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Did somebody try to get rid of the feed screw all together
and replace it with a hydraulic cylinder?
At the first glance it may present several advantages:
no backlash, big force at low cost.
Alex

al5502 wrote:

Did somebody try to get rid of the feed screw all together
and replace it with a hydraulic cylinder?
At the first glance it may present several advantages:
no backlash, big force at low cost.

We go through this every 6 months or so. It looks neat at first glance, and if you
have lots of hydraulic gear around, might even be affordable. Proportional (Moog)
valves are REALLY expensive, like $3000 and up! The hydraulic pumps are expensive,
massively power-hungry, and LOUD! The hazards of a hydraulic leak at 3000 PSI
are very serious. You get oil injected under the skin, and it poisons you. A flexible
line blowing off can also be dangerous. If there is any air in the system, it tends to
go crazy with oscillation. The performance can be iffy, too, with jitter and stick-slip
friction.

I think if you add up all the parts costs, even surplus, you could go out and buy a complete
ballscrew retrofit and servo drives new!

Jon

On Tue Nov 5 09:53:55 2002, Jon Elson, <elson@...> wrote:

al5502 wrote:

Did somebody try to get rid of the feed screw all together
and replace it with a hydraulic cylinder?
At the first glance it may present several advantages:
no backlash, big force at low cost.

We go through this every 6 months or so. It looks neat at first glance,
and if you
have lots of hydraulic gear around, might even be affordable.
Proportional (Moog)
valves are REALLY expensive, like $3000 and up! The hydraulic pumps are
expensive,
massively power-hungry, and LOUD! The hazards of a hydraulic leak at
3000 PSI
are very serious. You get oil injected under the skin, and it poisons
you. A flexible
line blowing off can also be dangerous. If there is any air in the
system, it tends to
go crazy with oscillation. The performance can be iffy, too, with
jitter and stick-slip
friction.

I think if you add up all the parts costs, even surplus, you could go
out and buy a complete
ballscrew retrofit and servo drives new!

Jon

All too true, but here's a question:

From the accuracy/resolution standpoint, what about slapping an encoder and
ballscrew/acme thread/timing chain onto a hydraulic motor, and as a way to
ditch some of the more complicated servo power supply/amplifier/control issues?
looks do-able, at least at first glance.

Bill

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Just want to address a cost issue: small power pack from
Princess - Auto -approx. $700 Canadian - $450 US.
Alex

In a message dated 11/5/2002 7:06:33 PM Central Standard Time,
toolingrus@... writes:

Some obvious negatives I can think of: Slow speed (?), piston seal leakage,
temperature may change volume of fluid (?). Just thinking out loud but I'd
like to hear what others think.

Key word here is "SOME..."! Just imagine how many negatives if you include
the ones you CAN'T think of! Unless the hydraulic concept would provide a
FORCE somehow unattainable elsewise, and you NEEDED that force, or the like,
or, perhaps you live in a land where "cleaning goop off everything every hour
or so is delightful", Steppers or Servo would be the way to go!

I've thought about hydraulic drive CNC for years. This may have
already been discussed but my idea is to use a closed system with
single shot pistons powered by solenoids or a motor driven cam. Each
stroke of the piston would pump a metered amount of fluid from one
side of the cylinder to the other kind of like a hydraulic jack. One
stroke of the piston would equal a fixed amount cylinder travel. A bi-
directional piston with check valves could be used to change the
direction. Example: 1000 piston strokes could equal 1" of
travel, kind of like a linear stepping motor. A stroke counter
could keep track of the position. This system should work with
stepper motor software. Some obvious negatives I can think of: Slow
speed (?), piston seal leakage, temperature may change volume of
fluid (?). Just thinking out loud but I'd like to hear what
others
think.

Also there will be temperature related problems as the cylinder position will
change with temperature of the hyd fluid.
bill

On Tue Nov 5 20:29:26 2002, JanRwl@..., <JanRwl@...> wrote:

In a message dated 11/5/2002 7:06:33 PM Central Standard Time,
toolingrus@... writes:

Some obvious negatives I can think of: Slow speed (?), piston seal leakage,
temperature may change volume of fluid (?). Just thinking out loud but I'd
like to hear what others think.

Key word here is "SOME..."! Just imagine how many negatives if you include
the ones you CAN'T think of! Unless the hydraulic concept would provide a
FORCE somehow unattainable elsewise, and you NEEDED that force, or the like,
or, perhaps you live in a land where "cleaning goop off everything every hour
or so is delightful", Steppers or Servo would be the way to go!

Then again we're likely not to hear much about setups that allready exist.
Seems that the Quackers don't like electricity, but do just fine with hydraulics
in all sorts of things, including machine shops.....


--
----------------------------------------------------------------------------
RKBA! Blessings On Thee, Oh Israel! 4-19!
----------------+----------+--------------------------+---------------------
An _EFFECTIVE_ | Insured | All matter is vibration. | Let he who hath no
weapon in every | by COLT; | -- Max Plank | weapon sell his
hand = Freedom | DIAL | In the beginning was the | garment and buy a
on every side! | 1911-A1. | word. -- The Bible | sword.--Jesus Christ
----------------+----------+--------------------------+---------------------

Constitutional Government is dead, LONG LIVE THE CONSTITUTION!!!!!

----------------------------------------------------------------------------

Bill Vance wrote:

From the accuracy/resolution standpoint, what about slapping an encoder and

ballscrew/acme thread/timing chain onto a hydraulic motor, and as a way to
ditch some of the more complicated servo power supply/amplifier/control issues?
looks do-able, at least at first glance.

