Fadal 4020 Z axis Stalling issue

BretSpegel · Oct 7, 2019

Hi everyone, I have a Fadal 4020 built in 96 with DC servo motors that's having an issue where the Z axis will stop moving if I'm feeding it down at feedrates less than 10 IPM. It always seems to be at the same Z heights in different spots on the sxis, possibly in the same degree of revolution in the servo motor. When it happens the readout will show the z axis still moving but the axis will be physically stalled/not moving. The only way I can get it to resume moving is to press the E stop then manually grab the ball screw by hand & back it off a half a turn then reset the E stop. I can then bring it back to the cold start position, go through the cold start procedure & the Z axis will be calibrated to the same height it was before the issue so it's not like the resolver is slipping on the shaft. Does this sound like maybe an issue with brushes and/or maybe a dead spot in the motor? Any help or tips on where to look is very much appreciated!

ifixcnc · Oct 7, 2019

What happens if you do not turn the screw?

BretSpegel · Oct 7, 2019

The Z axis stays where its stalled at & if I attempt to jog the machine either up or down in the Z axis it eventually throws a Z axis motor overload alarm. But the ball screw turns easily by hand so it's not like anything is binding up.

Tedger · Oct 7, 2019

Swap the x axis motor for the z axis motor and see if the problem follows the motor. You can also do the same with the amplifier

Hoss710 · Oct 7, 2019

is there a Z axis way cover that could be binding up? I have seen that before on other machines.

aarongough · Oct 7, 2019

More than likely the coupler on the servo is stripped... There is an oldham coupler that links the servo to the ballscrew and on many older Fadals these are secured using just a little set-screw. If that set-screw loosens then the servo and coupling will turn, and the machine will think it's moving, but the axis will just stall in place.

Verify whether the ballscrew is actually turning when the machine stalls. If it's not turning then it's likely the coupler. If it is turning but the axis isn't moving then it's likely a bigger issue lol.

ITS have replacement couplers that seem very sturdy, or you could drill/tap your existing coupler for a beefier setscrew or for more than one setscrew.

BretSpegel · Oct 7, 2019

Hoss710 said:
is there a Z axis way cover that could be binding up? I have seen that before on other machines.

There is a cover between the Z axis ways that covers the ball screw but it's moving very freely.

aarongough said:
More than likely the coupler on the servo is stripped... There is an oldham coupler that links the servo to the ballscrew and on many older Fadals these are secured using just a little set-screw. If that set-screw loosens then the servo and coupling will turn, and the machine will think it's moving, but the axis will just stall in place.

Verify whether the ballscrew is actually turning when the machine stalls. If it's not turning then it's likely the coupler. If it is turning but the axis isn't moving then it's likely a bigger issue lol.

ITS have replacement couplers that seem very sturdy, or you could drill/tap your existing coupler for a beefier setscrew or for more than one setscrew.

This sounds like a good possibility, the ball screw definitely stops moving. However if the coupler connecting the servo motor to the ball screw were slipping wouldn't I see the Z axis zero shift up or down when I reset everything? I would think the axis would shift one direction or the other much like when you're adjusting the resolver?

Tedger said:
Swap the x axis motor for the z axis motor and see if the problem follows the motor. You can also do the same with the amplifier

I actually have a couple used spare servo motors on the shelf I can swap in to see if they fix the problem although I think I'll try swapping the amplifiers first since that's a little less involved swap. Like I said it always does it at the same Z heights & only when feeding relatively slow which makes me think its a dead spot in the servo motor. I was just hoping someone else had heard of this issue before so I could pin down a place to start rather than just swapping parts till the problem goes away..

aarongough · Oct 8, 2019

If you're un-jamming the ballscrew by hand, jogging to the home positions and then cold-starting when you 'reset' everything then you won't see a zero shift, that would only happen if the resolver wasn't coupled to the servo correctly.

What happens to the 'following error' display next to the machine coordinates when the axis jams? If you remove a drive from the machine you can still 'jog' the axis and the coordinates will move but when the following error gets over a particular limit the the control will throw a fault. If after the axis 'jams' the coordinates keep changing but the following error goes up as well then this would be an indication the servo itself is not turning, either due to a mechanical issue or some strange electrical issue.

You need to get eyes on the coupler and servo shaft and verify whether or not the servo is still turning when the ballscrew stops. This is quick to do so I'd recommend verifying that before starting to change out drives or servos.

EDITED: because I made a statement that was incorrect regarding resolver feedback to the control

BretSpegel · Oct 8, 2019

aarongough said:
If you're un-jamming the ballscrew by hand, jogging to the home positions and then cold-starting when you 'reset' everything then you won't see a zero shift, that would only happen if the resolver wasn't coupled to the servo correctly.

