John Biretta said:Robert Apruzese said:
My question is: how much clearance
should there be between the cell and the lens to allow for expansion and contraction. Assume I will be using the lens in temperatures from +10F to +90F.
Why not measure it directly? Use a digital caliper
to measure the lens elements and cell at room temperature.
Then put them (but not the caliper) in the freezer
at 10 F and measure again. Easy to calculate how
much everything contracts at 10 F, then contraction
difference between cell and glass, and finally required
room temperature cell ID to avoid pinching at 10 F.
Problems generally arise since metal contracts
faster than glass, and hence there is pinching at
cold temps.
John and Robert:
The Coefficient of Thermal Expansion of BK-7 Crown glass is 7x10^-6 /DegC. With a 60mm lens, a 30 degree C temperature change would result in dimensional change of 0.0005 inches. Even a good man with calipers can't make that measurement, that requires a micrometer and then it is at the limit.
TE = D x CTE x Delta T = 60mm x 0.0000007/degC x 30C = 0.013mm = 0.00050 inches.
The CTE of aluminum is about 23 x 10^-6/degC, it expands and contracts much more, for a 30 degree change in temperture it would expand or contract about 0.0017inches. This is measurable with calipers if you are good with them.
The difference between the two is right at 0.0012 inches. This is for a 30 degree C temperature change, the difference between 70F and 10F is 33deg C.
It looks to me like for the Crown Glass, 0.0012 inches would be sufficient. That is going to be tough cut, the machinist will have to chuck the cell and align it in the lathe to within a "few tenths" and then cut about 0.0006" off the radius, the diameter changes by 0.0012 but the depth of the cut is only 0.0006". The machinists I work with in my research could do it but they would give a look and roll their eyes and let me know I owe them one.
I did look up one Ohara Flint Glass, S-NPH1, I have no idea if it is used in lenses but it has a CTE of about 9x10^-6/degC so it is between the Aluminum and the Crown glass.
Jon
