[Xeno]

I"m running an aluminum job with a 1995 VF3 Haas.

When milling occasionally it will not go to the correct Z height. I 'm running 3 parts at a time for about 2 weeks on a fixture and everything was fine.

I 1st noticed it when form tapping about 1" deep when it messed up the top 1/8" of all the threads. I tried to work around by changing the R and Z values but I kept doing the same thing.

I have a set of operations where I am undercutting circles with a T slotter on the 3 parts one at a time. The 1st part comes out fine but the 2nd and 3rd the it cuts at the wrong level, and at the same place.

The day before the operator forgot to screw a SS Bolt down all the way and trashed the T Slotter insert. It cut into it with a G3 move, it did not come down on it in the Z direction.

Also it seems jumpy/jittery when STOPPING while moving down in the z axis both when moving manually and when it's running on it's own. Moving up it's fine.

I did replace the X and Y axis motor control card a couple of years ago but z axis card I believe is original.

Any help would be appreciated.

[haastec]

Do you have a weight or nitrogen counter balance. If nitrogen, check the charge. If it is a weight, it's possible the chain is stretched, but not sure if that would cause your symptoms though.
Try running at 5% or 25% and see if the issue goes away, could be an encoder if not the counterbalance.

[Xeno]

I have a weight.
I'll look to see if the weight is moving smoothly.
I'll try the running the program rapids and feed speed slower....

[Xeno]

I swapped the Y and Z axis motor driver cards but it made no difference.

I put a dial indicator under the z axis and moved the z up and down the screen shows a change of up to .045" without the indicator moving. The ball screw is rotating the whole time.

So I believe it's the ball screw nut. It's possible it could be the ball screw itself. I have a feeling they are going to suggest replacing both. I wonder how much they go for?

[the_gentlegiant]

Hey Xeno,

Before running out to buy anything I would do all the obvious stuff that costs you only time and not money. Meaning check the bolt tightness of the nut to the Z casting. Not sure on Haas whether the ball nut flange is bolted on or cast integral with the body of the casting, but check those bolts too if you find them. Also check that the nuts clamping both the fixed and free end support bearings haven't come loose. And finally check that the fixed end outer race flange is tight too. That's a huge amount of backlash you're taking about. If that was true wear you would have been noticing trouble a long time ago. If that screw is belt driven, there may be a timing belt pulley gone loose or a keyway banged out of shape. Just things to consider. Good luck with it.

[Xeno]

Wise advise from gentlegiant...
The problem was from the ball screw became loose. There are 5 screws which bolt the ball screw Nut onto it's housing. 3 of the 5 screw heads were broken and the remaining 2 were loose which explains the .045" slop in the z axis.

I was able to take out the nut housing, extract the broken screws, inspect and then reassemble with new screws. Disconnecting, reconnecting and avoiding the lubrication line was a challenge. But I was able to get it all together. The slop is gone and I believe it's fixed though I'll only be totally sure after a week of machining.

Those ball screws are impressive. Though not in the picture I saw the manufacture was THK. After over 20 years of use and no maintenance the screw and nut are in perfect condition.

[haastec]

Wise advise from gentlegiant...
The problem was from the ball screw became loose. There are 5 screws which bolt the ball screw Nut onto it's housing. 3 of the 5 screw heads were broken and the remaining 2 were loose which explains the .045" slop in the z axis.

I was able to take out the nut housing, extract the broken screws, inspect and then reassemble with new screws. Disconnecting, reconnecting and avoiding the lubrication line was a challenge. But I was able to get it all together. The slop is gone and I believe it's fixed though I'll only be totally sure after a week of machining.

Those ball screws are impressive. Though not in the picture I saw the manufacture was THK. After over 20 years of use and no maintenance the screw and nut are in perfect condition.

Wow, good catch!

On a side note, how is the noise level of the thrust bearing (located in the black casting in your first picture) when Z axis rapids up/down? If it is at all noisy, now is the perfect time to replace. It's a few hundred bucks I believe, but good insurance to avoid failure any time soon. Plus you can keep the one you take out as a back up, because s$%t always fails on Friday afternoon right before a big deadline.

Just a thought is all.

Good luck!

[Xeno]

I did quite a bit of turning the ball screw by hand with the machine powered down I don't remember any noise from the bearings or anything else. It's all packed up now but with a 1995 machine I'm sure I'll be back in there sooner or later!
Thanks

[the_gentlegiant]

Wow Xeno... super great find. Not a penny spent either. Well maybe a couple bucks for some new bolts. Looks like someone got carried away tightenning those bolts originally. Good thing you caught it before the last two gave out. An unrestrained Z axis head slamming itself into the table would be a nasty and dangerous event. Let alone expensive. Hope everything runs fine once you're making chips again.