Acme feed screw threading

Heres my attempts at making replacement acme screws for my South bend, which had quite bad back lash but nuts seemed ok.

These are my very first attempts at lathe threading of any kind.

First attempt on the left hand side of the long bar,really messed up ,decided to tighten the talstock centre half way through and removing half the crests.

Second attempt in middle turned out ok but i took slightly too much off .

Third attempt with nut on i prefect ,rubbed slightly but after some oil and working the nut back and forth it seems excellent.

very pleased with myself ,now onto the 7/16" x 10 left hand cross screw.

I intend cutting off the old screw portion and putting the new threaded bit in with loctite and probably a pin as well.

Edited By mark smith 20 on 01/06/2016 16:04:55

Good grief, they're excellent looking threads, first time or not!

Andrew

Your threads do look great, well done.

The nut though looks quite a bit narrower than a stock SB one, and even the stock SB ones wear pretty fast. Be sure to use a good bronze for the nut, like SAE 660 or something, to slow the wear down. JD

Thanks

Thanks, what do you mean by narrower? It fits normally in the lower casting of the top slide,so couldnt be wider unless the casting hole is bored out wider. The nut is from a 1941 9A im rebuilding and i intend doing the ones on the 1963 one i have which also have alot of back lash in the screws .I cant recall if the nut on that one is wider.

Many British lathe manufacturers never actually made their own leadscrews because it was so specialised

There was a place around the Bristol area which made huge numbers for the UK industry, Britannia? I can't remember now

Hi Mark, on my 1949 model 9B, and many others I've seen, the nut is a T shaped part which has the central round bit as you show but the nut extends out from centre along the thread on each side of centre. Sorry I don't have a photo. I'll see if I can find one. The extensions give a few more threads in contact with the screw to spread the load over and reduce wear. JD

The nut tends to be sacrificial, to preserve the accuracy of the leadscrew

On my Drummond M series I had two original nuts and it only took me about a year to burn through both of them, they were very soft

Hi, I think your refering to the crossfeed nut which is as you describe ,the one in the photo is for the top/compound slide screw,which is just a straight cylinder with a hole through.

This is the original cross feed screw which has bad back lash and i,ll be making a new screw for this. This is the t shaped nut i think Jeff is refering to. Boxfords have the same style  ..

Edited By mark smith 20 on 02/06/2016 10:24:10

Edited By mark smith 20 on 02/06/2016 10:26:35

Edited By mark smith 20 on 02/06/2016 10:26:56

I here differing suggestions for what nuts should be made from. I know the south bends screws tend to wear as well as the nuts . Bronze is supposedly better for the nuts but does it cause more wear on the screws than brass. Some south bend nuts are cast iron or steel i believe.

I think the ones above are bronze but may be brass, quite hard to tell as they are quite yellow looking for bronze.

I have some cast round gunmetal bar would that be good for nuts??

When i do the cross feed screw it is a bit longer and i will try the follower rest, which means for the left hand thread i`ll have to go from left to right.

The rest mounts on the right side of the crossslide.

Do you have the rest jaws ahead of the cutter or before, having never used one ??

To make sure the jaws werent damaged it would make sense to me to have the jaws ahead of the cutter,is that correct??

Also im aware of a nasty burr that gets thrown up when you first start the thread which will damage the soft brass/bronze jaws. I can clean with a file after the first pass. Is this the correct way??

Thanks

Just got a quote for 2 off trapezoidal (Acme) 1/2" x 10tpi nuts for my vintage milling machine !!!!!! £175.00

I have found that the lead screw wears the most, but I found the reason on a Myford 7, abrasive grit had got embedded in the softer(looked like a zinc alloy) metal of the nut. The Myford lead screw (for the cross slide) was supplied with the nut.

Ian S C

With a bronze nut on steel screw the screw wears faster than the nut. It also wears unevenly giving the usual old lathe slack in the middle tight at the ends issue.

For all practical purposes the nut wears evenly so if making a new screw its sensible to adjust the male thread width to fit the worn, wider, spaces in the old nut. Next time round the old nut will probably have worn too much to re-use. The over-width threads on your shop-made feed screw won't fit a new nut made proper acme size but it should be easy enough to trim them down to fit.

Clive.

15" Clausing cross slide nuts=$750 or 565 pounds. Not happening.