M6 backlash nuts, homebrew, advice plz.

[JImmyhackers]

ive known for a while my cheapy machine doesnt have and kind of backlash mitigation. (each slide has about 0.15-0.2mm play/backlash in the lead screws)

however it looks like there is space for some anti backlash nuts./device.

my idea is
the 9mm channel (see pictures), could be filled with some 9mm brass square bar with an m6 threaded hole through it, held in tension with a spring.

just wondering if anyone could think of a better way?

 

[RJSakowski]

On a relatively new machine, without undue wear, a better solution is a split nut with a means of applying bias to the secondary side to take up free play. This is commonly done with a conical point screw to force the two halves apart or a screw with a through hole in one half and a threaded hole in the other to draw the two halves together.

When there is uneven wear in the lead screw, your proposal of a floating secondary nut and a spring applying bias to take up free play. I used this method on my Atlas/Craftsman 6 x 18. I used a compression spring and put the floating nut inboard of the fixed nut so normal cutting pressure wouldn't work against the spring. If an extension spring is used, you will want the floating nut outboard of the fixed nut.

 

[RJSakowski]

An anti-rotation device has to be used with the floating nut. In my case, I milled a slot for a boss on the nut to ride in. In your case that rectangular slot looks like it was made for that purpose (it probably was). Depending pn where the slot is relative to the fixed nut will determine whether a compression or an extension spring is used

 

[JImmyhackers]

sorry, i meant a compression spring, that will keep the lead screw "in tension" between the nuts.
its my understanding that the springs "strength" determines how much force the slide can handle before backlash starts to happen.
i.e. a sprung backlash nut isnt that good of a solution.

i could probably drill and tap a grub screw into the frontside of the main dovetail to push the square brass nut away from the one thats imbedded into the slide.

 

[RJSakowski]

If the floating nut is inboard of the fixed nut, any backlash will be removed when the carriage is stationary, as in turning an o.d. or being advanced in on a facing cut as cutting forces are working with the spring. However, if turning an i.d. as in boring, the cutting forces are working against the spring forces and depending upon the load, they can overrun the spring force. This can result in chatter and imprecise cuts. It isn't an ideal solution and I would use it only where I had uneven wear on the lead screw.

Your idea about the grub screw could work. Getting it right could be a bit tricky as the spacing between the floating nut and the fixed nut will depend on how the threads in nuts are clocked relative to each other. Another possible would be to use a wedge between the nuts to take up free play. The grub screw would bear on the upper surface, forcing the two nuts apart and removing free play. If the situation allowed, the thickness of the shim could be increased to allow for two threaded holes on the side of the adjustment screw to work against it to fix the shim in place.

Your adjustment range only needs to be about .5mm but depending upon the clocking, you could have to take up as much as the otch of your lead screw as well. A workaround would be a second tapered shim, inverted, which would have a hole rather than a slot and a thickness to take up the necessary space to accommodate the that caused by the lead screw nut clocking.

Finally, the wedge could be inverted so the adjustment screw(s) are pulling rather than pushing and a third screw threaded into thecarriage pushing in opposiion to lock the shim on place.

 

[homebrewed]

I have a mini lathe and a mini mill and have taken two different approaches for each of them. For the mini lathe, I cut the cross slide nut in half and inserted a spring between the two halves. With no machining load that would work OK but not so good otherwise. But the mini lathe's cross slide nut adjustment setup permits a different approach, where I can lock the spring-driven nut down to greatly reduce the backlash without being affected by machining forces. When, due to wear, the backlash increases, I just loosen the screw. The spring pushes its nut out to the other side of the thread shoulder, so when I tighten down the screw again the backlash is gone. So far it's working fine.

For my mini mill I have a setup on the X axis that essentially replicates the Sherline approach to minimizing backlash. They use a second nut on the feed screw that's exterior to the table, unlike the mini mill feed screw nuts. The exterior nut is lightly tightened down against a plate on the end of the table and held in place with a gear-like setup so it stays in place. To accommodate wear the exterior nut assembly can be loosened, the nut run up against its bearing surface, then tightened down again.

Using my version, I don't have a problem climb milling steel on my mini mill.

 

[JImmyhackers]

thanks for the info....spring loading the nut could be a good idea.
however im somewhat limited in access to whereever i put adjustment screws.
they can only go at the front. The top, bottom and sides are not viable/accessible when the machines put together.

i generally dont have a problem with my machines and their backlash......except on the lathe when parting.
when i get down to the end/middle/centre....the backlash means the slide can get pulled towards the work peice, the work peice will flex up and the parting tool goes under the work :S

not ideal

im hoping my additions will stop this