Re: [CAD_CAM_EDM_DRO] Z Axis Mechanical Layout Options - Bridgeport M Head

natchamp_87 wrote:

>Well, I got my new (to me!) BP M head mill. I have already started
>taking it apart for the clean up, ball screws, etc. One of the
>things I have not resolved is the best way to accomodate the Z axis
>travel. I am hoping someone can shed some light on the best way to
>accomplish this.
>
>If I move the quil, it appears there are a couple ways to do this.
>1) mount a motor to turn the same shaft as the hand lever. This to
>me has a problem of very long travel for a given rotational travel of
>the motor, unless I severly under drive the pulley which still might
>not be enough?
>2) mount a motor to turn the fine feed shaft and worm gear. This
>would fix the long travel per motor rotaion in 1 but the worm gear
>isn't fixed to the shaft, it works on friction. I could key-way the
>worm gear onto the shaft? This, to me, seems like the best approach
>for moving the quil.
>
>

Both of these are awful. The rack and pinion has a lot of backlash, and
no guarantee of accurate
linear motion for equal amounts of rotary motion. The added backlash
between the fine feed
worm and the pinion shaft will give you over 1/10" backlash! I can't
imagine milling anything
with that!

>If I move the knee I would need a bigger motor and could still under
>drive to address the higher torque required. However, there doesn't
>appear to be any good mounting locations for this method.
>
>Can anybody shed some light on this? I know people have done this
>many times and would love to hear some insight. Thanks a bunch.
>
>

The quill has only 4" of travel, maybe, which is a definite limitation.
Check the difference in
length between a 1/2" jobber's length drill in a Jacob's chuck, and a
1/8" end mill in a collet.
It is WAY over 4". So, you have to move the knee anyway. If the
vertical knee ways are worn,
as one would suspect in an older machine (although mine is even older)
then you get unwanted
movement in the Y direction when you move the knee. This is easy to
check with a dial
indicator. It is about .003" on my machine. I minimize it by always
lowering past where I want
to go, and then raise it slightly. This presses the knee back against
the column.

Yes, you'll need a much bigger motor to lift the knee. Gas springs, air
lift cylinders or chains and
counterweights are the alternative.

So, there are only 2 practical ways. Forget the rack and pinion - no
way this will ever so any
accurate work. You can either mount a ballscrew to the front of the
quill housing and drive it
from where the depth stop ring is attached, or drive the knee with some
sort of retrofit there.
One possibility is to modify the knee lift so the nut is mounted atop
the base pylon, and the
screw is fixed inside the knee. You might have to carve a little on the
hole in the bottom of the knee
to clear whatever turns the nut there.

Jon