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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1. Finished Part 2. First Op: Before 2. First Op. After 3. Machined Fixture 4. Second Op: Bearing Seat 5. Third Op: Finished Part

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.

I regularly follow practicalmachinist.com forums.
Especially the cnc machining section of it.

I notice when CNC Speeds and Feeds questions come up people often suggest my HSMAdvisor Machinist calculator.
A referral by a satisfied customer is the best referral in my opinion. Thank you to everyone doing this great favor to me and my prospective users!

Other times users of HSMAdvisor question speeds and feeds it generates and instead of going to me, they ask on forums.
Which is always fine, because extremely often "wrong" results mean something wrong in users expectations or the data he feeds the calculator

In the process of discussion it usually turns out that the calculation results were correct, but because user decided to use a depth of cut or tool length, larger than he should have, HSMAdvisor compensates and gives a very conservative feed rate.

Just like in this forum post over here: Engraving with a 1/32 ball mill machinist wanted Read More 

One of the most versatile ways of clamping irregular -shaped parts is with use of soft jaws.

In this one I had to machine a triangular-shaped part from two sides.

It is going to be some sort of a part holding jaw for a robot.

So step one: Machine one side of the part in vise. hold on to 1/8" of stock. So make sure to cut your part on a bandsaw oversize.

Step Two: Bolt soft jaws to your vise and machine a pocket using outside contour of your part.

Be sure to relieve corners.

Step three: Clamp your part in the soft jaws and machine the second side of your part.

One important thing to consider is: this method is not very accurate. depending on the size and a shape of your part you may be able to hold it within 0.001" though.

See attached photos of the steps below.

1. Machine one side 2. Machine pocket in soft jaws 3. Clamp the part 4. Machine the second side

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. 

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