1. #1
    V8kid's Avatar
    Lives in stewarton, United Kingdom. Last Activity: 2 Weeks Ago Has been a member for 2-3 years. Has a total post count of 8.
    Hi Chaps,

    I want a nonstandard 2005 ballscrew length (266mm) and it occurs to me that I could simply cut it to length with an angle grinder :) (with 1 or 2 precautions)

    To overcome the problem of re-machining the floating end to accept a BF15 bearing block can I use a BF20 bearing block with the ballscrew threads in direct contact with the bearing inner sleeve? As it looks as if the BK20 bearing is 10mm thick there would always be two threads in contact with the bearing. Of course I would not be able to use a retaining circlip but they are superfluous anyhow as the BK holds the shaft in position and the application is axial (compressive) load only.

    Seems too simple am I missing something?

    Any comments on the need for a BF block in the first place for such a short length and axial load would be appreciated.

    Cheers
    David

  2. #2
    Quote Originally Posted by V8kid View Post
    Hi Chaps,

    I want a nonstandard 2005 ballscrew length (266mm) and it occurs to me that I could simply cut it to length with an angle grinder :) (with 1 or 2 precautions)

    To overcome the problem of re-machining the floating end to accept a BF15 bearing block can I use a BF20 bearing block with the ballscrew threads in direct contact with the bearing inner sleeve? As it looks as if the BK20 bearing is 10mm thick there would always be two threads in contact with the bearing. Of course I would not be able to use a retaining circlip but they are superfluous anyhow as the BK holds the shaft in position and the application is axial (compressive) load only.

    Seems too simple am I missing something?

    Any comments on the need for a BF block in the first place for such a short length and axial load would be appreciated.

    Cheers
    David
    On a Z axis or even a short axis it is quite often the case that you will not need a floating BF bearing at all.
    ..Clive
    The more you know, The better you know, How little you know

  3. #3
    V8kid's Avatar
    Lives in stewarton, United Kingdom. Last Activity: 2 Weeks Ago Has been a member for 2-3 years. Has a total post count of 8.
    Are there any rules of thumb relating to acceptable ballscrew lengths without BF bearings? I guess its a function of the ballscrew diameter and the applied load?

    I'm using 20mm diameter and 266mm length with a NEMA 34 1700ozin stepper with a 2:1 drive reduction.

    Cheers!

  4. #4
    Many people would not use a floating end as said above, but many like me would use rigid both sides. So depends of what you do exactly and for what is meant exactly. Also if you dont say explicitly angular contact bearings, all chinese ball screw mounts come with simple bearings
    project 1 , 2, Dust Shoe ...

  5. #5
    Quote Originally Posted by Boyan Silyavski View Post
    Many people would not use a floating end as said above, but many like me would use rigid both sides. So depends of what you do exactly and for what is meant exactly. Also if you dont say explicitly angular contact bearings, all chinese ball screw mounts come with simple bearings
    Really. I don't think that most people use rigid both ends, I would say at the most 5%. Also all Chinese ball screw mounts do not come with simple bearings as you put it. In fact Fred at BST will generally ask what type you want.

    It would be fair to say that for the Z axes on a small mill or router would almost never have a fixed bearing on the other end
    ..Clive
    The more you know, The better you know, How little you know

  6. #6
    My second machine did not have a machined floating end on the X axis ballscrews and I had some 16mm ID bearings that were a perfect fit so used those. The machine ran well for many years and both mk3 and the current machine are like that. So it can be done and works ok but you wouldn’t see it on a commercial machine.

    On the unsupported end question all my machines had supports even on Z but I couldn’t fit them on mk4 as they ran inside the side members of the Z axis. It works fine and I think up to 300 mm is OK unsupported. After that you need to think about the free end starting to whip at high speed.

    Not many machines (DIY) have double support but it should help with longer screws rotating at higher speeds. My mk4 effectively has something similar on the X axis to give tension.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  7. #7
    On my Chester conversion I though there may be a chance of whip at higher speeds for the X axis so I have mounted the screw between bearings in tension. Y is too short to worry about but I might do the same for Z as that's also quite long.
    You think that's too expensive? You're not a Model Engineer are you? :D

  8. #8
    Quote Originally Posted by V8kid View Post
    Are there any rules of thumb relating to acceptable ballscrew lengths without BF bearings? I guess its a function of the ballscrew diameter and the applied load?

    I'm using 20mm diameter and 266mm length with a NEMA 34 1700ozin stepper with a 2:1 drive reduction.

    Cheers!
    What's been said is all good advice, but for a bit more insight look up "critical speed calculator" or something like that. There are a number of sites around which give graphs and so on which give an estimate of the critical speed for leadscrews/ballscrews based on diameter, length, and end support (fixed/unsupported, fixed/floating, fixed/fixed, etc). Critical speed is the maximum speed before whip in the screw becomes evident/becomes unusable.

  9. #9
    V8kid's Avatar
    Lives in stewarton, United Kingdom. Last Activity: 2 Weeks Ago Has been a member for 2-3 years. Has a total post count of 8.
    Thanks for all the good advice I've taken it on board wherever I can.

    Unfortunately due to the space available on a Bridgeport series 1 head there is insufficient room to mount a BK bearing and pulley at the bottom of the head so the ballscrew will have to be in compression rather than tension. In addition as its a milling machine rather than a router the axial loads are much higher so I'm a bit worried about skipping the floating bearing.

    There isn't enough room as I propose to mount the ballscrew only 25mm from the quill sheath rather than the usual conversions which are 75mm from the quill as there will be less side loading and less chance of buckling. This is also relevant regarding the coupling to the quill which in the Bridgeport is a single screw (3/8 fine IIRC) (reduced leverage)

    Yes I do need to do some calculation rather than supposition and the Hiwin site was a mine of useful info I just need to shut myself away for a couple of hours :) .

    So in conclusion I'm still undecided! Its awkward to fit a floating bearing in the limited space available and as I'm already machining the maximum amount of non stress bearing metal from the Bridgeport head casting I don't want to machine more off to provide a bed extension to fit a standard BF bearing block.

    In the best of bodgers tradition I'll probably do the calculations and unless they are too bad try the single support and if it doesn't work make up a custom floating bearing support.

    Wish me luck.

    Cheers

  10. #10
    V8kid's Avatar
    Lives in stewarton, United Kingdom. Last Activity: 2 Weeks Ago Has been a member for 2-3 years. Has a total post count of 8.
    Well that wasn't as difficult as I thought :)

    From the formulae supplied in the Hiwin manual the permissible (buckling) load for a 20mm ballscrew 250mm long is 2061Kgf and the critical speed is 10,000 rpm. After I did the sums I spotted graphs giving the same info on the next page - Doh! At least they were the same.

    So with a factor of safety of 50% I'm good for 1 tonne axial load and 5k rpm - I sure I wont be anywhere near that but after Hogmanay I'll rig up the scales and measure the Z axis axial load in practice.

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