I recently acquired a used Rong-Fu RF-30 Mill-Drill (unfortunately, no room
for a full-sized mill). The drawbar that came with it had been modified by
welding a flanged nut to the top. The problem is that the flange is larger
in diameter than the splines in the spindle so I cannot get full vertical
travel out of the quill. I can modify this drawbar in the lathe (and will
likely do so) but I also figured it would be a good idea to get a new
drawbar, particularly since Enco had some Bridgeport-style drawbars on sale.

I bought the Enco's #505-2179 US-manufactured J-Head 20-1/4" draw bar for
$9.95 (regularly $17.45), and I have to say it's very well-made. I knew that
it would be too long for my Mill-Drill, but I figured it would be simple to
modify. Now that I have it in my hands and can examine it, I can see it
would be very easy to simply shorten the bar: drift out the pin that secures
the hex-head to the top of the bar, shorten the bar to fit, cross-drill, and
re-pin the head.

But here are my questions:

1. The new drawbar has a nice, machined steel bushing that fits under the
hex head. This bushing has a lip that fits into the hole in the top of the
spindle. Seems simple enough: the bushing helps center the drawbar, protects
the top of the spindle, and acts as a bearing surface while tightening the
drawbar. But what's puzzling me is that there's a fairly fat neoprene o-ring
that's fitted over the lip. This o-ring is considerably larger than the
through-hole in the spindle and thus prevents the bushing from fully seating
against the top of the spindle. If I were to put the drawbar in the spindle,
insert an R8 tool, and tighten the drawbar, the elastic o-ring would simply
compress and would prevent the drawbar from locking the R8 tooling in the
spindle. So why is the o-ring there? Is it because true Bridgeport mills
have a larger through-hole in the spindle that allows the o-ring to fit down
inside the hole? This would make sense as the o-ring would then "hold" the
bushing in the spindle hole, keeping it centered, but allowing for some
centering "play" in the drawbar.

2. The RF-30 (and it's various "knock-offs") do not have a spindle lock to
hold the spindle from turning while tightening the drawbar. Do Bridgeports
and/or other larger mills have spindle locks? If not, how do you
sufficiently tighten the drawbar?

Regarding this last point, I was thinking about threading a section at the
top of the drawbar and adding a "tightening nut" below the hex head. Then I
would insert the R8 tool, spin the drawbar by hand (using the hex head) into
the R8 tool until snug against the top of the spindle, put a wrench on the
drawbar hex head to stop rotation, and then use a second wrench to turn the
"tightening nut" down against the top of the spindle to put tension on the
drawbar and lock the R8 tooling in the spindle taper. To remove the tooling,
again use two wrenches: one to stop the drawbar from rotating and one to
back the "tightening nut" away from the top of the spindle. Then whack the
top of the hex head to free the R8 tool from the spindle taper. What do you
seasoned pros think of this plan? Or is there a better way?

Regards,
Michael