Re: [CAD_CAM_EDM_DRO] Re: replacing feed screw with a hydraulic c ylinder

Van Der Sandt Coert wrote:

>What if you had one big pump to keep up the pressure and some kind of a
>rotary valve on each axis that just lets a metered amount of fluid through
>every time its activated.
>
>Alternatively you could have a pump on every axis and calculate the pump
>speed so that every axis reaches its goal at the same time.
>
>

And, what is going to drive those pumps? Electric motors? Clutches?
How can this be better
than just driving the machine directly with the motor? If you are going
to hook a servo motor
to a reversible hydraulic pump, then drive the table with a cylinder,
that has to be the
Rube Goldberg method of replacing a leadscrew!

Just as a reference point, hydraulic systems, especially hydraulic
motors driving a leadscrew,
was a VERY common system on larger CNC machines, in the 1970s. I
believe it was the
most common axis drive system on the larger ones, in fact. Direct
electric drive cam in
VERY quickly after larger power transistors became available. The
makers of big machines
never looked back, and you'll be VERY hard pressed to find any larger
CNC machine with
hydraulic drive today. The only machines still using hydraulic drive
that I can think of are
surface grinders, where they just have a steady back and forth feed
between limit switches.

The buyers of these machines must have a REAL good reason for making
this choice.
The reason, of course, is they had HORRIBLE experiences with the high
maintenance
costs of hydraulic systems! Another side light, the commercial aircraft
industry is trying
to desing their way away from hydraulics, for the same reason - high,
VERY high, maintenace
costs.

Jon