A different sort of anti-backlash scheme for an X2 mini mill, and serendipity

Over the last year or so I've been (slowly) working on testing a different type of anti-backlash system for my SX2 mini mill.  This approach does not use traditional spring-loaded nuts, split nuts or heat-formed acetal nuts.

The original approach involved two nuts -- the standard, as-installed one, that is installed into the X or Y axis table, and a second "traveling" nut.  The traveling nut is constrained inside a cage that is bolted to the table.  In use, the feed screw is turned and the traveling nut is carried to one side or the other of the cage.  Once it fetches up against the cage, the slop or backlash of the system is removed.

I tested this idea using some all-thread, two nuts and two aluminum angle brackets, and it worked.  I did find that the system was sensitive to misalignment, to the point that a little misalignment would cause the traveling nut to "freeze up", making it impossible to turn the feed screw (in that case, the all-thread).

Here's a photo of my test setup:

You can see that I used the table of my mill as the test bed for this.

So I proceeded to the next step, making a bracket for the traveling nut cage, modifying a nut to fit in the bracket, and drilling/tapping two 8mm holes to mount the bracket on the right side of the saddle (I was going to mod the X axis first, because that looked to be a lot easier than the Y axis).

The final results are shown here:

The bracket is on top and the traveling nut is between the two 8mm bolts.  The bracket has a slot to accommodate the traveling nut.  In use, the idea was to insert the nut in the slot and then advance the bracket down the feed screw until it could be bolted to the saddle.

However, when I did this I found that the traveling nut would seize up once the feed screw had been turned a revolution or two.  I thought the problem was a combination of the rather tight fit of the nut on the feed screw, and machining tolerances of the bracket (i.e., my so-so machining skills).  I tried shimming but there was no improvement.

Here is where the serendipity comes into play.  I started thinking about a variation on this, similar to an anti-backlash scheme I has seen on a Sherline CNC-ready mill.  They have a nut that is on the exterior of the table, held in place by what appears to be a fine-tooth gear.  The gear is loosened enough to rotate the nut so it snugs up against the table, then tightened.

So I took a second bit of my modified nut (I had cut it into two pieces) and fed it up against the exterior of my bracket.  I had removed the traveling nut at this point.  The idea was to use the bracket to implement something like the Sherline approach, and I needed to know the orientation of the nut when it fetched up against the bracket.  But to my surprise, when I brought the nut up to the bracket, the feed screw did not seize up.  And the backlash disappeared!  Well, this was interesting.  More interestingly, when I used my original traveling nut, it DID seize up.  Whoa.  That seemed to eliminate tolerance variations on the exterior surface of the bracket.

So I took a closer look at my two nuts (please, no jokes here ... alright, I can't keep you from going there).  The main difference was a ridge left on the edge of the second nut.  Maybe it was acting as a little buffer to ease the mechanics of the system.  To test this hypothesis, I sanded the end surfaces of the second nut smooth (I had already done this on my original traveling nut) and re-installed it.  It now behaved the same, locking up with a turn or so of the feed screw.

With the "buffer" hypothesis in mind, I fabricated a rubber gasket out of some rubber sheet, maybe 1/16" thick.  The gasket ID was the same as the feed screw OD, and the gasket OD matched the external nut.  Bracket installed, followed by rubber gasket and nut:

And this works!  The nut fetches up against the gasket with enough force to prevent it from backing out when the feed screw motion is reversed, and the backlash goes to zero. 

Now, I don't know if this nice combination of items and effects will be robust or not -- it's possible that lube grease from the feed screw will find its way between the gasket and external nut so it will loosen up.  If so, I can use a scheme akin to the Sherline approach to fix the nut in place.  I'm thinking of a hole drilled in the nut, pin installed to match a little arm that engages the pin on one side, slotted on the other (for adjustment) and fixed in place with a screw that eventually screws into the bracket.  This approach won't apply any axial force to the nut/feedscrew combo, just keep things from rotating.

 CONCLUSION:

Don't give up!  If something doesn't work out as you expected, analyze the setup, experiment, make an hypothesis and test it.  Repeat as needed.  Don't forget to gratefully accept what lady Serendip gives you.