On 11/6/14, 1:34 AM, nestork wrote:
micky;3304935 Wrote:

The graphic shows a screw with arrows pointed at the threaded part from
both sides, and to the left of that a circle with a diagonal slash
through it.
.....
Circlle/with/slash + 3 mm and then a drill bit with arrows
pointing from both sides, towards the bit. So the second part is

Circle +3 mm Bit.

Does that mean what I think it does?


I agree with both Mayayana and Clare Snyder here. Plastics have the
largest coefficient of thermal expansion. The lowest coefficient is
with ceramic materials like brick, glass, concrete, mortar at 5 to 12
parts per million per degree Celsius. Then metals at from about 12 to
40 parts per million per degree C. And finally plastics, which for PVC
is about 200 ppm/deg. C. if memory serves.

I hadn't thought about that. I looked it up. It's 70 ppm / C for
Lexan. Glass-filled Lexan has a much lower coefficient (21 ppm/C).
http://boedeker.com/polyc_p.htm

But, you should keep in mind that the 3 mm is for the largest dimension
of the sheet of plastic. (4'X8'?) If you're only using 1/4 of a sheet,
you may only need 1/4 of that 3 mm to allow for thermal expansion and
contraction. But, since you're using rivets which won't allow for any
sliding of the plastic as it expands or contracts, you need to allow
some way for the plastic to move or it will buckle when it expands. You
may want to use nylon screws and nuts instead of rivets. Most places
that specialize in fasteners will be able to order nylon screws, nuts
and washers for you. That way you can fasten the plastic down, but
still have some slippage of the plastic between the nylon washers to
allow for thermal expansion and contraction.

Based on 70 ppm/C, Lexan would expand 3mm/m for each 43 C. It seems
funny that the manufacturer would specify 3mm without mentioning size or
temperature range.

This page talks about Plexiglas, whose coefficient is a little bigger
than Lexan, and which is affected by humidity. It says to avoid
inflexible fasteners such as bolts, and adhesives should allow movement.
http://www.eplastics.com/Plastic/Plastics_Library/Coefficients-of-Thermal-Expansions-of-Plexiglass

Nylon screws and washers sound good to me. I used them on Plexiglas
windshields. It didn't occur to me to drill oversize holes.