Re: [CAD_CAM_EDM_DRO] Servo sizing

Ethan Vos wrote:

> OK, I know I should have paid more attention over the last few weeks but:
>
> How do I size a servo to replace a 337 in-oz stepper?

What is the size of the machine? What rapid traverse speed do you want?
Was the 337 In-Oz stepper adequate? At what speed?

These are the kind of questions you need to start with. Because steppers
are severely speed limited, you can't gear them down too much. Servos
can run pretty fast, so you can gear them down more.

The other thing to know is that steppers are rated by HOLDING torque, so
their
torque falls off rapidly as the speed rises. Servos are rated at stall
torque,
but their torque curve is practically flat from stall to full speed, so you
get
full torque all the way up the speed curve! Plus, servos are rated at
CONTINUOUS torque, but can deliver peak torque often about 5 times
the continuous rating. (You have to be careful to never exceed the peak
current on a permanent magnet motor, or the magnets can be 'erased'.)

> How important are built in brakes?

Generally not needed. You can usually rig the emergency-stop circuit to
switch in drain resistors in place of the power source to decelerate the
motors during a high-speed emergency stop. The one place these may
be needed is in vertical-axis drives (quill or knee) on machines with
ballscrews. The vertical weight can back drive the ballscrews, causing the
knee, especially, to fall when power is off. You can also make up some very

crude sprag brakes with a slotted wheel and a small solenoid if you need
it and the motors you want to use don't have them.

Jon