Work Holding: Screw Small Parts Onto a Fixture for CNC or Manual Milling Operations

The dirty little secret of machining is that it is (at least on my bad days :-) 10% operating the tool, and 89% trying to figure out how to hold the work. And 1% of the time trying to find your part after your attempt to hold the work has failed.

The easiest way is to use a machinists vice. But what if you want to do a profile or perimeter cut?

We are working on a small (1" x 3") rectangular part which needs to have the edges finished and the corners radiused. So the answer was to make a fixture to which we screw the part.

Here is the completed fixture. A 5/8" thick block of Aluminum with two threaded holes. The tool marks are from the first time we used the fixture and cut just a tiny bit deep.

Note the red Dykem in the upper right of the fixture. This lets us pull it out of the vise and put it back with the correct orientation. Also note the alignment rod. The mill is zeroed so that the part is at zero when it is in the fixture, and the fixture is snug against that rod.

The important things about a fixture like this are:

• Mark an index corner
• Make sure the fixture is snug against your alignment
• Tighten the fixture in the vise, but don't over tighten
• tighten the screws holding your part on the fixture, but again, don't over tighten
• If you are using CNC cut some air first. Zero your z axis well above the part and run your program a few times.