Re: Stepper Motor vs Servo Motor... Jon

> 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?

other considderations aside, steppers have higher torque at low speed
than at high speed, so your slow deep roughing cuts mirror that, and
your light fast finishing cuts mirror the lower power
requirement/delivery.

And your cutting speed, even at the highest feed rates will be much
less than your (no load) rapid transit speed. So you will be well
within the power curve of a stepper.

As far as descrete steps as low speed/high accuracy tool paths, you
need to determine if a step to tool movement is a large enough ratio.
I mean who cares if one step is geared down to 0.0001" when your
specs are 0.001"

Dave

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
> 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