LMS 3990 mill with the HeavyMetalCNC covnersion kit - but this isn't necessarily kit/mill specific


(I have also asked HeavyMetalCNC for advice, but while I wait for a response, I though I would ask for some opinions here.)


I have finally installed everything, dialed in all my axes, and have no binding and smooth moves.

BUT – I have a big backlash problem on my Y-axis.

X and Z – both have about a half a thou of backlash. Y – has .0331 !!!

I tore everything down, set the Y axis gib perfectly, reinstalled the Y ballscrew and nut, and reattached everything to the frame. There is zero detectable play anywhere in the mechanical system. But when driving the axis – there is .0331 measurable backlash.

If I tape a flag onto the stepper shaft, .0331 equates to about 1/8th of a turn before the ballnut engages. At 1/16th microstepping – that is about 400 steps…

Test procedures -
I measured by manually taking up the backlash, setting a test dial indicator to zero – then commanding a move away from zero, and return to zero – adjust the values until the dial does not move at all when taking up the backlash.

i.e.
-G0 Y -0.0331
-G04 1000 (pause for a second so I can see the dial)
-G0 Y0

.0331 – needle does not move <- taking up the backlash in both directions.
.0332 – needle moves a thou, returns to zero <- takes up the backlash, moves 1 thou
.0431 – needle moves to 10 thou, returns to zero <- takes up the backlash, moves 10 thou

I didn’t take any pictures or video, but I can if it will help explain better. (I am a total noob.)

So I believe it to be entirely a ballnut/ballscrew issue – don’t know which of the two or a combination of both.

What do you suggest?
-Send the ballnut ballscrew back to vendor, and see if they can pre-load it better?
-Try to pre-load it myself? (I don’t know how, or what size balls, etc…)
-Buy a new ballnut/ballscrew pair, and hope they perform better? (X and Z are fine – so it may just be a one-off part issue)
-Something else that I don’t know about that I can adjust?