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Often times CNC programming tutorials only teach you how to create the tool-paths and not enough attention is paid on showing how to properly hold parts being machined.

At the same time efficient workholding is an art in it self and mastering it could drastically improve shop productivity and accuracy.

Without further ado let's jump into the workflow.

Step 1. Analyze the Drawing and the Model

We would have to look at the drawing, tolerances and the CAD model to develop the machining strategy.

This particular part has tight (+/- 0.001) tolerances between the features located on the top and the bottom sides. In addition to that it has a 2.5 degree draft angle on external walls.

Thus I decided to not use the soft jaws approach and machine it in a fixture. Soft jaws are generally OK for tolerances down to +/-0.001" but because of the draft angle the part would always want to pop out of the jaws.

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Mind the Gap!

March 29, 2016, 3:22 pm

Don't you hate when "that guy" cuts your stock a little too short?

Holding on to just 0.060" here. The part is 1.5" tall made out of 4140, which helps it stay in place and not collapse under machining pressure. Pretty sure it would have moved if it was made out of mild steel.

PS. Did I eay I love those CR vise? I do. Can't lift that jaw with a lift truck.

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What to do when you can not clamp it

September 27, 2012, 11:51 pm

If you cant, then dont clamp it at all!.
Skin it!

In here i have to program and machine several sets of roundish aluminum pieces with +/-0.001" outside tolerance. and within 0.002" thickness repeatability.
Instead if fixturing it one by one i decided to skim cut a 33" x 23" x5/8" to within 0.0015 flat. And then machine each piece completely leaving .005" outside to holt everything together.

Worked out great. 

for pics

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Attached file: IMAG0019.jpg

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Sing In

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