What happens to the 'following error' display next to the machine coordinates when the axis jams? If you remove a drive from the machine you can still 'jog' the axis and the coordinates will move but when the following error gets over a particular limit the the control will throw a fault. If after the axis 'jams' the coordinates keep changing but the following error goes up as well then this would be an indication the servo itself is not turning, either due to a mechanical issue or some strange electrical issue.

You need to get eyes on the coupler and servo shaft and verify whether or not the servo is still turning when the ballscrew stops. This is quick to do so I'd recommend verifying that before starting to change out drives or servos.

EDITED: because I made a statement that was incorrect regarding resolver feedback to the control

I'm still a little confused, I thought the resolver was connected to one end of the servo motor while the ball screw is connected to the other end of the servo motor? If the ball screw were slipping in relation to the servo then the ball screw's position in relation to the resolver would also change right? And since the resolver always clocks to the same position when you go to the cold start position that would make the physical height of the spindle on the Z axis move up or down right? Although I also have never adjusted the resolver on the Z axis, only the X & Y axis. Is there a difference in how the Z axis locates to the cold start position?

I guess the next thing to do is try to get on top of the machine with the cover off the end of the servo motor while someone else sets the machine to feed the z axis so I can see if the motor is still turning while the ball screw is stopped, unless you can suggest a safer way to verify whether or not the servo motor is slipping? I was just thinking I could eliminate that possibility since the Z axis always locates to the same physical height when I reset it to the cold start position?

aarongough · Oct 8, 2019

To clarify: if you move the axis until it jams, then move it some more, then do 'SETCS' and 'HO' will it move back correctly to the cold-start position?

For inspecting the servo/ballscrew - One trick I use when something might be a bit dodgy/unsafe to do it personally is to set my phone to record video, place it in a position where it can see what it needs to (on top of the machine in this case) then go to the thing, shut down the machine and review the video. I also do this in cases where I need to see something on the back of the machine while running it or changing settings from the front. Very handy trick when working on machines by yourself.

Hoss710 · Oct 8, 2019

1 thing I just thought of. I ran into this about 20 years ago. if the machine has lead counter Balance system . we had 1 with a bad chain, and it would do a combo of tilting the weight. and bind from the stretched links. when the weight would tilt as it would come up as Z went down it would bind or grab in the column keeping the Z from coming down far enough. we replaced the chains and it fixed the issue. something to check.

BretSpegel · Oct 8, 2019

That's a good idea on using the camera on your phone! I still wouldn't say the axis jams, when it first happened it was during a countersinking operation, where the depth & retract height were close enough that the machine fed down in Z till it stalled then proceeded to rapid to the next hole location with the countersink still down in the hole. I was able to move the ball screw pretty freely by hand in either direction, I then manually jogged the machine to the cold start position & went through the cold start procedure then touched my first tool off to verify if it had moved from where it was previously set, it was as close as you could expect it to be(within tenths). Then I attempted the same countersinking operation & the Z stalled out in the exact same position. I can easily recreate the stalled Z axis at different Z heights when feeding it at speeds less than 5 IPM, I've attempted jogging it till it alarms out yet the offset heights of my tools I currently have loaded never change more than what you'd expect on a Fadal. It might also be stalling at the same degree of rotation of the servo motor but different heights, I haven't verified this yet though. Maybe that's another thing I'll do next is see if it happens at the same degree of rotation of the ball screw.

BretSpegel · Oct 8, 2019

Hoss710 said:
1 thing I just thought of. I ran into this about 20 years ago. if the machine has lead counter Balance system . we had 1 with a bad chain, and it would do a combo of tilting the weight. and bind from the stretched links. when the weight would tilt as it would come up as Z went down it would bind or grab in the column keeping the Z from coming down far enough. we replaced the chains and it fixed the issue. something to check.

I hadn't even thought about the counter balance weight yet, it does have one so I'll look at that too. Thanks for the tip!

mmurray70 · Oct 8, 2019

The couplers do tend to loosen up like Aaron said. Usually just cause more backlash though, not a problem like that but ya never know.

Id remove the brushes from motor and take a look at them, and also look down the hole at the commutator. Rotate it all the way around and look for a bad spot. Real easy to pull brushes out to check. Kinda sounds like problem is in the servo to me.

Hoss710 · Oct 11, 2019

was curious if there was any updates on this issue?

aarongough · Oct 11, 2019

Yeah, don't leave us hanging! :willy_nilly:

BretSpegel · Oct 14, 2019

It ended up being a super easy fix, pulled the first brush out & the spring on it was definitely bad. So I pulled all the brushes out & found one other brush with a bad spring so I replaced them all & now the problem is gone! Thanks for all the help & tips!

aarongough · Oct 14, 2019

Awesome mate! Glad you got it fixed! And thanks for coming back to let us know the resolution! Im sure this thread will help others with similar issues in the future!

Bobw · Oct 14, 2019

Sweet that you fixed the issue.

But I have a dumb question.. Shouldn't a '96 4020 have AC motors???

Fadal 4020 Z axis Stalling issue

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