Re: [CAD_CAM_EDM_DRO] motor speed/gear ration

nielsenbe@... wrote:
I am building my first CNC system and I can't seem to find what speed servo's
should be run at. I realize they can cover a wide RPM range but there must be
an ideal range.

/// You need to design the drive ratio so max speed of the servo which your power supply can keep up with equals your desired rate of rapid feed.

I am trying to determine my physical design so I need to
select a configuration/gear ratio that will work best for the motor.

I will be moving a 35-50lb carriage on THK linear bearings, travel will be
~40" @ 250 inches per minute. I have considered using a .2 or .25 lead ball
screw setup with a bayside reduction of ?? or possibly a rack/pinion setup due to
the high IPM rate if I can find a zero backlash option.

The pulling force should be rather low but I have no way of determining an
exact number.

I don't know if I should make the setup so it's running the servo at 200RPM
or 2000RPM???

Does anyone have an idea how much distance it will take to get the carriage
0-250 ipm and then back to 0 on the other end without beating the system to
death.

Thanks, Brad

Brad

First off to get a 50 pound gantry up to 250 ipm you are going to need a BIG servo for that. Probably 30in/lb or better or 3 N/m plus for torque. If you are using Gecko 320's you can run large servos at lesser voltage than they are rated sacrificing speed but not torque providing your power supply will deliver the required current for the servos rated torque.

Some of it is simple math actually. Let's say you go with 5 turns in 1" pitch for your ballscrew. Thats 5 rpm to travel 1". You want to go 250" per minute so, that is 1250 rpm at the screw. Which is too fast but, its your ballscrew. That means if you run a 2500rpm servo (again providing you power supply will deliver the current needed for that kind of RPM) you would need a 2:1 reduction by way of a toothed belt at the screw.

If you decide to go the rack and pinion route ( for a gantry router, its fine and cheaper). Calculate the circumference of the (important) pitch diameter of the pinion gear. And every rotation would equal the circumference in linear travel. Then calculate back to your servo for you reduction to the pinion gear.

As far as acceleration and speed go. Most CNC softwares like Mach2 have different rates that can be set for acceleration, velocity and steps per inch. So no matter the ratio you can synchronize your motors with the software.

Ron




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