Ned Simmons wrote:
On Thu, 31 Jul 2008 18:56:52 -0700, Tim Wescott
wrote:

I don't think that a hydraulic system could retain anything approaching
the rigidity that you get from lead screws, so I don't see it as being a
good candidate. I think you'd get such a springy feed that you'd be
constantly bouncing off of hard cuts, then digging in too far when the
cuts got light.

Notice that the cited examples (the mentioned tracer machines, the film
of the metal spinning, old old mill with hydraulic feed) were all things
where some fixture provided the rigidity, or where the precision of the
feed wasn't critical.


On the tracers the hydraulic fluid transmits the cutting force to the
tool. I don't follow what you mean by "some fixture provided the
rigidity." A properly sized leadscrew is certainly stiffer than a
column of hydraulic fluid, but the stiffness of the rest of the
machine structure is a bigger factor than the stiffness of either a
leadscrew or a hydraulic actuator of reasonable length acting in
compression.

I'll believe it if I see it, and you can pay me my going rate to do a
feasibility study if you want, but it's not something that I'd recommend
off the cuff.

Eric attested to the feasibility by virtue of the fact that the
machines he ran were able to hold tenths in production.

I missed that post -- hmm. Having read it I'll count it as 'seeing',
though.

I know that the hydraulic fluid itself isn't very springy, but even with
all-solid lines you'd expect there to be spring as a consequence of the
valving. I'd be interested in seeing just how the hydro-mechanical
system is put together so that it's errors are correctable by the servo
system.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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