Many of the really big machines from 20 years ago did this. Kearney&Trecker, Cincinnatti, etc.
But, you still need the exact same hydraulic power source and the expensive proportional servo
valves. My Allen-Bradley 7320 CNC was commonly used with hydraulically-operated machines,
and most used hydraulic motors, not cylinders. They had a special version of the DAC card to
feed velocity commands to the Moog valves. It had the small linear amp required, and added
400 Hz dither to the output to keep the spools in the valve floating.

The real killer is that even fairly small machines, like a Series-II Bridgeport, needed something
like a 15 Hp hydraulic pump, and if it used a constant-displacement pump, it drew 15 HP all day!
Even with the more expensive variable-displacement pumps, it would draw several Hp when
idling.

Jon

b564htf wrote:

I've thought about hydraulic drive CNC for years. This may have already been discussed but my idea is to use a closed system with single shot pistons powered by solenoids or a motor driven cam. Each stroke of the piston would pump a metered amount of fluid from one side of the cylinder to the other kind of like a hydraulic jack. One stroke of the piston would equal a fixed amount cylinder travel. A bi-
directional piston with check valves could be used to change the direction. Example: 1000 piston strokes could equal 1" of
travel, kind of like a linear stepping motor. A stroke counter could keep track of the position. This system should work with stepper motor software. Some obvious negatives I can think of: Slow speed (?), piston seal leakage, temperature may change volume of fluid (?).

Oh, this is GREAT! So, to move at 60 IPM (one inch/second) you need the pump to pound out
1000 strokes/second! How long is that pump going to last at that rate? What sort of device
is going to be able to drive the cylinder at such a rate? What would be the efficiency of a
fluid pump moving so fast?

Actually, it is totally impractical to try to make a liquid pump run at such a rate, cavitation
will cause it to be pumping vacuum, not oil. So, it won't be linear, ie. fast strokes will pump
less oil than slow ones.

Jon

People have built hydraulic-activated machine tools for decades. All of
them had positive control by means of servo-valves. You pointed out the
major problems yourself, with fixed ratio pumping. How can these be
surmounted?



At 01:04 AM 11/6/02 -0000, you wrote:

I've thought about hydraulic drive CNC for years. This may have
already been discussed but my idea is to use a closed system with
single shot pistons powered by solenoids or a motor driven cam. Each
stroke of the piston would pump a metered amount of fluid from one
side of the cylinder to the other kind of like a hydraulic jack. One
stroke of the piston would equal a fixed amount cylinder travel. A bi-
directional piston with check valves could be used to change the
direction. Example: 1000 piston strokes could equal 1" of
travel, kind of like a linear stepping motor. A stroke counter
could keep track of the position. This system should work with
stepper motor software. Some obvious negatives I can think of: Slow
speed (?), piston seal leakage, temperature may change volume of
fluid (?). Just thinking out loud but I'd like to hear what
others
think.

--- In CAD_CAM_EDM_DRO@y..., "al5502" <telecomt@t...> wrote:

Did somebody try to get rid of the feed screw all together
and replace it with a hydraulic cylinder?
At the first glance it may present several advantages:
no backlash, big force at low cost.
Alex

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I thought I'd made a mistake once, but I was wrong

True, if you buy them new hydraulic power packs are quite expensive, but
there is a lot of surplus/used stuff around. You generally need to replace
the hoses out of common sense, however.

Speaking of....I am currently in the process of CNC'ing a Moog Hydra-point
milling machine. This has hydraulic control like of which you speak. When
it worked, it was great. When there were "issues", well...'nuff said. You
can see what I have at:



I haven't updated the page in a while, but I will be doing so soon.

If anybody wants to buy any of the hydraulic parts (including the fiendishly
clever air logic stuff), contact me offlist and make an offer.

Gary

Date: Tue, 5 Nov 2002 14:51:18 -0500
From: "alex" <telecomt@...>
Subject: Re: replacing feed screw with a hydraulic cylinder

Just want to address a cost issue: small power pack from
Princess - Auto -approx. $700 Canadian - $450 US.
Alex
----- Original Message -----
From: Bill Vance <ccq@...>
To: <CAD_CAM_EDM_DRO@...>
Sent: Tuesday, November 05, 2002 2:49 PM
Subject: Re: [CAD_CAM_EDM_DRO] replacing feed screw with a hydraulic
cylinder

On Tue Nov 5 09:53:55 2002, Jon Elson,

<elson@...> wrote:

al5502 wrote:

Did somebody try to get rid of the feed screw all together
and replace it with a hydraulic cylinder?
At the first glance it may present several advantages:
no backlash, big force at low cost.

</snip>

Did somebody try to get rid of the feed screw all together
and replace it with a hydraulic cylinder?
At the first glance it may present several advantages:
no backlash, big force at low cost.

</snip>

I did some cad work for a hexapod mill and thought about hydraulic
cylinders. Spool valve parts are the type used in marine power
steering systems. With a little lathe work to eliminate the dead spot.
Planned on driving these with Gecko 320's and two way solenoids. The
pump is a PS pump. (400 psi). Encoder feedback from a gear rack
asmy. mounted on the side of the cylinder.

Thought about playing with the "old" style auto PS systems that had a
separate cylinder and valve, if you can find them anymore. They would
give these away.

Could use a newer style rack and pinion PS with a low cost
stepper/drive.


Any thoughts ?