[Dichotomy]

I am trying to rotary broach an internal Torx 20 on a Star SR 20 RII. When the machine broaches the stock is pushed back (Z-) and creates a short part. The machine peck broaches, recommended by Slater Tools, and I also have the Main Collet tight. I ordered a serrated collet, but that will take around 10 days to get to me. Are there any tips or tricks to get this thing running before then?

[MCImes]

Hm, a T20 isnt huge... what size is your stock? In general, it seems like the machine should be able to do this without too much trouble...

-First, I'd max out your main collet tension. Tighten it until your air cylinder wont close, then back it off so you can see it pause briefly before it closes.
-2nd, are you pre drilling to the maximum possible size?
-Do you have adequate space (drill point) for the chips to go into? If not, are you drilling out the chips after a peck or 2?
-Does the broach have an air escape hole? that might help if yours does not have one
-If you have live face tooling, you could pre drill the 6 lobes with a small drill to remove more material before broaching, thus lowering the tool pressure.
-Depending on your part geometry could you broach on the sub so that there is a shoulder acting as a dead stop?

[Benpen]

Are you Turning Off the Coolant before broaching? Did you double check and make sure that the Broach head is set-up correctly? It's only supposed to be .001 out of TIR. Lastly I would stop pecking at it. In my experience Slater's recommendations don't translate very well to swiss. I have a job that I broach a 3/8" hex .350" deep in 4140. I had to slow down from their recommendations to get it to work. Swiss generally doesn't have the HP or the rigidity you need to broach at those speeds. For that T20 I would be at like 800-1000 rpms, and like .002/ipr of feed going in and .015ish coming out.

[Dichotomy]

The stock is precision gound 17-4ss 0.1889". The main collet is also precision ground to to 0.1889". Slater told me to use a .114" drill, but right now I'm using an 1/8" drill. As for depth I have the drill hole .050" past the broach depth. Slater's technical support had me switch to 600 rpm with a feed of .0005". I think that there is not enough friction produced by the main collet and the stock to properly grip. I think the other collet right now might be the only solution.

[Benpen]

Give yourself another .1 of hole depth and see what happens. A serrated collet will definitely help. But at those diameters you can't be generating THAT much force. Like I said I broached a 3/8 hex in 4140 holding onto unturned .750 bar diameter in the sub spindle. I was making some pressure. You're doing a t20 and drilling to .125 already. The bottom of the hole is packing with chips. Also try turning the oil off and giving it a little dwell right before the broach. If the broach isn't vented you are creating hydraulic pressure and fighting it.


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[Benpen]

What condition is that 17-4? Is it the 908 stuff or whatever it is? Or has it been heat treated good and hard? That's a big part of it too. 17-4 is not always fun. .0005 of feed seems awfully slow too. Especially at 600rpm. Sometimes too slow is as bad or worse than too fast. Don't be afraid to move those numbers a little. .0002 of change or 200rpm is not going to make enough difference to explode a tool. But sometimes it makes enough difference to see a change one way or the other.


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[Dichotomy]

The hardness is 27-28 Rockwell C scale. The broach cuts about .030" in. After that it just pushes the stock. I changed the RPM to 1000 and the Feed to .002, and no change ocurred. The bar feeder on the machine is down, and I don't know if that is slightly contribuiting to the stock sliding. I checked the stock, and I can see wear marks from the collet after the broaching cylce. I'm starting to think that collet and stock have to clean of a surface finish to prevent the stock from moving. Well thanks for the help. I think I'll have to wait for that other collet to get here.