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squeezing the last bit of accuracy on a BP V2XT cast Iron

madmachinst

Stainless
Joined
Jan 15, 2007
Location
Central FL
This here is a retrofit and I did all the new electronics with machmotion controller 750 Watt closed loop servos. They have a calibration app where I put a DTI in my quill set it to zero off a 4" gauge block and then command a move( 4.007"), then I input into the screen what the move actually was, and that tells the machine how many steps per inch on that axis. Servos have like 10,000 PPR or something like that and between lead pitch and pulley ratio 10 rotations equals an inch. Now my problem is that throughout the axis' I have different steps per inch. One spot in X it can be 100,070 in another it can be 100,550 and so on and so on. OK, the machine is from 1991 ( last 5 years sitting in my garage collecting dust), but it is supposed to have NSK ballscrews and I also changed the regular bearings some stupid maint guy threw in there to thrust bearings when I originally retrofitted it back 5 years ago. I had decent accuracy back then only electronic problems, now with the machmotion system, electronics are better, but mechanically speaking I am getting different steps per throughout my axis motion. I haven't even started on backlash. ON Z axis I am getting like .0025" difference between 2" move and 4" move. driving me nuts. I was going off a parallel laying across the table. If I get screw mapping on this thing, you guys think I can rely on the knee to go down .25" increments and map that in the controller. Thanks for all the help.
Sincerely,
C. Wizman
 
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johansen

Stainless
Joined
Aug 16, 2014
Location
bainbridge island
The variability sounds more like backlash.

Variations in friction,etc, change where the table settles within the backlash. (Which isn't a hard value either, but depends on how hard you push on the screw)
 

madmachinst

Stainless
Joined
Jan 15, 2007
Location
Central FL
But I am only going in one direction. Zeroing the DTI on a gauge block by jogging onto it. commanding the app to move the axis .007" beyond the length of the gauge block and entering that dimension I get. say DTI shows only .0055 after zero then I enter 4.0055 ( If zeroing on 4" gauge block). So my question is how has that got anything to do with backlash? I am already taking up backlash when I jog onto the gauge block.

Now, regarding variations in friction and where the table settles in backlash. You think with an arduous warm up cycle and my manually activating the lube pump while that is happening that will help? Been so long since I took out the ball-screws, but do the ball-screws get their lube from the lube pump or do I have to move the covers and lube them manually?
 

Nmbmxer

Hot Rolled
Joined
Jun 22, 2008
Location
VA
Might be a servo tuning issue, any overshoot would have that effect if it was less than the total mechanical backlash. How much backlash do you have? I'd look at your tuning, the drives on my retrofit have a utility that will plot the command vs position and show the curve resulting from a step input, a little overshoot is ok as that is required for critical damping (min settling time) but if you have mechanical issues being underdamped might work better but can introduce following errors when profiling.
 

madmachinst

Stainless
Joined
Jan 15, 2007
Location
Central FL
Might be a servo tuning issue, any overshoot would have that effect if it was less than the total mechanical backlash. How much backlash do you have? I'd look at your tuning, the drives on my retrofit have a utility that will plot the command vs position and show the curve resulting from a step input, a little overshoot is ok as that is required for critical damping (min settling time) but if you have mechanical issues being underdamped might work better but can introduce following errors when profiling.

these are autotuned servos. seller Just told me to hook em up and let the servos do their thing. Now I am fixing on writing a program where the axis make the table move in diamond motion for a while as the diamonds get smaller and smaller to try and get the axis warmed up. If there were any buttons I should press on the drivers to get the set up to autotune would have been great to run that now. I'll get back to them on that and pray I don't get back a stupid response from them. I paid the bigger bucks expecting service otherwise I could have bought from Arturo.
 

madmachinst

Stainless
Joined
Jan 15, 2007
Location
Central FL
after running table on warmup program for 50 minutes I moved the covers on the ballscrews and they were shiny and it didn't seem they were wet/lubed. is removing the table a necessity to check those lines and see if there is a clog? Y axis I take it I just remove the box holding the motor pulleys and belts and then the way covers come off.

Also seemed like my X axis belt starting to tear. I'll post pics and see if you guys think it can last a few more miles.
 

Nmbmxer

Hot Rolled
Joined
Jun 22, 2008
Location
VA
Usually it’s the metering orifices than clog, you can take the oil lines off the manifold and see if oil comes out. Both of my boss and the v2xt had bad metering orifices. They can’t generally be unclogged and are $16ea from McMaster, you can probably find them cheaper. You have to unhook the lines from the ballnut and blow them out. If those are clogged, the ways are probably also not getting oiled and that means taking the table off…
 

madmachinst

Stainless
Joined
Jan 15, 2007
Location
Central FL
pictures of ball thread. I went back and forth with table a few times and hit lube, but the screws in my opine seemed dry. Here are pics. The cover for the crews OTOH were soaking. Worse case scenario if I need to make some parts on this thing before I can afford to send it over for planned on returcite, rescrape, rebearing reballscrew, and re do lube system. I'll just move table over to one side completely push the cover aside give it a lube with some kind of spray product, then do the other side. Suggestions on what I should use? Every 2 hour will be enough?
 

