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jdunmyer
09-18-2010, 07:05 PM
I'm helping a friend with his lathe, and we want to replace the cross-slide leadscrew and nut. Ordered a "precision Acme leadscrew" from McMaster that's 36" long; we need about 21". Machining a new nut from brass is trivial except for the tap. Buying one is about $200.00, so I'd like to attempt making it.

Was thinking of using a piece of the "extra" material and milling flutes in it. I don't think hardening should be necessary to do a single brass nut. My thinking is that the internal thread is going to be tight on the screw, but running the tap in a second time with a piece of wood in one flute to force it over a bit should do the job.

Any flaws in my thinking?

Because of physical space and mounting considerations, I don't think that a nut made by Evan's method will work.

portlandRon
09-18-2010, 07:18 PM
Check out Enco. I just got a 1/2 x 10 left hand acme. Had a 10% off code and free shipping code and got it for less then $60.

Evan
09-18-2010, 07:36 PM
It will work fine Jim. No hardening required. Tap the part and then freeze it, then run the tap through it again to get the clearance.

Here is one I made from the lead screw stock and the bronze nut I tapped with it. Only thing I would do different is to make the lead in twice as long.

http://metalshopborealis.ca/pics/lsnut1.jpg

http://metalshopborealis.ca/pics/lsnut2.jpg

Also, leave the shank longer and put a couple of flats on it so you can hold it with a wrench instead of depending on the tailstock chuck. It take a LOT of torque to run it in the brass/bronze.

PixMan
09-18-2010, 09:24 PM
I recently helped a friend make a new cross slide (2-piece) nut and left-hand Acme threaded screw for his Harrison M300. He got his 5/8"-10 LH taps out of some British supplier for $20 and had them (he gave me one) in hand in about a week.

I'll ping Mike to find out where he got the tap. I believe a 1/2"-10 LH will be even cheaper as it's smaller and...a standard pitch. Mike's 5/8"-10 LH, wasn't.

Photos of our work on Page 2 of this thread:

http://www.chaski.org/homemachinist/viewtopic.php?f=42&t=87345

Davo J
09-18-2010, 09:46 PM
I think I gave him the link to RDG for those. They are cheap, he bought 2 for $55 delivered.
http://www.rdgtools.co.uk/acatalog/ACME_TAPS__LEFT_HAND_.html

Dave

Robin R
09-18-2010, 09:52 PM
Reasonably priced ACME taps from the UK are probably from Tracy Tools, though they probably aren't HSS. http://www.tracytools.com/tapsanddiesacme.htm You should be able to get a matching bronze ACME nut from McMaster Carr. They come in different shapes, so you can likely find one you can machine to fit.

PixMan
09-18-2010, 10:36 PM
The link to RDG Tools looks familiar, or at least the prices do! And, the taps are in fact High Speed Steel. We made the two-piece nut out of 954 Aluminum Bronze.

Measured the overall length of the two existing assembled pieces while still bolted onto the cross slide. Measured the distance between them for the wedge used for backlash adjustment. Made a one-piece nut using the bought tap. Cut the new screw out of 5/8" ground Stressproof until it just barely threaded into the nut we'd made. Cut the nut in half, milled the thickness to match the OE parts, including the 15 angle for the take-up wedge.

Pin the fixed piece in place. Add the second half of the nut. Assemble complete, final adjustment to <.002" back lash. Perfect.

Davo J
09-19-2010, 12:12 AM
It was RDG that those taps came from, here is the thread about it to go along with yours above of making it.
http://www.chaski.org/homemachinist/viewtopic.php?f=4&t=87380

Dave

jdunmyer
09-19-2010, 09:24 AM
This lathe's screw is 3/4-8, LH Acme thread. Enco and McMaster both list an appropiate tap, in the $200.00 range. I could try single-pointing the nut, but that looks to be a PITA. Of course, I once made a new nut for my big planer that is something like 1 1/4"-8, LH Square Thread. That was successful, so I could probably do the Acme thread, but I'd like it to be "close" for obvious reasons.

JCHannum
09-19-2010, 09:33 AM
Jim;

Consider single pointing the thread first, even for a standard 60 degree thread, then use the tap to finish. Removal of the majority of material will reduce the force required as well as insuring the tapped thread does not wander.

I would harden the tap with Kasenit.

Evan
09-19-2010, 10:02 AM
Single pointing the thread is a good idea. It may not be necessary to use case hardening as the lead screw stock may be high enough carbon to harden.