# Z-axis moves when tightening (LMS 3990)



## ARC-170 (Jul 5, 2019)

The Z-axis on my LMS 3990 mill moves about 0.015", maybe more, (I'm eyeballing it) when I tighten the lock nut. Is there a solution to this?


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## darkzero (Jul 5, 2019)

I'm not familiar with that mill, since you mentioned locknut, does the Z lock work by squeezing on the gib? On square column mills that usually happens when the Z gib is loose. Try adjusting it. I have a RF-45 mill & mine only moves about .0002" when I lock the head.


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## Canus (Jul 6, 2019)

Are the gibs on the Z axis properly adjusted?


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## Ken from ontario (Jul 6, 2019)

The slop in my 3990  z axis is acceptable now, if you adjust the gibs so there's zero movement, then you'll have a hard time moving the head up or down so you have to have some play there. a half a turn should lock it completely.


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## Road_Clam (Jul 28, 2019)

ARC-170 said:


> The Z-axis on my LMS 3990 mill moves about 0.015", maybe more, (I'm eyeballing it) when I tighten the lock nut. Is there a solution to this?



It's definitely a gibb adjustment issue.  It's takes patients and multiple tries to get the gibbs adjusted "just right" so as to allow zero backlash and still allow smooth travel. When I lock my Z axis gibb I typically observe about a .002" change . I then back off the torque on the gibb and reset my Z with the fine adjustment knob.


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## hman (Jul 28, 2019)

Not guaranteed, but I've often had some luck by sticking a short (1/16" to ⅛") length of round brass rod into the lock screw hole between the screw and the gib.  Even something as dumb as a BB might work.

But first, as others have said, do a careful gib adjustment.


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## homebrewed (Jul 29, 2019)

Similar to Hman, I made a set of short brass plugs that are installed between the Z gib and screw.  The gib screw side is flat to match the end of the screw and the other is milled at an angle to match the gib.  The idea was to produce more-even pressure on the gib and make it easier to adjust the gib.  I scribed a line on the back of each one to make it easier to install them correctly.

To keep the gib from sliding back & forth, I milled some shallow slots on the gib.  The plugs fit into the slots.  I was making a replacement gib out of brass anyway so it was just a few extra machining steps.

I had thought about using bearing balls, actually have one underneath the gib-lock screw.  It changed the feel from a rather hard stop to a mushy one, probably because the ball can move up & down across the face of the gib.  Probably not conducive to a rigid gib setup.


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## ARC-170 (Aug 22, 2019)

The gib adjustment seemed to work, thanks. I noticed that the machine "nods" when I lock the z-axis. It goes up slightly. Tightening the gibs took almost all the play out, though.

How is the z-axis feed supposed to feel when properly adjusted? Mine feels a bit tight, but it does move. I lubed it as well.


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## Ken from ontario (Aug 23, 2019)

When tightening& locking the head, a slight nod is normal (~.001 max) ,the feel of the Z feed will never be like a drill press but it still should go up and down with some weight behind it, hard to explain but I also noticed the up/down movement will get smoother with use and lots of lubrication.
Consider installing a DRO ,at least for the Z axis, it is a  huge upgrade, I highly recommend it.


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