[marchantdice]

We use Kluber Isoflex NBU15. This is a high performance, high pressure synthetic lubricating grease, particularly suitable when extreme linear positional accuracy, precise repeatabality and low torque operation is required. It also offers exceptional stability in the presence of aqueous solutions, particularly water based cutting solutions.

Our bearing blocks are fitted with rubber seals as standard, these are fitted integral of the housing and locate on the assembly spacers. The bearings (matched pairs) are fitted with metal shields. All Ballscrew support blocks housings are certified for accuracy!

To view our bearing blocks http://www.marchantdice.com/ballscrews/bssalb.htm

[daloveshack]

I'm wondering if we can use software to bring cheaper rolled ball screws to within ground ball screw tolerances. Theoretically, if you had high repeatability, but poor accuracy, all you need is just the right amount of compensation (facilitated by stepper motors with enough "steps"). If you could temporarily mount a high precision measuring device (at C0 or better tolerance) to the carriage that your ball screw is moving and then take measurements throughout the length of the ball screw for each step that your stepper motor takes. You should be able to get a map of where the stepper motor thinks it is and how far the ball screw actually moved.

Step
1_____2_____3_____4_____5________6___7______8_____ 9____10
|=====|=====|=====|=====|========|===|======|===== |====|
0_____6____12____18____24_______33__37_____44____5 0___55
Distance in microns


If you reverse this mapping, you'll know how far to turn the stepper motors to get the exact desired position. For instance (using the example map above) if my stepper is at step 6 @ 33 microns and I want to move the cutting head 10 microns to the left to a distance of 23 microns. Since that is closest to step 5, I would only move the stepper motor one step left to step 5 @ 24 microns. In a non-compensated system, I would have assumed that each step was = 6 microns so i would have moved 2 steps over to step 4 @ 18 microns.

I'm thinking the problem here might be having to re-zero the stepper motor with your calibration scale before zeroing your piece with your cutting head. For this you would have to define your "calibration zero point" as a spot that you could always move your cutting head to with PERFECT accuracy. That is the point that aligns your map with your ball screw so if you get it wrong, your accuracy might be horrible. Then after this step, your system will have to be able to count the number of "steps" it takes to actually zero the object.
This lets the software know where along the map the you actually are. Because the calibration is relative to your "calibration zero point", the software correction will need to take place where you can get access to the number of steps between the calibration zero and the zero of the object. This puts it in the realm of the stepper motor driver software and not at the g-code level.

The beauty of this solution is that once you have stored your "map" you can remove your high precision measuring equipment. My guess is that these laser measuring devices are quite expensive so if they are only needed for calibrating then a single device could be used to calibrate several cnc machines and the cost could be distributed amoung them.

What do you guys think? I'm only a newbie at this so forgive me if I got this all wrong.