Tolerances on lead screw nut vertical play

Hi all!
I built a burly in 2020 and upgraded it to a primo shortly after that.
The machine and me had quite a lot of good projects and fun, but I was always fighting with feeds and speeds.
Example project: hardwood round box with colibri engraving:

The build size is moderate (446x437x76mm) but accuracy was never living up to my expectations.

I want to use the winter time to dust of my primo and get it back running and upgrade parts or size down where necessary to get better results / stiffness.

The play in the lead screw / nut holding the lead screw in the z-axis seems suspicious to me: I understand that it should not be totally tight, but the lead scew has about 0,5-1mm vertical play - is this okay or should i change the lead screw and nut? Is vertical play ok or should there be none?

Video of vertical play in the z-axis on my primo:

Thank you for any replies!

1 Like

The boxes look amazing, honestly.

The play in the leadscrew should not matter at all because the pipes are the factor that limit the movement of the core, not the leadscrew.

The boxes look amazing, honestly.

Thank you :blush:

The play in the leadscrew should not matter at all because the pipes are the factor that limit the movement of the core, not the leadscrew.

Is there no force on the z-axis from the tool going upwards? When the milling tool enters the material from above (even in an shallow angle) or “works” through it, are there no vertical forces pushing the whole gantry “upwards”?

I am sure something is a lot of room for improvement for me, i have seen other MPCNCs on youtube which yield much better results. So if the z-axis is okay this way, I will need to look for other imperfections in my build :construction_worker_man:

I never thought about this, sorry. I didn’t read your post thoroughly. The thing is that the vertical forces are marginal, you can clamp a workpiece from the sides without clamping it from the top and it’s not going anywhere. Besides the core including router is so heavy that it should be impossible for the leadscrew to actually go up. You can try loctite though or a bit of glue to fix the tiny screw or use two of the other holes.

@felixgonschorek , the nut is softer than the lead screw so I would try swapping the nut out first. It is a wear item so that is most likely the culprit.

The brass nut is supposed to wear much faster than the steel screw, so as indicated, the nuts are what you replace.

That said, the weight of the router and the steel tubes on the Primo hold the Z axis down. Upwards forces even from a downcut mill shouldn’t really be lifting the Z axis against gravity, unless your plunge eates are crazy, or your endmill is unusably dull.

@Tokoloshe @SupraGuy @Bichepin
Thank you all for your input, I will replace the nut
I see that this is not my main issue.
I will investigate further, thank you again for having thought about my problem and your valuable feedback! :heart: