Hydraulic lathes?

DaveB wrote:


What sorts of precision do you achieve? Is there anything special done in the hydraulic system to make its action more positive, or is just long rubber hoses and spool valves?
I'm glad I put weasel-words in my assertions, it looks like I'd have to stand corrected otherwise.
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Tim Wescott
Wescott Design Services
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wrote:

Tim, some machines we build have stainless tubing from the hydraulic power supply and some use steel under and over braid hose.
The servo loop usally consist of just a standard cnc control +-10 volt error signal and the position loop is normally 20 micron scales (quadrature) with a zero reference mark.
We use moog servo valves and servo amps
In some cases we use a frequency to voltage converter driven by the scale feedback and use this as velocity feedback.and of sum this with the speed reference.
Some machines have rotary hydraulic motors with encoder feedback and ballscrews.
Only downside of hydraulic loops is rapid traverse speeds, we try to run 400 ipm in rapid and can repeat to .0001 all day long, due to using scales.
If you walked in and watched a machine run you would be unable to tell any difference.
Daveb
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wrote:

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.

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

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Ned Simmons wrote:

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.
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Tim Wescott
Wescott Design Services
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One disadvantage of replacing the leadscrew on a lathe with a hydraulic cylinder would be that the length of the lathe would probably need to be doubled (close). I tried to find details of how the Sheldon tape lathes were made, but was unsucessful. Finding details about how such machines are constructed is difficult, but I was happy to find out that they exist.
As far as the criticism of stiffness between mechanical and hydraulic systems, I believe that one only has to compare mechanical and hydraulic presses. I think it's a matter of proper design to get what you need.
Finally, I found a 2006 description of a CNC machine made by a German company (Schutte) using electrohydraulic axes with a bed resolution of 0.1 microns (1 micron accuracy at the tool).
Here's a link:
http://www.productionmachining.com/articles/speed-accuracy-and-power-in-a-cnc-multi-world.aspx
Here are excerpts:
... In 1996, Schtte made the decision to go with electro-hydraulic axis drives for all end-slide and cross-slide actuation, replacing cams. Speed, power and accuracy were the motives for this decision and fit into the companys multi-spindle philosophy.
As the name implies, electrohydraulic drives use a combination of hydraulic fluid and electronic control to move an axis. Hydraulic fluid provides the motive force, and electronics, in the form of valve control and linear feedback, provide precision control of the motion.....
A key to successful use of electro-hydraulic drives is precise control of the valve that allows hydraulic fluid into and out of the cylinder. It controls the force, speed and distance traveled.
The valve, in turn, is regulated by a processor. The valve and processor system was created in conjunction with Bosch to give finite measurement of fluid movement in the shortest period of time. In addition to the hydraulics and electronic processor, each slide is equipped with a linear scale to give actual position feedback to the processor. The digital feedback loop operates at 800 Hz, feeding back the slide position 800 times per second.....
Multi-spindle applications have always been associated with high cutting forces, Jim says. Form tools and drilling operations are two prime examples of where feed power and cycle time intersect. The idea is to rough out the part as fast as possible and then bring in the semi-finishing and finishing operations. We believe the machine must be capable of power for the roughing and finesse, such as single-point turning for finishing. Electrohydraulic actuation gives us both.
Also, the system is capable of delivering the close-tolerance specifications necessary for shops to be competitive. Our electrohydraulic axes have a resolution of 0.1 micron. That resolution delivers a usable repeatability of 1 micron at the tool, Jim explains. That tolerance is necessary because of the changing nature of the work being run across the multi-spindle. Close-tolerance machining capability, along with processing flexibility, is in demand to eliminate the need for secondary operations. Customers want to drop parts off the machine complete, and that ability starts with accuracy.
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