Stationary steppers for X&Y?

I see this was discussed in 2018. Since then, has anyone tried relocating the steppers to be mounted to the base or corners rather than being carried on the moving carriages? The belts would be loops, anchored to the carriages, with a stepper at one end of the table and an idler pulley (tensionable) at the opposite end. This would reduce the weight of moving parts and thus increase the speeds esp. of rapids. When doing finishing passes of a 3D carving, it comes out better if all cuts are in the same direction. That means a lot of rapids back to the other side.
Stationary steppers would allow using much beefier steppers, to allow cutting steel and aluminum at a reasonable speed.
It would make sense to orient the belt so that its teeth and grooves are vertical, so that dust and swarf would tend to fall off it. That means the motors would point up or down.

I don’t think there is anything fundamentally wrong with stationary steppers, like a 3D printer, but it might not help that much.

Stiffness is related to belt length, and with a loop instead of a single stretch of belt, the belt will stretch more, so you would want a wider belt to offset that.

Also, the motors are not a big part of the weight of the gantry. Not insignificant, but not dominant.

Also, weight limits the allowable acceleration more than it limits the maximum speed. If you accelerate slowly enough you should be able to go just as fast with a heavy gantry as compared to a lighter one. The driver voltage will affect the maximum speed, so if you’re using 12V you could benefit from switching to a 24V motor supply. The motor current will affect the maximum torque which translates into a lower maximum acceleration for heavier gantries.

Wiring would be simpler with stationary motors, so that would be nice. Only the spindle and the Z axis would need to be wired through the drag chains.

As for beefier steppers, it has been discussed at length elsewhere but to summarize, it is only one of several factors that determine the limits of the machine, and to really see a benefit would require redesigning several other parts that are unlikely to tolerate the higher loads as-is.

I’m sorry, I don’t mean to dump on your idea. These are some things I would consider.

And if it’s primarily faster rapids you want, that might be possible already with a much less invasive change.

Thanks, Jamie, that is all useful. Changing acceleration profiles and using higher drive voltage are two easy improvements. Are they now standard?

I have heard of MPCNC users melting their motor mounts. Has this problem been solved by moving to 24V or made worse? One YouTuber installed cooling fans on his steppers. To me, this cries out for a bigger stepper motor. I will try to find the discussions about bigger steppers.

Better (wider, or more high-quality) belts seems like a cheap easy upgrade, if stretch is indeed a problem. Having a tensioner with a spring rather than just a zip tie seems like another easy upgrade. But if stretching is causing increased backlash, then a belt with better fiber reinforcement and/or different shaped teeth (and cogs to match) seems in order.

Not having built my MPCNC yet, I am curious what the weak points (that would break or bend with more powerful steppers) are that need to be re-designed. I can help do that as I go.

Fwiw, I get flexing and chatter before I miss steps. I really love the new Core design, but it’s still plastic and I’d expect it to give before the steppers do. Wish I could say for sure, but I’m still using a 611 on my primo and I can’t tell if the flexing I have is from the tool mount.