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  1. #16
    Join Date
    Apr 2009
    Location
    Kingswood
    Posts
    670

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    Loaded the nut and mounted on the screw, twice
    The complete process is simple and trouble free.
    The nut even balances on the transfer tube and spacer without need of a supporting stand.
    Easy to finger drop about 10 balls, then fish them around with the blunt end of tweezers until they either enter the return tube or go down the helix.

    First run was 108 x 1/4" balls.
    Used thread wires to estimate gap between balls at 2 mm.
    Feels 'loose' and the nut drops under gravity over full length of the screw.

    Second run was 54 x 3.65 mm on one circuit, and 47 x 3.65 mm plus 7 x 1/4" on the other circuit.
    The tread wires were marginally tighter to enter the ball gap, but still about 2 mm.
    Can definitely 'feel' that the nut is firmer, but still loose in my mind.
    The nut tries to drop down the screw, but is reluctant, especially near the ends, but it does rotate several turns before stopping.

    Note total ball counts:
    - I reported 103 originally, now 54+47+1(not used small one) = 102 (damnit),
    - originally 103 (or 102) as found, now 108 loaded by me.

    My judgement is that the 1/4" balls are too loose, but what size to purchase ?
    US price for a 6.37 mm ball is about USD 0.72, so 110 balls is a significant purchase, have to get it correct.
    Have no idea about the effect of a lubricant film on the fit, or the allowance for heat expansion.

    John
    Attached Images Attached Images

  2. #17
    Join Date
    Sep 2009
    Location
    Newcastle
    Posts
    166

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    Do the balls need to fill the ball path length? I don't think so.

  3. #18
    Join Date
    Apr 2009
    Location
    Kingswood
    Posts
    670

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    I think the ball size is determined by the fit to the screw groove, it affects the pre-load, backlash and friction factors.
    The balls in the path length simply need 'working clearance', currently about 2 mm.

    20 microns for lubrication plus 20 microns for heat over 54 balls is 2 mm, the whole current working clearance.
    If I increase ball size by 5 microns I would probably have to drop the ball count to 53.

    Note that I have to get new balls, the old balls are faulty and the 1/4" too small.

    I currently think 6.37 mm balls are worth trying.
    These may be too tight, then I can try alternating 6.37 mm with 1/4".
    But, the risk is that the screw root diameter plus ball diamater is too great, then I would have to re-order balls at 6.365 mm.
    Count will be such as to leave at least 2 mm in the path length.

    John.

  4. #19
    Join Date
    Oct 2011
    Location
    Norwood-ish, Adelaide
    Age
    54
    Posts
    5,289

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    Can you use a gauge pin to accurately size the groove? That would at least give you a maxiumum ball diameter. Not sure how loose is too loose but will some of that play go if the ends of the screw and the nut are constrained so that they can only move in one direction (that is mounted as they would be on a machine)?

    Michael

  5. #20
    Join Date
    Apr 2009
    Location
    Kingswood
    Posts
    670

    Default

    Ball size measurement was discussed in a parallel thread:
    https://metalworkforums.com/f65/t203...tal-micrometer

    The size determined was a distribution:
    - 1% bigger.
    - 12% 6.356 mm,
    - 84% 6.355 mm,
    - 5% smaller.

    Some sample balls from the largest group were measured independently at 6.357 mm.
    I think I should take 6.356 mm as the best estimate of the diameter.

    Loaded 54 balls from the 84% group into half the nut, a very neat fit.
    Installed the nut on the screw and clamped the screw to my small mill table with the table clamped.
    DTI on the nut showed total backlash of 25 micron when the nut pushed/pulled by hand.
    Tested rocking of the nut and table movement, both negligible effects.
    Moved the balls to the other half of the nut, tested as above, and got the same result.

    Loaded all 102 0riginal balls plus 6 new 1/4" balls in the nut and installed on the screw.
    Clamped the nut in the vice and measured the vertical looseness at both ends of the screw.
    Some simple maths reveals a vertical slack in the nut of 5 micron.

