Hurco VM2 backlash

[Haggismaximus]

In July I purchased a 2005 VM2 as my first VMC. Machine is in very good shape visually, control had approximately ~23K hours however there were only about ~5500 spindle on hours. Previous owner was Lawrence Livermore National Labs. The machine had clearly sat for a while as there was rust on the spindle nose and any of the uncoated parts, however it all cleaned up well. I pulled the way covers and there was some rust on the screws and linear ways but it did not appear to be anything major. Machine had plastic shavings under the covers and I suspect it lived a relatively easy life, the interior of the enclosure isn't missing any paint etc. Spindle runs perfectly and has less than .0001 runout tested with a Haas Gauge-Length Calibration Tool from another machine. The long winded version of it looked to be in good shape.

When running some parts I am getting an entry/exit mark when contouring and it looks like y axis backlash (see attached). Put some indicators on the spindle nose and jogged it and I'm getting about .0015 blacklash. Checked X as well and also got about the same thing. I also noticed that when jogging in .001 increments with the hand wheel I am not getting a consistent movement on the dial indicator. Some times it will move spot on for a few revolutions and then it will move from 0 to .0002-.0003 with one click and then jump to .0015 on the next click. Other time sit will advance .0008 or so. DRO is registering consistent movement but obviously the table is not doing the same.


I have yet to put an indicator on on the ballscrews to check if there is any fore-aft movement suggesting the thrust bearings are in need of replacement. The belts on the servo's appear to be in good shape and look/feel tight, everything is nice and clean. I did get the ballscrew error map printout that came with the machine for all 3 axis. When comparing it to what is in the control backlash mapping the figures are different than the paper printout. The paper has error readings in 1 inch increments whereas the control has them in .5 inch. I took a picture of the control settings and then changed them to what is on the printout but that didn't' have any affect. Other than checking to make sure all the mechanical connections are good and the thrust bearings aren't worn are there any other things that might be causing this other than a worn screw/nut? Could the encoders be to blame? Would servo turning potentially correct this? Ballnuts loose preload?

Would like to narrow down the issues before I have a tech come out and inspect it. Outside of this and a possible cabinet heat exchange issue the machine is in immaculate shape and has been working perfectly. I only paid $7500 for it so if I have to get the screws rebuilt I'm not that opposed to that assuming that cost is not bananas.

 

[MCritchley]

A good first step would be to check for lost motion with your ball screws. First check for axial slop in the thrust bearings with an indicator on the end of the screw, this reading should be close to zero. Second check you ball nut, place and indicator tip directly in the ball groove on the screw. mount the mag base directly on the axis being checked- jog back and forth and check the reading. Any slop in the ball nut will be detected with a reading larger than zero.

Also- checking axis backlash with the handle is bad practice- make a program- Doubtful servo tuning could help you

 

[ViktorS]

There are no obvious chattermarks as there use to be on excessive backlash. Are you sure that there actually is a problem before you go down this rabbithole?

Another possibility might be that your programming is not on point. As in, tool deflects and appears as lead-in/out marks and you need to change strategy or repeat the finish pass/reduce the feedrate to minimalize following error.

I mean, that much backlas as there appears to be in the picture would make a screeching sound when cutting.

EDIT: Just thinking to myself, looking at the floor where the endmill has cut, it does not look good. Seems like the tool was cutting more on the leading edge, probably being pushed over by the tool load and deflected. Maby not, I dont know your feeds and speed with what tool but from the picture that is what I would assume. Also, on the circular cut it does seem like it was made of G1 moves instead of G2/3, considering the facets on the surface. Still no chattermarks though, so maby bad programming/program.

 

[Haggismaximus]

Also- checking axis backlash with the handle is bad practice- make a program-

I plan to make a program and double check before I really get into it. This was around 9:30 last night so I was just doing the fast and dirty way, clearly not the correct way.

If it still exists I will put the covers and check the screw. Thank your for the advise.

 

[Haggismaximus]

There are no obvious chattermarks as there use to be on excessive backlash. Are you sure that there actually is a problem before you go down this rabbithole?

Another possibility might be that your programming is not on point. As in, tool deflects and appears as lead-in/out marks and you need to change strategy or repeat the finish pass/reduce the feedrate to minimalize following error.

