Re: steppers or servo?

One thing people often miss in their consideration of stepper motors
is that most widespread applications, like printers, see a constant
load, and for the most part, run at constant speeds. Also, if your
printer occasionally hiccups and re-homes to the beginning of a line,
you may not even notice it.

Machine tool control is an entirely different story. The loads vary
widely depending on materials, cutting speeds, and tool path. Also,
the motors are required to go through a full range of velocities
where
you become vulnerable to hitting resonances which will kill the motor
torque (especially when you are reversing across backlash). Lastly,
there is rarely an opportunity in machining where you can punt and
re-home an axis without having already having taken an errant cut out
of your part.

There are many good applications for stepper motors, but I do not
believe machining is one of them. Steppers, of course can be and are
used for machining all the time, and have traditionally been the only
cost-effective option for low-end systems. The price of servos,
however, is lowering dramatically, and as low-end software support
for
servos becomes available, I would imagine steppers will fade from the
low-end CNC market.

Jeffrey Kerr
J.R. Kerr Automation Engineering
www.jrkerr.com

--- In CAD_CAM_EDM_DRO@egroups.com, "Terry Ackland" <hexagon@o...>
wrote:

> Hi,
> Computer printers use stepper motors and they run forever with
> incredible accuracy. A hobby nc machine will also have the same
> repeatability as long the motors are not forced into missed steps,
> right?
> Servos can also be forced into missed steps so the advantage over
> steppers must just be the extra speed to return to the home
> position?
> Terry