Re: Low cost Servo Controller

Leslie Watts wrote:

> Even a few thousanths following error is a concern with coordinated
> moves in precision wood carving.
> 120+ ipm is typical for manual high speed routers and a few thousanths
> gouge or error means laborous secondary
> operations(sanding). Thosandths accurate woodworking may seem silly, but
> when mortises, tenons, dovetails and the
> like go together with a perfect light press fit it saves a lot of labor.
>
> I wonder how low I could get the error with EMC considering the cutting
> forces are very low. Most of the load
> is inertial- therefore predictable.

Well, I think you're nuts. Unless you are using a huge Vertical Machining
Center, your machine isn't stiff enough to give this sort of accuracy, especially
if you are accelerating hard up to 120 IPM. I know that this sort of accuracy
is not possible on a Bridgeport machine, because the machine itself bends
under its own weight. Unless you have built the most massive routing machine
ever, I suspect it is a lot less accurate than you think. Even if the machine is
really solid, the workpieces deflect under cutting loads, and also as stresses
inherent in the wood are relieved.

Now, admittedly, each component adds its own error, so keeping the following
error as small as possible is a desirable goal. I have 2-3 thousandths of an
inch backlash in my screws and associated mechanical components, so I'm less
worried about the tiny following errors. If I ever get the screws tuned up to
reduce that backlash, then I might want to go back and tune the servos up a
little better.

Anyway, you are not too likely to get gouges. The only way I would expect
that is if you contrived some toolpath that had both axes moving along slowly,
and then one axis takes off toward full speed while the other axis continues
crawling along. This might be able to have enough difference in the following
errors of the two axes to make a small gouge. But, mostly, you will find small
rounded-off areas on edges that were specified to be square. The amount of
this is controllable, too, in the way the max acceleration parameters are set in
the .ini file. If you increase the allowable acceleration, then the deceleration
ramp at the end of a move will be shorter, and the next move will start later,
giving less rounding-off. Finding the right balance between acceleration and
the capabilities of the machine will give the least rounding-off.

Jon

Jon