Ballscrew pre-tension

Hi,

My name is Jordi and I´m a fellow cnc builder based in Spain. I started a few years ago building a Momus and since then I´ve been bitten by the bug. I´ve been lurking the forum for a while and I´ve just decided to start participating on it.

I´ve got a machine at the moment fitted with twin ballscrew on the Y axis driven both by 3Nm nema 23 at 40v and I´ve got the feeling the ballscrew are at their limit being 1300mm cheap chinese RM1610 on a fixed-free configuration.

I would like to add some pre-tension to the ballscrew and I´m looking for advise on how to do it. Would something like a M5 threaded hole at the free end be enough to use a screw to tension it? Any better way of doing it?

Thanks,
Jordi

Hi Jordi and welcome to the madhouse (forum). Can you put up some pics of the machine as that would help. 1300mm should be OK for ball screws if they are 1610. Why do you think they are at there limit?

I don´t have any real pic handy at the moment but hope this cad model will help. It´s incomplete but the ballscrew are running on either side of the 100x100 steel beams.

The critical speed for 1610 should be about 1000rpm (using a root diameter of 14mm) and although the machine runs nicely at about 4m/min cutting speed, the motors stall every now and again if doing rapids at more than 4m/min.


Attachment 23910

I feel the ballscrews are quite long and I´ve read pre-tensioning them slightly might help...would like to give a try.

Quote:

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The critical speed for 1610 should be about 1000rpm (using a root diameter of 14mm) and although the machine runs nicely at about 4m/min cutting speed, the motors stall every now and again if doing rapids at more than 4m/min.

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On my router the screws are 1540mm x 2 with nema 23 motors driven by pulleys and belts. I can get 8-10 mtr/min

It would help if you could up your power supply to about 70V if your drivers can handle that. 40V is a a bit on the low side.

Quote:

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Originally Posted by jordiboronat

I don´t have any real pic handy at the moment but hope this cad model will help. It´s incomplete but the ballscrew are running on either side of the 100x100 steel beams.

The critical speed for 1610 should be about 1000rpm (using a root diameter of 14mm) and although the machine runs nicely at about 4m/min cutting speed, the motors stall every now and again if doing rapids at more than 4m/min.


Attachment 23910

I feel the ballscrews are quite long and I´ve read pre-tensioning them slightly might help...would like to give a try.

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Hi,
Perhaps you need to increase the driver voltage, perhaps the PSU current is not enough. What sort of power supply are you using? Switched supplies are not as good as analogue supplies because they don't like sudden power surges, and rapid moves require sudden change in current usage. Also, perhaps you have too high acceleration. Try setting it lower and see if that helps increasing the speed.

Another thing which came to my mind is the drivers and the micro stepping. What sort of drivers are you using and at which micro stepping? Also, more motor data is necessary to be able to say if 40V is enough or not.

Anyway, I think generally speaking, 4m/min is sort of a limit for DIY moving gantry type of machines with steppers directly driving ball screws. That's why I chose to build mine with moving table instead. Also, while 1610 offers a theoretical advantage in speed over 1605, I think the advantage is lost because it also requires higher torque from the steppers, so perhaps lowering the acceleration might be a solution. I don't know which acceleration values you have now, but try half of that just to see if it helps getting up the speed, if yes then try to increase until the steppers stall and set the new value about 10% below that.

Also, I don't think that increasing ball screw tension will help, but perhaps if you fix some sort of stabilizer in the middle of it. There are different solutions, I think the best would be to have one which gets out of the way when the gantry is in the middle. Other solutions for such long screw is to replace the fixed ball screw with a rotating one, but that is more complicated.

The machine is fitted with leadshine DM442 (fully digital) stepper drivers and 3N.m nema 23 from Zapp. Unfortunately the drivers only accept up to 40V and they´re currently powered to 36V. Set to 4m/min and 1500mms2 acceleration gives 0.15g. Upgrading to EM806 (or similar) and 70V PSU adds up quickly for 4 drivers.

I´ve been considering the Lichuan LCDA357H+LC57H3100 which are 3N.m 3Phase closed loop stepper, apparently 3 phase stepper runs smoother and being closed loop will help in precision jobs too, this combo works up to 50V and it´s about 80 euros each (driver+motor) and I can swap them easily as the frame it is the same.
Attachment 23911

Granty weight X+Z combined should be about 60-70 kg all combined. I´m very happy with the machine, just want fine tune it. Should have gone to 70V PSU from the begining but electronics where recovered from previous build....