Re: Custom made bearing for lathes

Hi Jon,

my lathe is converted to CNC and I will attach linear encoders with 1
micron steps. I presume this is fairly precise ( although I know at 1
micron there are vibrations, causing the encoder to signal them ). My
lathe has less then 9 micron shaft run-out. Do you think is enough?
At the "air spindle lathe" I want to be able to turn objects of about
5-6 centimeters (2 inch + ) in diameter.
I would need some plans or a solution somebody tried and worked, if
possible.
If this proves too hard. what about an oil bearing? What is the
typical run out / stiffness of it? Are they easier to build than the
air bearings?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@...> wrote:
>
> Matthew Tinker wrote:
> > This subject was discussed at length on the "Quorn universal
cutter and grinder" group. Nobody got it to work!
> >
> > Matthew
> >
> > Cristi <wtherapy@...> wrote: Hello,
> >
> > I wonder if it is possible to built with hobby machines a lathe air
> > spindle or a fluid spindle for a lathe. I am interested in this
> > because of the very high truth of running of these bearings (
> > especially air spindle / bearings ). I don't want the lathe
spindle to
> > run at high rpm ( 4000 - 5000 rpm would be the maximum ).
> > I know most of the air spindle tools are tiny and dedicated to small
> > and precision works, I guess a lathe chuck would be to0 heavy / big
> > for this?
> > Has anyone tried this thing? What would be the cost / effort to
do this?
> There was an article in home shop machinist several years ago.
> Some fairly crude procedures were used, and the author said it
> works. I think a good home shop guy could actually do better.
> This was for a tool and cutter collet holder, and fairly small
> diameter.
>
> As you go up, it gets harder. The thermal effects that would
> never be seen on a 1.5" spindle start to get troublesome when
> you get much larger.
>
> No, weight and size are not the problem. There are telescopes
> that use air bearings - RADIO telescopes! Many use fluid film
> hydrostatic bearings with hydraulic oild, but there are some
> with air bearings. A well-built air bearing, even of small
> size, can have a radial stiffness of a million pounds/inch.
> That means it takes a load of 1 million pounds to deflect an
> inch, or 1000 Lbs to deflect .001" A large air bearing can be
> quite a bit stiffer than that. This dwarfs the stiffness of
> ball bearings, and even oil-film bearings need to be made with
> insane tolerances to get this stiff. Of course, air bearings
> like this ARE made with insane tolerances. And, there comes the
> rub. So, you'd want to have a clearance per side of something
> like .0002" to get really good stiffness. Making such a precise
> fit on a 1.5" part is a challenge in the typical home shop, but
> with enough time to lap the mating pieces you can get there.
> Making a large lathe spindle and getting a reliable fit with no
> barrel-bulges in the middle gets a bit tougher. Finally, unless
> the bearing and the spindle are made of the same material,
> thermal expansion may cause it to get loose or bind badly as the
> room temperature changes.
>
> How big a part do you want to turn?
>
> Jon
>