What's causing my mill to do this?

[Involute]

When I rapidly move my mill's toolhead up (z axis), it makes an awful groaning sound and eventually binds to a stop. Moving up slowly, or down, are no problem. The video below illustrates rapid up and down. I've oiled the ball screw and ways, but I still have the problem. What might be causing it? Thanks for any tips.



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[lsces]

Sound simply that you are expecting too much from the stepper motor given the weight it has got to move. Since I see a Mach3 screen in the background, I'd start by setting jog speed to 10% and check a full up and down move, then increase the speed in 10% steps until t stalls. Back off the motor tuning to 20% below that setting. What can help is to reduce the acceleration setting. I normally go for 10% of the max speed, but you need to be in the 'comfortable movement' area as too close to the max is likely to loose the odd step.
You don't say what size motor or what is driving it. Bigger motor, higher voltage power supply or higher current may be the better fix if you NEED more speed.

[Involute]

Thanks, Isces. I’ll try that. I’ve had this machine for about 10 years and it only recently started doing this. So, if the problem is the motor, it was properly sized initially (I don’t know what size it is). Is this how steppers wear out, by gradually losing torque?

[lsces]

Magnets get weaker over time ... and newer motors have even more powerful magnets so simply replacing the motor should be better then it was when new

[Involute]

Actually, I just remembered there's a gas strut on the left side of the toolhead that takes some of the load off the motor when moving the head up. It hasn't been replaced since it was new, 10+ years ago, so I think I'll start with replacing that as it's a cheaper and easier fix.

[CS900]

before you go replacing anything I'd check to make sure your gib is adjusted properly. A loose taper gib will do exactly what you're describing, fine in one direction and bind up in the other.

[109jb]

It could also be something as simple as lack of lubrication on the dovetails, screw, bearings, etc.

[LeeWay]

I am betting on the strut. That is a long lifespan for one of those.