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Converting a Grizzly G8689 Mini Mill to 3-axis CNC, Part 1:
In mid-2015, I hooked up with another local DIY type, who had this Grizzly
G8689 Minimill he wanted to fix and improve. He'd bought it already-converted,
but the previous owner had cut several corners which reduced the quality and
accuracy of the overall build. I elected to help the new owner sort out the
hardware, in exchange for him helping me learn about the software.
One of the major issues was the use of these Lovejoy couplers, which use a rubber "spider" in between
two collars. They work fine for low-precision uses such as pump drives, but are too flexible and sloppy
for accurate CNC work.
And it wasn't just the couplers- even with the 'spider' removed, there was this much slop in the ballscrew itself.
A rough measurement with a dial indicator showed at least 0.010" slop, which is way too much.
After asking around to people that know more about this stuff than I do, it was determined these were cheap
ballscrews meant only for motion control- like actuating parts of a hospital bed- rather than precision actuation.
So the couplers have to go, and the ballscrews need to be completely
replaced. I also decided to add some 2:1 belt reductions to the drives,
to increase potential resolution without having to go too deep into
microstepping (which can reduce torque by quite a bit.)
So I stripped the tables apart, did a little figuring, and eventually came up with kind of a plan.
I ordered some 12mm ballscrews off eBay, along with some angular-contact ball bearings to fit the premachined
shafts, then picked up some timing-belt pulley sets from McMaster-Carr. The steppers that came with the built
machine were 570 oz-in, which is quite a bit more power than this small machine needs. I picked up a set of
more appropriate 270 oz-in motors, as well as this 120, shown, which may still be more than strong enough.
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