The Bronze-Loaded Acme Nut Experiment - Part 2

The third concept in the above video is the one employed in my lathe. The concept will not compensate for uneven wear in the lead screw, however.

What I did on my old and well used Atlas/Craftsman lathe was to add a second nut which was coupled to the original nut by a stiff compression spring. The nut is prevented from rotating by a key and the the spring forces the nuts against opposing faces, removing all backlash.

The nut that I used was actually an OEM nut left over from replacing it and I milled a slot in the cross slide apron to accommodate the boss on the OEM nut. The slot length only has to be longer the the boss diameter by one thread length. The backlash nut is located to the rear of the driving nut so that it works in concert with the cutting forces when the cross slide is feeding in. The spring compression force should be strong enough to overcome cutting forces when the cross slide is feeding out but not so strong as to make moving the cross slide difficult. A stronger spring will also result in increased wear.

The advantage of this setup is dynamic backlash compensation which results in uniform driving force. It also is possible to attain virtually zero backlash over the entire range of even a worn lead screw. One disadvantage is that there is an increase in driving force. Another is that the range of travel of the cross slide is reduced by the distance separating the two nuts.
 
RJSakowski said:
The third concept in the above video is the one employed in my lathe. The concept will not compensate for uneven wear in the lead screw, however.

What I did on my old and well used Atlas/Craftsman lathe was to add a second nut which was coupled to the original nut by a stiff compression spring. The nut is prevented from rotating by a key and the the spring forces the nuts against opposing faces, removing all backlash.

The nut that I used was actually an OEM nut left over from replacing it and I milled a slot in the cross slide apron to accommodate the boss on the OEM nut. The slot length only has to be longer the the boss diameter by one thread length. The backlash nut is located to the rear of the driving nut so that it works in concert with the cutting forces when the cross slide is feeding in. The spring compression force should be strong enough to overcome cutting forces when the cross slide is feeding out but not so strong as to make moving the cross slide difficult. A stronger spring will also result in increased wear.

The advantage of this setup is dynamic backlash compensation which results in uniform driving force. It also is possible to attain virtually zero backlash over the entire range of even a worn lead screw. One disadvantage is that there is an increase in driving force. Another is that the range of travel of the cross slide is reduced by the distance separating the two nuts.
Click to expand...
I made something a bit like that for the cross slide on my mini lathe. The OEM adjustment for the nut is pretty lame, using three screws spaced parallel to the axis of the feed screw. The two outer ones engage threaded holes in the nut and the middle one presses against the nut. The idea is to tilt the nut just enough to minimize the backlash using the outer screws and then lock everything down with the middle one. It never worked all that well. So I cut the nut in half, ending up with two nuts. Then I put a small compression spring between them so they are forced against the feed screw threads. The last step is to tighten down the two outer screws to lock the nuts in place. To accommodate wear, just loosen one of the screws so the spring again pushes the nut up firmly against the threads, retighten, done. It's not necessary to use a real strong spring since it only is used to take out the slack. I did have to make a shim to properly align the nuts to the axis of the feed screw.

At some point it may be necessary to elongate one of the screw holes to accommodate more wear but so far it works OK without needing to do that.

Unlike RJ's approach, unequal wear on the lead screw will result in variations in the drive force needed to move the cross slide, since the nuts are firmly locked in place.
 
homebrewed said:
I made something a bit like that for the cross slide on my mini lathe. The OEM adjustment for the nut is pretty lame, using three screws spaced parallel to the axis of the feed screw. The two outer ones engage threaded holes in the nut and the middle one presses against the nut. The idea is to tilt the nut just enough to minimize the backlash using the outer screws and then lock everything down with the middle one. It never worked all that well. So I cut the nut in half, ending up with two nuts. Then I put a small compression spring between them so they are forced against the feed screw threads. The last step is to tighten down the two outer screws to lock the nuts in place. To accommodate wear, just loosen one of the screws so the spring again pushes the nut up firmly against the threads, retighten, done. It's not necessary to use a real strong spring since it only is used to take out the slack. I did have to make a shim to properly align the nuts to the axis of the feed screw.

At some point it may be necessary to elongate one of the screw holes to accommodate more wear but so far it works OK without needing to do that.

