Re: Custom made bearing for lathes

If you want to find out what current practice is for high precision
lathes, google single point diamond turning optics.
These lathes, some of which are quite large, have precision and
surface finish in the few nm range, a micron is way too coarse.

They use hydrostatic bearings ususally. Air bearings aren't stiff
enough for most lathe spindles, although there may be some special
cases where that isn't true.

They have been commercially available for quite a while now, you
might even be able to find one on ebay. At the very low end, they
are used for turning contact lenses, at the high end they make large
aspheric/anamorphic mirrors. In the city I work in, Rochester NY,
there are probably a dozen of the high precision larger diamond
turning lathes. This place is a center of the US precision optics
industry.

BTW, they use single point diamond cutting tools, the cutting tools
have to be as precise as the lathe to get good results.

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@...> wrote:
>
> Cristi wrote:
> > 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?
> I didn't study the comaparative numbers for the oil film. The
> problem is a big oil film bearing self heats about 10,000 times
> greater than an air bearing! So, the air bearing doesn't need
> active cooling, but the oil film bearing needs a lot of cooling
> to prevent the oil from overheating and becoming too thin. This
> requires either cooling ports separate from the oil supply or
> much larger bearing clearance to allow the oil to flow faster.
> The greater clearance reduces stiffness greatly. The other
> downside to an oil film bearing is the whole scavenging system.
> And if the scavenging system doesn't do its job, the thing will
> spray the entire room with oil when the spindle starts.
>
> If you have the technology to do a high-quality tight clearance
> oil film bearing, you can do an air bearing.
>
> You need a toolpost grinder to grind the bearing journals to
> fine finish. You will need to make a guided barrel lap. It
> uses one bearing as the support while lapping the other bearing.
> You turn it around every once in a while and keep going back and
> forth. That way the main bearings are coaxial.
>
> You will also need to make a thrust bearing, which can be just a
> flange on the spindle, with two annular plates with tiny holes
> in them to admit the air. You have a spacer between the plates,
> and bearing clearance is set with shims.
>
> You can look at a university library for a book on "fluid film
> bearings" where I got a lot of this info. It had a couple
> chapters on air or gas bearings.
>
> Jon
>