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Sphinx lead-screws in tension

Discussion in 'CNC Mills/Routers' started by Andreas Bockert, Sep 8, 2018.

  1. Andreas Bockert

    Andreas Bockert Veteran
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    Hi

    An interesting discussion started in another thread (10mm Leadscrew in C-Beam Build) and I figured it would be worthwhile to break it out into a separate thread.

    Anyway, with a 1m lead-screw it doesn't feel great to put them in compression to eliminate backlash. Instead, putting them under tension would be better.

    My idea is to use cheapo thrust bearings like these (10pcs Axial Thrust Ball Bearings 8mm x 16mm x 5mm F8-16M Stainless Steel new. | eBay) in combination with a lock collar to achieve this. I want to use clamping collars like these ((4) 5/16" 2PC FULL DOUBLE SPLIT NEW SHAFT COLLAR BLACK OXIDE CLAMP SC31D SC031D | eBay) they are a lot easier to work with.

    I think this can be done with the extra length of screw that is shipped by default.. Barely! @Rick 2.0 suggested that I could simply push the motor coupling again the thrust bearing on the motor side.

    On nut to crack is how to actually generate the tension needed. I seem to remember a video about R7 CNC that used a nut to generate the tension. However, there will not be enough thread to do this with the current screw length. Any ideas?
     

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  2. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    The nut method shown in the R7 video really didn't create much in the way of tension. All it did was create a good snug-tight condition which is all you really need here. Personally, if I were doing this I'd assemble the front end (like you have shown in the photo) and then stand the unit up where it's bearing on these ends and then push down on the upper end with the coupler and tighten. This should be more than sufficiently snug. If you are determined that you must achieve tension though, drop the aluminum rail in the freezer and lay the screw out in the sun. Assuming about an 80 degree (F) differential when you assemble them you'll have about 1mm of pull when the temperatures normalize.
     
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  3. Andreas Bockert

    Andreas Bockert Veteran
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    I lime the idea of heat, albeit it seems a bit impractical.

    But let me see if I understand what you are suggesting. The idea would be to:
    1. Fasten the front thrust bearing and lock collar.
    2. Tilt the machine so it rests on the back. Letting the weight of the gantry ensure that the front is tight.
    3. Push the motor couplet against the rear thrust bearing and tighten it.

    Sounds like it could work to achieve a snug fit. I’m guessing that most of the gain of tension over compression is simply to avoid bowing caused by the compression.
     
  4. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    The reality is that the set screws probably wouldn't be able to handle the force induced as the 1mm differential settles out so it would be all for naught anyway.

    Actually I was suggesting rolling it forward and standing it up on the front ends of the screws. This will put the free end for the coupling at the top where it is easily accessible. You might want move the gantry to the front before you roll it forward else gravity may bring it forward unexpectedly.

    It's really more of an issue of restraint. Under the current system with the lock collars inside of the end plates as the screw begins to whip the diameter of the whip can increase because the ends of the screw can pull inward. With the lock collars outside of the end plates the ends can't pull in and thus the magnitude of the whipping is better held in check.
     
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  5. CNCMD

    CNCMD Veteran
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    You will see improvements with going with the clamping collar style. One of the issues that I found was unless you machine a flat onto your lead screw, you will not be able to get the set screw style to sit square to the bearing. That play is essentially slop in the movements. The clamping style eliminates the need for the set screw to sit perfectly. All you really need to do it use a block of wood and push against the leadscrew from the back side of the end bracket to put some compression on it, and then tighten down the clamping collar.
     
  6. Andreas Bockert

    Andreas Bockert Veteran
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    I agree that the clamping style collars works a lot better. The 5/16 ends up being 7.94mm so it’s a nice fit for the OB leadscrews. I think that they should stock these at the OB store and upgrade the the kit(s).
     
  7. CNCMD

    CNCMD Veteran
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    Agreed, I think you can also find them at Mcmaster.
     
  8. Giarc

    Giarc Master
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    Another option (what I did) was drill a hole through the setscrew hole into the shaft with the bit for a m3 tap. Then I tapped the hole in the shaft and put a regular, much longer, m3 screw in. I have zero backlash at that location. Well, at least no noticeable backlash. On the molecular level there may be some. ;)
     
  9. Andreas Bockert

    Andreas Bockert Veteran
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    Standing the machine on the screws seemed a bit unwieldy! I think just clamping a block of wood to the screw should be enough? Your comment about restraint makes a lot of sense. To keeping it snug enough to stop it from pulling...
     
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  10. ChadRat6458

    ChadRat6458 Journeyman
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    I flipped my plates around. I do like the thrust bearings. I will add them to my machine. I would like to try 10mm on my x axis. The y axis is fine with dual lead screws. 12mm might be too big for the c-beam. I almost have my 1m x 1m sphinx completed. I keep burning out the z axis driver. I will get another driver and re do my wiring.
     
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  11. joe williams

    joe williams Veteran
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    just ordered some clamping collars for my build too!
     
  12. GrayUK

    GrayUK Openbuilds Team Elder
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    Ditto: Thrust bearings and Collars :thumbsup:
    I think my new machine parts arrive tomorrow?? :nailbite::nailbite:
     
  13. Andreas Bockert

    Andreas Bockert Veteran
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    Just to be clear. I haven’t tried the thrust bearing approach yet. (I’m trying to keep to one project at a time). I would love to hear if it works out well.

    The clamping style collars works well and can also be retrofitted without tearing down the machine.
     
  14. CNCMD

    CNCMD Veteran
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    The clamping style is all that is needed honestly, I have done a significant amount of work when I had my Sphinx to eliminate all sources of backlash. There main 2 sources are the collars and the AB nut. The AB nut that openbuilds does the job. Like I said above the real issue is the set screw collars. If you don't have a flat, the screw will sit on an angle, causing the collar to contact at one point against the bearing, but as it rotates loses contact and creates backlash/slop.
     
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  15. Andreas Bockert

    Andreas Bockert Veteran
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    My concern isn’t backlash but rather whipping/bowing of the lead screw. Especially for 1m screws. Constraining it from this should make movement smoother.
     
  16. CNCMD

    CNCMD Veteran
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    Not to be a party pooper but I highly doubt this would do anything to reduce the whip. 8mm leadscrews are relatively thin in context to a 3 foot + span. Hence the discussion of 10mm leadscrews.
     
  17. Andreas Bockert

    Andreas Bockert Veteran
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    Tension just feels like a superior construction.

    But I suspect that you are correct. 8mm really doesn’t feel all that confidence inspiring at that length. I would like to upgrade to 16mm like your machine at some point.

    What feeds can you achieve? Is it whip-free?
     
  18. CNCMD

    CNCMD Veteran
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    My machine is a bad example to most, in the sense that I have a significant amount of weight in the machine which lends to the rigidity of it.

    I have yet to really push the machine. For aluminum with a 1/8" bit, I cut at about 750mm/min. I don't know if that is anything special, I usually go with .5mm depth of cut. Reality is that I could go faster, and lower the doc, but I go with what works for me.

    I do have my own limitations, running the small Nema 23's, so torque issues were actually recently resolved. Jogging the machine I can get up to 4000mm/min, but that's jogging.
     

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