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Helpful Member!  GrantR (Mechanical) (OP)
9 Aug 07 9:01
Hello

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.

PSE (Industrial)
10 Aug 07 7:30
A location for the formulae.

http://www.efunda.com/formulae/heat_transfer/conduction/overview_cond.cfm

This site below has numerous calculation applications.  If you look for/through the thermodynamics calculators you can perhaps find one that fits your needs.

http://www.martindalecenter.com/Calculators.html

Regards,
kenvlach (Materials)
11 Aug 07 2:30
Such a rule would account for volume & surface area & material of the part, contact area & type & mass of whatever it is sitting on, ambient air flow. Casting & heat treatment people must have the software...
The equilibration rate becomes asymptotically small approaching the ambient temperature. As a practical matter, any chance your plant has some large rinsewater tanks around?

Since you are familiar with A380 I presume you are using a linear CTE, of 21.1 µm/m-°C (20-100°C), from MatWeb
http://www.matweb.com/search/SpecificMaterial.asp?bassnum=MAC3800AF
rather than CTE of pure aluminum (23.6 µm/m-°C).

Useful magazine article: 'Compensation For Thermal Effects'
http://www.mmsonline.com/articles/030107.html

ALOBrazil (Mechanical)
25 Aug 07 22:17
I think the best way to find the temperature stabilization time is certainly to do a little test, measuring the part several times until the measurement results are stable.

I recommend also to compare the measurement results of the last measurement with the first one, after correcting the temperature. If your part has bended, maybe the simple linear temperature correction be not enough. In this case, it is better to wait the temperature of the part to normalize with the ambient temperature.  
GageDr (Automotive)
25 Sep 07 9:51
Grant,

Just a thought.

Have you ever considered Temp Comp Gages?  There are quality system that not only check the temp of the part, but also of the gage checking the part, as well as the part master.  This process that is widely used in the aluminum piston manufacturing, as well as the rail road industry.  

Another advantage to temp comp, is that it can be used direct on the manufacturing floor, leaving the cmm available for other projects.

Regards,    
gaging (Mechanical)
29 Sep 07 20:50
I routinely work with calibration labs that calibrate steel gages and masters. The rule of thumb they generally use is to allow masters to "temperature soak" 24 hours in the gage lab before calibration. Keep in mind that masters are generally fairly massive for dimensional stability.

Since liquids transfer heat more rapidly than air, it is common to keep a tank of machine coolant in the calibration room for "temperature soaking" masters more quickly if required. This is often used when lapping masters to size.

I'm sure the time to temperature stabilize your parts will far exceed 10 minutes in air, and probably even in liquid. It will depend on the material and geometry of the part. If there is a lot of surface area and not a lot of mass, the stabilization time will be shorter.

http://netgages.com/

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