On Tue, 1 Jun 1999, Elliot Burke wrote:
According to my material selection guide, polyester CTE is 4 E-5 /F, cast
iron is 6 E-6 /F. Most unreinforced plastics as quite similar in expansion
to polyester.
The difference in expansion works out to 3.4 E-5 /F, or 6 E-6 /C.
^^^^^
6 E-5 /C
The situation is somewhat better for stabilized polyester such as used for
printed circuit photo-tooling, such as Kodak Estar .007 inch thick material
* www.kodak.com/global/en/professional/support/techPubs/ti1814/ti1814.shtml#dimenstab *
CTE is 10 E-6 /F compared to cast iron at 6 E-6 /F. (Aluminum is 12 E-6 /F)
Using this material for scales works out to 4 E-6 /F, considerably better
than the 34 E-6 in your example above.
A 1 C change in temperature will cause a 0.001 inch relative movement in 16
inches between polyester and cast iron.
Using the same 16 inches, stabilized polyester moves .00028 relative to
the cast iron. Meanwhile the cast iron itself changed .00019.
A change in relative humidity of 1% changes the polyester about the same
amount as above.
A usual shop environment is not controlled anywhere close to 1 C, your
milage may vary.
Not to mention the heat imparted into the work by the machining changes
the part's size even if you control the shop.
Polyester also has a tensile modulus of around 500 kpsi. Thus a piece 0.004
x 0.5 inch (0.002 sq in) will stretch 0.001 inch in 10 inches with a 0.1
pound tension.
The other side of this stretchyness is that if held by its ends under a
little tension in a metal frame, its length will equal that of the metal
frame, as long as it remains under tension at the highest temperature of
use.
Moral of story: keep it under tension.
A good suggestion, and making the scale slightly undersized and stretching
it to dimension lets you calibrate the machine.
For many lengths stretching the scale should be practical, but for larger
machines the sag could be a problem. I still believe that constraining the
scale with a clamp or simple glue would keep the film scale in sync with
the machine due to the small forces required as you point out above.
-Ron Wickersham
