Himmy,
We're on the same page here. What I called an 'extension', you are calling a 'guide tube' . . same function . . even down to using the existing threaded pre-load as the base for the tube.
I'm not feeling the same concern as you and Vmax for the wobble. A machined concentric tube of less than 2" in diameter turning under 10k RPM that is under 12" long (all guesses) should not be trying very hard to get off the top of your mill, unless you cannot keep it aligned with spindle center for some reason. If it worrisome to you, just reduce the mass by making the guide tube thin and lightweight and rigid by using 7075. Keeping the draw bar centered in that tube would be an easy matter of a concentric bushing perhaps of a lightweight self lubricating plastic like Delrin, again very little weight and no balance issues.
It occurs to me that the draw bar extension would not need to be very large diameter beyond the collet threads. I find that pre-ground 'stressproof' steel rounds are very straight and I think the collet threads are 5/16" which would be more than needed for the rest of the shaft, methinks, so you could turn that down to reduce the mass above the spindle also, but I just am not seeing a problem with wobble since as far as I know, nothing is extending out from the tube or the drawbar shaft.
I guess you'll know soon enough! I'm anxiously awaiting pics of your next few pieces.
BTW, now that you have mentioned it again, I do recall you saying you had the CNC on the knee as well as the quill. Probably not many conversions are that extensive, one might imagine.
Simpson,
Well, due to an oopsie on my part, my first attempt at the guide tube did not seat properly, and ended up with 0.050" runout at the top - not good. At 8K RPM, it made some pretty good vibration! I corrected that, and I'm now under 0.005". With a more suitable lathe, I'm sure I could do better. But, the vibration is pretty small at this point, even at top speed. Probably tolerable as it is. Certainly good enough for now.
The collet threads are 7/16-20. I'm using a high-strength all-thread for the moment. Once the dimensions are all settled, I'll probably make something better. The Bellevilles and their supporting "top hats" stick out of the tube when the drawbar is up. They are partially exposed through most of the quill travel. But, they are always guided by the tube, so no problem there.
I first converted the knee. In fact, I still haven't done the quill. But, the knee is too slow, and there's too much wear and tear on the gears, screw, etc. for that to be the long-term solution. A quill drive is the next major project.
Terry,
My spindle is not a real BP, but is nonetheless identical to your picture. The front pulley is supported by two bearings fixed to the top housing. It is those bearings I am attaching the guide tube to. They, too, have to be concentric to the main spindle bearings, or you get a lot of rattling from the dog clutch and other things up there.
Regards,
Ray L.
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