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7x10, 7x12, 7x14 Mini-Lathe Information

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HOW-TO Upgrade To Ballscrews/Ballnuts In The X And Y Axes

in preparation for the plans and kits for upgrading the stock leadscrew and leadnuts to either ACME leadscrews/Acetal/Delrin leadnuts or ballscrews/ballnuts, we had to make sure the more challenging ballscrew/ballnut would work properly and not restrict travel or require material removal of the mill.

the anatomy of a ballnut
this is non-preloaded (standard, plain) ballnut from thomson/danaher purchased from mscdirect.

any catalog selling motion products will have these.

this picture is of the housing only. if you see a ballnut like this it means that the balls on the inside have been removed (and hopefully not all over your floor!)

here you go. all the pieces.

the two "c" shaped pieces are the two halves of the bearing race/guide that allows for recirculation of the chrome balls. they happen to be 0.1245" dia.

the two bags are of the same grade chrome balls - one set is 1/8" (0.1250" dia.) and the other is 3.5mm (.1379" dia.) intended for oversizing thereby reducing backlash.

turning a ballscrew and preparing the X-axis installation.
here's a turned shaft of the ballscrew.

they are hardened and are tough on the cutting tools.

notice the small chips. these were made at light cuts.

the bottom pile are too hot which were made at a high rpm (and a dulling bit) while the top pile is a better color.

here's a snapshot of the carriage. there's very little room to work with.

note the two acetal anti-backlash nuts currently in there.

here we're laying out the best positioning of the ballnut.

again, we're trying to maintain as much travel as possible.

an axial view. this is going to be a tight fit!
making a ballnut flange (with the cnc converted mini mill)
a prototype nut being cnc milled. this is the pocket for the thread of the ballnut.
here's a quick clip of a 0.020" depth of cut at 15ipm 1/4" ball end mill at ~1000rpm. click to see video
the hole has been milled. note the circle - no backlash artifacts detectable with the eye!

well, turns out the cutter was a 5/16" when it was supposed to be a 1/4"! oops!

here we go again...

same as before, no discernable backlash artifacts on the pocket hole for the ballnut thread

note the oops on the left.

here we're facing at 0.030" depth of cut this time around.

after a face and another pocket cut on the other side of the flange.
the z was probably 0.001" off since the plug was removed by simply pushing it through.
now we've contoured the entire nut flange with the 1/4" 2 flute hss end mill.
the cutter wasn't long enough so we had to redo the contour with a 7/16" end mill bit.

every time a plunge cut was made at 0.030" the table wobbled aggressively (a ball end mill would have been a better choice of cutter in this case, but would require a deeper cut to compensate for the radius of the tip).

this caused the motor to stall twice and "lost steps". all was not lost. we just re-zeroed the spindle using the dro mounted on the mill (not the control software's) and started from the last depth.

it's almost done! there are two "webs" or remaining stock left to hold the piece in place.

we'll take this off the mill and trim it with a bandsaw, then face it manually to finish it.

remember the motor stalling? the first time it happened, it was here...
here you can see a longer video of combined milling operations with the two cutters performing the operations mentioned above. click here to see video
here's the first (back) side
and the front side
a perspective shot showing why the piece had to be flipped over.

more to come..

how to reball the ballnuts to get better accuracy (e.g. reduce backlash!)
before we continue, here's a quick tip. if you have a glue gun in your shop, you can use it to quickly get dimensions of the thing you're working on.

this is the glue gun negative of the mini mill table. this gives us an idea of the dimensions we're working with...

one more thing. turning, grinding ballscrew material is tough stuff. this is a "dremel" sanding bit that lasted all of less than 1 minute of use.

supposedly the ballscrew is 4140 alloy steel that's hardened. it's very tough on your cutting bits for sure.

ok. here we go into "reballing" standard non-preloaded ballnuts.

why? this is an inexpensive way to address backlash in ballnuts. this is an effective way for machines with limited space (such as the x2 mini mill)/.

this picture shows a short piece of ballscrew to act as the jig to maintain proper hold for the grade 25 chrome steel balls.

the first step is to thread the ballscrew (no balls yet) into the nut just up to the top of the race finger (that makes sure the balls don't fall out all over the place).

then drop a few balls into the cavity.

btw, the stock balls are 0.1245" dia.

we reball to 0.125" dia. but 0.126" should yield better results.

after that, push your finger down into the top of the ballscrew while you thread the ballscrew toward your finger to force the balls into the race and back into the threads below.
look at the ball at the top. this is not good. the balls need to be between the fingers that make sure the balls don't escape.

after a few minutes, you'll get all 50+ balls back into the nut.

test fitting of the ballscrew upgrade
here's the ballscrew, ballnut, and flange installed within the table cavity of the x2 sieg mini mill.
here's a long shot. note the clearance of the ballnut with the saddle/carriage. it's close!
in the middle of travel no problems. however, at the extremes of travel X+, X-, the ballnut's edges scrape the underside of the table.
the simple, quick and only fix is to round the edges with a grinder.

these ballnuts are hardened as well. do not mill them! use a die grinder (even a file is no use) to soften the edges.

more to come...

we're almost ready to offer the plans!

we'll offer the acme leadscrew/acetal nut and ballscrew/ballnut kits as well as the plans as soon as we're happy with the results. almost there!


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