Re: [CAD_CAM_EDM_DRO] CNCing a big surface grinder ?

Ejay Hire wrote:

>Alternately, what's the feasibility of spending a few bucks on
>electrical hydraulic valves? You'd still need some form of encoder for
>positional feedback, but no need to tear into the treadmills.
>
>

Ugh! A bad choice. Back in the relatively early days of CNC, many
machines were made with hydraulic motors for brute force, and
proportional servo valves as the amplifiers. This required a hydraulic
pump running into a pressure relief valve, so that the system drew
maximum electrical power all the time, and dissipated the heat from
the fluid in big coolers. (Why they didn't use variable-displacement
pumps, I don't know.) The noise, power consumption and maintenance
headaches were huge, and as soon as power electronics got to the point
that large electric motors could be controlled with electronic servo
amps, all these machines were retrofitted or scrapped.

If you want a "project" to keep you busy for the rest of your life, go
ahead and do it this way. But, you'll be sorry!

First, good Moog proportional servo valves will run somewhere
around $5000 each. Used and repairable ones have a core charge
of something like $1000! Then, there are all sorts of hairy problems
when using some tyoe of servo driver to operate a servo valve-controlled
load. there are dynamic effects like fluid compressibility (actually,
I think it is the hoses that bulge) and sticktion in the servo valve
spool that really confound servo amps that expect linear performance
like an electric motor. The sticktion is dealt with by adding electronic
dither to keep the spool floating on oil. But, this wears out the valve
faster. The dynamic effects are dealt with mostly by lowering gain and
bandwidth a great deal.

Jon