How Bad is it Without Ballscrews?

[NC Cams]

You can map the table by using a set of JOB blocks as the standard - say 4" long.

Find the lead error at one end and then move the standard down the travel 4" at a time, rezeroing of the prior endpoint.

Thus, measure from 0.000 to 4.000 find error. THen, move the standard so the end that was at 0.000 is now at 4" and then see the lead error at 8" and so on down the table.

Time consuming but 4" is 4" and if you do it carefully, you should get lead mapped to within a few tenths or so all the way down IF YOU ARE CAREFUL AND DILIGENT. When it is all said and done, have somebody come in with a laser and have it done. Cheaper and faster in the long run and it is traceable as a standard if you get someone who is properly qualified.

You can also make a stepped gage and have it CMM'd. Faster to do but still involves $$$'s.

NOW, once you have the date, what do you do with it??? Unless you have the ability to input a lead profile into your software and for it to fit a curve to it, you're probably wasting time.

[jimbo]

nc cams,
thanks for the info. I figured it was something as simple as that. What about using a higher class ballscrew to map it, ie have a stage with a servo. move (via computer) the stage a certain increment, then move (via computer) the other ballscrew the same increment, and use a dial indicator to measure the difference. At least the mapping measurements couldnt be worse than the better ballscrew. backlash should be taken out if moved in the same direction. I imagine you could automate this with a digital indicator. Or I guess you could skip that and just use glass scales to do this as well.

[NC Cams]

Jimbo: you're mixing things up a bit which can get you confused

Backlash is lost motion, period.

Lead error is deviation from desired true position assuming you have NO lost motion.

Yes, if you go in 1 direction backlash SHOULDN"T be an issue - but it is.

Why? the tool is going to shove back against the cutter and size control is diminished as is finish, accuracy, etc. The tool moves as if the lead at any specific point were off.

Thus, you do need to eliminate backlash if you want high accuracy - this is where ball screw and ball screw bearing preload comes into play (no pun intended).

Once you eliminate backlash, you only have to deal with (localized) lead error.

Thus. if you have no backlash, and tell the screw to move the table 4.000" and you have a net lead error of +0.0005", you'll move 4.0005 when you think you moved 4.000.

Now, if you stack backlash onto that (say 0.0005), you now have to subtract the backlash from net movement because you expected to move 4.000 but you lost 0.0005 so you actually move 3.9995.

Yes you can comp the daylights out of B/L or lead error but, once you have a system that has both minimized, you get spoiled and you'll never be satisfied with one that is run of the mill... It is a dream to hit the numbers rather than to creep up on stuff and do the "oops, went too far".

[jimbo]

nc cams,
yes, I agree backlash still needs to be accounted for even after the ballscrew has been mapped. The backlash I was referring to was using the two ballscrews side be side during a mapping process, maybe with some tension against the ballnuts and always moving the same direction should eliminate (most) backlash from the mapping numbers.

My application was more for improving the lead error of rolled ballscrews over a large distance for a cnc router. With these cheaper ballscrews it is difficult to remove all the backlash or it will become difficult to turn. I do agree that backlash needs to be taken out of the equation or the mapping numbers will not be consistent.

thanks for the very useful info.

[NC Cams]

May have posted this before - please excuse the repost if it occurs.

First some basics:

Rolling friction is always less than sliding friction

Break away torque is always higher than torque needed to maintain motion in rolling environment.

Higher acceleration generates more force/resistance than and vice versa.

Gib clearance will create lost motion - tite gibs will generate huge back lash problems as screw needs to input high force and stretch until it finally moves stuff....

Hence, if you have a properly preloaded ball screw and a properly preloaded ball screw bearing and your gibs are properly adjusted, your friction will not be appreciably higher.

When you start to cobble up ball bearings and/or tapered rollers (ergo use them improperly) you will see a friction issue.

We did a moderate preload on our rolled screws via o/s balls and added true ball screw bearings (read that expensive). Oem bearings had 70-100 lb preload, the true screw bearings we replace them with had 480lb.

Result: our break away torque is nearly the same or lower than stock bearings but the continuous torque is LOWER (much smoother bearing due to internal load path of A/C versus deep groove ball bearings it replaced). Lead error is 0.001 worst case at 4 positions we mapped it at when we had machine tuned. WE can CLOSELY approach the accuracy of a ground part when we slowly and carefully cut a master profile with our cnc mill.

Backlash is miniscule (something like 0.0002 comp max on one axis and 0.0001 on the other). We see 0.0001 or so "flat" if we look hard via ultra precise dial indicator at direction change.

Lead error is 0.001 or less over 4 inch segment checked at 4 positions along the X axis. Almost but not quite as good as the neighbor's VMC but a fraction of the cost.....

Our mods were pretty much bolt-ins asside from the ball screw preloading. It wasn't cheap but they served our needs quite well.