Originally Posted by hybidder  What's the advantage of two motors vs one motor driving a full-width shaft with a pinion gear on each end? (if you're using a rack/ pinion design).
I don't see an advantage but several disadvantages come to mind, potential for missed steps, added expense/ complexity. Unless you simply have to go the two motor route due to design constraints ( clearances etc.). |
Advantages:
Double the torque to the heaviest moving part provides better accelerations
Independent mounting allows for better/easier Pinion to rack mesh
Possiblility for independant Homes to provide automatic gantry squaring
Squaring of gantry can be easily adjusted (more the home switch on one side_
No cross shaft and bearing/mounting hardware. More efficient
Disadvantages:
Requires one more motor and motor drive
Each motor needs it's own belt reduction
Possible racking if out of sync (rare)
We have multiple CNC tables (Routers, plasma, engraver) Some are stepper, some are servo and one is mixed. Two have dual drive gantry. In five years of operation we have had a couple of times when one motor would malfunction and cause a jam (non-destructive) but I have seen the same thing on a single drive with axle shaft (loose pinion gear on one side).
We were able to use smaller motors on the dual drive. The mountings allow the pinion to swing up into the Rack and are independently adjustable and tensioned so backlash on the R & P is minimal. If the pinions are fixed then you need to make the racks adjustable so you can get proper mesh. The adjustment process can be time consuming.
If your concern is over missed steps then it should focus on that problem because it will occur on a single drive design more frequently. Steppers lose steps because you attempt to operate them outside their RPM-Torque curve (The torque drops off with RPM so asking the motor to provide more torque than it can at a given RPM causes lost steps) Properly setup stepper systems do not lose steps unless the cutting tool or drive mechanism jams.
In a shaft based system, the spinning shaft needs to be large enough to counter any lead whip over it's length and have proper and correctly aligned bearings. It normally requires that the rack be mounted pointing up, which makes it more susceptible to dirt and grit in the rack teeth.
On smaller table top designs the pro's and con's are of less contrast but then R & P is more often used on larger footprint tables.
Whatever design you elect to embrace, the gearing of the motors (belt reduction) needs to be addressed. Rack & Pinion gives you multipled speeds with less torque and resolution.
An optimum stepper (proper voltage and signals) will spin about 750 to 800 RPM max. A 5/8" dia pinion gear moves the load 1.964 inches per one rev. Directly Coupled to the Stepper it gives you an impressive 1472 IPM rapids but costs you 1/2 the torque and 1/2 the resolution (potential accuracy).
TOM CAUDLE
www.CandCNC.com