A fixture holds a workpiece in a known, repeatable position so every part is machined the same. Two jobs define it: locating the part and clamping it.
The 3-2-1 rule
The classic locating principle uses six points to fully constrain a part: three points define the primary plane (stopping rocking and two rotations), two points define a second plane (stopping sliding and another rotation), and one point defines the third (stopping the last movement). This removes all six degrees of freedom with minimal, well-placed contacts.
Clamping rules
Clamp toward the locators so force pushes the part into them, not away. Apply force over supported areas to avoid bending, keep clamps clear of the cutting path, and use the least force that holds securely — excess force distorts parts that then spring back after release, throwing off dimensions. A good fixture lets an operator drop in part after part and get identical results.
Frequently asked questions
What is the 3-2-1 rule? A locating scheme using 3+2+1 points to fully constrain a part.
Where should clamps push? Toward the locators, over supported areas.
Why not clamp as hard as possible? Excess force distorts the part, ruining accuracy after release.