Re:Everything you need to know to design your own CNC router
by jacquesm on Tuesday January 19, 2021 @07:20AM
(#25832750)
Gear reduction gives you slop, if you need to reduce the better way is probably using a kevlar toothed belt and two pulleys. Still not perfect but better than gears. Finally, the best way (and most expensive way) is by using a properly pre-tensioned ball bearing driven spindle ('ball screw').
Re:Everything you need to know to design your own CNC router
by craftinator on Tuesday January 19, 2021 @10:07AM
(#25834068)
I work in the non-destructive testing industry, testing aerospace parts. We build CNC testing rigs fairly regularly, with x and y axes longer than 30 feet, and carry loads of >1000 lbs. These need to be able to center on holes with diameters of <1 mm, because that's what we calibrate our equipment on.
The most reliable build setups we have are kevlar belts coupled with 16:1 reduced stepper motors, and decoupled encoders that index using constant pressure rack and pinions. Even pre-tensioned ball screws give us enough slop to be a problem without encoders, and kevlar belts are an order of magnitude cheaper; going with a reduced stepper vs a servo takes the cost down by half.
Re:Everything you need to know to design your own CNC router
by jacquesm on Tuesday January 19, 2021 @10:10AM
(#25834113)
Ah good point, yes, at those lengths you really don't want ballscrews, they are impossible to support and will warp. But for short distance (up to 8' or so) with a support on either end they are fine.
Re:Everything you need to know to design your own CNC router
by craftinator on Tuesday January 19, 2021 @11:25AM
(#25835043)
Yeah, my brain is kinda tuned to those long distances. I also kind of harp on people who say they need really high precision systems, so use servos and spend bookoo bucks on slop-reductive hardware.
Even a cheap encoder, when decoupled from the motor, and directly coupled to the axis it measures, will give better axial positioning than a servo. It boggles my mind why people seem to ignore this in favor of a motor that knows where it is in space, but doesn't know where the axis it is driving is in space.
Servos are great if you're working with huge forces, need a lot of speed, need ridiculously high accuracy, and have a team of engineers to actually build and tune the thing. Ridiculously expensive and complex unless you're looking to go into busines manufacturing CNC systems.