View Full Version : Inside ACME threading tool?

07-27-2008, 09:01 PM

I am making a new tailstock quill, for my South Bend 13" lathe and have ran into a problem. I have only to make the left-hand 8 TPI acme thread and thought that I had the tool ground correctly. So, I was cutting the threads and it appeared that the threads were not getting deeper. I stopped and checked and found that the threading tool was only cutting the threads so deep and then dragging and boring out the hole. :(

So, does anybody have a picture or can direct me to someplace where I can find a picture, drawing, etc., of a working inside threading tool bit for acme threads?

I guess, I'll have to bore it out and make an acme threaded pressed-in insert to fix it. This is the last thing I have to do to finally get this South Bend 13" disaster fully fixed.:)

Thanks in advance.


J Tiers
07-27-2008, 10:17 PM
How's about more description of your procedure....?

If the tool had the depth ground in to do the cut, it should work. There isn't anything that "magic" about an Acme thread..... except that it is generally a deeper thread than a standard V... so there can get to be quite a bit of tool pressure if the tool is advanced very far on near-full-depth cuts.

I can sort-of see that possibly dislodging the tool and causing it to cut somewhere else than expected if it isn't securely mounted, but......

Lots of folks prefer a tap for that operation, and in smaller sizes that is probably the best way. The T/S ram isn't as critical as half-nuts, for instance.

07-27-2008, 11:45 PM
The problem is common. The thread is curving toward the tool instead of away from the tool after it cuts. This means the clearance angle to allow for the helix angle must be increased considerably. You can't use the same tool shape on an inside acme thread as you do on an outside thread. You need to grind in at least double the helix angle for clearance on the leading edge. The helix angle will vary for the same tpi depending on the diameter, ID or OD, of the thread. There are calculators available online to help you determine what the helix angle is.

07-27-2008, 11:54 PM
I would cheat and buy a bronze acme nut and machine it to mount in the quill. :cool:

07-28-2008, 12:46 AM
Hahaha I like Tattoomike's suggestion! Quick and easy.

You don't happen to have a picture for us, do you Evan? For us newbs, it can be hard to picture what it needs to look like. Maybe I'll grind up a cutter in the next few days and snap some pics if it works. I need to figure out how to get this pacemaker tailstock apart and I suspect I'll need to cut some acme threads for it.

J Tiers
07-28-2008, 08:26 AM
ALL boring operations suffer from the effect Evan mentioned. There is nothing about it that would make ACME threads different.

And, the worst that would happen is that the cutter would continue to pass through the threads, riding on the heel of the cutter, without cutting. Annoying, but generally does not ruin the work.

The OP stated that the cutter wiped off the threads ("boring out the hole"). That can only happen if the cutter shifts relative to the thread helix.

07-28-2008, 08:29 AM
No problem. Angle b should be sufficient that point B clears the crest of the thread. Again, this will also vary according to the ID.


07-28-2008, 08:33 AM
This is a photo from the Atlas Lathe book on Acme threading tools.


The tool needs enough end relief to clear the small ID of the finished thread and enough side clearance to clear the helix of the thread. Grinding on an angle to match the helix is another method to compensate for this.

This can result in a relatively small tool for internal threading.

The tool is either fed at 14-1/2 degrees or straight in, and small cuts are taken.

Another alternative is to thread to rough dimensions and make a tap of part of the leadscrew material, harden with case hardening compound and finish the thread with that.

07-28-2008, 08:33 AM
The OP stated that the cutter wiped off the threads ("boring out the hole"). That can only happen if the cutter shifts relative to the thread helix.

Nope. If point B doesn't clear the thread crest as the thread is cut deeper it begins to wipe off the thread crest if the side clearance isn't at least 2 x the helix angle.

07-28-2008, 09:02 AM
I think Evan is describing what is happening....at least it makes the most sense.

As far as Tattoomike's suggestion: I just checked, with McMaster-Carr and an acme-threaded bronze nut would cost about $33. However, a brass nut would cost $6.47. Would brass work for this application?

If it wouldn't, I'll re-grind the tool and try again using the information Evan and Jim H. provided.

Thanks again,


07-28-2008, 09:15 AM
I ground a homemade acme cutting tool for internal threads so I could cut a 3/8-12 internal acme thread. I had to stop the lathe so I wouldn't run the back of the tool in the opposite face while retracting the cutter. The tool would just barely clear the bore but I got it cut. The alternative was a $70 tap which I didn't want to buy or put an insert in like Mike said. That would work but I really wanted a solid part. It is a doable job though if you have the tool ground properly.

07-28-2008, 10:35 AM
For something that size I grind the entire tool from a 1/4" stick of HSS.


07-28-2008, 11:42 AM
In "Screwcutting in the Lathe" Cleeve suggests using a round bit so it can be rotated to the helix angle for coarse threads.

Cleeve provides plans for a simple sharpening jig which I built:

In addition, Cleeve provides a nice design for a boring bar to hold a round bit and also uses the bar to hold the bit in the sharpening jig. Of course, for small diameters a bar with an inserted tool may be too big to fit...

The procedure for sharpening Acme bits with this jig differs slightly from that used for normal threading bits because of the smaller clearance angle.

Cleeve's book is one of my favorites. Very practical -- every suggestion I've tried from this book has worked well.


07-28-2008, 11:48 AM
What you need is well shown in photo 26 on page 70 of the current issue of HSM (July/Aug 08):)

07-28-2008, 12:26 PM
Bad day, I posted to the wrong thread,

J Tiers
07-28-2008, 09:47 PM
If the heel is hitting the threads, it isn't a problem with the heel, it's a problem with the tool geometry in a different direction..... And it occurs with ANY threading.

Coarse threads at small diameters worse, of course.

With coarse threads, the helix angle becomes a lot more important. Try cutting square threads......

07-29-2008, 06:45 AM
Nope. If point B doesn't clear the thread crest as the thread is cut deeper it begins to wipe off the thread crest if the side clearance isn't at least 2 x the helix angle.

Evan, point B does not need to clear the crest, as long as it has sufficient clearnce not to rub on the diameter being cut and the leading edge of the tool is ground to the lead angle plus a few degrees the tool will ride in the groove and never go anywhere near the crests.

07-29-2008, 08:13 AM
Quite so. Ensuring it does clear makes it easier to be sure that it won't be a problem.