Unlike RJ's approach, unequal wear on the lead screw will result in variations in the drive force needed to move the cross slide, since the nuts are firmly locked in place.
Click to expand...
OK - as I understand it, you use the spring as a convenience to take up slack when you are deciding to remove the built-up wear, you secure it in the new position with the locknut, so you have "equal" wear on both sides of the screw.

Reading @RJSakowski 's approach, as I understand it, the screw is pushing on the nut, taking all the cutting forces when feeding in, or turning along X. If withdrawing the tool, as in (say) facing outward from the middle, or boring a hole with the tool on the operator side, then the stoutness of the spring is in play. Maybe I misunderstand some.

That same idea is behind using two (thin , flat) spur gears up against each other, one fixed to the shaft with a boss, and the other free to rotate. There is a peg or slot to fix a tension spring between them, to keep the teeth of the free gear trying to rotate "the other way", so taking up backlash. This scheme was common on the tuning dial arrangements of old radios.

I have also seen the huge pinions that rotate the 25m dish at Chilbolton. There they have a complete servo drive system, doubled! One rotates one way, and the other to drives against it. One drive gets to point, and the other has to follow the first, with enough force to be certain it can move things if it has to, but with a force just short of overcoming the other drive.

This whole repair thing started out because of inability to turn up a mounting for XRF radioactive sources! :(
 
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I recall the split gears used on radio dials. It's been a long time!

In feeding out as in boring or internal threading, the spring provides the cutting force so aggressive cuts would cause a deflection. A final "spring" pass should take care of that though. If desired, it could be avoided by flipping the tool and cutting from the back side.
 
RJSakowski said:
I recall the split gears used on radio dials. It's been a long time!

In feeding out as in boring or internal threading, the spring provides the cutting force so aggressive cuts would cause a deflection. A final "spring" pass should take care of that though. If desired, it could be avoided by flipping the tool and cutting from the back side.
Click to expand...
:) The Joe Pieczynski method!

My South Bends have 8TPI threaded spindle mounts for the chucks. Thus running in reverse risks the whole lot unwinding and dropping onto the bed at speed. Unless I can find a way to stop them unscrewing themselves, I can only use that method with a collet chuck. As it happens, the MT3 collet chuck set for my mill-drill (when it finds it's home), would fit. I need to work up a draw-bar for it. Doing all threading this way is attractive, because the tool travel is away from trouble. You can let the carriage keep moving to the right, and disengage at leisure (nearly).

Recalling the infinitely incremental automatic brake wear compensation in the front wheels of a French car I once owned, it seems to me that if the nut was split, and one half permanently encouraged to rotate so as to take up the slack whenever this became possible, with some arrangement to make the rotation "one way" only, one could know the nut was always without backlash, and yet able to transfer the full forces, whether shoving or tugging.

Also, it would be great if the dial end of the compound were a stepped bolt-on bulkhead, so that nut arrangements could be assembled without constraints from the dovetail slide. Another way might be to have as much of the nut as possible on the dovetail side to be made a female threaded blind hole, so that nut arrangements need not be confined to be within a circle perpendicular to the nut axis. When it is moved into place, the slide can be attached by adding the engagement cylinder, which has the mating male thread, looking like a bolt with a very tall cheese head! Duh - I know that many of us with 20-20 hindsight could probably design a better compound, and I suppose some do.
 
I know what a cheesehead screw is. They are uncommon here for some reason. I first encountered them on a trip to the UK in 1997. We have round head screws as well. I wasn't sure what the instrument head was. We have a raised head countersunk screw that is referred to as an oval head. Another type is a fillister head which would be similar to the cheese head but with a shorter head and slightly oval head with rounded edges. They are common in electrical connectors and sometimes have slight ridge on the underside for a better bite when making electrical connections.
 
To keep the chuck from unscrewing you need what I call a chuck keeper. Why do I call it a chuck keeper. Because it keeps the chuck from unscrewing when running in reverse. Simple to make. The headstock end is a very close fit to the ID of the chuck. Then there is a long bar that goes through the spindle with a nut and washer that holds it in place. As long as the threads on the nut are different from the 8 tpi on the spindle the two different threads work against each other preventing either from unscrewing. Both ends are machined to fit the spindle to keep it centered in the spindle. I did not come up with this. Another forum member told me about it.

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