Re: Z Axis Mechanical Layout Options - Bridgeport M Head

Jon, excellent points I didn't even think about. I am convinced that
moving the knee is by far the best setup, I just have to work on the
packaging. Thanks again.
Mark

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
>
>
> 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