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    Default Ball screw size

    Hi,

    I'm currently building a CNC router of 1.8x1.3 meter to cut wood. I need some advice to choose the ball screw size.
    I was thinking about using a Hiwin RM2005 with KL23H2100-35-4B 381 oz In. motor.
    I calculate critical speed that should be around 650rpm. Is it correct?
    What do you think about it?

    Thanks.

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    Default Re: Ball screw size

    Quote Originally Posted by BriceO View Post
    Hi,

    I'm currently building a CNC router of 1.8x1.3 meter to cut wood. I need some advice to choose the ball screw size.
    I was thinking about using a Hiwin RM2005 with KL23H2100-35-4B 381 oz In. motor.
    I calculate critical speed that should be around 650rpm. Is it correct?
    What do you think about it?

    Thanks.
    5mm lead, 600 RPM, 600 x 5 = 3000 mm/min = 118 IPM

    10mm lead, 236 IPM

    20mm lead, 472 IPM

    IMO, your choice is between 10mm and 20mm. It depends on the weight of the moving parts and the acceleration you want as to which one is better. Definitely do not use 5 mm on a machine with a 1.8m or 6' ball screw.



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    Community Moderator ger21's Avatar
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    Default Re: Ball screw size

    10mm should be the minimum pitch for a router.
    Some people use 5mm for the Z, to get away with a smaller motor.

    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Default Re: Ball screw size

    Thanks a lot for your answers!
    I think I will go for a 10mm.
    Smaller pitch means more torque and more precision, is it correct?
    20mm diameter screw is it large enough for 1.8m?



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    Default Re: Ball screw size

    I have 5mm screws as they can be found much cheaper. I have no trouble at all with speed at 1m long.



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    Default Re: Ball screw size

    Quote Originally Posted by joel0407 View Post
    I have 5mm screws as they can be found much cheaper. I have no trouble at all with speed at 1m long.
    I find it hilarious when people persistently recommend 5mm screws to others. His is 1.8m long, not 1m.

    Please remind me what your top speed and acceleration settings are?

    5 mm is a really bad choice for him. I have done the math for a few different machine parameters, and can say that confidently.

    Quote Originally Posted by BriceO View Post
    Thanks a lot for your answers!
    I think I will go for a 10mm.
    Smaller pitch means more torque and more precision, is it correct?
    20mm diameter screw is it large enough for 1.8m?
    More precision, yes, but keep in mind that my 2:1 gear reduction R&P router moves about 1.5" per rotation and is plenty accurate with 10x microstepping. That's about the equivalent of 38mm lead on a ballscrew for precision. So, it really doesn't matter to you as much as you think. A little runout from your spindle will cause more loss in precision than that.

    More torque, it doesn't work like that. Because you have to spin the lower lead ballscrew faster to get to the same speed with the same linear acceleration as a higher lead screw. Acceleration is the factor that is important. To see the comparison, all the factors need to be known and you need to do the math. Or you can just trust me that 10 is better than 5.

    If you had a motorized screw jack to lift a car, spinning and accelerating very slowly and with a small length, all compared to a ballscrew on a machine like yours, then the smaller lead would give you more force and be a better choice. But for a CNC machine it's more about acceleration than holding force.

    20mm diameter, I think should be fine, I went with 25mm because I thought it would be better with a rolled ballscrew for end machining purposes for 15mm ID bearings. I think I made a mistake.



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    Default Re: Ball screw size

    Quote Originally Posted by NIC 77 View Post
    I find it hilarious when people persistently recommend 5mm screws to others. His is 1.8m long, not 1m.

    Please remind me what your top speed and acceleration settings are?

    5 mm is a really bad choice for him. I have done the math for a few different machine parameters, and can say that confidently.
    I'll have to check what I have them set at. I have them backed off as it was capable of far too fast for my liking.

    I'm glade you've done the math because I have done the practice. LOL. Don't take offense, Just in jest.

    I had all these crazy recommendations and went a little less than most suggested as I built a hobby machine and wasn't worries about it being a little slow. Turned out so crazy fast that I need to slow it down.

    I agree that 5mm will give more accuracy than other parts of the machine but if you keep every part as accurate as possible then the total of all the inaccuracies will be less.

    I'll update with my current speed.

    Happy Days



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    Default Re: Ball screw size

    Yep. Currently set at 700mm/second squared for acceleration and 100mm/sec for velocity.

    - - - Updated - - -

    Just making a video of what that looks like.



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    Default Re: Ball screw size





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    Default Re: Ball screw size

    I must say after all above. I'm not saying 5 is better than 10. Just it's not as big of a deal as many make out. And as others have said, you want 1.8m and my example is only 1m. Whip will become exponentially more of a problem the longer you go. Also the longer it is the more you'll want it to have some reasonable speed. Maybe consider driving a ball nut rather than the screw. It's worth considering and checking prices. I'd even say you'll do with a smaller diameter screw then. It may become more economical way to do it.

    I'm commenting because I did get a little caught up in all the advise and I was a little worried my machine wouldn't be as good as it has turned out.

    Happy Days



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    Default Re: Ball screw size

    Yes 5mm is doable, but with it you're limiting yourself. Especially with 1.8m long axis.

    With higher pitch you can get away with higher inductance steppers, lower voltage PS, etc.

    It is also not true that 5mm is cheaper. Yes there are cheap 5mm screws all over internet but is the quality there?

    From a good vendor (BST, automation overstock, etc) the difference between 5mm and 10mm pitch is minimal.



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    Default Re: Ball screw size

    Yep. Currently set at 700mm/second squared for acceleration and 100mm/sec for velocity.
    That's a velocity of 236ipm, which I would find to be painfully slow.

    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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