Gunsmithing Cutting internal ACME threads?

[ken226]

I ground a HSS tool for cutting 1x8tpi internal acme threads but am having a minor issue.

I set my compound at 14 1/2 degrees and make get good looking threads, but the edges of the threads end up with a little ridge. the ridge runs along both the leading and trailing edge of the thread helix and protrudes about .002 above the crest of the thread, effectively reducing the minor diameter by about .004.

its not a huge deal, because I can set the coupound and y axis back to zero and take a pass to clean that ridge out in a single pass.

The problem is that I really like using the male acme threaded part to test fit between passes, and since that little ridge is in there, I can't thread the part in between passes to get a good 90%ish thread engagement. What could be causing the metal to be 'pulling' up along the cutting edges and creating this little helical knife blade along the thread edges.

the metal is 4130 cold roll. i'm threading using a threading dial, AXA qctp on a 12x36 lathe, cutting at 50rpm. Taking about .002 of depth per pass.

 

[338RUM runner]

Make sure you have enough clearance on your cutting tool and its not rolling the edge.

 

[ken226]

Make sure you have enough clearance on your cutting tool and its not rolling the edge.

which clearance are you referring to?

 

[338RUM runner]

The sides, sharpie them and make sure it isn't getting rubbed off.

 

[JBRC]

Sounds like there is not enough relief ground on your tool. Use the advice above about using a sharpie.

Are you using a cutting oil? Is there material build up on your tool?

 

[brwn bggr]

It does sound like a clearance
Is the 4130 anealed...
If you've been "rubbing" without clearance for too long,you may have a hard time cleaning it up...
Silly question here,but have you confirmed you tool is not moving...?

 

[LRI]

A .002" depth of cut is pretty conservative. Even for an acme thread which causes quite a bit more surface contact with the tool. Have you tried more along with higher rpm?

Chromoly likes to work harden and wants rpm along with some tool engagement. Your rubbing more than cutting and this may be causing the rolled up edge. The material is being pushed out of the way instead of being sheared off. I'd try .01/doc.

Last, simplest solution may be to just polish it out with emery and a stick as you go along just to make it easier to gauge your thread. Get the abrasives out of there before your tool passes though as they will chew up your tool pretty quickly.


C.

 

[ken226]

A .002" depth of cut is pretty conservative. Even for an acme thread which causes quite a bit more surface contact with the tool. Have you tried more along with higher rpm?

Chromoly likes to work harden and wants rpm along with some tool engagement. Your rubbing more than cutting and this may be causing the rolled up edge. The material is being pushed out of the way instead of being sheared off. I'd try .01/doc.

Last, simplest solution may be to just polish it out with emery and a stick as you go along just to make it easier to gauge your thread. Get the abrasives out of there before your tool passes though as they will chew up your tool pretty quickly.


C.

Thanks for the advice guys, its appreciated. I marked the tool with a sharpie and it does appear to be cutting off both the leading and trailing edges, as well as the front. Am I right in assuming I should be cutting off the leading edge and front only? Perhaps the trailing edge is pushing up the metal and causing this ridge.

i'll try grinding more relief on the trailing edge and adjusting the angle to make sure im only cutting on the front and leading edges.

 

[Victor N TN]

If you use abrasives to deburr with, be sure to put down shop towels or cardboard on the ways. The dust from the abrasives turns the way lube into lapping compound.

Good luck.

 

[Clark]

If you use abrasives to deburr with, be sure to put down shop towels or cardboard on the ways. The dust from the abrasives turns the way lube into lapping compound.

Good luck.

If only I could get others who use my lathe to do that

 

[Victor N TN]

I've seen some very good 10" lathes that were 60 years old that the ways were smooth as new. I saw another that was less than 10 years old with about 10" directly under the chuck and rearward worn badly. Never mind finger nails or an indicator. I held a 12" square blade on the ways and you could see 1/8" of daylight under it.