Flash cooling part to 68 Degrees for precision measurements

manofsteel1210 · May 24, 2017

We are machining some large aluminum castings with high precision bores (.0006" total tolerance) in a facility that is not temperature controlled. The temperature of the facility and coolant can reach temperatures of 90 degrees on a hot day. Due to the high coefficient of thermal expansion of aluminum and large bore diameters (approx 5") we can see quite a change from when we machine the bore to normalizing the part to take a measurement at 68 degrees. Due to the large size of the part it requires at least 12 hours to normalize when placed in an air conditioned environment. We are experimenting with dunking the part in an ice bath to reduce the temperature rapidly and take a reading for quicker feedback rather than waiting for the part to normalize for 12 hours. Has anyone ever tried this method? Are there any issues with doing this? It seems to work fairly well.

The real Leigh · May 24, 2017

So how do you equalize the temperature through the part cross-section?

Such a part would have a large thermal mass.
Cooling the surface does not cool the interior. It only distorts it.

- Leigh

Derek Smalls · May 24, 2017

Seems like it be a better idea to just compensate for temp by calculating what the expansion will be at the given temp. That's what I've always done on larger parts. Unless you've got some crazy geometry involved within a particular part, things usually expand pretty uniformly, and size change is easily predicted and accounted for. In fact, if you do have some crazy part shape that you are worried may not grow uniformly, I'd think that would make your way even more wonky.
Maybe I'm missing something, but like Leigh asked, how do you know what temp the inside of the part is at, and what affect the varying temp throughout it causes?

The real Leigh · May 24, 2017

One meaningful experiment would be to measure the part on the machine at working temperature.
Then repeat the measurement after 6 hours and again after 12 hours.
Measure the surface temperature each time.

Then graph the measurement and temperature at each sample time.
You could even do this on 2- or 3-hour intervals.

- Leigh

Ziggy2 · May 24, 2017

This is an application that screams for temperature control of the coolant. Don't need to run at 70F but at least maintain ambient. Everything will go better for you and the part will not run the risk of distorting.

DMF_TomB · May 25, 2017

Ziggy2 said:
This is an application that screams for temperature control of the coolant. Don't need to run at 70F but at least maintain ambient. Everything will go better for you and the part will not run the risk of distorting.

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i use 2000 gallon 70 degree temperature controlled coolant system. i got jobs where I shim a part 6 feet long .0003" higher on one ends and a program sprays coolant on one end and evaporating coolant has a chilling or cooling effect from the evaporation.
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part on one end goes down .0003" +/-.0001" every time. sure coolant temperature control helps but you still get some temperature problems.

IronReb · May 25, 2017

DMF_TomB said:
.
i use 2000 gallon 70 degree temperature controlled coolant system. i got jobs where I shim a part 6 feet long .0003" higher on one ends and a program sprays coolant on one end and evaporating coolant has a chilling or cooling effect from the evaporation.
.
part on one end goes down .0003" +/-.0001" every time. sure coolant temperature control helps but you still get some temperature problems.

Imagine the problems you would have if your coolant was running ambient temp of 94F.

There are many of us here that start our day at high 70s and finish the day at 98F. Hitting a bearing fit is done everyday in none climate controlled shops and expensive parts get chunked in the garbage because of 30 degree difference in the part at time of measuring.

With a 5" bore its obvious you don't have the luxury to do some trial and error, you cant screw this up and go big on the ID, going small means rework.
Your only and best solution is as mentioned above, by Derek post #3

DMF_TomB · May 25, 2017

IronReb said:
Imagine the problems you would have if your coolant was running ambient temp of 94F.

There are many of us here that start our day at high 70s and finish the day at 98F. Hitting a bearing fit is done everyday in none climate controlled shops and expensive parts get chunked in the garbage because of 30 degree difference in the part at time of measuring.

With a 5" bore its obvious you don't have the luxury to do some trial and error, you cant screw this up and go big on the ID, going small means rework.
Your only and best solution is as mentioned above, by Derek post #3

.
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you can calculate difference in metal expansion but values are approx not exact.
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reminds me of salesman who sold my old company a precision tape measure guaranteed to not change length with temperature changes. $30,000 made of invar metal
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trouble is everything you measure expands and contracts with temperature changes. it actually is better to use a steel tape measure to measure steel parts and a aluminum tape measure to measure aluminum. basically measuring tools expand and contract with temperature too. many a inside mic i have measured .001" off as i had held it in my hand too long.
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most reliable way is to change part temperature, didnt say it would be easy but its most reliable way.
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i have often seen whole buildings change length and floor height as temperature changed. even opening a outside door letting in cold air causes problems. i have even see sunlight from a ceiling skylight window cause problems

gustafson · May 25, 2017

If you had a nice round bar with a nice round bore I would think doing the math would be pretty repeatable.

