Re: Stepper Motor vs Servo Motor... Jon

John wrote:

> Jon, about how many RPM's should you look to get on larger machines (Larger
> than Sherlines)? I know this is totally dependant on how fast you want it to
> move and what TPI the screw is but could you give a few rough estimates? I
> have no idea so I will have a go at guessing......
> A very small knee mill, Harrison size, you would be able to crank the wheel
> at a maximum of 120rpm? So, 240rpms ish for a sort of reasonable speed?

Assuming a 5 TPI screw, 240 RPM would move the table at 48 IPM, which
might be fine for a smaller machine with, say, a 15" X travel. That would take
about 19 seconds for the full travel.

> There that's sort of working on low cutting speeds and higher torque but you
> need higher speeds for better finishes so even I can see a problem with
> that. Isn't there a risk if you aim to move too fast that the machine will
> start going nuts?

Why should the machine go nuts? If you ask it to go faster than the
maximum velocity that the control can handle, it should either tell you
it can't go that fast, or just go at the max velocity it is set for. If you set the
control to accelerate faster than the motors/drivers can handle, steppers
will lose steps. Servos will lag behind, and when the maximum following
error limit is exceeded, all axes will stop, and you'll get a following error
limits exceeded message.

> Also, can machines predict when to stop supplying power to the axis to let
> it drift to the correct place?

A servo never 'coasts', except when the E-stop is pushed. It is always under
control, although in some cases it may actually be coasting within fine limits
set by the CNC control. In other words, servo positioning drives are 4 quadrant
closed-loop control systems, where power can be applied to accelerate the load,
or power can be absorbed from the motors to slow the load. (Doing this in
either direction is where the 4 quadrants come from.)

Steppers work pretty much the same way, and energy is withdrawn there, too,
to slow a rapidly moving load.

The better CNC programs have acceleration and deceleration ramps that regulate
the speed changes so that the limits of the axis drive are not exceeded.

> My lathe seems to decelerate them click the
> last for steps on until it reaches the right place. Why doesn't this create
> problems if the carriage is moving at continuous speed and the top slide is
> doing this clicking deal when cutting a radius? Because the top slide's
> speed is not uniform as it decelerates to the correct spot.

This is the downside of stepper drives. A high-end velocity servo drive
does not have discrete positions, even though the shaft encoder does.
The tachometer allows motion to be smooth down to extremely low speeds,
where a stepper motor is moving in sharp, discrete jumps. Very high resolution
stepper systems pretty much alleviate this, but you usually suffer from low
top speeds in that case.

Jon