9 Aug 07 9:01
If this question has been already answered, I appologize but I could not find it.
We die cast, machine and assemble aluminum A380 parts. Tolerances are very tight, for example on diameters of aproximately 120 mm, we could have a tolerance of +/- 0.012 mm.
Dimensions on supplied customer prints are stated at ambient temperature of 20 degrees celcius. (reference ASME Y14.5M-1994)The temperature of parts after processing are on average around 37 degrees celcius.
After some basic calculations for part growth based on the CTE of the aluminum, and temperature delta from 20 degress celcius, it is obvious that temperature can significantly cause parts to grow, out of specification.
Most CMMs will account for this growth via temperature measurement and correction factors, so this is not an issue. But there have been individuals here that have said that it should take about 10 minutes for the parts to normalize to ambient temperature of the lab. I feel they are wrong, and I would like either correct them, or correct myself.
I have two questions:
1) If I take a part at temperature "X" (37 degrees celcius) and place it in a temperature controlled room at temperature "Y" (20 degrees celcius)is there are a rule of thumb for how long the part should "soak" to reach ambient temperature?
2) I am sure there is formulas that one could use to calculate the time, atleast theoretically. Could someone summarize the formalas to use to do this calculation? I know I have done this type of calcuation in Heat Transfer or Mechanical Measurement in university, but that was almost 10 years ago and the theory is a little foggy...
Ultimately, if I can't get an answer I will do a little test and physically time parts myself, but I would like to try this avenue first.
Thanks for any response.
Grant Ryan, P.Eng.