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:
- 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.
- 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