Adding a leadscrw to the rack&pinion.

I've been contemplating on adding a leadscrew to my lathe, and gasp maybe making it driven by a stepper/servo sometime in the future. So, I've spent about half a day in the garage today, and ended up making about 50% of the parts needed. Before any other things I want to record lessons learned: Lesson one: steel used in bolts at a harware store is not fun to work with. I made the splitnut out of a 1/2" diameter bolt, and boy was that not fun! Lesson two: Aluminum (at least 6061) flexes. A lot. Need to remember that (you'll see why later).

Project objectives.

• Minimum irreversible modifications to the lathe! (holes drilled, spots milled - whatever)
• Make setup rigid enough to make it possible to move the carriage to within .002" or better (not counting the backlash, of course).
• Since I am not sure about how my hardware-store-unknown-metal 1/2-20 rod will hold up, it needs to be easily replaceable with any other with minimum work. Same should be true about the splitnut as much as possible.
• The mountings for the leadscrew should allow me to adjust the location of it because I can not calculate where it should go (short of putting the whole project, including the lathe itself in a CAD drawing and doing it there).

These are all the parts for the splitnut assembly. Only 2 holes needed! Seems like objective #1 might be acheivable!

Here's how the splitnut parts fit together. The brass bushing (is that how you call it) was cut precisely so fits snugly touching the bottom of the carrieage. This allowed me to get away with just one 6-32 screw to hold everything in place.

Here's where the leadscrew goes. I really tried to preserver the long plastic chip shield that protects the rack from chips, but things just wouldn't fit. When the splitnut is unlocked, it would push both against the botton of the carriage and the shield. And I can't make the splitnut any smaller because I want to have at least 3 full threads on each half of it, which takes up a lot of space. Without the shield, however, I have plenty of space to spare.

After finishing the splitnut assembly, I decided to try and make the mountings too. Too ambitious... These things really take some time, especially if you are thinking things up as you go (as opposed to having a decent drawing with measurements and stuff). Notice that the mounting has slots milled allowing the ball bearing (that is press-fit into the mounting) to move up and down as well as back and forth by something like 3/4". That should allow me to position a reasonably straight leadscrew in such a way that it will not touch the splitnut when unlocked, allowing me to still use the rack&pinion. I'm really liking the ability to move the carriage fast, and I'd hate to lose that capability. Unfortunately, that's where aluminum flexing became a problem. The aluminum angle, though seemingly stiff, will flex a whole bunch under stress (I think something like >5 thou when you apply a kilogram or so of force along the leadscrew). That's really bad. I'll try to make the mounting on the other side a lot wider wider in the dimension along the leadscrew. Hopefully, this will compensate for the flimsiness of this mounting. I really don't want to re-do it.

Here's how it'll look like once I go to the hardware store and buy on of those cute rounded 10-32 nuts that will eventually attach the handwheen to the leadscrew. Yes, the handle looks ugly, but I have excuses - (1) it's functional, (2) I didn't want to waste more than a few minutes on it, so I made it without a single measurement, just looking at the handle on my crossslide, (3) it's my first one.

Still need to make the left mounting, assemble and adjust everything (includes bending the threaded rod into something that is reasonably close to straight) and, hopefully, figure out a way to graduate the handwheel. Anyone has an idea how to do that? No, I do not have a dividing place:(. I'da say another 1/2 day of work for me - I am still really slow at metalworking.