Parts is not Parts

The lathe is still driving me nuts. Actually, I think Fusion 360 is the culprit.

I took a 7×14 mini-lathe and turned it into a CNC machine. It should be possible to cut parts to within maybe three thousandths of spec on this thing. So far, that has not worked out.

I had problems adjusting the steps-per-inch figures on the screws. I more or less got past that. Then I had problems with backlash compensation. Now the screws work reasonably well, but I am having lots of trouble turning ideas into parts. CAD and CAM are holding me back.

If I had a mill, I’d be sitting pretty. There a bazillion CNC mill jockeys out there, and the people who make software bend over backward to help them. I would have lots of resources to call on. Because I have a lathe, everyone is pretty much hoping I will go away and die.

Perhaps I exaggerate.

Anyway, as I’ve said before, CNC lathes are not that common, and it’s hard to get help.

I can draw stuff in Fusion 360 CAD. It’s a non-intuitive, somewhat annoying program, but as CAD goes, it’s a breeze. I can also create tool paths and G code using Fusion 360 CAM. What I can’t do is get the x-axis to work right.

No matter what I do, the CAM program decides to move all the x stuff toward me by about a third of an inch. Here’s the result: if I design a cylinder 1″ in diameter, and I try to make it from a 1.25″ piece of stock, the cutting tool won’t even touch it. It will move back and forth maybe 1.30″ from the lathe’s axis, doing nothing.

Here’s something weirder: the x issue doesn’t pop up when I’m facing. The word “facing” refers to putting a flat surface on the right end of a part. To face a part, you have to push the tool halfway across it. If you’re facing inward, as I am, you have to start at the outside and push the tool all the way to the center. My lathe does that, and it’s an x-axis process. When it tries to reduce the diameter of a part–also an x thing–it misses the part completely.

The x-axis works for some things, and it’s useless for others.

People are telling me it’s because I have Mach3 set up for radius measurements and Fusion 360 set up for diameter measurements, but that’s wrong, because compensating for that when I start the lathe doesn’t help, and anyway, it wouldn’t explain why facing works perfectly.

I accomplished quite a bit today, even though I’m still not able to make Fusion 360 turn out parts. I learned a lot, and I managed to enter specs for my cutting tool into the program. One measurement is off by a few degrees, but it looks like it’s the best I can do in this program, and it will probably never make a difference, since the very tip of the tool is the only thing that touches the work. Turning the tool insert 3 degrees in the xy-plane amounts to nearly nothing.

I will keep poking people until they give me answers. If necessary, I will just annoy them until they do the work themselves to make me go away.

The big lesson, which is really being driven home now, is that smart people do not build CNC lathes. You buy a commercial CNC lathe, you buy a mill, or you build a mill. Listen and avoid my pain.