    As the nut has demonstrated excessive looseness, the new balls need to be increased in size.
    Vertical results suggest 5 micron, the backlash suggests 25 micron.
    Searched the web for hints on the effects of increasing ball size, without any definitive result.
    My personal guess is that 6 microns should be Ok, and doesn't seem excessive.

    If final backlash on the mill is about 20 micron, I would be happy, compensation should handle that.

    The new size is estimated to be 6.362 mm, with actual size determined by the stock of balls available.
    Balls are graded for dimension and range and are available from Grade 10 to Grade 100 (in micro inches).
    Some Grade 25, about 0.0007 mm, are listed with the lowest price offered (USD 0.79).
    Some batches have very high prices, so it is a matter of being selective on size and Grade.

    Spoke to Universal Bearings in Sydney as they list balls on their website.
    They only stock standard imperial and metric sizes from China.
    They were vague when I mentioned certification, batch number and grade.

    There is potential for this to come out badly.
    I am sure about my measurements, but not sure about the additional 6 microns.
    The firmness of the ball screw nut on the screw could still be loose, or too tight.

    Checked price for 110 balls with Bal-tec in the US.
    The price is horrendous with postage exceeding the price of the balls, total of almost AUD3 delivered per ball.
    You can find Bal-tec at website "precisionballs.com".
    I have contacted Bal-tec by email enquiring as to availability of cheaper postage options.

    John

  6. #21
    Join Date
    Jul 2016
    Location
    Melbourne
    Age
    31
    Posts
    1,070

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    I would try an SKF distributor maybe, they have a good presence here in Aus and I have bought some strange stuff through them no problem.

  7. #22
    Join Date
    Apr 2009
    Location
    Kingswood
    Posts
    670

    Default

    Michael,
    My understanding is that the groove on the nut and screw is two separate curves, and each may not be circular.
    Depending on the direction, the nut has single-point contact to a ball on one curve, and the screw single-point contact on the ball on the diagonal opposite curve.
    Then the curves in contact swap around for the opposite direction.
    It all sounds too difficult to measure or assess, I will have to go on the nut/screw assembled test results.
    John

  8. #23
    Join Date
    Apr 2009
    Location
    Kingswood
    Posts
    670

    Default

    Why is it so hard !

    Was always troubled by the discrepancy between the DTI and micrometers for ball diameter.
    So I re-did all the measurements with two inspection grade B&S and Mitutoyo bench-mounted micrometers and adjusted my estimate of current size upwards a few microns, to 6.365 mm.

    Placed an order on Bal-tec in California at 6.368+ mm for 110 balls at USD 0.79 each.

    Order seemed to go through, then link across to the credit card payment facility, to have my credit card declined.

    Tried two cards from one of the Big-4, and a card from a multi-national bank, all declined.
    Checked in my local branch, no problems found.
    The cards have 3, 5 and 7 months expiry.

    I will email Bal-tec today (Monday), then call them early in the morning (currently 18 hrs).

    Any suggestions ?
    John

  9. #24
    Join Date
    Jul 2016
    Location
    Melbourne
    Age
    31
    Posts
    1,070

    Default

    American banks are absolutely crazy, especially when it comes to debit cards that draw from an account rather than line of credit. Just call and see if they can do it over the phone, otherwise you might have to mail a money order or use some ancient thing like western union.

  10. #25
    Join Date
    Sep 2012
    Location
    North Yorkshire UK
    Posts
    2,763

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    Hi Guys,

    You might find that this is a "prevention of money laundering" issue ! Its starting to happen here with certain cards declined. I tried to buy fuel, in France, and had my UK card declined. No explanation. Yet a few miles further on, I had no such problem. It seemed to be one of the super store chains that did this, because it only got refused at their fuel stations.
    Best Regards:
    Baron J.

  11. #26
    Join Date
    Jun 2010
    Location
    Canberra
    Posts
    1,079

    Default

    Maybe see if they take PayPal?

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