On 06/24/2020 02:55 AM, Chris Albertson wrote:
I looked at the photos, can you tell me anything about how
the machine performed. If you milled a circle how
off-round has it?
Well, it has Acme (or square thread) screws with about .004"
backlash, so, not perfectly round.
But, actually, not real bad, either.
Yes, the Mini Mill does not have a bearing in the Y axis.
I installed a pair of thrust bearing that are held in
place by the drive pulley and the nut on the hand wheel.
It is really odd that Sieg did not put a bearing on the Y
axis when they used on X.
Yup, never understood that decision.
I wanted to keep this build VERY simple so I made a
plastic plate that screws to an existing hole on the mill
and then there is a pocket where the two bearings are
press-fit into the plastic. I did not want to cut any
metal as that would go against my "simple as possible"
goal. I wanted this to be do-able with only hand tools.
The goals is to say "look what can be done with a 3D
printer and a screw driver." I looked at your photos. VERY
similar design except I bought much larger X and Y motors
than you did so there was no need to gear them down. I
have torque to spare. They run 1:1 for X and Y. You wrote
that you found the smallest ball screw you could for the Z
axis. I took the other approach and used the largest one I
could physically fit in the space. Mine is a 16 mm
diameter. The difference is that your design spins the
screw, my design spins the nut. I had planned to use
aluminum like you did, then thought.... A 4 Newton Meter
motor can produce no more than 4 Newton meters of rection
torque even if it stalls. Plastic can handle a load like
that. All the cutting force is taken by cast iron dovetails
OK, the rotating nut might actually be a brilliant idea in
this case. But, I found that ballscrew on eBay for cheap,
and it was a perfect fit. I bored the Y axis bearing block
on my Bridgeport, which is my main CNC machine. It has less
backlash than the minimill, somewhere around .001 to
.0015". Part of that is flex in the mounting of the screws
and nuts.
Jon
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