Advice on turning a custom tap

I

ikoral

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The purpose of the tap I am trying to make is to cut threads into aluminum 6061 plates. I don't need a precise fit. The tool steel I have is W1 steel. The thread I need to make has about a 0.56" major diameter a 0.36" minor diameter a pitch of 6 and a flat at the bottom of roughly .050" . The particularly odd part of this thread is that it has a tooth angle of 32 degrees. I ground a HSS square bar to the 32 degree form and put a flat on the nose to just slightly narrower then the male thread. I set my top slide to slightly under 16 degrees and used a live center for support. I was using a 5C collet to hold the stock in the headstock. I started with increments of about 0.0025" depth of cut with a spring pass about every three cuts but by the time I was at about a total depth of 0.025 I noticed the stock was sliding back in the collet. So I changed to a 5C to ER32 collet adapter and added a stop to help keep the part from sliding. I also added more relief to the lead edge of my form tool in case i was rubbing. I continued turning and by the time I was at 0.040" depth I changed to 0.001" depth of cut. When I got to about 0.05" total depth the part rode onto my tool and sheared off.

I attempted to make the tap again but this tine I started with a 60 degree threading tool to remove material first so I made less contact when I changed to the 32 degree threading tool. But I again rode up and broke off, this time at about a 0.07" total depth and still using the 60 degree threading tool.

So at this point I am not sure what change I should make. I have no experience with turning tool steel any tips would be appreciated.
 
you may wish to hold the stock in a chuck, be it 3 or 4 jaw
the 5C may not be grabbing the W1 with sufficient force.
the 3 or 4 jaw chuck could be tightened with more force.
you could knurl a small portion of the stock to ensure the chuck holds too, if you were so inclined
 
Center bore the tool blank on both ends and cut your threads with the work well-supported between centers. You can always cut/grind down the cutting end to the profile you need after turning the threads. Besides, I don't know how you're going to cut flutes if you aren't between centers.
 
Ditch the collet chuck and put a 3 or 4 jaw on like ulma doc said above. I did almost this exact same thing trying to thread a part. The collet just doesn't have enough clamping force.

Joe
 
Liljoebrshooter said:
Ditch the collet chuck and put a 3 or 4 jaw on like ulma doc said above. I did almost this exact same thing trying to thread a part. The collet just doesn't have enough clamping force.

Joe
Click to expand...
Especially when using a form tool. A lot of extra loading. Same happens when trying to cut Acme, use the 60 degree to rough out and finish with your special tool.

Clamp well, with or without knurlling. Can you make a step on the steel to make a shoulder to stop the steel pushing back into the chuck? This will solve one of the problems.
Do you have a Weldon like end mill holder? You can make one and with a shoulder on you steel and a flat for the set screw, you should have good success.
Pierre
 
Now you've got me interested- why do you need to cut such an odd thread?
-Mark
 
Thanks for all the advice. =)

The tap is to make replacement plates for several vacuum valves that I need to repair. I'm guessing the manufacturer did not want customers fixing the valves themselves.
 
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