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Newer Z is gooder

A project log for Minamil 3dp: another minimal CNC mill

A very compact, very inexpensive, very DIYable, very precise little CNC mill. This one uses 3d printed parts.

paul-mcclayPaul McClay 07/03/2024 at 04:080 Comments

The first Z axis was ... good enough to not fuss with for a while while fussing with the X+Y axes. The second try (i.e. first attempt to improve) was meh. This one came out better.

The moving part & clamp changed more than the fixed part. I'll have to decide whether to make the next one slimmer or make the base plate behind it wider.


The most visible intent/reality gap with the first 3d printed (vs. 2d laser-cut) Z axis was the fiddly half-dozen M3 socket head screws in the tool clamp.

The clamp screws ran into single-perimeter holes. That "worked" because pull-out strength can be ok. But it didn't really work for the idea in mind because the hole threads were vulnerable to torque-out and wear from frequently turning the loaded screw  ̶k̶n̶i̶v̶e̶s̶ threads (call it "slice-out"?). That required attention and a tool to torque up the screws carefully, and to carefully torque all of them instead of just half (one side) to ensure even (i.e. minimum) tension.

Not the quick, casual, routine operation I had in mind.  


Reducing that mess to two thumb nuts:


A less evident fault with the first printed Z axis was sagging PLA around the warm motor.

left=up; right=down

The motor mount screw holes yielded some but didn't fully fail. I don't know if that's because it stopped creeping after I turned down the motor current, if it relaxed to some equilibrium and stopped there, or if it was ten minutes from falling apart.

In any case, it wasn't a big surprise that the PLA softened.

For the first version I opted to just print it and see what happened before getting too wrapped up in trying to anticipate heat effects. It didn't take long to confirm that the pulley(s) couldn't be PLA. PETG is working fine there (white vs. blue in the second pic). The motor attachment to the base part wasn't so obviously bad. It looked fine for a while. Distortion around the screws became evident after a while but wasn't a big deal. The screws continued to hold. It was "ok" to use but definitely needed some help.

I suppose it's possible that with two motors the motor current could be reduced enough to keep the motors cool enough for PLA. I haven't tested that. (A second motor/pulley/cord can be installed for cheap insurance against dropping the big angry part. That's more of a concern in the laser-cut version where the cord/s is/are hidden and run close by sharp edges.)


In the second version I tried embedding hex nuts at the surface -- middle in the pic below. They didn't help pull-out strength which still relied on the screws holding in the plastic part and I doubt they got any less hot than before. The idea was to try spreading the radial load over a larger area of PLA. I think they actually helped more than what I had in mind: by fixing the screws perpendicular to the motor tabs they spread the radial loads much more by requiring the motor to drag the whole screws sideways through the PLA if it was going to go anywhere. They also protected the threads in the PLA hole from stripping by providing a hard stop to torque the screws into. It looks like they worked because there's no indication that they moved. Caveat I didn't run that version very long or in hot weather.

While the nuts seem to have worked well as far as tested, that arrangement had a couple of drawbacks. If a nut were removed, I didn't have any great ideas for how to put it back in the right orientation to match the nut thread to the hole thread. And nuts would add a line to the BoM which I'm trying to keep short. 

For this version I'm trying PETG plugs in the PLA part to hold the hot motor mount screws -- right side in the pic above. PETG (or whatever can bear the temperature) is already in play for the pulley(s) so that adds nothing to the shopping list.



to the corner

Because I'm also chasing compact packaging, these 3d printed Zs have been designed to fit in a corner.

Part of that idea is that the closed-up box should be durable for casual storage. On one hand, I want to push the "spindle" as close into the corner as possible (i.e. pull the corner as close in to the "spindle" as possible). On the other hand, the transparent panels are less rigid that the hard frame and could get pressed in to some extent while jammed in a closet. The X&Y axes are well protected by the incompressible bottom part of the frame. But the upper parts of the "door" panels have no such support -- so I want to keep some buffer space between the inside of the panels and moving parts of the Z axis. But how much space is adequate "insurance" vs wasted excess?

A late idea as I was getting close to done with this revision was to turn the motors parallel to the (imaginary) corner walls. If I ever get around to matching this with an enclosure where it really is right exactly in the corner, the flat backs of the motors (or motor and a printed blank) will provide incompressible strong support at the tops of the less-rigid enclosure panels. That give pretty good protection for soft parts with much less "insurance" space. Optimistically, I think that might work really well.

(for my generic dremel-like tool, the corner of the big switch boss limits how close to the corner it can fit -- assuming symmetric Z-axis parts and simple orientation of the tool; that's ok because it's also near exactly the same limit location as for the 56mm fan/flywheel(?) that's apparently universally added to 52mm spindle motors)

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