1. #1
    Hey.

    So I've been a sleeper CNC builder for the last 6months. Built many machines ... in my mind ... sketched the best on paper ... and CAD drawn only a couple of the ones best compromise between material costs + parts.



    I've got a motley collection of parts (which above design is based on) and am almost ready to ask you guys for feedback on my build design ... but before I do ... im battling with some small details I would really appreciate some help with.

    BEARINGS
    What do most people do to solve the: "how to support the ballscrew + minmise axial play" problem?

    For instance what has HiltonSteve done in this machine?





    I appreciate you can use angular bearings like so:


    and fitted like so:



    but that is quite a bit of work + parts ... spacers, preload nuts, making retaining plate, etc. and angular bearings are very expensive

    I got quotes (Brammer) 12mm bore x32x10 28 but I would need 6 (xAxis=2x2 + zAxis=2)
    which is before the other parts


    SELF ALIGNING
    Is the axial missalignment of bearing block vs ballscrew an issue I should be concerned about?

    ie. Bearing housing not perpendicular to balscrew through error in drilling the bearing housing to base surface mount holes

    I am assuming not as I've not see any evidence of people using self aliging bearings be it in a off-the-shelf pillow-block housing or to a self-fabricated housing
    eg.


    -------------------------------------------
    So basically Im sure there is a simple solution but I can't tell what it is from looking at the designs.


    MANY THANKS!

  2. The Following User Says Thank You to williamturner1 For This Useful Post:


  3. #2
    Any missalignement can be a problem, it will put stresses on the bearings and the ballscrew and over time can damage the bearings and in some cases cause the ballscrew to break at the weakest point.
    Why dont you just use an off the shelf ballscrew support?

  4. #3
    Hi WilliamTurner1

    You only need the ballscrew axial support at one end, usually the stepper motor end. The other end is floating. By this I mean the thread is turned down to a plain shaft which sits inside a simple ball bearing, and can slide in and out if required (see some earlier posts about temperature change effects etc.). This cuts your axial support requirements from 6 to 3, since the floating end only needs radial support.

    I simplified this further and left my 16dia ballscrew unmachined and mounted it into a 16mm ID bearing. This only needs to provide radial support, so they are quite cheap. Works well for me.

    For alignment I would suggest mounting the floating end on a plate which is adjustable in as many planes as you can design in. You can then fine tune the position during running trials.

    As Gary suggests, you can buy these as kits. Search for BK12+BF12 which are the full support and floating support bearings as a set.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  5. #4
    Quote Originally Posted by routercnc View Post
    Hi WilliamTurner1

    You only need the ballscrew axial support at one end, usually the stepper motor end. The other end is floating.

    For alignment I would suggest mounting the floating end on a plate which is adjustable in as many planes as you can design in. You can then fine tune the position during running trials.
    Magic! Thank you for confirming. I had figured as much looking at a THK 1m 1605 I have which has a floating bearing at its end (10mm bore vs 12mm axial bore) which is free to move in its housing, the other end is a beasty axial with double bolt holes.

    Now I've got bearings and ballscrews sorted I need to sit down, do the calculations on motors + feedrates and also sort out drive (belts).

    When turning the ballscrews (12mm bore + 12mm thread) should I also design in a woodruff key?

  6. #5
    Quote Originally Posted by williamturner1 View Post
    Hey.

    BEARINGS
    What do most people do to solve the: "how to support the ballscrew + minmise axial play" problem?

    For instance what has HiltonSteve done in this machine?



    Let me explain what I did with my machine......!

    It is a very simple but very effective way of mounting the stepper motor and also overcoming the problem of endfloat in the ballscrew.

    I will try and describe my solution, hang on, actually it was JohnS's solution, credit due were its due!

    The design consists of a 25mm OD oldham style coupling, a piece of 2 1/2" square aluminium at 60mm long and 2 angular contact bearings (I used 7301's, 37mm OD 12mm ID).

    Step 1 -Face the 2 1/2" square and bore it straight through at 26mm, turn it round and face the other end then bore it 37mm to a depth of just under 24mm. The 37mm bore needs to be a push fit for the bearing and the 24mm depth needs to be slightly shy, from memory I think I did them at 23.90 - 23.95mm this is so the 2 angular contact bearings get clamped in position between the block and the face that the block is mounting to.

