Sciencey question

[khc3]

Okay, help settle a minor dispute, if only in my own mind. Bonus points if you are an actual engineer.

Suppose someone tells you that in the process of machining a bore in a part, let’s say a gun barrel, to final dimension, heat is created, causing the material (in this case, steel) to expand.

When the part has cooled, will the bore be larger or smaller than while it was still hot?

I am not an engineer, but I would think the material expands in all directions when heated, and contracts when cooled, making the bore larger.

Sorry if this is a stupid question.

ETA: The machining operation just creates the bore, not the rifling, that is a subsequent operation.

[rero360]

Not an engineer (yet) but I would think that proper cutting operations is not going to generate enough heat that would cause enough thermal expansion to make a difference. 7.2x10^-6 inches for every degree of temperature change for steel

[khc3]

Ok, lets say it is an abrasive cutting (honing) operation that makes the od of the part somewhat hot to the touch. And we are measuring to half a tenth.

[jpmuscle]

If left to cool on its own wouldn’t the change be zero?


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[GH41]

I once read that many post to Dear Abby were generated by the incarated. Are you incarcerated? Why are you asking the question???

[khc3]

Then again, we shrink fit gears and bearings on shafts by cooling the shaft with liquid nitrogen and heating the otherparts. Guess I am wrong. Seems counterintuitive though.

[rero360]

Ok, lets say it is an abrasive cutting (honing) operation that makes the od of the part somewhat hot to the touch. And we are measuring to half a tenth.

A 150 degree F change will result in a thousandth of an inch, so negligible.

[khc3]

I once read that many post to Dear Abby were generated by the incarated. Are you incarcerated? Why are you asking the question???

I’m asking because this very question occurred at work. After thinking about, I realized that from a physics standpoint, I didn’t know the answer.

Already apologized if you find it stupid.

[khc3]

If left to cool on its own wouldn’t the change be zero?


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Should have clarified the change is from measuring while still hot to when cool.

[khc3]

A 150 degree F change will result in a thousandth of an inch, so negligible.
We’re measuring a half of a ten-thousandth of an inch. The change would be from ambient temp, 73 degrees, to something around very hot tap water, maybe 140 degrees, on the outside of 1.063” material.

[rero360]

We’re measuring a half of a ten-thousandth of an inch. The change would be from ambient temp, 73 degrees, to something around very hot tap water, maybe 140 degrees, on the outside of 1.063” material.

Well the temp inside will be roughly the same, and since the expansion is in all directions, with that temp change you’re looking at 0.00025” of change max in the radius

[thopkins22]

The physics are that any two parts of an object being heated will be farther apart than when they are cooled, provided they are not restrained by an external force.

So while it expands in all directions, if you made a mark on both ends of the diameter of the bore, while they do expand in all directions, they still have to be farther apart.


I’m not an engineer by trade, but by degree.

The amount of heat generated will have an effect, because any amount of heat has an effect. Blowing warm breath on it has an effect. It’s almost certain that the impact is so utterly minuscule that I can’t imagine bothering to measure it. But it will definitely be smaller provided the barrel steel warmed during the process.


https://physics.info/expansion/summary.shtml

Edit:the part none of these answers are considering is any thermal expansion in the cutting tool itself...which will no doubt nullify some of the expansion of the barrel steel but probably at a different rate. Once again, at an irrelevant level.

[rero360]

The physics are that any two parts of an object being heated will be farther apart than when they are cooled, provided they are not restrained by an external force.

So while it expands in all directions, if you made a mark on both ends of the diameter of the bore, while they do expand in all directions, they still have to be farther apart.


I’m not an engineer by trade, but by degree.

The amount of heat generated will have an effect, because any amount of heat has an effect. Blowing warm breath on it has an effect. It’s almost certain that the impact is so utterly minuscule that I can’t imagine bothering to measure it. But it will definitely be smaller provided the barrel steel warmed during the process.


https://physics.info/expansion/summary.shtml

Edit:the part none of these answers are considering is any thermal expansion in the cutting tool itself...which will no doubt nullify some of the expansion of the barrel steel but probably at a different rate. Once again, at an irrelevant level.

Yeah, I didn’t bother dealing with cutting tool expansion, most likely difficult coefficient of expansion but still negligible with the range of temperatures we’re looking at.

[Dr. Bullseye]

I have an engineering degree in common sense. Of course the barrel is going to expand but it is going to expand and then contract in all direction BUT the engineers who design these things take thermal expansion into account and know exactly what they are doing. They have formulas for this. If this were easy, we would all be cutting our own barrels.

[elephant]

The idea is to drill a hole using lubricant/coolant to keep temperature constant. Generally speaking, (from a machine shop owner) you don't want to increase the spindle speed or feed speed to a point where temperature changes. Same goes for grinding and honing. If heat is generated during machining, your either running too high speed or too high feed. Heat not only effects your parts but also your tooling. So, in other words, if you generate heat, you doing it wrong!

[GH41]

I’m asking because this very question occurred at work. After thinking about, I realized that from a physics standpoint, I didn’t know the answer.

Already apologized if you find it stupid.

I don't find the question stupid at all. I just didn't think you would get an answer other than the answer you should already know. Sorry if my Dear Abby reference offended you. Your example was a rifle barrel. The answer would be the opposite if you were boring small hole in a massive chunk of steel.

[Failure2Stop]

The idea is to drill a hole using lubricant/coolant to keep temperature constant. Generally speaking, (from a machine shop owner) you don't want to increase the spindle speed or feed speed to a point where temperature changes. Same goes for grinding and honing. If heat is generated during machining, your either running too high speed or too high feed. Heat not only effects your parts but also your tooling. So, in other words, if you generate heat, you doing it wrong!

This is the correct answer.

[MegademiC]

Should have clarified the change is from measuring while still hot to when cool.

If a cylinder bore is measured hot then cold, the cold dimension will be smaller.

We shrink fit parts here- heat up the cylender, slide it on, and when cool, its tight.

From a practical machining perspective, see elephants post. When machining parts, they flood the work with lube/coolant.

[markm]

Anyone ever seent the movie "Heat"?

[thopkins22]

From a practical machining perspective, see elephants post. When machining parts, they flood the work with lube/coolant.

I think that’s the thing. Even with coolant there is going to be temperature variation, and to some degree it will be in the workpiece.

I don’t know how effective their machines are at cooling, nor do I know diddly squat about manufacturing. I do know physics. And any variation whatsoever in temperature, will have an effect on it...even if it is 1/1000 of a degree warmer(or colder or whatever.) And there will be some heat entering the barrel steel, even if it is almost immediately transferred to coolant.

I don’t think he was really concerned about the practical variance in bore diameters, or how machine shops actually do this so that things meet specs at the end of the day, but rather he had a physics debate with a friend/coworker.

[ramairthree]

This question has shown up on the first quiz of the first semester of the first year of physics with calc since the first time a course in physics in college was offered.

The answer is it depends.
What are you making the barrel out of?
There is a vastly most likely answer to this, but that would be assuming thermal expansion principle of the material you are making the barrel out of.

Tell me you are not making it out of, say zirconium tungstate, and trying to trip me up by making an assumption.

I am not an engineer-
But I made Calc, Phsyics, Bio, Chem, Organic Chem, P Chem, etc. my bitch decades ago.