Internal Butress Thread Turning

Uglydog

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I get to make some 5TPI Internal Buttress threads for an articulating arm DP.
Pic of similar DP below.
The nut controls the elevating mechanism.
I've measured up the remnants of the old nut with my mechanical mics and intend to take them to the local tech and compare my results with their optical comparator results.
I'm carefully studying the data in the Machinerys Handbook.

I'm posting here to identify if anyone here has any tips/tricks on turning buttress threads.
Is it just like turning Acme and V threads with HSS ground to match the buttress?
Ideas?
Suggestions?


Daryl
MN

This is the same make/model of machine. But is not the specific DP on which I'm working.Hammond DP.jpg
http://www.ebay.com/itm/SLS1A8-Radial-Arm-Drill-Hammond-Cleveland-7554SO-/230692498573
 
I've never dealt with butress threads, but I would imagine that as long as your tool has the right profile and the right rerlief/clearance, you are good to go. Note that the angle of advance is fairly steep, the advance side of the tool will have to clear that.
 
The one time I had it done (by a real machinist), he told me it was real simple, just used the same tool
for the internal and external threads and matched 'em. That was for a nut to back a ten ton piston
press, a vee thread could have made radial stress that was unwanted. Throat was a couple of
inches, plenty of room for a bit in a 1" boring bar. At 5 tpi, probably your project is similar sized.
I suspect he used the shop's optical comparator to blow up the profile while grinding the
bit, which may have been done freehand.

There's a standard 45 degree thread shape, in 'the book', which is what we used.
 
Having not cut buttress I can only offer a couple of untested thoughts.

If you feed straight in with the cross slide the cutting or shearing will occur only on the 45deg face.

The compound could be spun to feed 44deg into the flat working face of the thread and the tool could even be given a small
amount of back rake to reduce shearing pressure. At a 44deg feed the back of the thread will be given a small
cut to keep it clean and the angled face will help push the tool into the flat face.
You'll just need to figure the infeed at 44deg to get proper cutting depth using the compound.
 
I would not set the compound to 44 degrees. Leave it as is, at it's normal position, and just feed in using your cross slide. You have uneven thread flank angles and may cause threading issues. If it was me, I would feed towards the lessor angle, the 7 degree, non-load bearing flank. Doing this, should keep all cutting pressures against the load bearing flank while cutting the thread. This will eliminate all of the slack in the half nuts against the leadscrew when feeding.

All of experience I have cutting Buttress threads were done on CNC lathes in past lives. Of course, most all of them were special designed for the applications we used them in.
 
Feed straight in. You will have a lot of tool pressure. So, try and do your best and make your set-up as rigid as possible. Chatter is nobdy's friend. Set your compound at 90° so you have the option to clean the thread up. Good Luck…Dave
 
I would not set the compound to 44 degrees. Leave it as is, at it's normal position, and just feed in using your cross slide. You have uneven thread flank angles and may cause threading issues. If it was me, I would feed towards the lessor angle, the 7 degree, non-load bearing flank. Doing this, should keep all cutting pressures against the load bearing flank while cutting the thread. This will eliminate all of the slack in the half nuts against the leadscrew when feeding.

All of experience I have cutting Buttress threads were done on CNC lathes in past lives. Of course, most all of them were special designed for the applications we used them in.
The small 5-7 degree angle side IS the load bearing side.
The advantage of a buttress thread which is only loaded on one side is ease of machining over a square thread that will be loaded on both sides.
The buttress thread was mostly developed for gun breeches which are only loaded in one direction if you are lucky. Other uses include aircraft propeller shafts and hydraulic cylinder glands.
Easy to cut with a lathe, thread like you are making a 60 Deg. V thread.
 
The small 5-7 degree angle side IS the load bearing side.
The advantage of a buttress thread which is only loaded on one side is ease of machining over a square thread that will be loaded on both sides.
The buttress thread was mostly developed for gun breeches which are only loaded in one direction if you are lucky. Other uses include aircraft propeller shafts and hydraulic cylinder glands.
Easy to cut with a lathe, thread like you are making a 60 Deg. V thread.
In my neck of the woods, we use the 7 degree flank to push with and the load bearing is against the opposite angle, 45 degree. Yeah, it creates large hoop stresses in the materials, too. We design for that too!
 
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