Problems with finish using Mini Mill

Did you surface your spoil board? Does it cut thru the spoil board more in the back or front.

Part of my thinking is that maybe your Z-axis is tilted either forward or backward a bit (from exactly perpendicular). If you surface your spoilboard, this will show as ridges in the X-direction (if you surface along the X-axis).

 

Conventional or climb milling? Speeds and feeds? Coolant? Leadscrews or ballscrews? Spindle runout? Workholding? Upcut or downcut end mill? Carbide or HSS?

If I had to guess, I'd say you need to bump up the spindle speed and tighten up the anti-backlash on your y-axis screw, but there are a lot of possible issues at play here.

 

Yes, that's what I understood. The reason I thought about the tilt is because if the z-axis is tilted (forward for example), then the cut on the back would be different than the cut on the front. If you are doing small step-downs, this might cause roughness on one more than the other. If you do surfacing by taking passes from left to right, this tilting will show up as ridges on your spoil board. BTW, the forward/backward tilt wouldn't affect the cuts in the Y direction (as much).

Now, when I surface my spoil board, I actually take a rectangular spiral cut from the center to the outsides. This will show Z-axis tilt in both X and Y directions if it exists - but the MiniMill is kinda small. I never checked this on my MiniMill until AFTER I converted it to the larger MMXL (like a C-Beam machine). When I did, I had tilt in both X and Y. I've corrected it and it works much better now.

This may not be what is causing your problem, but it is worth checking.

 

Not sure what you mean by 0.002", but it sounds like you may have yours closer than mine. I just adjusted mine until my thumbnail didn't catch on a ridge on the spoil board.

I used a machinist square against the side of the Z-gantry plate to check for sideways tilt (making sure the spindle mount was square to the plate). To check for front/back, I used the square aligned with the front of the Z-axis C-beam (pretty much eye-balling it from the side since the square wouldn't fit against it). Now I didn't attempt adjustment while it was still the MiniMill, so I'm not sure what would be best to adjust. My Z-axis now rides on the X-axis (CBM style), so for front/back tilt, I shimmed the X-axis C-Beam (at the bottom in my case) with some thin sheet metal and aluminum foil. For sideways tilt I adjusted the gantry plate that the Z-axis C-Beam rides on the X-axis.

On the MiniMill, seems like the sideways adjustments would be more difficult (hopefully the supporting tower was cut and assembled square - the only place I can think that might be out of adjustment is the spindle mount - but you said your Y-direction cuts looked good). For the front/back adjustment, you may be able to shim the tower connections - but sounds like you got that one pretty good already.

 

I have a similar problem when cutting aluminum. I'm doing 0.05 inch width cuts with 0.05 inch depth with a 0.25 inch square endmill. Around 25 inches per minute feed with 13400 RPM speed (I have a Makita router), climb milling. I get good finish when cutting along the X axis, and poor finish when cutting along Y. Cutting along the X axis also sounds good. Cutting along Y I can hear the cutting causing vibrations, which resonate on and off and push the cutter deeper and shallower as it travels through the Y axis. So, I believe this is technically chatter, but it's not the high frequency squeal you hear on a big CNC mill when it chatters, it's lower frequency, probably due to the lower rigidity/mass compared to those machines.

Varying the speeds and feeds can help a little, as some speeds & feeds seem to resonate more strongly. Removing the manual jogging wheels from the lead screws also seemed to help a little, not sure why.

Bolting down the mill to the workbench in more locations didn't help. Adjusting the lead screw lock collars didn't help. Adding a second spindle mount and more securely shimming the existing mount didn't help. Conventional milling didn't help.

Stuff I want to try next: I want to try increasing the preload in the Y axis lead screw nut. I want to try rotating the mill 90 on the workbench to rule out interactions between the mill and workbench. I ordered some new lead screw end plate bearings and want to try hand fitting/shimming to get the fit tighter (there's currently some play in the bearing). I have noticed rotational play in the Z axis mounting plate relative to the C-beam, even though I have significant preloading on the z axis wheels, so I ordered some linear rail bearings and will rework the z axis to be stiffer.

 

I was able to get some improvements by increasing the preload on the X and Y gantry wheels. I noticed that I could see them moving slightly when I pulled on the gantry. After cranking the preload up, there's no more movement. There does seem to be a touch more resistance to movement when I crank the leadscrews by hand, but the motors don't seem to mind. I'm more concerned about rigidity than rapid movement anyway.

I'm trying to decide which is more of a problem: lack of rigidity, or lack of vibration damping. If it's rigidity, I could add half inch aluminum plate to the back of the extrusions. If it's vibration, I could fill the voids in the extrusion with sand.

 

Did you notice improvements after your column changes? If so, can you describe your new Y end plate?

I'm most worried about the Z gantry wheels, because having the wheels on the inside of the C beam seems like it makes them especially poor at resisting rotation on the Z axis. I'm planning on replacing the wheels with hiwin-style sliding block linear bearings, just need to wait a while for the parts.