lefthand acme thread

hi all I've purchesed a Mitchell lathe but the acme nut is badly warm I've tried to find a engineers to make one for me but with no success I live in wigan and would appreciate any help in finding someone how can make one

regards

frank

Might be worth thinking about making an acetal nut **LINK**

Rod

What thread is it? Someone might already have the correct tap.

Have you tried Kingston Engineering in Hull, Had two new acme lead screw nuts made for my Tom Senior milling machine late last year, great service but not cheap.

Before making a nut, I would check the screw for wear - the nut may be tight at the ends but loose in the middle. Better to find that out now than later.

I made a nut like the link in Rod’s post. Worked a treat. I made it from rectangular pieces of acetal - easier to hold in a vise. One can machine it down to a cylinder afterwards, leaving a collar on the end. The old nut can be bored and the two pressed together, securing with the collar - it just depends on what shape and size of your old nut. If of adequate diameter, one could add pins as well to prevent it turning, if needed. Loctite can help (groove to increase the glued area?).

The link is spot on re backlash - they are very tight and need easing off. Mine was a square thread and needed easing off on major and minor diameters to make it easier to use. I’ve so far only gently reamed it and it is still very tight. I’m going to put a nick on the screw (stone it off afterwards) and try to ease the major diameter in the nut.

This was to replace the spare brass nut, I already had, when mine failed, so not needed in a hurry.🙂 So more of an experiment than a required repair job. Backlash due to a worn screw, with these, may not be quite so important as with a metal nut?

Hi thanks for the the help it's a 4 t.p.I for a 7/16th's rod I contacted

A couple of places but some didn't do them and other would only

Do them in bathes that would cost around £900 to a £1000 I've checked the rod and there's no visible ware on it the nut is very badly warn though

Tracy tools sell some ACME and Trapezoidal taps:

**LINK**

Could you recheck the size? 4tpi seems very coarse for a 7/16" size.

What part is it?

Edited By old mart on 27/01/2020 21:28:51

Edited By old mart on 27/01/2020 21:31:27

7/16" 4tpi had 1/4" of thread and only 3/16" core. Are you sure it's not a 2-start thread with 1/4" lead and 8tpi?

Maybe stub ACME, but the Model A cross slide thread was 1/2 X 8 ACME and the core wasn't very big. 

The thread pitch, or lead can be checked by looking at the dial engraving, 4tpi would mean 1/4" , 250 thousandths, per turn.

Edited By old mart on 27/01/2020 22:39:33

Mart, the dial will tell you the lead but not always the pitch. If it's a 2-starter the lead will be .250" but the pitch .125". More importantly the double-depth will be 1/8" and the core 5/16" so much stronger.

I can't imagine how anyone would cut a 7/16-4 internal acme thread (unless as you say it's stub acme). I'm sure it's possible I just can't imagine how. Too small and coarse to single-point and a tap would be very weak. My milling machine has a 5/8" .400" lead 2-start square thread screw on it and that would be bad enough.

Hi I'll post some pic with mesurment's and check that I've got it right to be on the safe side. P's I work night's that's why my response's area a bit delayed thanks

Frank, nights is only relevant if you live in the same time zone. For those in Australia, it might not show at all!

hi all sorry its 7/8ths major diameter not 7/16ths as I stated earlier

Have a look at some of the standard lead screw manufactures like Automotion Components. There prices are really good. I bought a 1 metre length of 16mm x 4mm pitch left hand and a nut to suit for around £60 I had enough o make 3 cross slide screws.

If you have the means to machine a replica, I would look at Kingston Engineering ACME leadscrews and nuts. They do a 3/4 X 8 ACME left hand and a variety of nuts off the shelf, and you would be better off with new m and f parts even if the pitch was halved. The new threaded part could be coupled to the front part of the old leadscrew, or a new replica.

Edited By old mart on 28/01/2020 14:33:50

Hi all these are some pic of the screw and the acme nut as you can see in the second pic it is badly warn

Have never tried this, but since this is a one off.

Turn a male thread, 7/8 x 4 tpi,, in hardenable steel, such as Silver Steel

Gash it longitudinally, so that becomes a Tap.

Harden and temper.

Select the raw material for the new nut, and at the appropriate time in the sequence of machining it, (so that it can be held ) drill core diameter, or a couple of thou oversize, and Tap your Left Hand nut..

As long as the Leadscrew is unworn, your lathe should be able to do the machining.

Howard

7/8" 4TPI is eminently do-able. Last week I turned a 3/4" 5TPI last week that I was a little worried about because of the 1.8" depth but in the end it was straightforward.

Frank if you send me your leadscrew and nut I'll have a crack at it.

Pete.

From the ACME thread calculator that I use, the core will be at least 5/8", or a bit more, certainly achievable. **LINK**