I mean, that much backlas as there appears to be in the picture would make a screeching sound when cutting.

EDIT: Just thinking to myself, looking at the floor where the endmill has cut, it does not look good. Seems like the tool was cutting more on the leading edge, probably being pushed over by the tool load and deflected. Maby not, I dont know your feeds and speed with what tool but from the picture that is what I would assume. Also, on the circular cut it does seem like it was made of G1 moves instead of G2/3, considering the facets on the surface. Still no chattermarks though, so maby bad programming/program.

Feed rate was 48ipm, 8K RPM, 3/8 3flute YG1 in a Tendo hydraulic. Had done some other test parts and the line goes away when I do a spring pass. So could be a programing error. For the rounded part I went parallel to the length vs perpendicular which would probably explain the faceting.

 

[metalmadness]

In the past I have seen a Hurco VMX42i go out on the Yaxis ballscrews. A ball bar test revealed massive deviation from the acceptable limits.

I would bet this is the exact same thing going on. You shouldn't need to replace much, but you will need to get the system dialed in and properly compensated. Getting a ballbar test is the best solution if it is in fact a backlash problem like you suggest.

EDIT when you say that you had a ballbar test readout on all 3 axes, when and by whom was that done? Did you pay for it? If it is 10 years old it doesn't mean much. Is it recent, as in "when you bought the machine" recent?

 

[metalmadness]

There are no obvious chattermarks as there use to be on excessive backlash. Are you sure that there actually is a problem before you go down this rabbithole?

Another possibility might be that your programming is not on point. As in, tool deflects and appears as lead-in/out marks and you need to change strategy or repeat the finish pass/reduce the feedrate to minimalize following error.

I mean, that much backlas as there appears to be in the picture would make a screeching sound when cutting.

EDIT: Just thinking to myself, looking at the floor where the endmill has cut, it does not look good. Seems like the tool was cutting more on the leading edge, probably being pushed over by the tool load and deflected. Maby not, I dont know your feeds and speed with what tool but from the picture that is what I would assume. Also, on the circular cut it does seem like it was made of G1 moves instead of G2/3, considering the facets on the surface. Still no chattermarks though, so maby bad programming/program.

It doesn't necessarily require chatter OR chatter marks to be a backlash or backlash compensation issue. With a machine that old, it is a vast rabbit hole of potential issues that could be the actual problem.

 

[Haggismaximus]

In the past I have seen a Hurco VMX42i go out on the Yaxis ballscrews. A ball bar test revealed massive deviation from the acceptable limits.

I would bet this is the exact same thing going on. You shouldn't need to replace much, but you will need to get the system dialed in and properly compensated. Getting a ballbar test is the best solution if it is in fact a backlash problem like you suggest.

EDIT when you say that you had a ballbar test readout on all 3 axes, when and by whom was that done? Did you pay for it? If it is 10 years old it doesn't mean much. Is it recent, as in "when you bought the machine" recent?

I have not had a ball bar test. Machine came with paerwork that listed the ballscrew error chart. Presumably to be entered into the screw mapping dialog.

 

[ViktorS]

Feed rate was 48ipm, 8K RPM, 3/8 3flute YG1 in a Tendo hydraulic. Had done some other test parts and the line goes away when I do a spring pass. So could be a programing error. For the rounded part I went parallel to the length vs perpendicular which would probably explain the faceting.

Alright! Then I would suggest not litsening to these nay-sayers and preachers of doom and go with a springpass as is more or less customary practice anyway, or leave less stock for finish pass. Besides the few hydraulics I've used was quite flexy, might try a different holder too.

Starting to change the ballscrew is not something that is done light heartedly as a good screw is quite expensive. So if the only problem is that line and no other issues I would seriously not recommend you to consider it.

It doesn't necessarily require chatter OR chatter marks to be a backlash or backlash compensation issue. With a machine that old, it is a vast rabbit hole of potential issues that could be the actual problem.

Yes. Yes it would. You mean to tell me that a table that is loose and can rock back and forth what looks like close to 1 mm would not bounce around like a rubberball during a cut? I call that bullshit, no matter what year of the machine.

