GD&T 3-2-1 Principle

The concept of 321 principle is important to understanding the basics of GD&T and datums. A brief explanation on its definition

Every component/part has 6 degrees of freedom in space.

If the part must be constrained in all degrees of freedom considering it has a reference system of three mutually perpendicular planes formed by axes x,y,z

At a minimum

It needs three points of contact on the primary plane.
2 points of contact on the secondary plane
1 point of contact of the tertiary plane

Consider the blue block

There are three pink pins which support the block .

The three pins constrain 3 degrees of freedom – 2 rotational degrees of freedom and 1 translation degree of freedom

Then two green pins are brought into contact – this constrains additional 2 degrees of freedom

Finally one red pin is brought into contact with the perpendicular face to both.

This constrains all degrees of freedom.

321 principle is a key principle in fixturing of parts to constrain all degrees of freedom .
Depending on the weight and type of surface the contact of the pins may increase.
Each pin indicates a point
The first three points are lying on primary plane
The second two points are lying on secondary plane
The third point is lying on the tertiary plane

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