Lathe Cross Slide Backlash

[Matt C BeechParticipant @mattcbeech]

Hi, new poster here, so apologies if this is not quite the right forum.

I recently acquired a second hand lathe cross slide from a garage clearance, looks like it may have been for a Myford ML4 possibly?

Anyway, it's generally in good shape and is perfect for my limited needs. However it has one problem.

The screws in the cross slide have a load of backlash (1/8 " if not more) mainly because the studs that the screws are threaded to which fit into the moving slider are really loose.

They seem to be made of brass or some soft metal and should be tightly fitted, but they are not. I have a few questions:

1) Is there a straightforward way to fix this problem?

2) Why are they made of brass or some softer metal rather than steel?

3) Is it worth machining a new set with some sort of rawl-like fitting so they stay really tight?

Any thoughts very welcome,

Matt

[Matt C BeechParticipant @mattcbeech]

[Matt C BeechParticipant @mattcbeech]

Oh – I was going to share a couple of photos too. You can see the brass (?) thread on the screw here and it has already been center-punched somewhat in what I assume was a previous attempt to tighten it.

This one comes out in your hand when you dissasemble the slide.

[Tony RayParticipant @tonyray65007]

Matt,

I had an ML4 a long time a go but your lump does not look familiar.

I'm not sure what you mean by "mainly because the studs that the screws are threaded to which fit into the moving slider are really loose."so I'm going to guess a bit.

Excessive backlash will occur as a result of a worn feed nut ( the yellow bit) and the feed screw and by any end float associated with how the feedscrew is 'anchored' to the non-moving part ( sorry I'm not an engineer so my terminology may not be correct). I seems to me that endfloat is the the main problem – this can be reduced by shimming but a better method is to fit thrust bearings ( Arceutrotrade sell them) This is a common improvement made to such mechanisms. It looks like you would need one between the boss at the handle end and the casting and one between the docuble locknuts an the casting at the other end. Once fitted you take up the backlash by tightening the double nut and locking them when you are happy with the feel – too tigh will feel lumpy.

Your feed nut is most probably bronze rather than brass. Bronze is a lot tougher than it looks and has been used with steel feed screws for a very long time as it is a good combination. The feedscrew thread is probably ACME or butress and making a new nut is best left to the experts.

Hope this helps if not do come back.

Tony

[Matt C BeechParticipant @mattcbeech]

Tony,

To summarise – two possible causes:

1. End float – due to loose anchoring of the feedscrew to the static base
2. Feed nut – bronze fastening.

It's definitely the second problem. The feed nuts are both (a) a bit worn – which would account for +10 thou of movement and (b) very loose in their fixings which would a account for +100 thou of backlash.

Thanks for the pointers regarding the thrust bearings – they are a really useful tip.

Given that making a new feed nut sounds tricky (why do you think that?) what sort of experts do you think need to be tracked down to look into this?

Also, do you know why they made feednuts out of bronze rather than steel?

Matt

[roy entwistleParticipant @royentwistle24699]

Matt Feed nuts from bronze so that you get two dissimilar metals rubbing together basic engineering practice

Roy

[Sub MandrelParticipant @submandrel]

Making a feed nut is tricky because acme threads are difficult to cut (due to the large cross section of metal that needs to be removed) particularly in bronze. If you do anything less than a spot-on job you will end up with a worse nut than the original worn one. Also, you may well find there is more wear in the screw than the nut (usually around the middle) – a good bearing bronze is very resistant to wear.

10 thou backlash in the cross slide is rarely a serious problem – it will always be there and be relatively constant. Don't worry too much unless you go CNC, as 99 times out of 100 you will be feeding in only one direction. You should check the fixings for the feednut though as if it is wobbly it will affect accuracy less predictably. Sort this. have a little practice and see if you still need to replace the feednut.

Post a picture of the whole lathe and someone will identify it for you – that will make finding sources of a replacement easier.

Neil

[Matt C BeechParticipant @mattcbeech]

Roy, thanks for that insight – these are the things that someone fairly new (like me) is not aware of. Is the reason for using the dissimilar metals to do with preventing corrosion etc?

Neil, yes. I can see that would be difficult, I read up a bit following your reply, and after taping a 3/4 inch recently I can see that it would be tough! I don;t know what the original lathe was for this x-slide. It fits the small lathe I have, and this is a good thing, since the cross slide that I picked up with the lathe has a damaged fitting where the compound pivot is bolted on. The casting has broken and it cannot be tightened in some orientations.

So any opinions on my thinking at this point welcome: I think I can solve the main problem if I can tighten the bronze feed nots where they fit in the cross slide – I was thinking of the following:

– drill the spurs on the feed nuts to the depth of the spur. with a small bore drill bit (1/16 or so)

– use a slitting cutter to slice the spurs vertically through the drilled hole to effectively create a rawl plug

– insert the feed nut into the casting and then screw a short self-tapping metal screw into the top of the feed nut so that the spur is expanded to fit the hole tightly.

It feels a bit "hacky" as an approach, but the only other way I can see is to try and use a counter-punch to flare out the stub and tighten it in situ which sounds just as nasty, and the evidence is this has been tried in the past without much success.

Any other ideas from you more experienced folk?

Matt

[Michael GilliganParticipant @michaelgilligan61133]

Matt,

If I have understood the contruction correctly

[ and that may be a big if ]

The bronze nut incorporates a dowel pin, which fits a plain hole in the casting. … This is a common arrangement, and the parts should be a "bearing fit" [i.e. the dowel pin should move freely, but without slop, in the hole]: This is essential, to allow for minor errors in construction. … However; they get into the condition you have found, because the "wear-mechanism" escalates [the pin wears, so the nut tilts, so the pin wears more … etc.]

My suggestion would be:

• turn the pin to a convenient diameter, a little smaller
• ream the hole in the casting, a little larger
• make a bush to fit the pin, and Loctite it in place
• skim the [now oversize] pin to fit the reamed hole correctly

This should return things to the "as-designed" configuration.

With the gibs nicely adjusted, it should give years of service.

MichaelG.

[Matt C BeechParticipant @mattcbeech]

Michael – absolutely spot on in your understanding. I like your superior solution too, very elegant, and it confirms my nagging doubt about them being fitted too tightly (which would be the outcome of the rawl hack).

Many thanks!

Matt

[Michael GilliganParticipant @michaelgilligan61133]

Matt,

Just a quick P.S.

It's worth getting the alignment of the feedscrew with the dovetails as accurate as you can. … A bit of precision "fitting" will give things an easier ride.

MichaelG.

Edited By Michael Gilligan on 16/01/2014 11:00:51

[Gary WoodingParticipant @garywooding25363]

You might consider making your own leadscrew nuts as in the following article..

**LINK**

[BazyleParticipant @bazyle]

Other methods for adjusting a nut are to slit one side lengthways and have a couptel of screws to tighten the gap. The sloping side of the acme thread allows some take up of slack. Technically awful but has been done commercially. Next optionis to slit 90% way through crossways and again tighten up. This keeps the two halves connected but the tread is tilted so only in partial contact. Again poor but used on many smaller chinese lathes to make it look like they are more advanced in actually having an adjustment and allows sloppy production to be corrected.
Finally slit 100% at about 1/3 length and tighten up with precision adjustment method – the professional way.

[Matt C BeechParticipant @mattcbeech]

Bazyle – thanks for the tips, the feedscrew nuts are less than 1/2″ across so it sounds like it could be fiddly, I’ll look into it though.

Gary – interesting, but will probably try the other approach first.

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