jim rozen writes:

Plain PTFE would be a poor choice for a way bearing
surface IMO.

"Turcite" (not turcite) is just a brand name. My understanding is
that
Turcite B is PTFE, and Turcite A or TA and TX or X are acetal-PTFE
copolymer. If you have some other information, I'd like to hear it.

The only information I *do* know is that pure PTFE cold-flows - a
lot. It would be a poor choice for way sliders.

Yes, we've all seen that, but: do you have actual experience with PTFE
cold flowing in the low pressures of a machine way? I believe you are
confusing a high-pressure cold-flowing property with conditions in a
machine way.

The references I can find (such as the _Merck Index_ entry for PTFE, and
http://www.glenair.com/conduit/choose.pdf) state that PTFE cold flows
under *high pressures*. An example would be highly compressed regions
in gaskets. The pressures contemplated are 1000 psi or more.

Cast iron machine ways are designed with a meximum of 100 or 150 psi.
For example, a Bridgeport Series 1 saddle has about 27 sq in of bearing
surface, and worst-case loads would therefore be well under 100 psi, at
which level cold flow is a non-issue. The reason for this pressure
limit (well below the strength of the bearing itself) is to maintain oil
film lubrication.

I plan to run tests to prove this by just putting test masses on top of
test squares, and seeing what happens. I've already been testing the
material from MSC (vs Turcite B which is hard to get and exorbitantly
priced). For this I have been using cast iron angle plates, which I
scrape to 0.0001" flatness, but so far I have just been proving the
process of bonding with epoxy and getting a bearing-quality flatness on
the PTFE surface.

The manufacturer is cagey about just what Turcite B is, but it does
exactly match plain ol' PTFE's peculiarities in the published mechanical
properties. So my suspicion is that it is simply PTFE at a premium
price. There are no other candidate substances possible to my
knowledge. It isn't like there is some secret Coke formula involved.

I also speculate that some case of what is called "cold flow" are merely
ordinary plastic deformation. If you look at the low numbers of the
mechanical properties for PTFE, and considering the uniquely vanishing
coefficient of friction, then you can see how it would happen in many
applications. But not in machine ways by my analysis.

Another difference in machine ways is that the thin layer is bonded to
the relatively unyielding metal way. This eliminates any stresses from
forces accumulated across a long distance.