Re: [CAD_CAM_EDM_DRO] BYO CNC Lathe ?

Chris L wrote:

> Ok,
> I've taken an interest in purchasing a small lathe to convert to CNC.
> Everything I look at though, spooks me off a bit. The Taig...To Small.
> The Sherline, To Pricey. The Imports ? Well, we picked up a Jet 9x20 for
> a quick and dirty machine and I was not very impressed. I did not think
> one should build a machine like that out of such soft metal. You should
> see how wobbled out the wrench holes are in the spindle from taking the
> chuck off ! Scary! I do not think I would ever cnc that thing.
>
> So, I began to think about building my own. (BYO)
> Is there any reason why I could not use some good sized THK-IKO style
> bearings for the rails ? Why do all lathes have the typical beds with
> sliding gib concepts anyhow ? Linear bearings should work, No ?

For rigidity. The cutting forces are surprisingly large, even on pretty
small lathes. Secondly, and tendency to vibrate will set the whole
machine ringing like a bell, and cause horrible chatter marks in the
part, or even rip the part out of the chuck. There has to be an important
reason my Sheldon lathe weighs 3000 Lbs! Even the bed of that lathe
can be made to vibrate without a great deal of effort, and it is about a
foot square in cross section (two 2" wide vertical beams with cross-
struts in between) and weighs at least 1500 Lbs for a 6' length.

The Unimat was made like this, and I have heard horrible stories about
vibrations in that machine.

>
> I could even put the Tailstock on the same rails with a separate
> leadscrew and handwheel instead of having the tailstock made in the
> conventional way. That sounds alright.
>
> I could come up with a really neat spindle with a really large thru hole
> instead of all this too small stuff. (I knew I would someday find a use
> for front wheel drive cars... The front hub and spindle ! I could keep
> my GM lathe ALL GM !)
>
> I thought I could probably use big chunks of Billet aluminum for the
> headstock and tailstock but was worried about not getting enough mass.
> But I suppose it would still be as good as a Taig or Sherline.

No, I suspect if you use unsupported rails, it would actually be worse
than a Sherline! With supported rails, it might be a little better, but you might
have rapid wear on the rails or bearings.

Jon