Originally Posted by
HimyKabibble
Phil,
It's all trade-offs. CNC'ing the quill is mechanically rather complex, but due to the smaller mass, lesser friction, etc., allows use of a smaller motor, and allows greater acceleration and speed. CNC'ing the knee is mechanically very simple, but requires a very strong motor, and speed will be limited, due to the enormous weight and friction in the knee. As an example, my X/Y axes will move well over 300 IPM. The same motor on the knee, even with an additional 4X reduction, is only good for about 70 IPM. I originally intended to do my quill, but doing the knee was so easy (I did it in a single day), it was the quickest means to my end. Eventually, I will do the quill as well. When I do that, the knee will be used for tool length offsets, and most of the actual "work" will be done with the quill. There are relatively few jobs where a single tool is required to move more than 5" in Z. In fact, I've never had one that came close.
Unless you have a tool changer, CNCing the quill makes perfect sense, since you will have to manually set tool offsets anyway, and you can do that just fine with a manual knee. Some will argue the quill will provide better accuracy (due, presumably, to inaccuracies and slop in the Z axis ways), others will argue that the knee will (due, presumably to slop in the quill bushings when the quill is extended). Really, it depends on your specific machine, and your specific requirements and expectations.
There are a number of threads on the Bridgeport forum where you can read all the arguments for and against both methods. As far as I can tell, it comes down to personal preferences as much an anything else. I don't think either approach is inherently better or worse than the other.
Regards,
Ray L.