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  1. #11
    Ok thought you meant the linear bearings on the rails

    For ballscrews my machine is 5mm pitch all round but if I was starting again then 5mm Z and 10 mm for X and Y. This gives better rapids in wood and should be Ok for aluminium but if you drive the X and Y via belt and pulley you can swap out the ratio on X and Y to give 2:1 to give more torque (and less speed) if needed on the aluminium
    Not sure about 20 mm pitch - will give very high rapids but you loose some torque and could stall the steppers? Anyone tried this much pitch? May be ok with enough voltage on the steppers . . .
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  2. #12
    BriceO's Avatar
    Lives in Here, Switzerland. Last Activity: 2 Hours Ago Has been a member for 0-1 years. Has a total post count of 20.
    Not sure about 20 mm pitch too :) Belt and pulley can offer more possibilities, good idea! Do you know where I could find good ones?

    Someone made a graph for me for the same motor with different pitch considering the weight of the gantry, ballscrew type, motors specs,... :
    Click image for larger version. 

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    10mm seems to be a good compromise. What do you think about that?

  3. #13
    I think 10mm pitch will be plenty good :)
    Atleast I hope so as I have it on my machine that I'm building!

    Seems like you chose good motors but if speed is important to you they will perform much better with higher voltage psu and drivers. The G540 is a bit outdated I think :)
    If you have already bought it its okey because it probably offers good stable motion. But you can get better performance!

    Skickat från min SM-N910C via Tapatalk

  4. #14
    BriceO's Avatar
    Lives in Here, Switzerland. Last Activity: 2 Hours Ago Has been a member for 0-1 years. Has a total post count of 20.
    Ok, I took a quick look a your CNC, it's a nice work Nr1madman! It seems to be finished soon :)

    I can't figure out why is it better to build the rails on the moving plate and the bearings on the stationery side. Maybe someone could explain me.
    And is it necessary to have ballscrew with preload torque?

    I already bought the G540 for my 1st machine but I really like it, it just works fine.

    I'm finishing to update my design and will probably order linear rail Hiwin HG20 (1800,1380,300mm) and ballscrews RM2010(1795,1375,295mm).

    Really excited to start to build it!

  5. #15
    The reason to mount the rails on the Z axis plate is part that the rails actually strengthens the plate. And if you have the bearings on the plate you always have the same distans (leverage) from bearing to the tool tip. If you have the rails on the plate you can raise the part that you are machining closer to the spindle and shorten the distance from bearing to tool tip for higher rigidity.

    Most people seem to buy standard ballnuts without preload. What I've seen some people do is use 2 ballnuts and preloaded these against each other for backlash free operation. Or are you talking about ballscrew mounting with 2 fixed bearings instead of a floating bearing in one end?

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  6. #16
    Z axis bearing arrangement
    Arrangement A: With rails on the moving plate (Z axis) the lower bearing is always as close as possible to end of the tool.

    Arrangement B: With rails on the stationary plate (Y axis) the lower bearing is much further away from the end of the tool (vs arrangement A) when cutting close to the home position / tall parts. This is less stiff an arrangement Vs arrangement A. However, when cutting at full stretch near the table than they are the same.

    So sometimes they are the same, and sometimes A wins in terms of stiffness, depending on the height of the part you are machining.

    I've seen arguments made that arrangement B is OK, probably on small Z travel machines, and that it could be said to be a bit easier to build. Jazzcnc put forward an argument for B, can't remember what it was now. My personal preference is A.

    Ballscrew 'preload'

    "Ballscrews with preload torque" ? You put this right after the sentence over rails and bearings so assume you mean preload on the bearing carriage- the part which slides on the rail? If so ZA is better than Z0 if you have the choice:
    http://www.mycncuk.com/threads/9373-...r-a-diy-router

    If you mean preload on the ballscrew - the part which rotates and drives the axis then for general DIY machine the ball nuts as supplied from China are fine. They typically have 30 - 50 um (0.03 - 0.05 mm) backlash. C7 grade is fine for DIY machine, but if you really want there is a C5 grade:
    http://www.mycncuk.com/threads/1558-...curacy-V-Price
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  7. #17
    Oh!
    Don't think you need 2010 screw for Z axis.. 1605 should be fine. Might even be okey with 1204 for Z if you have a smart mounting strategy :)

    Have you calculated the length of the ballscrews from desired ballnut travel + ballnut distance + end machining or from what is easy to mount in your design?

    Skickat från min SM-N910C via Tapatalk

  8. #18
    @routercnc
    I should just shut up, you explain so much better :D

    Skickat från min SM-N910C via Tapatalk

  9. #19
    BriceO's Avatar
    Lives in Here, Switzerland. Last Activity: 2 Hours Ago Has been a member for 0-1 years. Has a total post count of 20.
    Thanks guys for the explanations ! :) I'll probably use arrangement A and 1605 for the Z axis.
    I have 6082 T651 aluminium plate, do you think is it flat enough to use it?

  10. #20
    Chaz's Avatar
    Lives in Ickenham, West London, United Kingdom. Last Activity: 20 Hours Ago Has been a member for 4-5 years. Has a total post count of 963. Received thanks 67 times, giving thanks to others 42 times.
    Quote Originally Posted by BriceO View Post
    Thanks guys for the explanations ! :) I'll probably use arrangement A and 1605 for the Z axis.
    I have 6082 T651 aluminium plate, do you think is it flat enough to use it?
    Nope, try get tooling plate. www.clickmetal.co.uk

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