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  1. I would have thought that the hollow section, being less stiff than the solid one would start whipping at a lower speed.... like for like on external diameter. I am sure the critical speed is related to the stiffness in some way.

  2. #22
    Quote Originally Posted by irving2008 View Post
    I am sure the critical speed is related to the stiffness in some way.
    That would seem logical since tensioning the screw sort of makes it stiffer, and also raises the critical speed.

  3. #23
    Critical speed is related to stiffness, but dynamic stiffness not static stiffness. The difference is the mass term. Nat freq is related to SQRT(k/m), so although the stiffness drops for a hollow section, the mass drops more. Therefore the frequency increases, as does the whirl speed.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  4. #24
    Cant help thinking of my ex-girlfriend now... One christmas she put on over 2 stone in weight... Then tried to startve herself slim but after a couple of days kept whining on about feeling hollow...

    Whether she was full, or hollow, there was definite sagging in the middle.. And it did take a huge amount more force to get her moving...!!

    All this interesting stuff about hollow lead screws aside, I think i had better check the real critical aspect to this.. Cost! Had originally thought of dual drive rack & pinion, which would have allowed less powerful motors (was looking at which is a mechmate european recommended motor), but the rack itself look expensive.. If anyone can point me to good sources for R&P, and I am assuming Zapp is as cheap as anyone on lead screws and ballnuts..?

    Better get my calculator out and work out some prices..


  5. #25
    You could always consider chain drive...

    A chain and sprocket solution could work out very cheap to do.

  6. or belt drive?

  7. #27

    Am using a chain drive now, which works ok, but as this is my 3rd build I was looking to upgrade wherever I could.. Have a feeling that motors and electronic are going to be the biggest buy but those will be very dependent on the type of drive mechanism. One thing I will say about chain drive, get a good quality chain! I was shown the difference between a cheapo one (which I have) and a decent make and was very surprised at the accuracy difference.. 2nd thing is, keep them tucked away cause they attract dust like flies to poo..


  8. #28
    Why not use ballscrews on the X-axis, but spin the ballnut instead of the screw. I know it's a bit more work to set this up, but it removes the resonance issues and greatly reduces the rotating mass...

    I'm doing this on my router, just with threaded rod not ballscrews:

    Click image for larger version. 

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    One of those on each side of the gantry.

  9. #29
    Hi Jonathan..

    That looks well funky.. I would like to see the completed unit included how you attach the motor..


  10. #30
    I'm attaching the motor just to the side of the gantry on a sliding mount thingy....much like I have on the Y-axis (see by build log). I'll mill a slot in the gantry side for the belt to pass through. The outer rings of the tapered roller bearings will be held in some 20mm aluminium plate, with bolts between the plates to apply preload to the bearings.
    It'll be a while until I make that.

    Note how I've made the delrin (might use brass...) nut in 2 parts, squashed together to eliminate backlash. That's the theory anyway.

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