Re: [ATM] Public Terrestrial Telescope: Re: Replies Regarding FocuserBearing Material

["Don Clement" <don_clement@charter.net>]

Dave S wrote:
>
> "And Don Clement suggested Turcite, another polymer-based self-lubricating
> plastic. But alas, Turcite is made of Delrin and molybdenum sulfide, so it
> comes with the shrinkage problems common to Delrin."
>

I don't believe that Turcite is made using molybdenum sulfide. I believe it
is similar to Delrin AF that is PTFE filled. I have used both Delrin AF and
Turcite and Turcite is a much better material. Turcite rod is manufactured
or used to be manufactured by Shamban in Newbury Park, Ca. (Any remember the
Tally rocket ship on the 101 just before the Camarillo grade? )  In my
experience Turcite is very stable over time and machines well. (I use a
vortex  air cooler when machining Turcite) Turcite is oven annealed to
reduce internal stresses. Ultra high precision Hardinge lathes (also my CNC
mill) use Turcite in the ways so Turcite is a proven stable material.  I
found that the trick to using Turcite is to accommodate the material
dimensional changes due to differing COE, water absorption, etc. with the
design. Also adhesives do not work well with Turcite. I now broach and mill
a keyway and use a 1/16" square metal key to prevent Turcite leadscrew nuts
from rotating with my focuser. I have had no problems with Turcite Nuts
since using this improved design with the broached keyway. When using
Turcite with other materials such as aluminum I design for it. For example
if making a large threaded part I would not make the large part using solid
Turcite but would drill a series of axial holes in an aluminum cylinder and
press in a number of small diameter Turcite plugs. The threads would be
machined in the assembled Turcite/aluminum part.  The aluminum holds the
small diameter Turcite plugs and dimensional changes of the small diameter
Turcite plugs have little effect on the large threaded part. The small
diamter Turcite plugs make the actual threaded contact though. If the part
were turned from solid Turcite the dimensional changes of the solid Turcite
would be significant.



Don Clement

Running Springs, California

(Thank you Firefighters)

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