such forces may be attained by friction.

Some basic principles or rules need to be followed while planning for locating blanks in fixtures, such as;

  • One or more surfaces (preferably machined) and / or drilled / bored hole(s) are to be taken for reference
  • The reference surfaces should be significant and important feature(s) based on which most of the dimensions are laid down
  • Locating should be easy, quick and accurate In case of locating by pin, the pins and their mounting and contact points should be strong, rigid and hard
  •  A minimum of three point must be used to locate a horizontal flat surface
  •  The locating pins should be as far apart as feasible
  • Vee block and cones should be used for self-locating solid and hollow cylindrical jobs.

Types Of Locators used In Jig And Fixture For supporting :

The location arrangement depends on the type of operation, degrees of required accuracy.

  • Flat locators: These are used for the location of the flat machined surfaces of a component.
  • Cylindrical locators: These are used for the location of components with drilled holes.
  • Conical locators: These locators are used to support the work pieces with cylindrical holes.
  • Jack pin locators: These locators support rough work pieces.
  • Drill bush locators: These locators hold and locate the cylindrical work pieces.
  • Vee locators: These locators hold circular and semicircular work pieces.
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