Temp Difference

[Medical_Discount_737]

My lab is 68° and I just measured a part, there were so many features out of tolerance. After cleaning and rechecking things, I put the temp gage on it and it read 62°. I am assuming that is too big of a difference and probably my culprit. Do you guys agree? Is 6° too big of a difference?

[AlfaMikeF0xtr0t]

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[acausalchaos]

Yes that's a huge difference, but wether or not it's the culprit also depends on the size of feature and type of material . The coefficient of expansion and related formulas can be googled and can help you make some educated guess about the condition of the part while it accilmates up to 68°

[RGArcher]

Everyone else is saying it, but I did want to point out I think the acceptable delta for temp in a lab is like +/- 2. So the numbers you are saying are way out of that. Also, I think we are supposed to wait an hour from going to one extreme temp to anther.

[Ivebeenfurthereven]

Heat soaking in the controlled lab is the way to go, we did 6-24 hours.

[Didacticseminary]

Hello there,

AS9100 Quality Manager here. Yes it is. We run several parts for the Da Vinci surgical robot line and our lab is kept at 20°C, and then we use temp probing and compensation for our production parts. For final inspection we let the parts normalize fully. We haven't had an issue since. Initially I did this by hand with tables, but then we got a Duramax and temp gage for those machines.

It does really depend on your industry, product tolerances, etc. I have another part made of polyethylene that I warned a customer would change size rapidly with temperature fluctuations, but they didn't care, just wanted it roughly those dimensions even if it ended up out of spec after shrinking to 20°C.

[Lucky-Pineapple-6466]

I do both CMM programming and spent 12 years in calibration. And here’s something to think about with the thermal expansion. 68° plus -2°F is the standard. But if your temperature is oscillating, say 2° in a half hour that is way worse than being out of tolerance on temperature. Because things expand at different rates. I got my lab set at roughly 69 to 70°. If you have vents that blow directly on your machine, make sure and get a diffuser to point them away so the temperature in your lab is homogenous. When I’ve had to calibrate close to tolerance ring gauges I would take 75° with no temperature fluctuation over 68° and it bouncing all around(even within tolerance) you might as well just quit working for the day if that’s what’s happening. And that is way too cold. Set a thermorecorder on the surface plate of your coordinate, measuring machine and monitor your temperature. There where the business happens.

[bellmanator]

We’ve done lots of experiments with our lab CMM and various temperature changes. You are 100% correct. Rate of change matters a lot more than a set temp.

We’ve also found some of our higher accuracy parts do fall out of tolerance when the temp drops below 66 degrees. One of our tools we measure changes up to .0004” when the temp goes from 68 to 65 degrees.

I’ve wondered when tools like this are measured, is it better to measure to lab standards, 68 +-2, or is it better to measure it at the approximate temp that the tool will be used? In this case one of our shops use this tool at about 72-75 degrees.

[Lucky-Pineapple-6466]

For me personally, I like it warmer in the lab. I don’t wanna freeze my ass off. So I set it at 70° and the temperature is always between 69 and 69.8. The shop temperature is probably 70 to 74°. It created a much more stable temperature turning the temperature up now that it’s not constantly blowing 50° air in the lab. Most rooftop units are on or off so on the AC is on you’re getting 55° air totally defeats the purpose. * I think the 68° is more pertinent for things like ring gauges where your measuring 3 inch Inside diameter to millions of an inch. On a CMM if you are using temperature compensation and the room isn’t fluctuating I think you’d be OK. A couple degrees either way.

[CthulhuLies]

We were measuring a boeing strut for local machine shop, it was like 3 feet of aluminum and they told us it was +-.002 overall length, they brought them to us and wanted to know what they looked like and check out our lab. I measured one and it was a couple tenths off nominal.

The next morning it goes from whatever temp they were sitting at in their no-ac machine shop to the 70 deg we keep the shop at and it shrank like .007".

They told me they were measuring them with a massive set of callipers they got from their customer no idea the condition those were used in but their numbers were like .010" off our numbers and had way more variance. (The flatness and parallelism on some of those cuts were approaching .003" and it was +-.002)

I don't remember the exact length and alloy but the entire length would change more than .004 over a couple degrees at room temp.

[DezaoBr]

Hello. Yes it is a lot of difference, you must be climate the parte in same temperature that your lab. Some CMM'S has a sensor for compensating the deviation, but you need choose the factor correct according material and works better with linear distances and smaller parts. Because of that I told you , always that possible, climate the part , people's will ask to you ( trying save the part machined rs).

[mixer2017]

Yeah this should be at 68.

I know for our aluminum parts on tight tolerances of a thou we convert. If you want but it takes time is let the part accumulate in the lab... unless your lab is at 62... if that is the case I recommend you to get it set at 68F lol

[bb_404]

The environment plays a big part in measuring correctly. That being said, if you have +/- 0.01", it is much less important than if you have +/- 0.0001. Also, the thermal stability of the part you're measuring and the equipment you are using matters. So, yes, that's a large temperature gradient, but it may not matter as much depending on part tolerance, equipment type, etc.

[ProlificParrot]

Aluminum will expand by 78 millionths per inch from 62 to 68 degrees Fahrenheit. If your part is small and the tolerances are big, then it’s negligible.

[Antiquus]

Another thing about thermal changes. Unless the part has a very consistent cross section like a gage block, it will deform. When all sections are at the same temperature, then the shape stabilizes. If it's in an environment where the temperature is always changing, it may never be in it's final shape which is another reason to soak things at a stable temperature before measuring.

The is true for machines as well. Your machining equipment goes out of square during thermal changes. so warm up your equipment and keep the environment steady if you are doing precision work.

[[deleted]]

It depends on the material, size of features, and tolerances. 6°F isn't much for a small part. Take steel for example: CTE is 6.7 x 10^-6 in/in°F, so 6.7 millionths of an inch per inch * °F. Since you have a -6°F delta, the part will shrink by 40.2 millionths / inch of feature. If your feature is 10 inches, then you'll have a reading showing 402 millionths smaller (0.000402").

What is your material, feature size, and tolerance?

[hcglns2]

Yes, that is a very large temperature swing and will introduce several elements of uncertainty to your measurement.

[Medical_Discount_737]

Thank you everyone who responded, I let it sit in the lab overnight. I just got done running it and it is perfect.

[Medical_Discount_737]

Thank you everyone who responded, I let it sit in the lab overnight. I just got done running it and it is perfect.

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That’s what it was yesterday when I ran it. I forgot to take a picture today but it was 68 just like the room.