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  1. #1
    Join Date
    Apr 2009
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    Kingswood
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    662

    Default Ballscrew Nut Re-fit

    My CNC re-furbish project on a Kondia turrett mill has languished for a few years.
    But, I can now devote some time to it and the first issue has arisen.

    The X-axis ball screw nut was separated from the screw after some problems.
    The shaft leading up the helix is Dia 25 mm, and I 'assumed' that diameter would allow the nut to simply slide off.
    Not so, the nut jammed immediately.
    Tried working it back and forth and eventually removed the ball return tubes and extracted the balls.
    Not easy because they are heavily greased - got 103 balls.
    Quite a few gouge marks in the shaft, but of no consequence because this area is non-utilized.

    Intend to clean it all up and re-fit with a central oil system.
    Included is new brush wipers if they can be obtained.

    Previously, the Y-axis nut but was removed by sliding onto a Dia 27.2 mm tube, retained on the tube while
    machining the screw, then re-fitted without any drama.

    Looking for suggestions:
    - source for brush wipers,
    - ideas on on how to get the nut back onto the helix.

    Thanks,
    John.

  2. #2
    Join Date
    Jan 2012
    Location
    Melbourne
    Posts
    88

    Default

    I managed to put back together a ballscrew once by cleaning everything up and then feeding the balls in through the tube hole. Keep going until they start coming out the other end, keep letting out and feeding new ones in until you think you’re letting ones out that are properly packed. The first few in won’t be well packed because you’ll be struggling to keep the screw and nut aligned while you feed the balls in. Then fill the return tube with balls and bolt it back on.

  3. #3
    Join Date
    Jan 2012
    Location
    Melbourne
    Posts
    88

    Default

    I managed to put back together a ballscrew once by cleaning everything up and then feeding the balls in through the tube hole. Keep going until they start coming out the other end, keep letting out and feeding new ones in until you think you’re letting ones out that are properly packed. The first few in won’t be well packed because you’ll be struggling to keep the screw and nut aligned while you feed the balls in. Then fill the return tube with balls and bolt it back on.

  4. #4
    Join Date
    Apr 2009
    Location
    Kingswood
    Posts
    662

    Default

    Thanks for the suggestion, I wondered whether that was a viable method.
    My nut has 2 return tubes, with each end buried within the nut when the retaining clamp is fitted.
    It will need quite a bit of dexterity, possibly some ingenuity, but worth a try.
    One could possibly fill the nut with one tube installed, fill the the other nut at the work bench, hold the balls with a grease, then invert that tube and install.
    The nut will be full when all 103 balls are inside it and it revolves smoothly around the screw !

    I have started looking at Youtube clips and searching for suitable descriptions.

    John

  5. #5
    Join Date
    Apr 2009
    Location
    Kingswood
    Posts
    662

    Default

    Attached is one of the ball return tubes showing the damage inflicted by a previous worker (!). Ball Nut Return Tube Compr.JPG This tube, and its mate, would still operate but they are clearly not ideal. The other parts required are the end wipers - some wear/distortion. Got on to the Australian representative of THK ballscrews, they have no spares and claim trying to contact the factory would be pointless. Posted a thread on LinuxCNC Forum to see if anyone there can suggest a spares supplier. This post is just to see if anyone here might have a suggestion. As an aside, placed a query through Alibaba for bellows to keep swarf out of the nut in future. John

  6. #6
    Join Date
    Oct 2011
    Location
    Norwood-ish, Adelaide
    Age
    54
    Posts
    5,255

    Default

    What's the bore of the tube - it looks like the sort of thing that may be able to be bent around a mandrel.

    Michael

  7. #7
    Join Date
    Apr 2009
    Location
    Kingswood
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    662

    Default

    Michael,
    The problem with a new fabrication is the special machining at the ends - it would take multiple attempts to get it correct.

    One small step forward was a quote/order on a Chinese company through Alibaba for protective bellows.
    They were not too expensive, but could easily be not suitable.
    Report with photos when I get them.

    I found a Korean address/site that sells redundant and used electronic and associated mechanical components, including ballscrews.
    They do not have an English version of their website and it appears they do a lot of business after a walk-in inspection of the goods, with prices in Korean WON.

    Found on their listings a used ballscrew of the same type as mine.
    Price is not too high, but so far, they have not answered my email.

    Any suggestions on how to get assistance with contacting a Korean company ?

