Another Acme Thread Question!

I am in the process of restoring a Taylor-Hobson CXL Engraver from 1955, like this one:

The leadscrew nuts (there are obviously 3 of them) are worn. On inspection the leadscrews themselves look to be in great condition.

My question is how do I go about cutting an internal thread on replacement brass nuts? The thread looks to me to be a single start 1/2" x 5tpi Acme.

Thanks in advance.

Edited By Nishka on 03/08/2016 12:58:17

Edited By Nishka on 03/08/2016 12:58:49

Edited By Nishka on 03/08/2016 13:17:39

An ACME thread that small would normally be tapped rather than screwcut due to the limited space inside for the tool and carrier bar. If I had to do it I'd be getting creative using a live bar supported by the tailstock with the nut blank mounted up on the cross slide. Minimise the active length of the length of the small diameter portion of the bar which actually carries the tool and passes through the job. Use a support / driver bush in the headstock drilled to take the extra length of the smaller diameter with a lock screw to hold it on a flat for drive. Larger diameter going back to the tailstock. Need to ensure that the bar always resets to the same depth in the driver. Obviously the Acme tool is made thin so final cut to size is by moving saddle a fraction.

Monumental faff! ACME taps are expensive though so probably not worth it for one time use.

The delrin replication technique is worth looking into as loads are light and the screw good.

Clive.

Thanks for the quick response Clive. I am not sure I understood 80% of it but thanks anyway. That's why I added it to the bigginers questions.

If commercial taps are too expensive it is always possible to make them:

These taps were made for a 1/2" square thread, but the same principle applies for Acme. Just a different thread form.

Andrew

It is not so much the cost (athough that may not be viable for a one of use) it is finding a tap for 1/2" 5 tpi Acme thread.

I like the home made tap idea though and that could be do-able with a 5tpi thread cutting tool.

If you only use it for engraving not light milling the backlash is of little consequence. If you want to take it up leave the nuts intact but make an add on nut to work with the existing one.
 However if you find buying taps etc expensinve and daunting look up 'evanut' which will find instructions on making a delrin nut without machining.

Edited By Bazyle on 03/08/2016 13:34:20

Hi

This method is effective

http://bbs.homeshopmachinist.net/threads/43645-Making-Acetal-leadscrew-nuts-the-easy-way

I asked a similar question in May of 2014 and was directed to this link, which I had read previously but forgotten all about.

It is an article by Evan Williams for creating a delrin nut the easy way - most likely the same 'evanut' referred to by Bazyle.

I ultimately used this method to produce a replacement cross slide nut for my Mellor lathe - details of my making it here

Edited By Zebethyal on 03/08/2016 14:32:31

+1 for the delrin nut!

David

Are you sure the original isnt square threads?

I'm pretty sure it is Acme and not square, what do others think?

If it is Acme then it is not a 'standard' thread, being 1/2"-5tpi.

Edited By Nishka on 03/08/2016 16:12:43

You may be right that they are acme ,look in very good condition. The acme tap doesnt even appear on US ebay so you unlikely to find one unless very lucky.

Even if you made a tap for a nut (i presume you have a lathe ),theres no guaranteeing that it would be a good fit on the screw.

Do you have replacement nuts as i noticed a place called pantograph services appears to sell similar ones and screws with 0.2 " pitch . Nice prices though! Approx £200 for 3 nuts.

http://www.pantograph.co.uk/pdfs/spindles_accessories.pdf

Edited By mark smith 20 on 03/08/2016 16:37:47

Edited By mark smith 20 on 03/08/2016 16:39:15

Thanks for the link, those are the parts I need but as you say a tad expensive. They may be my last resort though. I can get an awful lot of brass for £200 and spend some time cutting them on my Myford.

If I go this route what is the best method of cutting a 5tpi internal thread and should the nut be brass or bronze? It looks like brass on the originals, at least to my very untrained eye!

An alternative would be to fabricate both the leadscrews and the nuts and changethe pitch to 1/2-10tpi. If I also fitted a graduated handwheel I could have measurable movement!

I made new screws for my southbend in link below, my first attempts at threading and just used a acme profile hss bit that i made up. I just used leaded free cutting steel.They came out quite good and a big improvement in the backlash.

The worst part cutting threads like this is the burr that forms on the outside which wrecks your rest jaws . I filed it off after each pass to help with that. I made them to fit the orginal nuts which were slightly worn.

The southbend nuts were bronze i think and it appears to wear to screw more than brass does.So you`ll get people suggesting brass as the worn nut is easier usually to replace and others will suggest bronze but it is harder and more likely to wear the screw.

Ive never done internal threads . But the taps are readily available from RDG tools.(dont know how good they are)

**LINK**

If you start by making a backlash adjusting additional nut in brass or delrin (acetal) it can be loose as you like. See if that work for you and only if not go to the added complication route.

Hi, have you tried pantograph services they have loads of stuff i have a similar engraver and have used them in the past , no connection i have just used them in the past

cheers

.

It looks much more like a square thread to me !

Which, if I recall correctly, is standard for Taylor Hobson.

MichaelG.

Nishka,

Although you say you are "restoring" the machine, if you aren't averse to converting to metric screws, Automotioncomponents.co.uk will sell you one metre of 12mm diameter x 3mm pitch leadscrew for a smidge over £6 and three cylindrical bronze nuts to suit at £7.12 each. These prices might be plus VAT and postage, but still fairly cheap in comparison to your other quotes. It would, of course, involve a certain amount of lathe work and drilling the old threaded section out and replacing it with the new bronze nuts, suitably reduced in diameter and attached into the remains of the original nuts with Loctite or silver solder etc. (Have a look at Stefan Gotteswinter's youtube stream - cross slide mod - to see how he did a similar modification to his lathe. That'll give you some idea of what I'm trying to convey. )

John

P.S. I have no connection with Automotion, not even as a satisfied customer!

Thats what i thought about the square thread, very difficult to tell from photos due to the turns giving the illusion of a slope to the threads .