Re: BYO CNC Lathe ?

There is a lot to be said in favor of mass. There is also distance
involved with the torque calculation. a wider spread will yield
higher torque. I think the boxy, squareish bed of the lathes is
there for torque and strength. But they design those units for any
work, not a product specific design.

There is a link regarding a CNC lathe, linear shafting, about 1.5
inch dia I think, spread about 18 inches apart. very well made
unit. but I am sure the mass of the table is there to HOLD that 18
inch spread, or vibration and the harmonics will surely make
themselves known.

This week, I took a 0.01" deep cut on a 2" solid cold rolled steel
bar 22 inches long. If I took 0.015" the vibration took over.
9"x20" South Bend lathe, chuck and a live center to hold it.

Dave

--- In CAD_CAM_EDM_DRO@y..., "bswim_99" <bswim_99@y...> wrote:
> 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