Re: BYO CNC Lathe ?

People don't realize how much torque lathes put out or the amount of
cutting pressure generated.
Unlike a mill where your tool gets some relief as the cutter spins, a
lathe is always under stress of cutting. If you're making a 6" long
cut, that pressure is going to build as you cut until you disengage
the cutter from the part.
I could see stuff bending or flying apart unless you used some heavy
cast iron for the base.
The smaller Mazak CNC's lathes use replaceble slides for the ways.
But they sit on a tremendous amount of cast iron. They cruise too.
The one I ran did 1200ipm.

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
> 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