What to do with the KNEE on a Bridgeport?

[Aaron Moss]

Soooo, I ended up with 2 Bridgeport Series 1's one is actually a a clone called Exacto? Which is actually pretty nice, from the 70's I guess, good shape with an integrated oiler. That oiler made it the future doner for a CNC conversion. X/Y is easy, I already have a ball-screw servo kit for that.

But the Z axis is a little more of a question. I like to leave the free quill, so the knee it is. Looked at a few kits online and they never appear to swap the screw. The Acme screw that's with the machine is actually pretty nice, the handle and gear mesh feel nice, few thou of play, but overall pretty good.

I can't really find a reason NOT to do the knee, leave the existing screw, get a massive NEMA stepper, reduce it a little with a HTD belt and be done. Make sure the program understands the backlash and go for it.

Most parts are 2.5D, and some kooky thread milling thing I'm going to have to do with a 4th axis, but I think I can do that with plenty of space to get the knee lash out before the cutter engages. It will have to be done in a single pass, but fairly straightforward. (See my other post for that one)

I already own the mill, got a spare kit for x/y, just need to do the Z and get an Acorn and move on.

So is there any real reason this is a bad idea?

Thanks
Aaron

[Jim Dawson]

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.

[Aaron Moss]

Jim I would really like to see the pics, but something isn't letting me, CNCzone vs the new site, can you PM me something. Air spring sounds very interesting.

Thanks
Aaron