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Explain 3 - 2 - 1 principle of location with neat diagram

3-2-1 Principle of Location used in Jig & Fixtures:

[1] It is also known as six pin or six point location principle. In this, the three adjacent locating surfaces of the blank (work piece) are resting against 3, 2 and 1 pins respectively, which prevent 9 degrees of freedom.

[2] The rest three degrees of freedom are arrested by three external forces usually provided directly by clamping.

The 3-2-1 principle states that the six locators are sufficient to restrict the required degree of freedom of any work piece. In this, motion is restricted using clamps and locators. A three pin base can restrict five motions and six pins restrict nine motions

Methodology of 3-2-1 Principle: For this, refer the below figure;

[1] The work piece is resting on three pins A, B and C which are inserted in the base of the fixed body.

[2] The work piece cannot rotate about the axes XX and YY and also cannot move downward.

[3] In this way, the five degrees of freedom 1,2,3,4 and 5 have been arrested.

[4] Two pins D and E are inserted in the fixed body, in a plane perpendicular to the plane containing pins A, B & C.

[5] Now the work piece cannot rotate about the Z axis and also it cannot move towards the left.

[6] Hence the addition of pins D and E restrict three more degrees of freedom, namely 6, 7 and 8.

[7] Another pin F in the second vertical face of the fixed body, arrests degree of freedom 9.

The above method of locating a work piece in a fixture is called the 3-2-1 Principle.