On 2010-11-27, Bob La Londe wrote:
My Taig is wearing out. I've had it for just over two years and I would
guess its seen well over 1000 hours of run time. Probably over 2000. I
should probably save my time and energy and just jerk the spindle and
steppers for a new machine, but I hate to waste it completely. Its biggest
problem since day one has always been the quality of its Z-axis.

Since you are talking about a Z-axis -- I presume that you mean
the Taig mill, not the Taig lathe (which is what I have)..

I think I have a design for a Z-axis retrofit for it all worked out, and I
have a question or request for clarification from some of you guys who have
built, retrofit, or modified your own machines. Everybody says you need a
brake on the Z-axis if you use a ballscrew or it will backdrive. How much
force does it take to back drive? I know the answer is probably "it
depends", but I was looking for a more general answer.

A. Next to nothing. (stop driving it and the weight of the ball nut alone
will fall with no load)
B. A gentle push. (it takes some force, but the weight of the ball nut
isn't going to back drive.
C. A firm push. (you can get it moving with the ball of your thumb and
your carriage weight will definitely make it fall.)

Yes -- "it depends".

A ball nut on a vertical ball screw will move under its own
weight -- and accelerate rapidly.

However -- this is not counting the various frictions involved.
How good are the ways -- and how tight are the gibs? What about how the
stepper motor forces get to either the ball screw or the ball nut? (I
know that the X-axis and Z-axis on the Bridgeport BOSS-3 rotate the ball
nut around a stationary ball screw. And in the Z-axis, the ball screw
is hollow around the quill so it pushes the quill centrally on axis.

Depending on the size of the stepper motor -- it may have enough
torque just from its internal magnetism to resist the falling weight of
the Z-axis -- even with no power applied. Certainly power applied to
one pole should be sufficient to prevent motion unless something else is
seriously wrong.

My other thought is that with a very light weight spindle and carriage would
the power of a 380oz stepper motor be adequate to hold it in place? Then
just apply a simple relay operated friction brake or even a manual one when
the power shuts off for the motor.

If there are no cutting forces, probably the internal magnetic
fields will hold the motor stationary enough -- even without power.

Ok, upon afterthought that last part kinda answers itself. If the motor can
move the carriage adequately (steppers are rated in holding not moving
force) with out stalling under cutting load then it should have no problem
holding it while idling.

Yes -- and as I said above -- likely with power off, the
permanent magnet fields in at least some steppers may be sufficient to
resist the force of gravity. First off -- with no Z-axis leadscrew and
nut installed, does the weight of the head move it on the ways? If not,
you should be fine with no power one. But once cutting forces come into
play, things like the spiral of the flutes can affect how much feed is
applied to the Z-axis by the spindle. (And, of course, a left-hand
spiral would generate up forces instead of down.)

Note: A 380 oz stepper motor is a very adequate drive for most
applications. It takes real force with your hand against the table to stall
one, and you can't do it by grabbing the shaft with your thumb and
forefinger except maybe at speeds in excess of its rating.

Then probably good enough to hold the ball screws. The usual
reason for warnings about ball screws is for a manual machine where
cutting forces can back-drive the ball screw unless you have a hand on
the wheel for each axis. (Hmm ... three hands ... ??? :-)

Enjoy,
DoN.

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