Re: [CAD_CAM_EDM_DRO] Re: replacing feed screw with a hydraulic cylinder

----- Original Message -----
From: Jon Elson <elson@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Wednesday, November 06, 2002 12:21 PM
Subject: Re: [CAD_CAM_EDM_DRO] Re: replacing feed screw with a hydraulic
cylinder


>
>
> 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
>
>
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