Thread: Rotating Ball nut
The moment of inertia of one of your RM2010 ballscrews is just over half the inertia of my RM2510 spinning nut, so if paulus used that design with the same motors and drivers he can expect to get about half the accelleration you get, assuming the mass of the gantries is similar. Similarly, if he uses the design pictured in the first post, then the acceleration would be less than 1m/s^2, which is a bit too low really. Either way if drumsticksplitter has put the numbers in the motor calculation spreadsheet, then he will have found that the 4Nm motors are ideal so long as the inertia of the rotating nut assembly is minimised.
For this size machine, then since RM2510 ballscrews are being used the only realistic option is to rotate the nut, since the critical speed would otherwise limit the feedrate to around 5m/min.
The Following User Says Thank You to Jonathan For This Useful Post:
I'm curious how you have screwed into the end of the sttel box section on your gantry?
To Paulus, That indeed is very interesting and also worrying for my 12Nm motors. I was looking at the 150v drives to start with, but I got a guy in china to spec the drives and power supplies to my motors, like he did on a plasma table I recently built and he recommended a 68v 400w system. He also sold the 150v drives btw... However, I do think myself these would work best and would save having to buy power supplies for every drive.
This whole thing with acceleration and critical speed is going way over my head... I was hoping to achieve a 2:1 ish reduction to my 2510 screws to buy a bit of resolution and torque. I've sourced taperlock pulleys, because I was sick to death of damn grub screws coming loose on my last build. Therefore the smallest pulley available is a 34 tooth, which with a 72 tooth pulley gives me 2.12:1 reduction. The problem being that the 72 tooth pulley is 114mm, and going by Jonathon's recommendation to keep the moment of inertia low with small diameter components, I'm faced with another problem...
I've seen this:
Its from a Techno LC router (google search), which is a similar kind of design I was looking at. I know you can't see any detail of the ballnut and bearings, but there is a quite a big ass pulley... This particular machine is servo driven, I wonder if that's why this design works for them?
I tend to put two grubscrews in each pulley, with a small brass cylinder piece under each grubscrew so that they don't mark the shaft. Do them up tight and use threadlock and the difficult bit will be getting them off, not keeping them on. Also your problem with the 12Nm motors is getting enough speed, so adding a 2.12:1 reduction is only going to reduce that speed (although it will improve acceleration). With the 4Nm motors then I'd go for 1:1 if you're mainly cutting wood since you don't need the extra resolution. If you keep the motor mount and ballnut mount as separate parts then it's easy to change the size of the motors, but more difficult to cover the belt.
'The placement of the ball screw in the center of the axis of travel eliminates the possibility of racking.'
So they've tried to justify it by saying it avoids a problem which doesn't exist with two motors done properly, and in fact they've made racking become a potential problem by using one. They've also used THK SR bearings and made out that they are excessive, when in fact they have lower load ratings in the direction they're using them and in the first pages of the datasheet is says you're better off using different bearings with equal load ratings if they're mounted upside down, as is the case here. You can do better than this company...
Last edited by Jonathan; 05-03-2013 at 09:43 PM.
I was searching the last two days for Nema34 stepper for my machine and found some weird things. I will post tomorrow on my build log.
05-03-2013 #16If you keep the motor mount and ballnut mount as separate parts then it's easy to change the size of the motors
They've also used one ballscew on the X-axis, which is pretty poor as it will cause the gantry to deflect/rack when the tool is near the ends.
Would I still have problems with my ballscrews being slightly bent if going for then rotating nut? I think I can straighten them up to almost perfect with a garage press, I know its going to be a long and tedious procedure and making sure I don't make them worse along the way.
I think you may find the best straightening device would be something like the one we used to use to straighten motorcycle fork tubes. If you don't know what that looks like, it is a bit like a very large bearing puller, but with only two legs at 180 degrees, each with a hooked end to hook over the item to be corrected. Find the centre of each part of the bend and apply gentle pressure to that point, with the leg hooks positioned to suit. The device does not need the legs to be pivoted. I hope that makes sense. G.
Geoffrey, that's essentially the same as using a press with some V-blocks.
It may be possible to carry out that procedure with the ballscrew in place on the machine, something that would be impossible with a press. It could also be helpful to others who have bo access to a press. G.
Last edited by GEOFFREY; 06-03-2013 at 03:33 AM. Reason: addition
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