    John

  8. #8
    Join Date
    Apr 2009
    Location
    Kingswood
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    662

    Default

    From the amount of views, this subject is of some interest.
    Not much progress to date, stalled out trying to contact the Korean web site.

    My web searching tells me the best approach to mount the nut is to pre-fill it with greased balls, then insert a suitable holder for transfer to the screw.

    For the photo enthusiasts:

    Ball Nut End Brush Compr.JPGBall Nut End Compr.JPG

    Photos shows the nut with edge of a track broken away, and one of the end brushes
    This nut is hard, so it was some sort of whack, or jam-up of swarf.
    Perhaps it was track debris pieces that caused the scavenging effect on the ball return tubes.
    Note that the fracture is on an unused part of the ball track.

    So, I have a nut that has:
    - lost at least one ball (2 circuits needs an even count, not 103),
    - damaged ball return tubes,
    - fractured edge on one track,
    - has distorted end brushes,
    - operated with swarf mixed with grease.
    Hence my interest in pursuing replacement parts.
    A new nut is about AUD 450.

    Followed up with a Google Translate version of my inquiry to Korea a couple of days ago - no response.
    The website is dgfa1.com: ?? ???/????? ?? ??? ??????

    All contributors,
    Do you know someone fluent in Korean that might be interested in assisting me ?

    The whole job could be done via email and telephone.
    Happy to pay a reasonable fee.

    Job would be some/all:
    - inspect the site and translate the contents,
    - determine communications channel,
    - call the listed number to discuss (in Korean).
    - determine the correct method for requesting a price with delivery to Sydney,
    - determine payment method,
    - assist in ensuring correct details submitted.

    John
    Last edited by electrosteam; 28th Jul 2019 at 11:45 AM. Reason: webpage display

  9. #9
    Join Date
    Apr 2009
    Location
    Kingswood
    Posts
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    Default

    Never got a response from the website.

    Spent a couple of weeks looking for a S. Korean speaking person to translate the site for me, and eventually found one.
    Called the listed number and confirmed that they would supply quantity one, but did not deliver internationally.

    My new friend suggested we could use her parents address in S. Korea for delivery, and they could forward to me.
    I did not want to involve unknown people in handling a 850 mm long package weighing quite a few kg, so I did not accept the offer.

    Spent some time looking for a S. Korean version of the US Shipito without success.

    Then my friend discovered a S. Korean company that appears to be modelled on Shipito, but primarily delivering to S. Korean expats.
    I have contacted them with a view to initiating an account, but no response at the moment.

    But, when I checked the supplier's website, the desired item is no longer listed !
    Too many weeks trying to get there.

    So, back to Plan A - a general clean of what I have, a small amount of dressing and a re-assembly.
    I hope my ball count was off and that no balls have been lost.
    Cut the damaged ends of the ball return tubes back to re-establish the original tip strength.
    Checked the fit in the nut and rolled balls into and out of the tube/nut interface - seems Ok.
    The balls are 6.365 diameter on both my micrometers, the tubes are about 6.55 internal diameter.

    I will ask my friend to call the website to confirm nil stock remaining.
    And also open an account with the freight forwarder 'just in case'.

    John.

  10. #10
    Join Date
    Apr 2009
    Location
    Kingswood
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    Default

    Checked all the balls, 102 x 6.365 and 1 x 6.355, so at least one ball has been lost.
    I will reload with the small ball in the 52 group, the 51 group being all large balls.

    In addition to loading the nut onto the screw, I have the problem of positioning the ballscrew in the saddle.
    The sequence is to install the Y-axis ballscrew to the saddle (simple shims), and then the X-axis ballscrew to the saddle, without the table.
    Then slide the table on to engage the ballscrew bearing seat into the table endcap bearing.

    Easy (relatively) to get the X-axis ballscrew parallel and level with the x-axis sideways, but difficult to ensure co-axial with the expected bearing in the endcap on the table.
    The endcap has two dowels.

    Presumably in the factory, with the original trapezoidal leadscrew, the endcap is offered up to engage the bearing, the endcap is bolted to the table, then the dowels added.
    But I need to position the ballscrew in alignment with the endcap bearing in an endcap pre-located by dowels.

    The options appear to be:
    - leave the dowels off, just bolt the endcap on,
    - fill the existing dowel holes and re-drill and fit dowels,
    - go to extreme metrology and/or jigs to get the ballscrew in the correct location.

