Re: [CAD_CAM_EDM_DRO] microstep accuracy was what comprises as step

Carol & Jerry Jankura wrote:

> What about the controllers that accept microstes but have a real servo on
> the back end? What kind of accuracy can you attain with these?
>

It depends on a WHOLE lot of things. There are deadbands inherent in
closed loop control, but they can be pretty narrow in some systems.
The encoder resolution is the basic unit here. but, this can get quite
high at reasonable cost. 1000 cycle/rev encoders are not terribly expensive.
These give 4000 counts/rev when all encoder transitions are counted.
If you put this encoder directly on a 5 TPI ballscrew, then you get
5 x 4000 or 20,000 counts/inch. So, the unit of resolution is .00005",
yes that's 50 micro-inches! Now, depending on how flexible your
machine is, how accurately the screw is ground, how accurately
the table ways were made, what class of couplings you have between the
leadscrew and encoder, etc. the accuracy of the machine can be
low or high. I have such a setup on a VERY old and worn Bridgeport,
and I can usually hold tolerances of .003" or so on modest sized
parts. If I need to hold it tighter, I can adjust the program after a
test part. having a machine with absolutely perfect accuracy still
won't guarantee accurate parts. You still have to contend with
thermal expansion of the part, mechanical flexing of the part and
possibly fixture, and flexing of the tool. These can be considerable.

Jon