    Step 2 - 16mm x 5mm pitch ballscrew, turn one end down to 12mm OD, this again needs to be very accurate +0.00 -0.02 as it needs to be a good fit on the bearing. Turn it down for a length of approx 35mm, this length depends on the type of oldham coupling you are using. Screwcut the end at M12 x 1.25 (metric fine) to a length of approx 11mm, leave a length of 24mm untouched at the original 12mm dia as this will be were the angular contact bearings fit.

    Step 3 -Take one half of the oldham coupling and drill and tap it M12 x 1.25, basically turning it into the nut for the ballscrew.

    Step 4 - Take the other half of the oldham coupling and 'adjust' it to fit the stepper shaft.

    Step 5 - Slide the angular contact bearings onto the 12mm dia on the ballscrew then screw on the half of the oldham coupling which is tapped to hold the 2 bearings in place, tighten it just enough to take the backlash out of the bearings but do not over tighten it. Tighten the 2 grub screws to hold it in place.

    Step 6 - Slide the ballscrew with bearings into the block, bearings should be very slightly proud - 0.05-0.1. Mount block with screw onto machine, when tightened you should have no endfloat on ballscrew but it should rotate freely.

    Step 7 - Put the other half of the oldham coupling onto the stepper and offer it up to the block, you should find that the stepper shaft is too long and stops the motor from meeting the face of the block, cut the shaft down to suit. Dont forget to put the coupling disc in.

    Step 8 - Look up the address for John Stevenson Engineers in Long Eaton on Google and post 1 packet of hobnobs for each mount that you make, come on it was his idea so I think its only fair...!

    Drawing attached which should help you understand the above a little better.
    Attached Thumbnails Attached Thumbnails Motor mount assy without mount plate.pdf  
    Last edited by HiltonSteve; 23-09-2010 at 12:02 PM.

  7. #6
    Whatever you do don't mount the stepper on pillars.

    For one it's the most flexible design known to man or beast and secondly it looks like a grandfather clock on steroids.

    Don't forget to do as Hilton Steve says and send the hobnobs, don't bother looking us up in the phone book, like all the best whores we aren't in there as we are ex-directory.
    It's done like this to stop customers bothering us.

    .
    John S -

  8. #7
    I found John through a link on the ArcEuro website (help/quote for machining). I had not joined this forum at the time, otherwise I would have gone prepared with an assortment of biccies, including the aforementioned. Didn't see any bats either, but it was day time and they were probably hiding.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  9. #8
    Quote Originally Posted by williamturner1 View Post
    I got quotes (Brammer) 12mm bore x32x10 28 but I would need 6 (xAxis=2x2 + zAxis=2)
    which is before the other parts
    28 for 1 x 7201 bearing...!!

    I should try looking elsewhere, should be paying no more than 2.50-7.00 each depending where you get them from and the quality you are after. You will not need anything that's high quality as the speed that it will be working at and the forces involved will both be quite low compared to what the bearing specs are.

    If ringing round then ask for the Chinese bearing prices, they should be more than good enough for what you will be using them for.

    And there is always fleabay.... http://cgi.ebay.co.uk/7201-SINGLE-RO...item2eab04d5bb

  10. #9
    Quote Originally Posted by HiltonSteve View Post
    Let me explain what I did with my machine......!

    It is a very simple but very effective way of mounting the stepper motor and also overcoming the problem of endfloat in the ballscrew.
    ...
    Step 8 - Look up the address for John Stevenson Engineers in Long Eaton on Google and post 1 packet of hobnobs for each mount that you make, come on it was his idea so I think its only fair...!

    Drawing attached which should help you understand the above a little better.

    Thats brilliant! Thank you very much for taking the time to explain this!!

    You mentioned
    2 angular contact bearings (I used 7301's, 37mm OD 12mm ID).
    What tollerances did you use (Normal=BE, P7, P5, P4, P4A)?
    Did you run them back to back?

    ...
    Just noticed your latest post about sub 7 ... I will keep hunting

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