Temperature a factor?

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Mr Ron
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Temperature a factor?

Post by Mr Ron »

When turning or milling, heat is generated in the work piece. How much does temperature affect dimensions? I know it makes a difference when machining to very tight tolerances (gauge block size changes by hand temperature), but for everyday machining when tolerances are ±.001", is temperature an important factor? To put it another way, does the dimension change after the material cools down?
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whateg0
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Re: Temperature a factor?

Post by whateg0 »

The temperature coefficient of steel is between 6-7 x 10^-6 in/(in*F), so if the temperature of a 3" diameter part rises by 100F, it would be ~0.002" bigger than before. Some materials, like aluminum, will change more and others will change less.

I use this site for a lot of data like that.
https://www.engineeringtoolbox.com/line ... -d_95.html
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liveaboard
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Re: Temperature a factor?

Post by liveaboard »

Yes; even in my simple work, temperature affects the size of the workpiece significantly for things like interference fits for bearings or pivot pins.
I've learned to let it cool down before final cut and polish. Just being hot to the touch can make enough error to ruin a job.

Of course a bore will shrink when it cools so you get a chance to correct the error.
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Bill Shields
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Re: Temperature a factor?

Post by Bill Shields »

Unless you are working invar 😀
Too many things going on to bother listing them.
Rich_Carlstedt
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Re: Temperature a factor?

Post by Rich_Carlstedt »

Used it at work in the die shop all the time,
You want to remember one phrase
" Five Ball 63" Memorize that !- say it over and over
The word Ball means the number of Zero's after the decimal point
That is the expansion rate of Steel per degree F per inch !
And it's related to your Mike temperature ( which is normally room temp ..say 68 degrees)
So you are cutting a chunk of round steel and it is 200 degrees and you measure it at 2.501
What is it's real size ?
Well 200 is 132 degrees over the mike temp ( 68) and the part is 2 1/2 " in diameter
So 132 x 2.5 x .0000063= 0.002079 so that means 2.501-.002079= 2.4989 " at room temp ( Micrometer Temp)

See how that works ? It is simple if you remember 5Ball 63 ,,
For Aluminum, its 4Ball 11
For Brass, it's 5 Ball 99

Hope this helps
We were always working with interference fits for multiple different metals so I Trained our machinists in using their calculators

Rich
Mr Ron
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Re: Temperature a factor?

Post by Mr Ron »

How did machinists deal with temperature when working on press fits. Did they wait for the temperature to drop before the final fit was made? During wartime machining, time was at a premium. How much could a machinist wait for temperature to normalize before continuing? I would assume he would be working on multiple pieces and would allow time for the piece to cool down while he was machining another part. Once cooled down, he could get back to finishing that piece. If that was the case, it would require at least 2 lathes (or mills); one cooling down while the other was working so as not to tear down a setup.
Mr.Ron from South Mississippi
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Harold_V
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Re: Temperature a factor?

Post by Harold_V »

Mr Ron wrote: Mon Jul 11, 2022 12:24 am How did machinists deal with temperature when working on press fits.
Can you say "coolant"?

The home shop rarely emulates industry, so it might be hard for the home taught machinists to understand how things may be done.

Exterior round precision fits are (were) often ground, where holding tight tolerances is a breeze and temperature plays only a minor role (thanks to coolant).

It is also relatively common for parts to be rough machined before finishing (which is good shop practice). The finishing operation generally doesn't heat the part much.

Tight work always requires an observation of temperature. Inspection labs are not only temperature controlled, but humidity controlled as well.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
Mr Ron
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Re: Temperature a factor?

Post by Mr Ron »

Harold_V wrote: Mon Jul 11, 2022 12:48 am
Mr Ron wrote: Mon Jul 11, 2022 12:24 am How did machinists deal with temperature when working on press fits.
Can you say "coolant"?

The home shop rarely emulates industry, so it might be hard for the home taught machinists to understand how things may be done.

Exterior round precision fits are (were) often ground, where holding tight tolerances is a breeze and temperature plays only a minor role (thanks to coolant).

It is also relatively common for parts to be rough machined before finishing (which is good shop practice). The finishing operation generally doesn't heat the part much.

Tight work always requires an observation of temperature. Inspection labs are not only temperature controlled, but humidity controlled as well.

H
Thank you, Harold. I forget about coolant. I certainly have a lot to learn and it's people like you that make it possible for us amateurs to learn a trick or two.
Mr.Ron from South Mississippi
Rich_Carlstedt
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Re: Temperature a factor?

Post by Rich_Carlstedt »

Mr Ron
We held Very close dimensions in our shop and it was not air conditioned and we had no inspection lab or inspectors
Our machinists were highly skilled and never trusted one source of measurement and we did not use grinders,
Our round die parts went from 4 inches to 96 inches and if you want to have fun, try to hold +/- .001 on a 96 inch diameter part (12,000 # )
Harolds comment about coolant is exactly rght on and there are other methods. Measuring temp when checking size is critical
and notice I did not say room temperature when I pointed out the formula....room temp means nothing..It is the part temp and the measuring instruments temp that is critical . One example is when My machinists were cutting a critical press fit, when done, they would place the Mike on the work piece and cover it with a thrermal blanket and go get a cup of coffee. Then when returning the mike was the same temp and they could measure. We also did not trust one mike and another technique is using a Pi Tape , which is extremely accurate as the tape immediately matches the temperature of the part and it can do ID and OD measurements. Another technique is to do all part turning
except critical until the following morning when the part has stabilized in temp so measurement can be done. Another technique.
is to have ALL lathe chucks measured and stamped with OD size. and if the chuck runs out, take a finishing pass on the OD before stamping. Now the chucks warm up just like the work piece and a machinist can using his indicator measure the chuck OD and the part OD and his DRO will give him a reading so he can calculate the OD.. Lots of tricks in getting super precise sizes without expensive building construction. But always pay attention to temperature !
Rich

https://www.pitape.com/
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Harold_V
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Re: Temperature a factor?

Post by Harold_V »

Yep! We used to use a Pi tape to measure the finished diameter of the bulkhead rings for the Sergeant Missile, which I recall to be 32" (this was back in the late 50's, early 60's. My memory has grown dim) in diameter, with ± .005" tolerance. They were magnesium castings. Shop was not air conditioned.

H
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liveaboard
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Re: Temperature a factor?

Post by liveaboard »

Back in the home workshop...
I rarely use coolant because my lathe isn't set up to drain, and I'm just a DIY amateur.
So if I need accuracy for an interference fit, I let the work stand for a while.
Sometimes I unscrew the chuck with the work in it and screw on another chuck to do other work while I wait.
Rich_Carlstedt
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Re: Temperature a factor?

Post by Rich_Carlstedt »

liveaboard wrote: Tue Jul 12, 2022 1:57 am..............................
just a DIY amateur. So if I need accuracy for an interference fit, I let the work stand for a while.
Sometimes I unscrew the chuck with the work in it and screw on another chuck to do other work while I wait.
Well Liveaboard , you may call yourself an amateur, but you are using professional techniques !
Congratulations !

Rich
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