[craynerd]

I've just got a second hand x3 mill and I noticed the z axis jumping! It wasn't smooth... It would move nicely down and then you could feel and physically hear it jump! The hand wheel didn't actually move the head and then it would fall all of a sudden.i took a video and this was a particularly good example if it working badly! Normally, I can move though 1mm very smoothly and then it happens...in this video, it was doing it often! If you turn the volume up you can hear me cranking the hand wheel but no movement on the dro and clunk when the head does fall.

X3 milling machine - z axis not smooth - YouTube

I've got to sort a few things out now but I'm going to have to come back down in a short while and try and sort this out. Anyone any thoughts on why this would be happening. A quick inspection and it seems the bevel gears are aligned ok at the back. When I turn the graduated hand wheel, the bevel gears are always meshing and the vertical lead screw is moving, just for that short time the head doesn't move with it and falls off a sudden. :-(

I also noticed this x3 model doesn't have a gas strut :-(

[handlewanker]

Hi......the weight of the head is trying to overcome the friction of the slides that are tightened up to keep it square to the column.

You are now trying to pull the head down against this friction and it will fall in short bumps as the weight takes over and overcomes the friction, stopping occasionally as the friction grabs the slides.....that is a characteristic of dovetail ways and nothing except counterbalancing it upwards against the weight of the head with positive pressure will cure it.

You will find this problem on surface grinder heads too and they cure it by having springs or counterweight to pull the head up against the screw thread so that you always have positive pressure pushing the grinding head down.....it never falls down by the force of gravity.

Gas struts are used to reduce the weight of the head, but as the machine is designed to position the head on the column and be clamped so that the quill can do the Z movement, with CNC the head now becomes the Z axis and so the gas strut carries most of the head weight to help the Z axis ballscrew.

You can have the head tight to the column slides for accuracy and prevent " head nod", but the Z stepper will not like it, or have the slides slack to allow easy travel but the head will " nod" back and forth when it moves up and down.

No matter what gas struts do you still have a slide method not designed by the manufacturers for Z axis movement because it was designed as a manual mill and the quill to do the Z axis work manually.

The only REAL solution is to mill off the dovetails on the column and fit linear ways, but that is totally re-engineering the machine big time, and not for the casual user or those with little machining experience.
Ian.

[109jb]

It could be much simpler. It is a used manual mill, so these are the things to check.

Do the ways have lube?
Are the gibs too tight?
Is the z axis nut mount to the carriage loose?
Is the Z-axis nut worn out?
Are the thrust bearings on the Z-axis screw adjusted properly?

That's a start.

[109jb]

oops. double post