El Beast - Initial design phase, comments and critique welcomed!

[Zeeflyboy]

Looks like it will be an extremely high quality machine!

I have a question which you will probably find quite noobish but I am interested in your glass scales. Specifically, how do you use the glass scales in a feedback loop to control your position? Does the position from the scales get fed back into the motor driver and that corrects any error? Or is the position fed back into your break out board and on to your motion controller?
The reason I ask is if you have position feedback​ from a linear scale any backlash in your drive system wouldn't matter?? Then theoretically one could use c3 screws instead of c7 for instance. Or, standard gearboxes instead of low backlash units. Am I thinking correctly here??

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I simply use the glass scales for calibration purposes. They were mainly the answer to my current machine's less-than-optimal screws - I simply adjusted the steps per unit to match the scales. In mach3 you can actually go deeper than that and map the ballscrew to compensate for variation along the screw itself too, but I never went that far.

Certainly it is possible to link a positional encoder such as a glass scale and a CNC controller to give absolute position feedback, but I haven't looked into it too far as for my needs the calibration was sufficient. I think linuxCNC has a way of using it.

It certainly would be a good way of compensating for lead inaccuracies and variation - I don't think it would do a great job of controlling excessive backlash though, but then most semi-decent ball screws don't have much backlash anyway. If you had a lot of backlash it would have to be very quick to arrest movement to stop chatter and poor edge surface finish, there is never really a good solution for backlash other than to remove it.

BTW I think you have the C's backwards, the lower the C number the more demanding the specification... so in your example it would more be a case of it allowing you to use c7 rather than more expensive c5/c3/c0. Of course good glass scales aren't cheap either so it's debatable as to how much saving you would make.

Edit - EMC2 can do it too apparently.

Edit 2 - also came across this quote which pretty much describes what I was thinking regarding it not being a cure for backlash:

But, a caution: Knowing position via the glass scales doesn't
eliminate the backlash problem. The real problem is that the
position of the table is not constrained in both directions by
the servo. it is only constrained in one direction at a time,
and the motor cannot hop from one side of the backlash to the
other instantly, therefore cutting forces and inertia can flip
it from one side to the other faster than the motor can
compensate. This can lead to messed-up parts, broken tools and
general foul language around the shop. So, don't think the
glass scales are some kind of panacea that allows you to do
precision work with sloppy leadscrews.