Boxford AUD Cross slide

[Speedy Builder5Participant @speedybuilder5]

The crosslide screw and nut are well worn on my Boxford AUD lathe and plenty of backlash.  I was just wondering the idea of replacing with a ball screw and stepper motor and “replacing” the handle with a digital “thingy” which would “generate” the pulses for the stepper via a controller, Arduino with display etc etc.

I have no intention of going CNC, but living in Metric land, it would be useful to be able to dial in metric or imperial at the flick of a switch.

I welcome your views on this idea.

Bob

[Michael GilliganParticipant @michaelgilligan61133]

Seems very logical to me, Bob

… most of the caveats have been well-aired in recent discussions.

[but will almost inevitably be reprised here]

MichaelG.

[Clive Brown 1Participant @clivebrown1]

Is the backlash causing a problem in producing accurate work?

[BazyleParticipant @bazyle]

NO. Not a good idea. The subject of ball screws not being a ‘magical’ improvement was covered in a recent thread wrt doing it to a round column mill.
Normally it is the nut more so than the screw. Is it ‘jumping’ yet, so worn that it slips? If not you have some way to go. If the idea of making a full new nut is daunting just make a short section two threads long. It doesn’t need to be accurate at all, just enough to thread on the screw. Add it to the current nut on the screw (off the lathe) until it acts as a lock nut. Do this at the unworn outer end of the screw. Release it by say 1/8 turn or enough to give 5 thou of play and join the nuts together with a bit of brass shim soldered across them.

Most work involves pushing the cross slide inwards so the main nut handles this. (except boring) The small nut is enough to pull it back and engages with less backlash.

[John HinkleyParticipant @johnhinkley26699]

Bob (the speedy builder),

A slightly more sophisticated version of Bazyle’s suggestion is demonstrated in This YouTube video.  And it’s adjustable, too. This is the way I’m going to reduce the backlash on my new mini lathe cross slide.  Not because I need to, but because I want to!

John

 

[bernard towersParticipant @bernardtowers37738]

I would be very wary of fitting a ball screw to a manual lathe as they are known to reverse under load if not held as it would be with a stepper.

[Speedy Builder5Participant @speedybuilder5]

OK, I hear.  Backlash is variable depending upon which part of the cross screw is being used and can be between 0.010″ – 0.025″ .  So typically for the first roughing cut, the diameter varies quite a bit. Subsequent cuts are normally OK,  but when screw cutting and backing off between cuts it presents a challenge.

Do you need such a large stepper to prevent it jumping when the cutting load is applied as obviously CNC lathes face the same challenge ?

I appreciate that the simple answer is a new screw and nut,  but just asking ….

Bob  (Speedy Builder as in the 5″ SPEEDY Loco)

 

[HopperParticipant @hopper]

Might be easiest to buy a length of Acme threaded rod and make a new screw.

[Clive Brown 1Participant @clivebrown1]

The Acme thread needs to be LH. A seller on Ebay lists Boxford cross-slide repair kits @ ~£105. Might be worth considering as it includes screw and nut.

Consistent backing off and resetting when screw-cutting is greatly helped by a cross-slide stop. I posted a picture of mine in an old thread; https://www.model-engineer.co.uk/forums/topic/cross-slide-dept-stop/ That should neutralise wear and variable backlash.

[Michael GilliganParticipant @michaelgilligan61133]

Quietly amused to see my prediction coming-true

… most of the caveats have been well-aired in recent discussions.

[but will almost inevitably be reprised here]

Note … I still think Bob’s original idea has great merit.

MichaelG.

[Speedy Builder5Participant @speedybuilder5]

For those interested, page 69 of the Boxford “Know your lathe” book explains the cross slide stop used in screw cutting, but this does not avoid backlash “jumps”.

Thanks Clive for your tag to an earlier post.

But to put my question a different way, how would you convert the cross slide to make the lathe a CNC lathe?

I have seen several blogs on the net, but the ones I have seen are using the original screws and for me, that would not be any different to what I have.

[BazyleParticipant @bazyle]

If you are going full CNC you can take a different approach, getting a cheaper? smaller (in order to fit) ballscrew with essential non backlash end bearings mounted off the back of the slide with only a (3D printed?) position indicator dial at the front. Note possible problem of ballscrew nut being too big to fit in current location.
The next decision would be how you CNC in the Z along bed direction – short range converting the topslide or heavyweight full saddle movement.

[Michael GilliganParticipant @michaelgilligan61133]

Returning the question as first posed:

This is a pretty good demonstration by Tony Foale:

https://youtu.be/3aW7tl91rq0?feature=shared

… well-worth half an hour of viewing, I would say.

MichaelG.

.

.

Edit: __ Yes, that Tony Foale

[Michael GilliganParticipant @michaelgilligan61133]

Too late for an edit, so here is the overview:

https://youtu.be/WyG4LOG8EfA?feature=shared

MichaelG.

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[Speedy Builder5Participant @speedybuilder5]

Thanks for the video link, extensive machining of the cross slide and saddle to get the ball screw in, but at the 24 minute mark an interesting comment. Practically zero backlash and no comment about the cross slide moving due to pressure of the cut.

I don’t think the Boxford would have enough room for the ball nut which would be the limiting factor.

 

[Speedy Builder5]

On Speedy Builder5 Said:

[…] no comment about the cross slide moving due to pressure of the cut. […]

Although back-driving is a real risk in some situations, I think it is mostly hypothetical/trivial in cross-slide application with a pitch of about 5mm.

For a run through the required calculations, have a look here:

https://youtu.be/7w6lJm245w0?feature=shared

MichaelG.

.

[Pete RimmerParticipant @peterimmer30576]

I know two people who have fitted 5mm pitch ballscrews to their cross slides and neither have back-drive issues. One thing to remember is the pitch is not the only function of helix angle, the diameter also affects it. Larger diameter = smaller helix angle for the same pitch. Smaller diameter = larger helix angle and greater chance of back-drive.

[John HaineParticipant @johnhaine32865]

I’ve CNC’d my S7 and IIRC I did find that with the 5mm x 12mm ballscrew fitted but no stepper it would back drive if I leaned hard on the slide.  Mine is a PXF S7 which has a larger tunnel for the feedscrew so it will take a 12mm one.  The nut had to be fitted “back to front” though as I couldn’t machine out a recess for it.  The Z feed is via the main leadscrew, too hard to fit a stepper on the topslide.  Anyway who needs a topslide when you have CNC?  Would be nice to have a ballscrew but I stuck with Mr. Myford’s standard screw – eliminating backlash isn’t so critical on Z.

Having “dual standard” electronic dials is a nice though but maybe not so easy.  Simple encoders just generate pulses – easy to arrange these to advance the slide a small metric amount every pulse, but need some software trickery to generate some subdivision of 2.54 pulses out per pulse in.  Of course all that is in effect done in a CNC controller where you could just switch to a different “imperial” profile.  Then you’d also get all the benefits of CNC like flexible screwcutting, wizards for things like tapers and ball turning etc!

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