W.E.T.[CAD_CAM_EDM_DRO] Re: Z axis drive

Gary and All:
Hi, I have a question on one of you excellent comment.
When you were talking about faulting the machine in acc or dec did
you not mean "without faulting the machine"?
Those Farrel machines a usually all BIG ONES are they not?
In Michigan upper penn. they have a hoisting drum that is 40 feet in
dia. and a large pieced together 40 foot pump engine flywheel. I
wonder what machine they were turned on back in the 1800 w/o
electricity or electronics? (If you know please reply directly to
me)
GOD'S BLESSINGS
Bill


Many machines even with mechanical counterweights use an electric
brake on the Ball Screw that is tied to the Servo Drive "Drive OK"
output. This brake is engaged in its default state to prevent the
weight of the head from back driving the ball screw and free falling
and requires power to be applied to disengage. This power is
controlled by the Servo Drive "Drive OK" and sometimes by the CNC
"Drive Enable" outputs in series. If either output goes low the brake
voltage is dropped and the brake mechanically engages preventing free
fall. Usually this will occur so quickly that the brake is set before
the drive output has fully decayed and the head will nt drop at all.
Similarly when the machine is restarted the drive is fully energized
and in control of the axis before the brake is released.

On large machines that I have retro-fitted where the head could be
over 7,500#, we generally use a hydraulic counterbalance. One example
of this was a 24 Foot Farrel Vertical Boring Mill. The z-axis ram was
20" square, have a 50 HP Spindle inside of it and would extend out 9
feet, it weighted 16,000#. This used a simple ram circuit with the
cylinder mounted parallel to the ram and connected to the end pushing
up on the ram. We apply a variable hydraulic pressure to the
hydraulic
cylinder ram to take a major part of the head weight and allow the
cylinder to be back filled by the pump as the head rises maintaining
constant load. When the head stops the cylinder ram maintains a
constant load on the head. When the head is driven downward a check
valve closes the supply line from the pump cylinder and the ball
screw drives against the cylinder increasing the hydraulic pressure.
A
separate pressure relief valve in he circuit then opens to vent the
cylinder back to the tank. Again there is usually a normally closed
solenoid valve in this circuit behind the relief valve that is held
open by the Drive OK/Drive Enable Signal which closes if the signal
goes low maintaining counterbalance in the event of a power failure
or
machine health problem.

The biggest reason that we use a counterbalance however is to balance
the dynamic reactions of the machine in the up and down direction. On
a heavy head without a counter balance it is not unusual for the
servo
drive to be firing upward all the time even when the machine head is
going down or at rest. It is easy to understand that when the head is
being raised that the drive has to fire upward, what is not so
obvious
is that as the head feeds downward the ball screw essentially offers
no resistance to back driving and the head tends to free fall. The
servo has to act like an engine brake on a truck to maintain
control.
You can see this if you monitor the current up and down. The
counterbalance allows us to balance the drive current and performance
independently for both up and down travel. The pump pressure is
varied
to assume a large portion of the head weight to keep the up currents
and accelerations reasonable. The pressure relief valve in the return
circuit is then adjusted separately to prevent head free fall and
offer enough resistance to match the down current and accelerations
to
the up travel. This allows us to run a much tighter error limits and
higher acc and dec ramps with faulting the machine with a following
error.

For lighter weight heads we have also used a hydraulic ram connect
directly to a nitrogen accumulator where the nitrogen pre-charge
maintains the load.

gary

skykotech wrote:

>>All the commercial VMCs I've worked on use hydraulic or pneumatic
>>counterbalencing on the Z axis, usually hydraulic and with the
>>cylinder in a 2:1 compound arangment with chains like on a forklift.
>> Reduces the inertia problem since the counterballance is some chain
>> and a piston as opposed to a giant weight.
>>
>>Pete C.
>>
>>
>>
>
>Hmmm, my Shizuoka B-3V bedmill does not use a counterweight for the Z
> axis, and the z axis alone probably weighs 1000lbs. I wonder why?
>
>I think it has some clutch arrangement to prevent the z axis from
>falling when the servo is off.
>
>I wonder if adding a counterweight would be beneficial to my
>mill...hmmm....
>
>
>
>
>
>Addresses:
>FAQ: http://www.ktmarketing.com/faq.html
>FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
>Post Messages: CAD_CAM_EDM_DRO@yahoogroups.com
>
>Subscribe: CAD_CAM_EDM_DRO-subscribe@yahoogroups.com
>Unsubscribe: CAD_CAM_EDM_DRO-unsubscribe@yahoogroups.com
>List owner: CAD_CAM_EDM_DRO-owner@yahoogroups.com,
>wanliker@..., timg@... Moderator: pentam@...
>indigo_red@... davemucha@... [Moderators] URL to this
>group: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO
>
>OFF Topic POSTS: General Machining
>If you wish to post on unlimited OT subjects goto:
>aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach
>it if you have trouble. http://ww
>
>I think it has some clutch arrangement to prevent the z axis from
>falling when the servo is off.
>
>I wonder if adding a counterweight would be beneficial to my
>mill...hmmm....
>
>
>
>
>
>Addresses:
>FAQ: http://www.ktmarketing.com/faq.html
>FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
>Post Messages: CAD_CAM_EDM_DRO@yahoogroups.com
>
>Subscribe: CAD_CAM_EDM_DRO-subscribe@yahoogroups.com
>Unsubscribe: CAD_CAM_EDM_DRO-unsubscribe@yahoogroups.com
>List owner: CAD_CAM_EDM_DRO-owner@yahoogroups.com,
>wanliker@..., timg@... Moderator: pentam@...
>indigo_red@... davemucha@... [Moderators] URL to this
>group: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO
>
>OFF Topic POSTS: General Machining
>If you wish to post on unlimited OT subjects goto:
>aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach
>it if you have trouble. http://www.metalworking.com/news_servers.html
>
>http://groups.yahoo.com/group/jobshophomeshop I consider this to be
>a sister site to the CCED group, as many of the same members are
>there, for OT subjects, that are not allowed on the CCED list.
>
>NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING
>THEM. DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO
>EXCEPTIONS........ bill List Mom List Owner
>
>
>Yahoo! Groups Links
>
>
>
>
>
>
>
>
>

[Non-text portions of this message have been removed]



Addresses:
FAQ: http://www.ktmarketing.com/faq.html
FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
Post Messages: CAD_CAM_EDM_DRO@yahoogroups.com

Subscribe: CAD_CAM_EDM_DRO-subscribe@yahoogroups.com
Unsubscribe: CAD_CAM_EDM_DRO-unsubscribe@yahoogroups.com
List owner: CAD_CAM_EDM_DRO-owner@yahoogroups.com,
wanliker@...,
timg@... Moderator: pentam@...
indigo_red@... davemucha@... [Moderators] URL to this
group: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO

OFF Topic POSTS: General Machining
If you wish to post on unlimited OT subjects goto:
aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach
it if you have trouble. http://www.metalworking.com/news_servers.html

http://groups.yahoo.com/group/jobshophomeshop I consider this to be
a sister site to the CCED group, as many of the same members are
there, for OT subjects, that are not allowed on the CCED list.

NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING
THEM. DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO
EXCEPTIONS........
bill List Mom List Owner


Yahoo! Groups Links