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  1. Hi all, so we got some new TBI ball screws from Fred at BST Automation, to replace the bent 5mm pitch ones - as discussed on the forum here: http://www.mycncuk.com/threads/14508...r-build-pickle

    I have given them a little examination and there's a problem:



    As far as I can tell (and please do let me know if my measuring method is all wrong) one screw has a wiggle of 0.35mm and the other 0.15mm. The nut's flange seems very accurate as does the motion of the nut relative to the screw.
    So my question is how realistically straight should they be??? This is with the v-blocks about 1.8m apart, and the last ones were over 1mm out over that length... Would it be reasonable to expect the x rails and bearing attachment to push the TBI screws straight during use with this level of run-out? Or should I be contacting Fred to query this?

    They were not super cheap, and before I go ahead and modify the rotating nut attachments and fit them I would really appreciate any feedback you might have.
    Last edited by Bongo; 02-06-2021 at 07:53 PM.

  2. #2
    I wouldn't worry about that amount of error with how you are measuring it and over 1.8mtr distance just using V-blocks. It could probably deflect much of that 0.15mm under its own weight over 1800mm distance. Between your screw alignment to the rotating nut and the error you have in the rest of the machine, it will far outweigh that in the grand scheme.!

    Are you adding any preload to the screw.?
    -use common sense, if you lack it, there is no software to help that.

    Email: dean@jazzcnc.co.uk

    Web site: www.jazzcnc.co.uk

  3. The Following User Says Thank You to JAZZCNC For This Useful Post:


  4. Hey Jazz, thanks for taking the time man, appreciate that, and I do feel much reassured :D

    That's what I thought about the 0.15mm, I was less sure about the 0.35mm wobble - but I guess that is just 0.15mm deflection on either side of some theoretical centre line...

    I do plan to tension them, and they have threads on both ends for that reason.
    Actual details of how I mount them need working out though: the steel brackets on either side are substantial enough to put a serious bit of tension on them, and I don't want to pull them so much that I lose accuracy on the screw's pitch Any idea what kind of force I should be aiming for with these 32mm screws if tightening the M25 x 1.5 nuts with a torque wrench?
    I want to be absolutely sure that the tensioning nuts are pushing onto a perpendicular surface - I know it's not ATM, even by eye, and would probably 'pull' a bend into the screw if torqued up the way they are. I was planning to mount them loosely and use metalised epoxy to create a perfectly perpendicular & flat surface to tighten against...

    Well, thanks again, I guess I better get on and do some lathe work on the rotating ball nut spindles, so I can actually mount them ;)

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