z axis loosing steps

[ptjw7uk]

Hi All
Got back to playing with my cnc mill. Tried to do some engraving but as the engraving proceeds accross the x axis the cutting depth gets greater so I assume the z axis is loosing steps.
How is one supposed to test for this, I think I should try a dial gauge.

Peter

[tribbles]

That sounds to me like the surface isn't parallel to the X-axis. Does the cutting depth get less when moving the other way around (just an obvious thing to check!)

You could also try locking the Z-axis somehow (so it won't move).

[ptjw7uk]

Problem is if you lock the z-axis you wont be able to do engraving!

peter

[tribbles]

Yes, but you can check if it moves up and down - it's only for testing.

[John S]

Reduce the acceleration what is happening is that the Z axis is losing steps on the way up so when it lowers it goes deeper into the work.

May pay to also reduce the microstepping as more steps, less torque.


.

[Robin Hewitt]

There is a difference between up and down.

Up can be the start of two consecutive G0's with a 180 degree turn in the middle.

A sort of worst case scenario for losing steps if you're accelerations aren't quite up to scratch.

[ptjw7uk]

so what should the acceleration be!
I thought more steps was better, obviously not.

Peter

[John S]

Unfortunately no one can tell you as it's all machine dependant.

You need to alter the settings and try it under power.

Replace the tool for something with a flat end [ reverse the cutter ? ] and then get something like a 1/2" roller or just a piece of short scrap.
Put that under the 'cutter' and zero onto it. Remove roller and move head up and down a few times and see if the roller will roll back under when it's in the down position.
This will give you a bit of leeway for it to move without clouting into any work.

.

[ptjw7uk]

Got back to my X1 mill and tried JohnS suggestion to reduce the acceleration. Tried a search on the web, not a lot of help so just winged it. Also tried JohnS suggestion about the spaced under the z-axis and it definately showed it losing steps on the up direction.
Thought some more and have now tried a dial gauge, and old one I have had for some time and never found a use for in that it works by pulling up the plunger. So I fixed the plunger rod to a chuck in the z-axis and proceeded to raise and lower the head, could not detect that much change but it did alter over 10 cycles, ie gets lower.
I then thought about the gibs and tried to adjust them, this has made it worse.
I also noticed another strange thing in that if I jog up or down using the slowest jog rate the dial gauge shows the head to move in the same direction for a few steps after I have changed direction?
I have read on the web that some have had problems with the head gibs saying that are difficult to set due the weight of it. I may now have to consider a counter weight system as for what I want the mill for a poor z-axis is next to useless.
Peter

[irving2008]

Peter,

How are you driving the Z-axis and what is the drive to the quill motion, a worm and rack or pinion and rack?

Putting in a counterbalance arrangement will aid in supporting the weight of the quill etc, but at the expense of adding more mass to move = lower acceleration for the same motor torque