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  1. #41
    Quote Originally Posted by Jonathan View Post
    Perhaps, but you can't deny the simple calculations I did in the previous post demonstrate a significant torque disadvantage from using a bigger screw with lower pitch.
    No regards torque It's undisputable obviously, but I know from experience with actually using them not calculating or theorising.! . . they have more than enough torque even with the disadvantage.



    Quote Originally Posted by Jonathan View Post
    The calculations of critical speed may not mean as much as they imply, but I feel they mean more than nothing and give a good basis.
    Yes to a small degree but simply too meny variables to explicitly say that 16mm are the best by far, to the point of dismissing the others completely.

    I tend to work and put more value in experience than calculations and I know from my own experiences with helping others that long thin 16mm screws give more grief than the same length 20mm.
    If you want to go looking around the zone etc then I think you'll find more unhappy owners of long 16mm screws than you will 20mm.

    I know if asked to build a machine with 16mm screws of this length I would re-fuse and recommend 20mm with correct size pitch to suit the intend purpose of the machine.

    My proposal to use geared 20-05 was work around to no cheaply available 20-10 and wasn't saying they are the ideal solution just an easy one that I know works thru practicle hands on experience. . . . The right pitch for the job wins every time but theres always other ways to skin a cat.

    I'll take real world experinece over calculated theory any day . . . . IMO it's priceless and feel privilaged when someone shares there often hard learned experineces.

  2. #42
    Oops, sorry gents, dropped the quote somewhere along the line, what I should have posted is:


    Quote Originally Posted by Ricardoco View Post
    :confused:



    Ok let me show some diagrams..

    Attachment 4430
    With no load and Spindle central this is the look of the machine from above.

    Attachment 4429
    Now cutting brings in to play these forces and consequences.



    Rick
    Rick (anyone), A silly question perhaps... most of the utube vids I've looked at seem to position the work at 0,0 giving the forces you describe in the pic 2. Would I be right in thinking that if the work was positioned mid Y (above the ball screws) with x=0 or where ever, it wouldn't be so so much of a problem? And, would that imply that best accuracy / precision would be in the centre of the table rather than at the edges??

  3. #43
    Quote Originally Posted by JAZZCNC View Post
    No regards torque It's undisputable obviously, but I know from experience with actually using them not calculating or theorising.! . . they have more than enough torque even with the disadvantage.
    It's almost impossible to have too much torque since it so directly affects machining and is so useful, and the 20mm screws enable a much greater factor of saftey than the 16mm ones, or higher feeds/acceleration if you choose that way.

    When I did the motor tuning on Chip's router we got perfectly good feedrates (can't remember exactly what, long time ago). That's with similar length 16mm screws, but only 5mm pitch so nowhere near as good as 10mm pitch if you're after speed.

    Quote Originally Posted by JAZZCNC View Post
    If you want to go looking around the zone etc then I think you'll find more unhappy owners of long 16mm screws than you will 20mm.
    Surely that is because the vast majority of people use 5mm pitch 16mm screws, which due to my reasons earlier are clearly going to cause grief.

    Quote Originally Posted by JAZZCNC View Post
    I know if asked to build a machine with 16mm screws of this length I would re-fuse and recommend 20mm with correct size pitch to suit the intend purpose of the machine.
    Yes, naturally so would I in an ideal world where it's a reasonable price to get 10mm pitch.

    Quote Originally Posted by JAZZCNC View Post
    My proposal to use geared 20-05 was work around to no cheaply available 20-10 and wasn't saying they are the ideal solution
    Gearing helps but you're still limited by the torque due to the high screw energy and potentially whipping. I will only ever use pulleys anyway as clearly it gets the most optimal torque and has the added bonus of reducing resonance.

    Quote Originally Posted by JAZZCNC View Post
    I'll take real world experinece over calculated theory any day . . . . IMO it's priceless and feel privilaged when someone shares there often hard learned experineces.
    Yes experience is of course great if the situation is similar enough for it to apply. I probably lack in experience to you, but however valuable experience is, experience backed up with mechanics is indisputably a lot more useful.

