3-2-1 Locating Principle. Why generally in base plane three locating pins are used?

 

3-2-1 Method of Location: 

3-2-1 method of location is the fundamental arrangement of locating system. A work piece may be positively and properly located by means of six pins,so positioned that collectively they restrict the work piece in nine of it’s degree of freedom.For this purpose a definite arrangement is used.This  arrangement is called 3-2-1 method of location.

 

Cause of using three locating pins in base plane:

A minimum of three points or pins or locators are generally used in base plane to locate the work piece, although four or more may be used to provide adequate support. It should be noted that a minimum of three pins will always theoritically establish the same location of the work piece. four or more pins(locators)don’t establish this type of theoritical location because if the work piece surface is not geometrically true, the position is still determined by three pins.

Three pining system create one surface which is geometrically true and this surface is parallel to the locating base surface whereas four or more pining system create surface more than one having non-parallel relation with the base surface. So the use of more than three pins or points causes surface deviation frequently.That’s why three pins are used in base plane.