Re: stepper accuracy compared to servos...

Everyone can go back and forth on stepper vs. servo motors but the
real fact is that neither motor, on it's own, is accurate. Several
recent posts have talked of Z axis creep. A little Sherline with
steppers will have ZERO accuracy the first time you try to cut a 1/4"
x 1/2" slot in hardened tool steel. The steppers will just buzz and
go nowhere with the computer oblivious to what's happening. A servo
might grind away until it gets to where it should but if you take a
servo and break the encoder feedback you might see the motor run and
crash the limit switch.

The "feedback" is what really gives accuracy on any system. With the
right electronics a stepper with a 2000PPR encoder can top a servo
with a 500PPR encoder. The same servo with the 2000PPR can top a
stepper with the 500PPR encoder.

Steppers are used a lot because the "feedback" usually is the FREE
prayer that under good conditions it will "step" when you want and
the stepper drive can be nothing but a few low cost transistors and
resistors.

Servos must have an encoder with relatively good electronics and thus
cost more to put on a machine. So, they're not as common on lower
cost (the smaller higher volume) machines.

Another down side for steppers is lack of power. Steppers can have
torque at a stall but it drops fast at high step rates. Servo motors
can crank a lot at speed, but need more expensive electronics to do
it.

What do the "Big Boys" use? A great full size mill will probably
have AC servo motors with Heidenein glass scales.

Al