Re: [CAD_CAM_EDM_DRO] Re: Custom made bearing for lathes

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