Lh Acme Nut

AlbertNakaji

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My LeBlond lathe needs a replacement cross slide screw and nut. Nut was made of "grey iron" or cast iron, very worn and broken. The screw seems to have "disappeared", along with several other items that were removed and put in a box (some 0ne must have dumped the box and the contents). Making a replacement screw is easy since threaded rod (even Acme LH threads) are available and reasonably priced. The nut is another story (special shape, looks similar to SB). Closest I could find was $80, not including shipping to Hawaii (which, if you've tried it, is horrendous since some won't use USPS even for small items).
My course of action was to cut the internal threads, but that special tap is also very expensive. Decided to take a section of the all thread and make a tap.
This is what I learned: First, cutting acme threads (5/8 x 10) using a tap is very difficult. I drilled a 1/2" hole in a piece of delrin for the minor diameter. Then the tap went in for the threads. The tap was very tapered with 3 flutes. Someone had a post about such a tap (someplace where I can't find it again) so I just followed the concept. After getting about 1/4 the depth on the taper, turning the tap became very difficult. It was difficult enough to cause me to stop and try starting the threads with an internal threading tool (regular 60 degree since I don't have the 30 degree acme shaped cutter). It seemed reasonable that once the threads are cut with the 60 degree, the rest would be easy. This is not true. There is still enough material left to be removed that turning the tap was still too difficult.
The second thing I learned with that Delrin is, indeed, a tough material. I had reservations about using it as a nut for the cross slide, so this was just a practice piece. Trying to cut the threads has convinced me that it's plenty strong for the nut.
The third thing was a reminder that just because it can be done, doesn't mean it should be done. Yes, with persistence, I will be able to get the threads cut with the tap. My decision is that if I am to continue along this line, I will grind a proper cutter for the boring bar and use that instead of a tap. I'll probably give that a try since I enjoy playing around with this stuff, but I have a feeling I'll end up buying a couple of the correct size/type of nuts and just braze/solder them together for the part. I'll be able to adjust the backlash to practically zero using that approach too.
If this was a project that had time constraints, I'd be very frustrated at this point. Since it's not, it's been enjoyable and a nice learning experience.
I hope this saves others the potential for frustration if confronted with a similar situation/need.
 
Albert, have you tried looking on ebay for a replacement cross slide Acme nut? Several guys make these nuts and they are cheaper than buying an Acme tap. I had a guy duplicate a cross slide nut for my Emco (Austrian) lathe but with a 1/2" LH Acme thread and the nut was a great fit.

I'm not sure what kind of Logan lathe you have but have a look on ebay. If you don't see what you need, contact one of the sellers and see if he can make one to fit your lathe.
 
The lathe is a LeBlond (very old one). One guy on ebay can cut a nut and it'll cost $80.

Albert, have you tried looking on ebay for a replacement cross slide Acme nut? Several guys make these nuts and they are cheaper than buying an Acme tap. I had a guy duplicate a cross slide nut for my Emco (Austrian) lathe but with a 1/2" LH Acme thread and the nut was a great fit.

I'm not sure what kind of Logan lathe you have but have a look on ebay. If you don't see what you need, contact one of the sellers and see if he can make one to fit your lathe.
 
Oops, my mistake. At least you can get the lathe up and running - worth it.
 
During my apprenticeship the LeBlond lathe in our shop lost it's crossfeed screw. The lathe had once been assigned to production and the clowns in production decided that they didn't want to do hand work, they fed the oil hole for the cross screw with CI chips (which eventually ate the screw). Anyway, the tool room got the lathe. I was given the job of replacing the screw and it's nut. The screw was easy, yeah, 5/8 10 Acme, left hand. I made a tap from drillrod, hardened it and sharpened it. I got a chunk of half hard brass and made the nut from that, Yeah, it was hard to crank that tap through the brass, but it worked nicely when I got done. Still have the tap, would send it to Hawaii if you paid postage both ways.
 
Tom,
Thank you. Will take you up on your offer. PM sent to you.

During my apprenticeship the LeBlond lathe in our shop lost it's crossfeed screw. The lathe had once been assigned to production and the clowns in production decided that they didn't want to do hand work, they fed the oil hole for the cross screw with CI chips (which eventually ate the screw). Anyway, the tool room got the lathe. I was given the job of replacing the screw and it's nut. The screw was easy, yeah, 5/8 10 Acme, left hand. I made a tap from drillrod, hardened it and sharpened it. I got a chunk of half hard brass and made the nut from that, Yeah, it was hard to crank that tap through the brass, but it worked nicely when I got done. Still have the tap, would send it to Hawaii if you paid postage both ways.
 
Look up Evanut on Google. It shows an easy way to make a lead screw nut out of plastic.
Jimsehr
 
There was a lot to read, and what is of concern is the final write-up in which Evan stated that after making 10 nuts, he found that they were not adequate to carry the load of his milling machine so he has discontinued their use.

My guess is that there are two things that affected his results that would not apply to a lathe cross slide nut: First, the nut was used (I think) to carry a lot of weight (quill or table or something else that's heavy). The cross slide, even if used to do facing cuts, should not be subjected to as much weight (my guess) on a consistent basis, as on the milling machine; Second, it appears that the nut was about 1/2 of the diameter of the screw (or about 1/2" with the screw being 1"). In the lathe screw, it will be about 1.5 times the diameter of the screw, or about 1 1/4" in length, for the 5/8" screw. Thus there should be lots more load bearing surface area. I could possibly make it even longer too.

I have some acetal to play around with while I wait for Tom's tap to arrive.

Look up Evanut on Google. It shows an easy way to make a lead screw nut out of plastic.
Jimsehr
 
Your tap in in the postal system, no bets on delivery, however.
 
Tom's tap arrived yesterday and was put into use today. Was easier than expected since I drilled out the brass and used a thread bit to do the initial cuts. Will go into the mail tomorrow and back to its home. Thank you, Tom.
 
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