Possible Y Axis encoder problem causing bad surface finish.

[jensen11]

Hey guys,

First of all, I am relatively new to machining so you may need to dumb things down for me at some point

I'm working with a 2017 TM1P, getting a strange surface finish. I'm guessing this HAS to be an encoder problem if I have the the correct understanding of the issue...

Here's a photo, the movements are always positive on the Y axis, and only occur during an X axis movement. I haven't noticed it when its using both axis. Its happened with my chamfer mill along the same faces so it cant be the tool. Its inconsistent between parts so it seems to me to be a mechanical issue. I'm guessing the encoder is seeing a false movement in the negative direction along the Y axis and trying to compensate resulting in a 3 thou ish movement positive in Y before jerking back to position. I can feel the machine jerking along finishing passes. I cant get this phenomenon to repeat itself by handle jogging.
I know that the problem cant be related to the spindle. I've been running the same parts for about a month now and nothing has changed, just decided to act up this morning.

Any ideas?

 

[jensen11]

It may also be helpful to know that this doesn't happen all of the time. some parts are only effected on one side.

 

[Garwood]

That's just standard Haas. You're totally fine.

 

[jensen11]

That's just standard Haas. You're totally fine.

Definitely wasn’t an issue yesterday.

 

[TAIWA NUMBA WAAN]

Here's what I think is happening, based on that photo:

You say these are happening when the Y axis is moving in the positive direction. That mean's it's actually the X axis servo that's doing it, meaning the X axis is oscillating left-right by a few thou at a somewhat repeatable rate, although not perfect. This tells me it's the Integral parameter in the PID algorithm that the servo driver is running.

I suspect there is too much static friction at some point in the X axis mechanical drive train - likely the way covers or something that's creating a high minimum amount of force required to move the X axis. So when the X axis moves, it overshoots the target coordinate by a tiny amount, but the PID can't rapidly pull it back to the correct position because there's too much mechanical friction, so when the I value (integral is always increasing to to correct error) becomes large enough, it overshoots the target position in the wrong direction again. These micro-oscillations are only a few thousandths of an inch in your case, but show up as square waves on the sides of your parts.

Check yer way covers for tight spots.

 

[BT Fabrication]

probably got slop in something, IE like the ball screws or the bed has something that crapped out on the ways.

 

[jensen11]

Here's what I think is happening, based on that photo:

You say these are happening when the Y axis is moving in the positive direction. That mean's it's actually the X axis servo that's doing it, meaning the X axis is oscillating left-right by a few thou at a somewhat repeatable rate, although not perfect. This tells me it's the Integral parameter in the PID algorithm that the servo driver is running.

I suspect there is too much static friction at some point in the X axis mechanical drive train - likely the way covers or something that's creating a high minimum amount of force required to move the X axis. So when the X axis moves, it overshoots the target coordinate by a tiny amount, but the PID can't rapidly pull it back to the correct position because there's too much mechanical friction, so when the I value (integral is always increasing to to correct error) becomes large enough, it overshoots the target position in the wrong direction again. These micro-oscillations are only a few thousandths of an inch in your case, but show up as square waves on the sides of your parts.

Check yer way covers for tight spots.

That makes sense. it is actually happening during x axis movements, and the Y axis is what is oscillating. The servo load is very low and consistent when the issue is happening, so I don't suspect its a friction issue. also the Y axis that is oscillating wouldn't be under load in this circumstance as it is a direct movement along the x axis.

 

[marsheng]

If it is like my machine, there are 2 angular contact bearings fixing the ball-screw to the table. I replaced mine and found I had to put a 0.1 mm preload washer between the 2 to get rid of the backlash. Saying that, if the steps are visible on only a part of the travel, then the ballscrews are worn in that section.

I have a servo screen that shows positioning errors. Compare the faulty one to the other axis. If its jumping about and the others are stable, it's probably tuning, if not mechanical.

 

[jensen11]

This is the best lead I have so far. I’ve been running the same parts on this machine for five years, so that sounds plausible.