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Random motion in Z on 2001 VF-5 (about .023" slip) between parts

Milland

Diamond
Joined
Jul 6, 2006
Location
Hillsboro, New Hampshire
Hi all,
My (mostly) trusty VF-5 has started to show a random slipping in Z that seems to be around .023" max. Today was the first time I saw this excess motion, and it's showing up as deep or shallow cuts on a series of parts that are fixtured pretty consistently in Z, so I don't think it's a parts location issue. Tooling is just three different smaller endmills, all secured in good toolholders with clean pockets and tapers. Cutting forces are low, this movement occurs even when rapids are turned down to 25%. When movement occurs it seems to be consistent per part, but I've only done a few pieces to avoid too much scrap.

I have searched for info on similar problems, not come across a definite failure mode, but could this be pointing to a slipping coupling between the Z servo motor and the ballscrew? My understanding is there's no key or other "fixing" element in the coupling, but if you know better please say so. With the encoder on the servo, coupling slipping will ruin your Z positioning, anything else it could be? I'm pretty sure the thrust bearings are OK, I don't hear anything on axis reversal or constant feed.

Gotta love these weekend failures...

Thanks for any help!
 
I-ve been lucky enough with my Haas's that I've never had to work on one, so I'm not sure how their motors are coupled. On past machines, I've encountered problems with the couplers slipping. One of the weirdest problems I ever came across, I had a machines which was doing something similar to what you described, but in "Y". On a lot of servo motor assemblies, the encoder is coupled through a bellows coupling. The bellows coupling had fatigued and was broken in the middle, but there was enough spring pressure to hold it together so it didn't just stop working, the two broken ends would catch, but then slip when it changed direction. If you put an indicator in the spindle and move Z up and down, it will look like inverse backlash.
 
Thanks, Larry - I'll be working on the machine later today, if I find anything else I'll be posting about it. I've not heard of Haas servo's having anything like a bellows for the encoder attachment, but it's possibility that something failed in that area.
 
Does this machine have a nitrogen counter balance system? If yes, check the pressure and fill if needed.

The coupling is a possibility, but I have not run into that particular scenario personally. Access to the coupling isn't to tough, make a few timing marks and run it some more to verify.
 
Does this machine have a nitrogen counter balance system? If yes, check the pressure and fill if needed.

The coupling is a possibility, but I have not run into that particular scenario personally. Access to the coupling isn't to tough, make a few timing marks and run it some more to verify.

Hi haastec, thanks, I was hoping you'd see this thread. Both my VMC's use nitrogen cylinders, and I have a small N bottle and refill coupling, so the machines are spot-on (the VF-5 is at 875psi with the head raised).

I'm going to run some tests with an indicator in a little bit, will see If I can access the coupling to put some marks on it. Have you ever heard of a failure of the encoder as Larry described for Haas servos? This really is out of the blue, I've not had a problem like this with any of the Haas mills I've run over the last ~20 years.
 
Hi haastec, thanks, I was hoping you'd see this thread. Both my VMC's use nitrogen cylinders, and I have a small N bottle and refill coupling, so the machines are spot-on (the VF-5 is at 875psi with the head raised).

I'm going to run some tests with an indicator in a little bit, will see If I can access the coupling to put some marks on it. Have you ever heard of a failure of the encoder as Larry described for Haas servos? This really is out of the blue, I've not had a problem like this with any of the Haas mills I've run over the last ~20 years.

I have never had one of the Haas AC servo motors apart to even see the encoder, so I cannot comment on it's construction.

It's probably worth checking voltages in the cabinet. If you are using NON smart amps, then your amps will have a yellow, white, and black wires going to each drive. Check to make sure you have +/- 12vdc at these wires. Check DC Buss voltage, and also check the voltages going to the pcb's stacked at the upper left of the cabinet. Again, make sure they each have good +/-12vdc and +5vd going to them. Bad voltages can cause strange symptoms.

You can also check to see what you have for backlash in Z-axis also. It's possible that a lock nut on either end of the ball screw is loose but usually you would hear a thud with this condition as it slaps going back and forth.

You should be able to access the z motor coupling when the head is all the way down, it will be behind a gold colored cover.
 








 
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