On Thu, 31 Jul 2008 12:56:31 -0700 (PDT), "Denis G."
wrote:

Sometime recently I read about someone who acquired an older milling
machine that had hydraulic feed on the table. I wondered about how
these tables might be controlled and if any lathes have been designed
using hydraulics to move the carriage or crossfeed instead of lead or
feedscrews. (After all, there are mechanical, hydraulic and pneumatic
linear actuators.)
I don't know if you could control hydraulics as precisely as
feedscrews (with respect to headstock spindle rotation) for cutting
threads, but you'd never have to worry about worn screws and metric/
inch conversions. I imagine that the big problem is in designing the
feedback system and getting it to respond well.
I've seen descriptions of hydraulic systems made by companies like
Enerpac that control the balancing of bridge sections to keep them
level while being positioned with cranes. I know that hydraulics can
be controlled with electronics and proportional or servo valves.
Probably it's difficult to beat mechanical feed and leadscrews because
it's a simple and accurate system, but I was just curious.
Over 30 years ago I ran Sheldon lathes that were hydraulicaly
controlled. We could hold .0001" easily with these lathes. They were
15 inch swing machines, I think. They had an adjustment called
"dither" which kept the valves always in motion. This was to avoid the
problem with "stiction". I don't remember if it was the valves, the
actuators, or both that needed the dithering. Interestingly, I have
two older CNC lathes with Fanuc controls that also use dithering. The
servos always move a little. The repair man had to adjust one of my
lathes when the dithering was out of whack, it would lose precise
position and the finish would suffer. He told me the dither was left
over from hydraulic systems. My newer CNC controls no longer have a
dither adjustment.
ERS