"DoN. Nichols" wrote:

On 2012-03-17, Tom Gardner wrote:

[ ... ]

It would be nice to have a CNC mill but I do so very few jobs that a CNC
would shine that I just can't justify it. Besides, doesn't a CNC setup make
it difficult to use the mill in manual mode?

It depends. The first CNC mill which I encountered -- a
Taiwanese clone of a Bridgeport retrofitted by Anilim to CNC had Dished
handwheels with spring-loaded folding crank handles whose balance was
such that the handle would stay folded in at high speeds. (This was a
servo conversion, not a stepper one, FWIW, and the rapid motion was
scary. :-) Anyway -- stop the CNC control, and you have just enough
current through the servos to avoid self-feeding of the ball screws, and
you can easily use the handwheels to feed it -- with proper dials still
present.

However, my Bridgeport BOSS-3 (being converted to servos) is
quite different.

The X-axis ball leadscrew is rigidly mounted to the right end of
the table, and the ball nut is mounted between a pair of opposed tapered
bearings with the proper preload to minimize backlash). That ball nut
is rotated via a stepper (or servo) mounted well below the table on the
right side of the knee. No access by hand to the timing belts used to
connect them.

The Y-axis ball leadscrew is rotated, but by a timing belt from
a stepper below it which is recessed into the knee, and it is all
covered. (There is a Veeder-Root counter to display the X and Y axis
positions.) But there is no access for a handwheel.

The Z-axis is actually a hollow ball screw which surrounds the
quill, so the feed is truly on axis (the Anilam conversion above used a
ball screw in place of the feed stop screw, so it is pressing off center
and would hasten wear of the quill in the headstock.) However, again
the ball nut is mounted in bearings and turned via a timing belt and a
motor off to the left of the headstock. The original stop screw is used
to actuate the feed limit switches instead, and can't be used to read
the position. There is a dial on the bottom of the pulley so you can
tell where in a rotation the motor (and ball screw) are, but not which
rotation.

The control, when the CNC is stopped, has jog modes which allow
motion of 0.001", 0.010" 0.100" and 1.000" -- rotate a switch to point
to the desired size of the jog, and then push in the switch (which is
also a pushbutton) to cause the motion. I've actually done a little
machining with this, but it was a serious pain. :-)

So -- all in all, I would have to say that this one was not
suited for manual machining, and that the Anilam conversion (or some
other custom conversion of a manual mill) would be the better choice,
with the folding handle handwheels, so you don't get whacked by the
handle when you stand too close just when it is going into a rapid
motion. :-)

Most any CNC mill can be used manually if it has an MPG. Just select the
axis and a suitable step rate and crank away. I can't really see why you
would want to though when you can just "manually" machine one line of G
code at a time in MDI mode.