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madmachinst

Stainless
Joined
Jan 15, 2007
Location
Central FL
rest of pics.
 

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madmachinst

Stainless
Joined
Jan 15, 2007
Location
Central FL
The variability sounds more like backlash.

Variations in friction,etc, change where the table settles within the backlash. (Which isn't a hard value either, but depends on how hard you push on the screw)

Now, say I took a torque wrench 1/4" drive and tried to rotate the ballscrew to move the table. I understand from prototrak they say no more than 15 in lbs. So basically if it takes less than that to smoothly move my axis', You think I can safely rule that out? At this point I just want to know what is causing my problems. if I get more than 15 in lbs, then I'll start loosening stuff. Now, before the servos where on there I could rotate the axis' by wrist muscles only ( me no tough guy). I am wondering when I reintroduced the servos maybe the belts ended up getting too much tension? Had to redesign servo adapter plates, maybe that did it?
 

neilho

Titanium
Joined
Mar 23, 2006
Location
Vershire, Vermont
You mention that the x axis belt is tearing. I've known that to cause inconsistent angular spindle position on a lathe. Or are you having problems with the y axis, too?
 

madmachinst

Stainless
Joined
Jan 15, 2007
Location
Central FL
You mention that the x axis belt is tearing. I've known that to cause inconsistent angular spindle position on a lathe. Or are you having problems with the y axis, too?

Whne you say problems you mean repeatability or tearing belts. Belt only tearing on X axis. It only happened with those long rapid moves seen in video. Maybe the other axis just as bad only didn't tear because their motions are not that long on rapid?

Be honest with you, all of them are giving me problems. Like I originally said, I had to make new servo adapters to go from the old SEM DC servos to the new rectangular AC servos. I should have made the bolt holes for the bolts holding the plate to "box" ( I don't know what they call those things where pulley and belts go in) rectangular so I could adjust belt tension. I tried rotating
#2 torn belt.jpg
basically it is the pulley attached to X axis servo , with a flat screw driver. It took more than just my wrist to do it. Took more like my arm. Not a lot of force, but took some force. Will get a 1/4" torque wrench to that and the other axis' to see how much it takes. Then will remove belts, and try rotating things with torque wrench ( don't know how will do the quill) and see if the belt tension was excessive. If so will try to see about slightly longer belt of same manufacture. Joe Pie from advanced innovations told me might be the belts and I will seriously pursue that.
 
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CJD

Aluminum
Joined
Aug 28, 2019
I have one of the old Boss Bridgeports. Even though it's old, I cannot detect any backlash with the ball screws. Once the weighs are lubed, moving the bed is effortless. If you have different distances with equal turns, then it has to be a backlash error...due to broken ball screws or defective parts. Have you checked to make sure you do have ball screws?
 

neilho

Titanium
Joined
Mar 23, 2006
Location
Vershire, Vermont
Whne you say problems you mean repeatability or tearing belts. Belt only tearing on X axis. It only happened with those long rapid moves seen in video. Maybe the other axis just as bad only didn't tear because their motions are not that long on rapid?

Be honest with you, all of them are giving me problems....

Yes, I meant that a torn belt resulted in repeatability problems. Tensioning a timing/cog belt properly is quite an exercise, esp if you take the mfgers recommendations seriously.

Seems to me if all axes are having a repeatability problem, and there's only one torn belt, it's less likely that belts are the problem. Did you change belts?

Has the oil metering problem been addressed?
 

madmachinst

Stainless
Joined
Jan 15, 2007
Location
Central FL
Yes, I meant that a torn belt resulted in repeatability problems. Tensioning a timing/cog belt properly is quite an exercise, esp if you take the mfgers recommendations seriously.

Seems to me if all axes are having a repeatability problem, and there's only one torn belt, it's less likely that belts are the problem. Did you change belts?

Has the oil metering problem been addressed?

Haven't addressed oil metering problem cause I don' have the time to tear it apart. All the belts have different tension since I have had to make adapter plates to mount the new servos. Y has known .003" backlash, But I am not dealing with backlash yet. I just want a repeatable steps per inch. Mathematically should be 10,000 steps per rotation on servo times 5 rotations per inch leadscrew times ratio 2:1 on pulleys = 100,000 steps per inch in system. using app on machine I get numbers more like 99,894 on Z 100,090 on X and those are just averages. Different points on screw giving different numbers with variations as much as 250 steps at different locations.

BTW, it is just the edge of the belt that is torn off, say about 1/16 of inch on about 1" wide belt. have no way to measure stretch on belt cause manual says with 7lb pull should be about 1/4" deflection. Fine can measure Z and Y but X once I take cover off, servo and pulley are removed and no way to measure belt. Servo and pulley permanently attached to the cover.
 








 
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