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Help to stop backlash

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glenn thomas17/11/2015 07:19:32
8 forum posts

Can anyone offer me advice on obtaining a Acme profile 3/4 10tpi left hand thread single start nut? Its for the topslide on my Qualters & Smith 1324 lathe. I have tried several u.k companys only one would help at a cost of £370 as they would have to make the tooling to turn the thread, and that was for one plain nut (i would have to sape it and weld a lug on).

I have found a company in U.S and have sent several emails which they have not replied to, so i have taken the hint.

I have no expectations of finding the correct nut, i would be happy just to get a nut i could make fit, just to get rid of the exessive backlash which is annoying me!

Please help.

James Alford17/11/2015 07:37:37
377 forum posts
73 photos

Glenn,

I cannot help with making a metal nut, but I had a similar problem with the main carriage nut on my Flexispeed. I used polymorph to make a replacement, moulding it around the leadscrew and then shaping it to fit. So far, it is working well. Others have used Delrin to do the same job.

This my original thread, which contains links to the Delrin idea and several thoughts by others on alternatives to polymorph.

Regards,

James.

glenn thomas17/11/2015 07:44:53
8 forum posts

Thanks James, hadn't thought about that, will look into it!

Ian Parkin17/11/2015 08:15:51
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793 forum posts
194 photos

I would have thought that i can cut you a nut

lets see a diagram first but I should imagine £50-60 will do it in bronze

Chris Evans 617/11/2015 09:40:57
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1657 forum posts

Have you googled lead screws ? Several UK companies list lead screws and nuts at very reasonable prices. I will soon be stripping my lathe to replace the cross slide screw and nuts. A length of acme or trapezoidal threaded bar a pair of nuts and some machining should see the job done. Mine will take a little longer as I intend to make a new Cross slide at the same time.

Chris Evans 617/11/2015 09:45:07
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1657 forum posts

Just checked the places I bookmarked. Auto motion in stock and Kingston engineering. Bothe list screws plus nuts in various materials.

Hopper17/11/2015 10:45:26
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4530 forum posts
94 photos

Anyone with a lathe can make one.

Ady117/11/2015 10:49:18
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3687 forum posts
514 photos

Aluminium is also a good option, T6 has very good wear properties and is easy to cut

Bazyle17/11/2015 13:11:33
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5215 forum posts
201 photos

As an alternative to James' polymorph above look at moulding warmed Delrin around the nut. Search for "evanut" which will find a long series of reports of success on the HSM site.
Are you sure it is 3/4 in for a topslide. That seems a lot even for a heavily built lathe like that. There are also a number of ways to improve backlash other than a new nut depending on the particulars of the design.

Ajohnw17/11/2015 13:41:14
3631 forum posts
160 photos

It might be best to ask what degree this excessive back lash is in terms of % rotation ?

John

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glenn thomas17/11/2015 16:26:02
8 forum posts

I'm loosing about 15mm on the dial of the topslide which on some things is causing ripples in the work! I sent the actual nut to Abssac who identified it and were very helpful and suggested Kingston or Halifax, all these companys and many others i have tried cannot help as all their machinery is metric, hence the high cost to make a one off.

Thanks for all your help so far though!

Ajohnw17/11/2015 18:07:41
3631 forum posts
160 photos

Are you sure you mean 15 mm ? One complete turn of a slide dial doesn't usually move the slide that much. That's why I asked in terms of percentage of a turn or fractions eg 1/10, 10% etc.

The reason I asked is that sometimes people expect too much in terms of backlash. One turn of yours would move the slide by 0.1" or circa 2.5mm but best stick to inches if that's what you have. In real terms on a lead screw with that pitch and new parts might give you around 0.010in back lash maybe up to 0.015in measure on the scales.Lathes with far more than that are still usable.

The answer to cope with backlash is to always wind into a cut. That way the back lash is taken out. If for some reason you wind in too far wind out say 1/2 a turn or more and then wind in again to the correct setting. If the nut was well worn say 1/2 turn 0.050in backlash then wind out over a turn and then back in again. Even with that amount of play the lathe is still usable but most people would be thinking maybe it's time to change the nut. Problem though - the lead screw will be worn as well - if you do organise a tight nut some how it will be too tight in places.

There isn't much on the web concerning adjusting gib strips on lathes. Best done with the lead screw out or disconnected and the slide pushed by hand. What's wanted is very slight resistance to movement and no binding due to the oil film breaking down. People use all sorts of oil but some have changed to slideway oil and found it to be better - including me but your gearbox oil is probably suitable.

There is a bit here but a bit myford based as are other bits and pieces on the site which is now archived.

**LINK**

If you still don't get a decent finish it would be best to post some photo's including one of the tool you use.

John

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Edited By John W1 on 17/11/2015 18:09:14

mark costello 117/11/2015 18:08:42
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589 forum posts
12 photos

Roton in the USA has them. 1-800-GO-ROTON (1-800-467-6866)

glenn thomas17/11/2015 18:22:36
8 forum posts

Thanks for reply John, no its not 15mm, you can tell i'm a novice! theres about a quarter of a turn where nothing happens. You are right of course that the lead screw is worn too in one spot but the nut is twice as bad and i was hoping that if i could find a nut with a good thread i would reduce my problem, even better if i found a longer nut it would have extra contact on the better threads?

I have emailed Roton 3 times with no reply!

Thanks again.

Bazyle17/11/2015 18:26:25
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5215 forum posts
201 photos

Did you get the nut back? photo? Are you definitely talking about the topslide and not the cross slide? A common kludge is to slit the nut and force the gap to close or apart to take up the slack. Depending on how much room there is around where it fits there are some other solutions.

Tim Stevens17/11/2015 18:46:56
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1170 forum posts

It might be helpful to remember that there are generally two places where backlash will be created. The thread is one, but the other is where the feed screw is located. The nut wears a bit all along but mainly in one section, but the locating bit wears continuously wherever the slide is.

Cheers, Tim

Ajohnw17/11/2015 19:52:46
3631 forum posts
160 photos

You have a usable lathe with 1/4 of a turn of backlash and as some is always there best get used to it and later when you have learned to use it make a new nut and leadscrew yourself. This aspect is very unlikely to cause your finish problem. Loose slides can plus other aspects.

Sometimes it's not play in the nut that causes a lot of the backlash - it's the bearing that locates the lead screw. If that's the case you will see the parts round the dial moving backwards and forwards as you try winding in and winding out. On a lathe like the Kerry it's probably adjustable.

John

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frank brown17/11/2015 19:54:58
436 forum posts
5 photos

Take apart the compound to extricate the nut, because If its like my 70 year old British lathe the leadscrew is steel and the nut is bronze and extremely worn. On my one, the tops of the visible threads are almost pointed, they are so thin. You can check for this by screwing the compound right out and check its "loose" movement against a fixed position. A clock dial is good for this. Repeat when the compound is screwed right in. These two extreme positions are not often used in daily life so the leadscrew will hardly be worn, but the nut could be be. In mid travel of the compound both components will be worn.

Frank

Breva17/11/2015 23:54:43
73 forum posts
7 photos

Glenn,

Have a look at this site. It should answer some of your questions and gives the full sequence on making an acetal /delrin nut.

John

**LINK**

glenn thomas18/11/2015 06:20:43
8 forum posts

Thanks everone for all your help and some really good points, much appreciated.

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