    The perfectionist in me says I should use the jigs and metrology, the home hobbyist light duty user says 'leave out the dowels'.
    The hollow ribbed design of the cast aluminium endcap prevents alternative positions of the dowels.

    John

  11. #11
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    Jul 2016
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    Melbourne
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    Default

    I would leave out the dowels unless there is room to put just a small 1/8 one in just to locate it?

  12. #12
    Join Date
    Apr 2009
    Location
    Kingswood
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    Default

    The table end cap has solid metal in line with the T-slots (probably to prevent damage).
    I can fill the table T-slot ends and then drill for dowels after fitting the end cap and testing.

    Made a transfer tube to enable desktop loading of the balls into the nut.
    1214 bar125 mm long turned down to 27.1 mm swapping ends in a collett.
    Held bar in the collett, equal overhang inside/outside.
    Got a 3/4" boring bar at McJings and made a holder for my ancient 4-way toolpost.
    Bored, carefully left to right, to 25.2 mm, so 0.95 mm wall thickness.

    Made a locating bush to hold the transfer tube concentric in the nut while loading balls.
    Will make a cap to cover the end of the transfer tube during ball loading.

    Both ends of the screw had severe gouging on the shaft lead-ins.
    These were Dia 25 mm dia when they should have been just under Dia 27.2 mm.

    The screw was mounted in the lathe and lead-ins filed and emery cut.
    Simply aimed for a good finish and fit to the transfer tube.

    Mounted the screw to the fixed steady and used 5 mm sash cord and oil to clean/burnish the ball tracks.
    Also wool thread for the groove in the helix peak and emery tape over the top.
    The screw came out looking good.

    New thought on ball loading - use a suitable hand-held tube and a vacuum pump to hold the ball while loading.

    Checked all the balls under a 200x microscope.
    Half the balls are worn, but probably re-usable.
    The others either show scuff marks or pitting.
    The nut has two separate circuits, so one half may have been the problem
    I need new balls.

    I measure 102 x 6.365 mm and 1 x 6.355 mm (last digit is midway between marks on the micrometer scale).
    The micrometer measures spot-on at 5.1 mm and 7.7 mm on slip gauges.
    My inspection grade DTI station puts the diameter at closer to 6.37 mm.
    Probably 1/4" nominal oversized to fit.

    Can anyone comment on how to assess the required ball diameter ?
    And a source for 110 balls at not too high a price ?

    John

  13. #13
    Join Date
    Apr 2009
    Location
    Kingswood
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    Made the transfer tube end plug.

    Went down to CBC in Penrith for a chat, left with 110 x 1/4" balls for under $14.
    Apparently special sizes are ordered through their system and I could do better myself directly overseas.

    Knocked up a simple nut holder and found a cheap (acquarium ?) vacuum pump with matching rubber tubing.

    Tried one of the new 1/4" balls and did a trial (nil lubrication).
    Positioned the transfer tube in the nut with its end just below the ball return tube capture zone.
    Simple to pick up the ball and release it opposite the capture zone.

    Worked a treat !!!!!!
    Fascinating watching the ball disappear down the helical tunnel presented to it.

    Will provide further update and photos tomorrow.

    Plan now is to load the nut with the 1/4" balls, move the nut to the screw, and try the 'feel' of the nut on the screw.
    Then re-load the nut with the old balls (+ at least one 1/4" to make up the numbers), move to the screw and re-try the 'feel'.
    Depending on the results of these 'feels', a decision will be made as to acquisition of oversized balls.

    John

  14. #14
    Join Date
    Jul 2016
    Location
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    Default

    This thread is great thanks for all the detail.

  15. #15
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    Apr 2009
    Location
    Kingswood
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    Continued the trial fit, no need for the vacuum, just drop the ball into the vertical jigged assembly and let gravity do its job.
    Can even drop multiple balls and stir with a small screwdriver !
    Filled up one circuit with 54 x 1/4" balls, so the old nut may have lost 5 balls.

    With the present jig, cannot get an accurate view of the fit of the last ball.
    My assessment is that the gap left between the first and last ball is about 1/10 of a ball, say 0.6 mm.
    With 54 x 1/4" balls, each ball could be 0.01 mm larger to fill the helical path length.
    But the transfer tube is slightly smaller than the target screw minor diameter, so no point assessing fit/size at this point.

    Plan to fit the nut to the screw without end brushes and circlips so that a view of the balls on the screw might be available.

    Busy at the moment, but will try and get a photo of the process and result later today.

    John

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