Help needed on 4 start thread

[extanker59]

We are making a 5/8"-18 4 start OD thread on a Mazak quick turn 100. Material is 17-4PH. Coolant concentration is slightly high at 11%. SFM tried 30-70.
Made our own shim at 6.4 deg (should be at calculated 6.89 deg).
Major tearing at front of thread.
Should the shim angle be closer? Did I do the math wrong?
Driving me crazy.

[txcncman]

I get 22.8 degrees. Maybe I do not understand what you mean by "shim".

[extanker59]

On laydown thread holders they have an angled shim (also called anvils) that goes under the insert. You change shims depending on the diameter and TPI.
For example using the SECO chart 1/4"-28 thread would take a #3 shim which is 3 degrees. Our part is outside the chart so the calculation, according to them, is arctan lead/(pitch dia x 3.14)

On our part the lead is a really fast .2222 SECO doesn't sell the proper shim so we had to make our own.
We tried all the shims we had on hand (up to 4 degrees) but they were much worse.

[extanker59]

Just an update/final word.
Got it to work by increasing the SFM to 100 and grinding more relief on the threading insert.
Still not convinced we had the right shim angle but we got the parts out the door.

[txcncman]

The relief on the cutting tool was what I was thinking you were getting at. Of course It would significantly weaken the insert, but I guess you made it work.