  4. #44
    Quote Originally Posted by mocha View Post
    Rick (anyone), A silly question perhaps... most of the utube vids I've looked at seem to position the work at 0,0 giving the forces you describe in the pic 2. Would I be right in thinking that if the work was positioned mid Y (above the ball screws) with x=0 or where ever, it wouldn't be so so much of a problem? And, would that imply that best accuracy / precision would be in the centre of the table rather than at the edges??
    Ok understand better now.! If using just a single centrally placed screw then yes the most accurate position is in the centre of the machine and as you approch the outer edges the gantry will flex and twist~(racking) reducing precision.
    How much racking depends on several factors but obviously the wider the machine the worse it gets, wide gantry or wide placement of the bearings the gantry sits on helps reduce this to a degree.
    Basicly if you have central screw on narrow machine with wide bearing placement then the racking affect can be controlled to acceptable levels. Wide machines are better suited driven from both sides of the gantry using 2 screws.

    Regards work positioning I think you'll find when you see a referanceto 0,0 position it will often relate to work coordinates which are often differant to Machine coordinates.

    Excuse if you know this.! . . . Machine coordinates define the overall cutting area of the machine. .Work coordinates define a referance point on your material that you want to start work cutting from.
    So say you have 400mm * 400mm cutting area with machine coordinate 0,0 defined in the lower left corner (looking down from the top) and place 100mm x 100mm material in the centre with the cutting start point in the lower left corner of the material.
    Then the work coordinate is at 0,0 but it's actual position on the machine is at 150,150. Where you position the work 0,0 coordinate will be determined by where abouts you position the 0,0 in you cad program and the part your drawing.
    Often Machine coordinates are positioned in the lower left corner so you'll never have negative machine coordinates, so If your work as negative coordinates IE Circle with centre defined as 0,0 work coordinate then you need to offset the 0,0 work coordinate far enough away from 0,0 machine coordinates other wise the machine will crash into the limits.

    Hope this helps if i'm teachng dad to suck eggs then i'm sorry.!

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  6. #45
    Quote Originally Posted by Jonathan View Post
    experience backed up with mechanics is indisputably a lot more useful.
    Ok Jonathan we'll have to agree to disagree on some things but I'll agree with you on the above.!
    (Provided your experienced in mechanics.! . . I know I am got papers some where that tell me so.:whistling:)

  7. #46
    Thanks for your time with the answer Jazz, most helpful! No, not sucking eggs even slightly, I'm in the happy position of knowing enough to know I don't know enough! :-)

    I might have clarified what I meant better by substituting the 0,0 for a "corner of the work area" But, yes, I see what you mean. I'm interested in this thread as the design (Rev 872) I'm playing around with uses steel too, most helpful!

  8. #47
    GTJim's Avatar
    Lives in Coventry, United Kingdom. Last Activity: 2 Weeks Ago Has been a member for 8-9 years. Has a total post count of 110. Received thanks 3 times, giving thanks to others 20 times.
    Just an idea and probibly silly, but how would this concept work as a rotating ball nut?

    Click image for larger version. 

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    Ball screw fixed at each end, pulley machined to fit ball nut (green) and motor mounted to cross brase of gantry under the table with another pulley and belt.

    Jim

  9. #48
    I can't see a problem
    but your cable and may be vibration on the rail.

    maybe box section instead of flat bar

    but I'm no expert

    James

  10. #49
    if I'm reading the drawing right, is the belt the only thing that will transfer the movement to the gantry, for the gantry to move on the x axis? that would be a problem.

    Caveat as above, I'm no expert! :-)

    max
    Last edited by mocha; 14-09-2011 at 06:06 PM.

  11. #50
    I'm afraid that wouldn't work. As soon as you put and tension on the timing belt it will bend the ballscrew which is obviously bad. The ballnut needs to be supported with bearings and it's own mount. The best option is a pair of preloaded angular contact bearings with the pulley in between so that both bearings resist the tensioning force of the belt. This is what I used. Angular contact bearings are best as they accept radial and, importantly, axial loads whereas standard deep groove bearings don't take much axial load. Having said that I've seen one on youtube that uses deep groove bearings, but it's only cutting foam...

    Quote Originally Posted by mocha View Post
    if I'm reading the drawing right, is the belt the only thing that will transfer the movement to the gantry, for the gantry to move on the x axis?
    Yes!!

    Here's mine:

    http://www.mycncuk.com/forums/showth...t-design-ideas

    I've almost finished designing one similar to that for 16mm and 20mm pitch screws with 7207 bearings.

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