Tom Wait wrote:

A Bridgeport M head is a common addition to a Hardinge TM. In fact theres
one up for auction on Ebay all set to go on a Hardinge mill, but I'm afraid
the bids will go to the moon. Someone lucky SOB just won a Hardinge mill
with an M head on it for $310. It was in NY, to much frieght for my blood.
That's why I was curious about Gunners J head. I'm not savvy about the
various head designs Bridgeport offers. Just looking for information, and a
bargain.
The M head is MUCH lighter! 1/2 Hp, higher speeds, but no lower speed.
The M does not have a back gear or a power quill feed. The J head has
both, as well as a VERY much larger quill and spindle. The M is
available with Bridgeport C taper, MT3 and B&S #7. That limits you to
1/2" tooling, and not a lot of other special arbors are available, like
flycutters, boring heads, end mill holders, etc. for these tapers. You
can get almost anything ever invented in the R-8 taper.

If the M head is just about "right" for the Hardinge TM, I think you'd
find the J head to be too large in some dimension. The distance from
the mounting flange area to the bottom of the quill, and to the quill
centerline are both several inches larger on the J head. And the weight
increase must be 2-3x, also.

On the other hand, on a machine which CAN handle the weight and size
differences, the J is a much more versatile head. I changed my
round-ram Bridgeport from M to 1J, and have been VERY happy with the
upgrade. The tooling advantage I mention above is part of it, but the
performance of a boring head, and the ability to run at much slower
speeds is also a plus. The slow speed is great when boring, using
jeweler's slitting saws and flycutters. I could never get a boring
head to work with the M head, without massive chatter marks. I had
to grind down a boring head arbor's 3/4" straight shank to 1/2" to fit
the M head, which may have been part of the problem. I replaced the
shank with an R-8 one when I got the J head, and had no more chatter
trouble.

Jon