new build z axis not flat and screw to long

It’s been a long time since a bought my MPCNC here (Nov?). Finally have had some summer vacation time where the kids schedule and mine lined up to do the build (or at least start). I have two questions:

I loosened and tightened several times the z-axis assembly. The z rails are not entirely flat. I’m checking them on the edge of a glass table as shown in the instructions. I’m wondering how accurate this needs to be. putting three sheets of notebook paper under the end of one rail removes the rocking. My cheap, digital, Harbor Freight calipers show equal gaps between the mounts. My best guess is that the holes are far enough off (handheld drill) that when I push down to make them flat it deforms the metal and springs back slightly even after tightening. Do I need to disassemble and make the rails again?

Second issue - my lead screw is too long. Not sure if I calculated wrong now, or when ordering, or just forgot what I did. If I need to go back and redo the z-axis, I guess I might make it longer. If not, I’m thinking I’ll just cut the lead screw to the bottom of the z-rails (hack saw/angle grinder). Anything I should be cautious of there? Cut the top or bottom? Round the end slightly? Or should I go back to the calculations and remake the assembly?



Cut the end that goes into the coupler. Rounding the end or at least taking off the burr is a good idea as well.

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The Z rails can almost be twisted by hand to get them perfect (unless you actually have bent rails). Get is as close as you can before you get too irritated with it and then call it good!

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I had trouble getting the rails flat too, then I realized the holes I drilled in the tubes were too small. Since the tube is clamped between the nuts inside and the motor mount outside, the hole can be a bit bigger. I’d try reaming the problem hole out a little bit or even drill it the next size up. Once I “embiggened” the holes I had no problem flattening the assembly.

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