But I do agree that it can be a rabbithole of potential issues. But in my experience, when troubleshooting start with the easiest solution. New ballscrew is not that.

 

[Brian]

"I also noticed that when jogging in .001 increments with the hand wheel I am not getting a consistent movement on the dial indicator. Some times it will move spot on for a few revolutions and then it will move from 0 to .0002-.0003 with one click and then jump to .0015 on the next click. Other time sit will advance .0008 or so. DRO is registering consistent movement but obviously the table is not doing the same. "


Sounds like it could be stiction, double check your way lube situation, make sure the ways/rails are getting the proper type and amount of lubrication.

 

[metalmadness]

Alright! Then I would suggest not litsening to these nay-sayers and preachers of doom and go with a springpass as is more or less customary practice anyway, or leave less stock for finish pass. Besides the few hydraulics I've used was quite flexy, might try a different holder too.

Starting to change the ballscrew is not something that is done light heartedly as a good screw is quite expensive. So if the only problem is that line and no other issues I would seriously not recommend you to consider it.



Yes. Yes it would. You mean to tell me that a table that is loose and can rock back and forth what looks like close to 1 mm would not bounce around like a rubberball during a cut? I call that bullshit, no matter what year of the machine.

But I do agree that it can be a rabbithole of potential issues. But in my experience, when troubleshooting start with the easiest solution. New ballscrew is not that.

Well we don't need to fight about it, but I have seen a ballscrew issue I have experience with did not experience chatter.

Programmed radius in 4140 HT. Full Slotting. Table executes what it thinks is the correct motion. Results show otherwise. Table was so loose that the tool was sucked into the material, resulting in horrible faceting as well as a slot that completely violates the specified motion/tolerances, scrapping the part. There was no chatter in this case because the tool was fully engaged in the material. If you watched the machine, you would see huge table jerking motions as the servos are basically fighting against the tool pulling itself into the material, but because there was so much mechanical backlash there was no hope.

BOOM goes the dynamite, there is an real world example of a ballscrew issue with ZERO CHATTER. Solution? Run a ballbar test and see how fucked the table is, and then fix it!

Either way, if you require a finish pass you require a finish pass. If that solves it then that is a nice easy solution. BUT a tight machine would not cause that issue OP is seeing, even if it was slot milling a profile and had a lead in lead out. You would see a very small vertical line where the tool entered and exited. You would not have a huge gouge in the part.

 

[MCritchley]

Well we don't need to fight about it, but I have seen a ballscrew issue I have experience with did not experience chatter.

Programmed radius in 4140 HT. Full Slotting. Table executes what it thinks is the correct motion. Results show otherwise. Table was so loose that the tool was sucked into the material, resulting in horrible faceting as well as a slot that completely violates the specified motion/tolerances, scrapping the part. There was no chatter in this case because the tool was fully engaged in the material. If you watched the machine, you would see huge table jerking motions as the servos are basically fighting against the tool pulling itself into the material, but because there was so much mechanical backlash there was no hope.

BOOM goes the dynamite, there is an real world example of a ballscrew issue with ZERO CHATTER. Solution? Run a ballbar test and see how fucked the table is, and then fix it!

Either way, if you require a finish pass you require a finish pass. If that solves it then that is a nice easy solution. BUT a tight machine would not cause that issue OP is seeing, even if it was slot milling a profile and had a lead in lead out. You would see a very small vertical line where the tool entered and exited. You would not have a huge gouge in the part.

Why an expensive ball bar test when you can do an indicator check as outlined in post two? Most service techs do not carry a balbar in their tool box. Its a total waste of money with out doing a 30 minute lost motion check with a dial indicator.

Sure the issue could be tool holder or cutting toll issues-that should be worked out separately

 

[Haggismaximus]

It would appear that I am the problem. Need to get rid of that Jump to Conclusions Mat. Tested X as well and it was about the same

Backlash - Album on Imgur

 

[ViktorS]

I realize I came across as a mean jerk, I apologize @metalmadness!

Good to hear you found a simple solution @Haggismaximus

 

[metalmadness]

I realize I came across as a mean jerk, I apologize @metalmadness!

Good to hear you found a simple solution @Haggismaximus

lol I have thick enough skin, but thanks for saying that! cheers