Nema17 motors driving 1204 ballscrews ...that ok?

**Jonathan** · 23-06-2013

The efficiency of a ballscrew is typically over 90%, compared to maybe 30-40% for a threaded rod which relies on sliding friction, so you would expect a big increase in feedrate. The torque required to accelerate the ballscrew would be greater due to the increased diameter, however since the pitch is greater the required angular acceleration is actually less, so overall you'll be much better off with the ballscrew.
The main reason for ballscrews having higher efficiency is they rely on rolling friction, not static friction, so you may want to consider trying the method I demonstrated here, where the screw nut is replaced with a bearing at an angle resting on the threads. That converts the drive to mostly rolling friction, so the efficiency is greater, plus it's also easy to get very low backlash. It's a very cheap way of upgrading, but I'd still stick with ballscrews if you can fit them in due to the accuracy (among other things).

Perhaps by using timing belts and pulleys to drive the screws you could fit in bigger motors? You could also use a favorable ratio to get higher resolution for milling PCBs. Incidentally, what feedrate and spindle speed do you currently use?

£66+15 for that spindle seems a lot to me when compared to the cost of the small brushless motors and controllers from hobbyking. Granted they're not that quiet, but I'm sure you'd find ways round that. The substantially higher RPM should make up for that too, since you could cut the PCBs faster.

**HankMcSpank** · 23-06-2013

Thanks for your input Jonathan.

Originally Posted by Jonathan

Perhaps by using timing belts and pulleys to drive the screws you could fit in bigger motors? You could also use a favorable ratio to get higher resolution for milling PCBs. Incidentally, what feedrate and spindle speed do you currently use?

Well my previous spindle only spun at 5,000rpm (which was my limiting factor wrt pcb milling feedrate), so I kept the feedrate at about 200mm per minute (which is painfully slow).

The 300w chinese spindle turns at about 8,000rpm @ 32V - the spec suggests it can spin as high as 12,000rpm at 48V...but I wouldn't want to run it that high, cos I'm pretty sure it'll be stressed & its life will be dramitcally reduced. But even so, I've won a bit of feedrate (up to 320mm per minute I guess). My main win moving to asbo's chinese 300w spindle is noise (it's unbelievably quiet) & also weight ...because including bracket, the spindle chinese 300w weighs just over 1kg (vs 2kg for the Proxxon bfw/40e plus bracket)...therefore I can push my rapids faster too.

The spindle I've now also just bought off ebay (pictured further above) spins at 10,000rpm @24V, so I guess I can up the feedrate to 400mm per minute with it (btw, I have no idea what a normal feedrate is for pcb milling?!)

Originally Posted by Jonathan

£66+15 for that spindle seems a lot to me when compared to the cost of the small brushless motors and controllers from hobbyking. Granted they're not that quiet, but I'm sure you'd find ways round that.

I must be one of the few folks on here whose main priority is noise (I'm not blessed with a shed in the garden, garage, workshop etc)...so noise is of major concern.....and I've precious little time enough to be working to address any noise issues introduced by a noisier, more powerful (or faster) spindle.

Re the cost of the spindle...well a basic 300w motor & mounting bracket (which drives the spindle) costs about £47 alone (delivered) on Ali-Express , so an extra £34 I paid, gets the milling spindle, backplates, pulleys & belts including delivery from Asia.... personally, I think that's great value.