Everything that deviates from that is going to make math less reliable.

I would try to keep the part as close to 68 degrees as is practical, it seems like temp controlling the coolant might be a good way to try.

You get a 30 degree temp difference on an asymetric part I just don't know how you would keep measurements consistent.

Paolo_MD · May 25, 2017

Assuming we're discussing producing at least small runs of a specific part, I would suggest to pick a couple of them with measurements closest to the extremes at 68° and put them in a more or less controlled environment with temperature close to 90°. Repeat the measurements at that temperature and use that hot controlled temperature environment to measure the other parts based upon the hot measurements of your samples: if the measurements are in between, you're surely within tolerances. If they're very close to the limits, they'd be better verified at 68°.
This way you cut down on the parts that need to be checked at 68°, unless you need to provide a full measuring report with each given part.

I suspect that the investment in a temperature-controlled coolant tank would pay itself in a rather short time, since it would minimize the differences between parts machined early in the morning and the ones machined at the peak of the heat.

Paolo

The real Leigh · May 25, 2017

On the subject of temperature...

What is the operating temperature of the final complete product in normal use?

How is that reflected in your production measurement of the parts?

- Leigh

Chip Chester · May 25, 2017

And start with 68-degree stock. Even if that means building a pink/blue foam shack with a window air conditioner in it. Then the 'shock cool' doesn't have to penetrate to the core; it's already at temp.
And even it you can't AC the whole production line, you can get cheap ducted roll-around ACs and condition the machine enclosures. Starting with at-temp stock, and machining in an at-temp enclosure with at-temp coolant just leaves heat gain from stock removal process. A few passes should pinpoint that, and allow you to adjust up-stream so it's al dente when complete.

Chip

manofsteel1210 · Jun 1, 2017

Thank you for all the responses. We are investing in a coolant chiller as suggested several times in the posts above. This should help the problem. Since posting this I have done extensive research and found numerous instances where companies "soak" the part in an air conditioned environment to cool to 68 Degrees but nothing where anyone actually soaks the part in water to cool it. If anyone has tried this method or knows someone who has, I would love to hear feedback on this. The reason of using water would be to speed up the cooling time and get quicker feedback. Also someone mentioned keeping the stock cold, I would question if this is necessary or would you get the same effect by flushing it in the machine with chilled coolant before proceeding with the machining.

manofsteel1210 · Jun 1, 2017

Unfortunately we are a job shop so we have little to no information on the temperature at final use. We are really only concerned with providing a part the measures to print at 68 Degrees per the standard.

The real Leigh · Jun 1, 2017

Chilling is no better than heating.

You need the part to be at 68 degrees uniformly. That requires removing heat during the fabrication activity to prevent the part from heating in the first place.

- Leigh

CarbideBob · Jun 1, 2017

Take a part.
Check at shop temp and record the numbers.
Let it soak in the inspection room for 3-4 days and measure it.
Take it out back out in the shop, let soak, measure again. Try at some different temps.
Go through the numbers and build a calibration offset accounting for your part and your gage temps as both grow but differently.
You end up with a comp chart or spreadsheet. You can get well inside most gages capability this way.

I once ran a grinder with decent sized aluminum core wheels CBN wheels in a non-air-condition shop.
Being a number crunching guy bored to tears I found that I could calculate the shop air temp rise during the day from the offsets I put into the grinder as the wheel core grew.
Bob

CalG · Jun 2, 2017

There is NO SUCH THING AS FLASH COOLING when it comes to metrology.

Measurements need to be made within "agreed upon" conditions.

The real Leigh · Jun 2, 2017

CalG said:
There is NO SUCH THING AS FLASH COOLING when it comes to metrology.

The part being measured must have uniform temperature all the way through.

That would certainly invalidate any rapid change of surface temperature.

- Leigh

ortho · Jun 22, 2017

"Maybe I'm missing something, but like Leigh asked, how do you know what temp the inside of the part is at, and what affect the varying temp throughout it causes?" (Post #3)

I agree with what Smalls has to say. What temperature (hot or cold) is the part soaked at? One of the key words is "soak".

offyman1 · Jun 22, 2017

Machine an aluminum plug at the morning temp. Might take a couple of days so doesn't heat up. You can then measure it in the pm to get the warm size.

Flash cooling part to 68 Degrees for precision measurements

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