Rotating Ball nut

[Shinobiwan]

The inertia of my design for RM2510 is equivalent to 1250mm of RM2510 ballscrew, hence much better acceleration is obtained than if the screw was rotated.

But what is much better acceleration? I use 1.5m RM2010 and happily achieve 4000mm/sec^3 at 16.5m/min with just Nema23 4Nm(video posted in my Strike thread) I don't run at that because its harsh as *$&£ but if a standard setup can achieve that I have to wonder why bother increasing it further when its already excessive?

Surely all this talk about critical speed and higher acceleration is, in some cases, misleading given practical real world evidence that flys in the face of it?

[GEOFFREY]

But what is much better acceleration? I use 1.5m RM2010 and happily achieve 4000mm/sec^3 at 16.5m/min with just Nema23 4Nm(video posted in my Strike thread) I don't run at that because its harsh as *$&£ but if a standard setup can achieve that I have to wonder why bother increasing it further when its already excessive?

Surely all this talk about critical speed and higher acceleration is, in some cases, misleading given practical real world evidence that flys in the face of it?

Cubic acceleration - WOW!!! G.

[Jonathan]

But what is much better acceleration? I use 1.5m RM2010 and happily achieve 4000mm/sec^3 at 16.5m/min with just Nema23 4Nm(video posted in my Strike thread) I don't run at that because its harsh as *$&£ but if a standard setup can achieve that I have to wonder why bother increasing it further when its already excessive?

If the inertia of the system is reduced, less torque is required to achieve a certain acceleration. To reduce the inertia you can either make the gantry lighter, or reduce the mass/size of the rotating components. Clearly optimising the rotating components is preferable since reducing the mass of the gantry will be detrimental to the overall rigidity of the machine. Since reducing the inertia of the rotating ballnut assembly to be less than the screw is not difficult, it's logical to do that as even if your motors will achieve the required acceleration you gain a greater factor of safety which is always good when it comes with no added cost. So to answer your question directly, it's to get the inertia low enough, or if it already is, then increase the factor of saftey on the torque requirement to make the system more reliable, or reduce the cost of the electrical components.

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.

[drumsticksplinter]

To tension,

I'm curious how you have screwed into the end of the sttel box section on your gantry?

I had 6mm plate profiled out and tapped with 4 x M10 holes. These were then trued up to the ends of the box section and for the moment tacked into place. They will be fully welded once I know this layout works...

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?

[Jonathan]

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.

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?

It looks like what's happened there is they've designed it with a large pulley, realised that that requires more power to drive it and just stuck a reasonably large servo motor on to compensate. Not the greatest design. 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. Lets look at how they justify it on their website:
'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...

[drumsticksplinter]

If you keep the motor mount and ballnut mount as separate parts then it's easy to change the size of the motors

I think that this might be the way forward, I already have 3 x 4Nm nema 23's, which I deemed not powerful enough to spin the ballscrews... Now that option is out of the window then I might look favourably on these again, especially given that I can source the drives much cheaper for them.

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.

That is a major concern because even moving the gantry by hand there is some racking and I've tried to create a fair balance for rigidity vs weight! I think now I have a clearer Idea of what I need to achieve with my motors and a rotating nut design...

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.

[GEOFFREY]

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.

[m_c]

Geoffrey, that's essentially the same as using a press with some V-blocks.

[GEOFFREY]

Geoffrey, that's essentially the same as using a press with some V-blocks.

Yes, but with far more control and the screw could be supported on centres or V-blocks to enable regular rotating to check for staightness. G.

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.

[Jonathan]

That is a major concern because even moving the gantry by hand there is some racking and I've tried to create a fair balance for rigidity vs weight!

I'd be wary of compromising rigidity for weight at all. It's much more important to have a strong machine than a fast machine.

Would I still have problems with my ballscrews being slightly bent if going for then rotating nut?

The rotating nuts would still work well with a bent screw, however it will shorten the life of the ballnuts since you'll be applying a radial force to the nut which is not something they're designed for. How much it shortens the life depends on how much it's bent and the length, since that determines the force the nut has to apply to straighten it as it moves along. It's hard to quantify. How bent are they?