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Thread: Ballscrew Basics

  1. #25
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    Lightbulb Ground Precision for Rolled Price?

    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____50___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.



  2. #26
    Community Moderator ger21's Avatar
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    Mach3 can do screw compensation. The hard part is the measuring.

    Gerry

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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


  3. #27
    Registered DieGuy's Avatar
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    I just bought one of these for $29 + shipping off ebay

    If aligned to your axis of motion 1 inch incremental steps could be measured with a dial indicator. Also they are highly accurate for lesser distances. More than adequate for mapping the first order errors in the screw.

    I just love a bargain!



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    I was thinking of something more high-tech like the AR600 shown here: http://www.acuityresearch.com/products/index.shtml
    I would imagine that a computer interface is a requirement since you don't want to manually make the thousands of measurements it would take to measure the entire length of the screw and then input them into a computer. (Ex 200 step motor x (12" screw/.200" travel per turn) = 12,000 steps/1' of screw, w/ 1/8 driver thats 96,000 steps!).

    With that device you could zero your blank and cutting head, measure the distance of the cutting head to some reference pt, then step your motor through the various positions it will make during its cuts and make measurements to the ref at each step and input them into the computer. Still a very tedious task.



  5. #29
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    I think laser displacement measuring is the dogs danglies as the brits would say, but most of the DIY machines here hardly support that kind of accurate geometry that would indeed make use of the linear displacement accuracy. Certainly not for $29



  6. #30
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    Here's another idea I came up with. I'm not sure if my math here is correct so let me know if its not.
    If you have a 1600 dpi optical mouse (like this http://www.logitech.com/index.cfm/pr...ONTENTID=10121) then I'm assuming that you can accurately measure movement as little as 1/1600in or .000625".
    C5 is .0005" error but its over 12" so the mouse may not be very good at correcting screw errors.
    Especially since mice are probably not so good with absolute positioning to begin with, but I bet they do real well at relative positioning.
    That is if you move the mouse a small distances it will be fairly accurate, but over long distances it will probably loose track and start accumulating error.
    With this in mind, you could use the optical mouse to create your own backlash compensator! It already senses in 2 axes and you already got your computer interface.
    As long as your stepper motor's steps translates to movement >= .000625" you should expect to sense at least one tick from the mouse driver for each motor step.
    If you don't sense the tick, then the stepper motor is taking up play in your system and you should take another step until you sense the appropriate number of ticks.



  7. #31
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    Is it backlash or screw compensation your after?



  8. #32
    Gold Member Bloy2004's Avatar
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    Quote Originally Posted by DieGuy
    I just bought one of these for $29 + shipping off ebay

    If aligned to your axis of motion 1 inch incremental steps could be measured with a dial indicator. Also they are highly accurate for lesser distances. More than adequate for mapping the first order errors in the screw.

    I just love a bargain!
    What IS that device?



  9. #33
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    Hi Swede,

    thanks for such a good write up on ballscrews. I have a question. I have allways been taught that when tensioning bearings (like the front wheel bearings on a car) you tighten the nut until it bottoms out and then back it off slightly to allow clearance in the bearing for the grease to move between the race and the rollers, otherwise in a very short time the bearing will be damaged. Doing this allways left a very small amount of movement.

    When tensinoning the bearings on the fixed end of a ballscrew should this rule still be applied or are shims used to control the clearence between the two inner races?

    Cheers
    Splint



  10. #34
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    I'm posting a reply to get this to show up again as a current thread.

    It bears bookmarking.



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    Regarding post number 32, is that a master height guage? Also called a Hommel if I remember corectly.

    If so, how in the hell was that highly accurate instrument only $29? ( or did they charge $1000 shipping )

    I used to use them in the toolroom on a surface table for component measuring. Using a dial indicator on a normal height guage you would zero the indicator on the part being measured then move the indicator over to the Hommel and place it at the nearest ledge ( proper name???) and use the dial on top of the Hommel to move the column of ledges up or down untill the dial indicator was back to the zero point.

    Each ledge was 25 mm apart and the micrometer dial at the top only had a travel of about 25 mm. Obviously if the dial read 12.7mm and you were on the 100 ledge you would be at 112.7mm from the feet/table


    Please correct me if I am wrong but it has been a while since I last used one.

    (And if it's not a Hommel the jury will disregard what i just said)

    Regards all M



  12. #36
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    Question Trapezoidal?

    Hi all. I have come looking for info on ballscrews for my next machine, and this thread jumped out and bit me!

    I know the thread is fairly old, but I have one question that maybe Marchant Dice ( Who I intend to buy from ) or another knowlegeable person can answer:

    On the Marchant Dice website they have Ground, Rolled, and Trapezoidal ballscrews. Im thinking that Trapezoidal may just be science talk for 'Multi-Start'.....is this correct?

    Thanks.



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