Another Acme Thread Question!

[NishkaParticipant @nishka]

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

[NishkaParticipant @nishka]

1/2″ x 5 Acme thread boring

[Clive FosterParticipant @clivefoster55965]

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.

[NishkaParticipant @nishka]

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.

[Anonymous]

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

[NishkaParticipant @nishka]

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.

[BazyleParticipant @bazyle]

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

[hthParticipant @hth]

Hi

This method is effective

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

[ZebethyalParticipant @zebethyal]

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

[David ColwillParticipant @davidcolwill19261]

+1 for the delrin nut!

David

[mark smith 20Participant @marksmith20]

Are you sure the original isnt square threads?

[NishkaParticipant @nishka]

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

[mark smith 20Participant @marksmith20]

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

[NishkaParticipant @nishka]

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!

[NishkaParticipant @nishka]

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!

[mark smith 20Participant @marksmith20]

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**

[BazyleParticipant @bazyle]

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.

[Mike LightfootParticipant @mikelightfoot72419]

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Nishka on 03/08/2016 16:12:10:

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

.

It looks much more like a square thread to me !

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

MichaelG.

[John HinkleyParticipant @johnhinkley26699]

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!

[mark smith 20Participant @marksmith20]

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 .

[Michael GilliganParticipant @michaelgilligan61133]

Posted by mark smith 20 on 03/08/2016 18:20:52:

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 .

.

A quick impression, in plaster, wax or plasticine, would soon confirm.

MichaelG.

[KettrinboyParticipant @kettrinboy]

I,m definitely a convert to plastic nuts , my Dore Westbury MK1 mill leadscrew nut stripped out and rather than try the moulding a new nut from acetal method i just made a whole new nut from acetal billet and got a new 1/2" x 10 TPI ACME tap from RDG tools and it was easy to tap a 40mm length of thread , with bronze unless you can rough the thread out in the lathe before you put the tap up it to finish the last few thou then it will need a lot of force to tap a 5TPI thread just with the tap alone as bronze is not the easiest stuff to tap a thread in , the plastic nut on my mill has lasted 3 yrs now and done a lot of work the backlash is still 10 thou which is the same as when i first made it.

regards Geoff

[HopperParticipant @hopper]

Stick the internal measuring jaws of a digital/vernier caliper down in the thread "valley" and you will soon see if it's square or Acme by holding it up to the daylight. You can then measure the width of the flat at the bottom of the valley and the width of the flat on the crest of the thread.

Looking at those nuts, you might think about turning up brass cylinders threaded internally and then boring out the old nuts and soldering the newly threaded cylinders in place. Something like common procedure for repairing Drummond lathe half nuts.

Brass will be a lot easier to machine than bronze, which is going to be important when using a 1/4" diameter boring bar with a .100" wide or so Acme/square thread toolbit sticking out the end. You might do well to rough it out with a V tool then finish it off with the final Acme/square tool. To maximize the boring bar diameter, you may have to start with the tool bit not fully sticking out and then move it out once half depth of cut has been reached.

[NishkaParticipant @nishka]

Thaks for all the answers and suggestions guys, a lot to think about. As to the thread being Acme or square – my understanding is (and I amy well be completely wrong on this) that in a square thread the Root and Crest widths would be equal at 1/2 the pitch. Given that the pitch is 0.20" then this figure would be 0.10".

On the other hand, an Acme threads Root and Crest is equal to 0.3707 x pitch, giving a Crest width of 0.0754" and a distance between crests of 0.1246".

I realise that the method od measurement shown below is far from ideal or accurate but I believe to shows enough information to determin that the thread is an Acme one.

The distance betwiin the two yellow arrows is close to 1/8" and the distance between the two blue arrows is significantly less than 1/10". To me this looks like being closer to 0.1246" and 0.0754" than to equal distances of 0.10".

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