My "fill in" project for the summer has been a ball screw and air
cylinders for my knee on the Excello CNC mill. Still got several parts
to fabricate but I was close enough to fit it all together today to
check for problems.

I found a good one. Drop the air and power to servo and the table
drops like a rock. I knew it would go down, but the speed was totally
unexpected. I'm going to need to add some sort of "dead man" brake to
the servo. That is, drop power and the brake closes.

Any good suggestions for what to use? Keep in mind I'm a cheap skate.

Karl

On Tue, 09 Aug 2011 14:13:28 -0500, Karl Townsend wrote:

My "fill in" project for the summer has been a ball screw and air
cylinders for my knee on the Excello CNC mill. Still got several parts
to fabricate but I was close enough to fit it all together today to
check for problems.

I found a good one. Drop the air and power to servo and the table drops
like a rock. I knew it would go down, but the speed was totally
unexpected. I'm going to need to add some sort of "dead man" brake to
the servo. That is, drop power and the brake closes.

Any good suggestions for what to use? Keep in mind I'm a cheap skate.

A solenoid that can stand 100% duty cycle, rigged to hold a dog clutch or
a very self-energizing brake open when the power is on. Dunno where
you'll get the solenoid, but there's got to be something out there for
cheap. (Unless Iggy has bought the world's supply of them, surplus).

Note that you need to be careful about the dog clutch -- you want it to
positively catch, even if things have already spun up a bit. So you want
it more like a ratchet on a come-along, rather than a square dog clutch.
If the clutch just bounces instead of engaging, then you'll just have an
audible alarm when your table drops. That's probably not what you want.

You may also want to consider some way of parking it cleanly -- i.e.,
drop the power to the brake hold-off solenoid first, then drop power to
the servo.

Depending on just how disastrous it can be for the knee to drop, you may
also want to consider some purely mechanical safety device, like a
centrifugal thingie on the ball screw such that if it spins up too fast
some dogs fling out and catch in a rack, stopping the works up until you
or your servo motors actively apply torque to the ball screw.

--
www.wescottdesign.com

On Tue, 09 Aug 2011 14:13:28 -0500, Karl Townsend
wrote:

My "fill in" project for the summer has been a ball screw and air
cylinders for my knee on the Excello CNC mill. Still got several parts
to fabricate but I was close enough to fit it all together today to
check for problems.

I found a good one. Drop the air and power to servo and the table
drops like a rock. I knew it would go down, but the speed was totally
unexpected. I'm going to need to add some sort of "dead man" brake to
the servo. That is, drop power and the brake closes.

Any good suggestions for what to use? Keep in mind I'm a cheap skate.

Karl

I've used this type of brake sandwiched between a servo and an
overhauling load in a situation similar to yours. Warner Electric and
Electroid make similar brakes, but they're much (like 2 or 3X) more
expensive. You do have to consider how to coordinate application of
the brake with both intentional and unintentional loss of motor
torque.
http://www.ogura-clutch.com/products...y=2&product=61

--
Ned Simmons

I would do two things.

First, I would implement a counterweight to ease the load.

Second, I would get one of three:

1) A servo motor with a build in brake (brakemotor).
2) A servo motor large enough to hold the table in one place against
weight, without overheating.
3) A geared servo motor.

If you recall, I also have a motorized knee on my mill. I have a
geared servo motor that does not slip when powered off, due to gear
reduction. When I bought it, it had no encoder, but it had a little
extra shaft, so putting an encoder on it was easy.

I had to enhance EMC2 to turn it off when not instructed to move for
more than 10 seconds. I wrote that component in C. Without it, the motor
would overheat, trying to push the knee "one last hair".

It works great, as of right now.

i

On Tue, 09 Aug 2011 14:13:28 -0500, Karl Townsend
wrote:

My "fill in" project for the summer has been a ball screw and air
cylinders for my knee on the Excello CNC mill. Still got several parts
to fabricate but I was close enough to fit it all together today to
check for problems.

I found a good one. Drop the air and power to servo and the table
drops like a rock. I knew it would go down, but the speed was totally
unexpected. I'm going to need to add some sort of "dead man" brake to
the servo. That is, drop power and the brake closes.

Got an extra air cylinder? Fab up some brake shoes with discarded
disc brake pads and spring-load them to clamp on the ball screw by
default. When air is applied to the servo, it's applied to the spare
cylinder which opens the brake. Kill the air and the brake
auto-engages. A 1/2" stroke cylinder would do it.


Any good suggestions for what to use? Keep in mind I'm a cheap skate.

Aye, iffen it's cheap you'll be wantin', laddy, just keep a pair of
vise grips handy.

--
Fear not those who argue but those who dodge.
-- Marie Ebner von Eschenbach

On Tue, 09 Aug 2011 16:28:42 -0700, Larry Jaques
wrote:

On Tue, 09 Aug 2011 14:13:28 -0500, Karl Townsend
wrote:

My "fill in" project for the summer has been a ball screw and air
cylinders for my knee on the Excello CNC mill. Still got several parts
to fabricate but I was close enough to fit it all together today to
check for problems.

I found a good one. Drop the air and power to servo and the table
drops like a rock. I knew it would go down, but the speed was totally
unexpected. I'm going to need to add some sort of "dead man" brake to
the servo. That is, drop power and the brake closes.

Got an extra air cylinder? Fab up some brake shoes with discarded
disc brake pads and spring-load them to clamp on the ball screw by
default. When air is applied to the servo, it's applied to the spare
cylinder which opens the brake. Kill the air and the brake
auto-engages. A 1/2" stroke cylinder would do it.


Any good suggestions for what to use? Keep in mind I'm a cheap skate.

Aye, iffen it's cheap you'll be wantin', laddy, just keep a pair of
vise grips handy.

I may just use a variation of the vise grip. Tighten gibs as soon as
move complete, Just put an extra limit switch on this. But, it makes
machine a one man show, nobody else could run it.

Karl

Karl Townsend wrote:

My "fill in" project for the summer has been a ball screw and air
cylinders for my knee on the Excello CNC mill. Still got several parts
to fabricate but I was close enough to fit it all together today to
check for problems.

I found a good one. Drop the air and power to servo and the table
drops like a rock. I knew it would go down, but the speed was totally
unexpected. I'm going to need to add some sort of "dead man" brake to
the servo. That is, drop power and the brake closes.

Any good suggestions for what to use? Keep in mind I'm a cheap skate.

Karl

Visit surpluscenter.com and find a suitable failsafe power to release
brake pack. Option 2 is to add an air cylinder acting against a spring
brake so when counterbalance air is on, the brake is released.