Re: What to do with the KNEE on a Bridgeport?
The Exacto machines are top quality, I have owned 6 of them over the years, 5 of them I bought new. Sold the last one about 6 years ago.
I have heard of using the knee for the Z axis, and I don't know if anyone has replaced the Acme screw with a ball screw. I have also seen pictures of the knee counterbalanced with gas struts or air cylinders. I think some BP factory built CNCs came with a counterbalance cylinder. Overall no reason it wouldn't work, but you would be moving a lot of mass. The shear weight of the knee should almost eliminate any backlash.
When I bought my Eagle (BP clone) it was a 2 axis machine and of course I wanted 3rd & 4th axis. I went through the same thought process as you, knee or quill? I didn't want to lose the manual capability on the quill, but I didn't really want to use the knee either. I looked at a number of solutions and kits for a quill conversion, and there was nothing on the market that would do exactly what I wanted. Also I did not want to lose any of the quill travel. There is one kit available that will allow manual operation but requires opening the cover and removing a couple of screws to use manually. Nothing was acceptable to me, I wanted to be able to switch back and forth in a couple of seconds with no tools, just flip a lever and turn off the Z axis for manual operation. I just designed my own system and now I can switch between full manual, and 2, 3, or 4 axis CNC in a few seconds.
What I came up with is a gear train that is driven by a stepper. 15:1 reduction to the quill pinion shaft, and use an air spring counterbalance to eliminate backlash. This system has 0 backlash. One lever flip disengages the gear train for manual operation, and a mouse click turns off the Z axis in software.
Jim Dawson
Sandy, Oregon, USA