View Full Version : Cutting a 1/2" 10 left acme thread

12-20-2012, 12:50 AM
So I am trying to make a new cross feed nut for an atlas shaper, not having a very good time. After all the work making the blank and then centering started to cut and something's gone wrong may have engaged at the wrong time or there's too much flex in the tool, anyway thread looks awfull and I can't seen to test the leads crew to it as it won't start. Anyone had luck single pointing something like this. Taps a 100 plus shipping, might try and make my own did it once befor. http://pbckt.com/pL.QFkrr6. http://pbckt.com/pL.QFkDdS.
What I making and what I've made. Anyone in Edmonton with a tap? :)

12-20-2012, 02:00 AM
The cutting tool needs to have the flanks ground for clearance, which means taking into account the helix angle of the threads. Brass can be a chore to cut internal threads in. Tool needs to be sharp. Also, it's pretty easy to have the bottom of the tool rubbing- again, this is a clearance issue. Sometimes you can help this situation out by raising the tool a bit above the spindle axis.

big job
12-20-2012, 06:35 AM
Of what I see looks like you are getting there. A nice sharp properly ground
tool is key. However any internal under an inch is tough as far a flexing.
I think the only way is many many light cuts. My old eyes, what I can see
looks like you need more root. Fiddle with tool height like Darryl said. Im also
thinking even if you go a tinch more I dont think it matters its not that too
critical on a shaper like as half nuts on a lathe. Ps Myself I would make a
second one.

+ or - Zero
12-20-2012, 07:20 AM
I usually just make an acme internal threading bar out of an Allen (hex) wrench of the largest size that will fit (you know cut off the excess L part and grind the cutter shape close as possible to the main, longer bar). Follow the tips already given by darryl, and big job, then do many, many passes, often at the same depth of cut until actually clean threads all the way through the depth of the part, then move on to a deeper cut --lather, rinse, repeat. Eventually it gets done... eventually. Don't hurry it, won't work.

12-20-2012, 09:40 AM
I had to replace the Acme threaded rod and nut on my older lathe a few months ago. Since the lathe was broken there was no possibility of making one so I went to McMaster Carr for the rod and nut. They have precision left hand Acme rods and nuts, but be sure you're sitting down when you look at the prices for the nuts.

The price of the precision left hand threaded rod isn't bad. A 3' section is $34.40.

The precision platform nut however is a bit pricey at $118.43.

Hopefully you don't need that fancy a nut for your application. Mine had to be machined and fitted into the compound. It ended up about 1/2 its original size. All is well now with the repairs. It's as accurate as the day it was new.

I'm willing to bet that if the nut is worn to the point it needs replacing the rod also has a fair amount of wear.

12-20-2012, 09:40 AM
You can also gash the end of an old acme rod and use that as a "clean up" tap in brass.

12-20-2012, 09:54 AM
I'm not sure if acetal's up to it, but you might want to try this: http://bbs.homeshopmachinist.net/threads/43645-Making-Acetal-leadscrew-nuts-the-easy-way


PS: I'm in Edmonton, don't have the tap, but wouldn't mind meeting other metal munching minds.

12-20-2012, 11:48 AM
Single-pointing the internal thread is difficult because there's a lot more deflection force cutting an Acme. I suggest you follow the other's tip about using a piece of the leadscrew to make a tap, but first single-point it most of the way first. That will keep the thread concentric to the bore and will make tapping much easier.

Jaakko Fagerlund
12-20-2012, 01:29 PM
Just yesterday made eight pieces that had a nice trapezoidal thread - 12.71 mm pitch, 40 mm length. Best result was obtained by first opening up the thread with a grooving tool and only after that I used my form tool.

Here is a nice trick:

Use a form tool that is narrower than the final thread. Take a touch from the bore of the thread and move the cutter to the final diameter, but put a suitable bolt and extension nut between the lathe cross slide and the cross slide dial (or similar place), so that it stays firmly in place when you wind the cross slide to the required diameter.

Start cutting as normal, but advance the tool with the top slide (that is parallel to the ways) and thus work open the sides of the thread.

This is way faster than cranking to such-and-such diameter over and over and also easier on the tool as it can be narrower than the final width. The only downside is that this requires opening up the thread first with a grooving tool to the final diameter.

