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