Re: Bridgeport Mill head Q's

> From: "Fox, Dan" <dfox@...>
>
> In Digest #109, Jon Elson said:
> <begin quote>
> > Well, almost. I had a 1941 M-head machine, SN 1388. I replaced
> > the M head with a J, and it was a huge improvement. The great
> > availability of R-8 arbors and end mill holders is a big advantage.
> >
> <end quote>
>
> Can anyone point me to some information concerning the differences between
> the M head and the J head? I know essentially nothing about milling
> machines, apart from being able to distinguish them from lathes. Which is
> the earlier design? Which is more desirable? (I'm guessing J here.) Also,
> the same info about round ram vs not-so-round ram.

The M head is older, smaller, lighter, and has less features. It uses one
of 3 somewhat rare spindle tapers (for milling) and so end mill holders
and arbors for things like boring heads are hard to find. It also lacks the
very slow speeds available on the J by way of the back gear.

The J comes in variable speed or step pulley, both have a back gear for
very low speeds - very useful for fly cutting and boring. The J head
has a reversible power quill feed with an automatic depth stop. The
standard R-8 taper makes an enormous number of arbors and other
special tool holders available, as well as a wide selection of end mill
holders. The advantage of end mill holders over collets (in addition
to reducing 'tool suck' and improving rigidity) is that they can place
a tool at a repeatable location, great for CNC. The J head has an
additional inch of quill travel, and is MUCH more rigid than the M.

>
> What part of the machine is considered the ram vs the head? The head is the
> part that has the motor and quill, right? The ram is the part that allows
> translation motion of the head in the y-axis, right? (moving closer to and
> farther from the face of the operator). Are they swappable among Bport mills
> of a certain age, i.e. do the bolt patterns that need to match up match up
> etc.?

There are either 2 or 3 versions of the turret/ram assembly. There is the
old M head machines, up to the late 1940's. There were some J head
machines made in the late 40's and early 50's with a round ram, and no
knuckle to 'tilt' the head. These rams may have been a larger diameter
than the M head round ram - it sure looks bigger in the picture I have,
but I've never seen one of these machines in person. Then, the dovetail
ram was developed. The tilt and roll knuckle was brought back, although
much more massive than the M head knuckle. The dovetail ram advantage
is that the head doesn't need to be re trammed, as the dovetail ram cannot
rotate in the turret, as the round ram invariably will when unlocked.

Jon