Rich Carlstedt
12-20-2012, 02:46 PM
Good point !
I have used square tools to rough the Acme thread to reduce the cutting load, but having a narrow tool with the 14 degree angle makes good sense

100 years ago I lived in Canada and also had to make a 1/2-10 LH Acme nut , for my lathe crossfeed.
I did the best I could with a form tool, being careful to not go oversize, and then I took the leadscrew off the lathe and ground cutting flutes in the first 3-4 threads, then used the leadscrew itself as a tap. Usually the end is big as it gets no wear.
Took a bit of in/out, but I was able to tap with the screw and got a beautiful sliding fit in the middle of the screw when assembled
Basically the leadscrew was a finish tap for the job


Optics Curmudgeon
12-20-2012, 03:24 PM
I made a cross feed screw and nut for my Atlas shaper 6 years ago. The screw is 1144 stressproof, and the nut is just a brass cylinder, like the original. The screw was threaded between centers, with a follow rest. No way you'll get a consistent thread depth without one. 1144 cuts beautifully, and doesn't warp. The nut was also threaded in the lathe, in the four jaw.

12-20-2012, 03:57 PM
I'm not sure if acetal's up to it, but you might want to try this: http://bbs.homeshopmachinist.net/threads/43645-Making-Acetal-leadscrew-nuts-the-easy-way


PS: I'm in Edmonton, don't have the tap, but wouldn't mind meeting other metal munching minds.

Can anyone post a reason why an acetal nut, formed using Evan's method wouldn't be able to handle the stresses of being used on a small shaper? I'd buy a new length of left hand Acme rod and form the nut as described in the above thread. Maybe form several nuts. Even if they only lasted a year between failure, you could simply replace them.

12-20-2012, 04:27 PM
your exactly thinking my way rosco. its not for making rockets or anything else thats requiring high scruinty. . . whatever works and is easy.. .. .. even should chime in here. . .
oh hell, what do i know ? ? ? ? ?

Optics Curmudgeon
12-20-2012, 04:28 PM
In this application (the cross feed) the stresses are low, so the acetal should be fine.

12-20-2012, 07:08 PM
I've got some hard mystery plastic, mayby I will make one from that. I would like to have one more try at the brass one though, my tool was a little spindly as it was made for a 3/8 acme screw for a myford. Didn't have much luck that time either so I made the tap which worked fine. Thanks for the suggestions.

John Stevenson
12-20-2012, 07:24 PM
1/2" x 10 tpi Left hand tap 13.50


12-20-2012, 08:13 PM
I have an Atlas 7b shaper that had the same worn crossfeed nut problem. I purchased a steel hex nut with the proper thread for a very reasonable price from McMaster-Carr in the US. I welded it to a 1 inch diameter body. A lot cheaper than a tap and a lot easier than chasing the internal thread!! I do not know your sources in Canada. The original nut was brass but steel should be ok for many years.

Bob Fisher
12-20-2012, 08:38 PM
Not too difficult except grinding the tool. Made mine from an old tap, takes awhile but the steel is first class. getting the tooth shape is the hardest part without a shadowgraph , but with an acme thread gage you should be able to get very close. As has been mentioned, the clearance is the hardest part on such a small dia. Once you set up make a few for spares. The actual machining time is the least of the job. Bob.

12-21-2012, 09:46 AM
Hmmm......it's too early in the morning for me to be making major decisions, but in your first link to photos it appears your tool is oriented for cutting a right hand thread not a left hand as your title says .

12-21-2012, 01:52 PM
I have use a drill and machine the shank end to shape of the thread and then harden the end
The other way is just buy a one past tap. It is not cheep but you fine this one is use a lot on machine tools


12-21-2012, 02:15 PM
I had to make a new tail stock quill nut for my Sheldon Lathe. What I did was make a stepped tap from drill rod and hardened it. Then made up a boring tool and cut the ID threads on the lathe say 80-90%. Then took the tap and finished it up. Worked like a charm.

12-21-2012, 04:35 PM
Hmmm......it's too early in the morning for me to be making major decisions, but in your first link to photos it appears your tool is oriented for cutting a right hand thread not a left hand as your title says .

I didn't have a photo showing the setup. Haven't cut a left hand thread (successfully ) before but I believe I am correct in having the tool setup the same for a right but feeding from left to right (away from the chuck) is that not correct?