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  1. #1
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    Jan 2007
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    87

    Chinese Ballscrew upgrade Question Series 1

    I have a 1965 Series I Bridgeport 9X42 Mill that I have converted to 2 axis CNC. I am presently using the old Acme leadscrews, which have about 10-12 mills of backlash near the center of travel. I have split the nuts, and tried all sorts of adjustments, but the wear especially on the X-axis results in too much drag away from the center of travel if I tighten the nut further. I have a 2 axis DRO on the machine.

    I am contemplating converting to ballscrews to reduce backlash and increase motor efficiency. I can deal with metric conversion in software, so I have been looking at some rolled ballscrews on e-bay, from a chinese company; the store name is linearmotionbearings. I know several people have used their products in homemade CNC applications and seem fairly happy with the products.

    I solicit comments on my idea to see if perhaps I am missing something. Has anyone tried this, and compared it to the Rockford conversion kits? Another question is whether I need to use 30 mm diameter ballscrews or if I could downsize to 25 mm ballscrews. My machining does not require heavy cuts, so I am tempted to use the smaller screws. Another question is if I really need left hand threads on the Y-axis screw. I can deal with the direction reversal in software easily. I can also compensate for linear errors within reason. I do not feel the need for higher precision from ground ballscrews.

    I do not want to spend too much money on this project since the mill is used for hobby work, and I am retired on a fixed income.

  2. #2
    Gold Member
    Join Date
    Feb 2006
    Posts
    7087
    Those screws should work fine, lots of people have used them with good results. Though they will still often have *some* backlash (on the order of 0.001" or so).

    A 25mm screw should also be fine, that's what I used on mine. LH or RH is completely irrelevent to a CNC conversion, since you can reverse the direction of the motors.

    Regards,
    Ray L.

  3. #3
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    Jan 2007
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    87

    Re: Chinese Ballscrew upgrade Question Series 1

    It turns out I found a precision ball-screw kit someone had purchased and never installed. I was able to purchase it for about the price of Chinese ball-screws, and the ones I got have double nuts with pre-load capability. On my initial set-up I did not know how to determine pre-load and I set it too high. The torque to turn the ball-screws was excessive, I did not measure it, but guessing several ft-lbs. I finally found a source describing how to set pre-load. According to Steinmeyer.com the dynamic capacity of a 1 inch diameter 0.2 inch lead ball-screw is between 5000-8100 lbs. The preload is usually set to 10% to 20% of dynamic load (500-1620 lbs). According to Thompson ball-screw company, the additional torque (inch-lb) to turn a ball-screw with preload will be .007 X the preload. For my ball-screws the torque therefore should be between 3.5 to 11.3 inch-lb. I purchased all new balls since I likely damaged the original balls with excessive pre-load, and am setting it up now with the proper pre-load. I should know the results in a few days.

    Another useful tip from Thompson; on a single nut ball-screw with backlash, increasing the ball size is commonly used to reduce backlash. Thompson claims a .001 larger ball will reduce backlash by .003, good to know.

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