# How does this happen, stresses in metal?



## Superburban (Feb 11, 2021)

I watched this a couple of times (Ok, I skipped a lot the other times), He does everything right, even doing most of the cutting at the tail stock with a center. Then suddenly (on the vid that is), everything is way out of round. He says it is due to stresses in the metal.  It does not make sense to me, I think it would be round, or have chatter if there is spots of different hardnesses. What do I know.

You can skip a lot up to the 21 min mark.






He sends it out for heat treating, and is able to save it in part two, still does not make sense. But I never got into the differences of different metals.

Since the video was done in 2020, might explain it all.


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## sycle1 (Feb 11, 2021)

This was a hardened metal bar that the customer supplied that was not heat treated or hardened properly, it does happen.
He resolved it to the customers satisfaction.


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## mmcmdl (Feb 11, 2021)

Material when cut relieves stress which makes the part move , especially in large shaft work . I did a lot of Worthington Pump shafts in the past on the big lathes and cylindrical grinders . We would leave approx .030 on all diameters . I had to lap many of the centers so they would clean up on the grinding operation .


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## SLK001 (Feb 11, 2021)

Cold rolled steel builds up a lot of internal stresses that get relieved by cutting (turning) the metal.  The material should have been heat treated to a dead anneal to prevent the deformations, then heat treated to the required hardness, but even annealed bars can deform.  If you cut a keyway in a piece of O1 tool steel (annealed), the bar will warp.


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## Superburban (Feb 11, 2021)

I guess I am just not grasping it totally. Does this warping/ stress releasing happen while turning, or hours later?


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## mmcmdl (Feb 11, 2021)

Could be both . We had large parts stress relieve while transporting them on a truck to the customer . Stainless is the biggest culprit . If you've ever cooked on a stainless grill it bends to heck and returns to a somewhat original shape . Stress relieving was always an operation in the process after roughing and before finish machining .


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## Superburban (Feb 11, 2021)

Interesting, thanks. 

I guess I never caught anyone talk about as a standard part of making parts.


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## FOMOGO (Feb 11, 2021)

While turning, and shortly after, would seem to be the logical assumption. I'm not a metallurgist, but I find it really interesting. What little I know I've gleaned working with metal over the years is relegated mostly to repairing my own equipment, and that of a few friends, welding, grinding, a little design work to solve problems that I'm too cheap to have someone else do for me. A little actual machining from time to time. Metal certainly does move, and after you've watched inducing heat through welding, and see your nice squared up project go off in all directions, you pretty quickly learn to make the heat work for you when possible, or at least minimize it's negative affects. Annealing, and hardening are an important part of making the metal do what you need it to do, and have it come to the level of accuracy, and wear resistance that you require. All right gentleman, start your lathes, and fire up your your heat treating oven or rosebud torches. Nothing like the smell of sizzling motor oil in the morning. Smells like victory!!! ( blatant  Robert Duvall  ripoff ) OK, no more posting while indulging in evening cocktails and such. Cheers, Mike


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## mmcmdl (Feb 12, 2021)

Superburban said:


> everything is way out of round.


It actually is not out of round , but out of concentricity to other datums . Out of round would be checked with mics or snap gages .


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## matthewsx (Feb 12, 2021)

Superburban said:


> Interesting, thanks.
> 
> I guess I never caught anyone talk about as a standard part of making parts.


This is a very interesting topic and for those who haven't read much on metallurgy it probably sounds like some kind of voodoo black magic. But, it is extremely important for making parts that are both accurate and strong. 

As I understand it when you look at a piece of steel what you're really looking at is a frozen mixture of elements (alloy) which has different types of crystalline structure depending on how it was formed and what heat treatment it received afterwards. Different stresses can form depending on many factors and as with anything under stress it will deform or break given the right circumstances. 

Although I've only read a little and never heat treated anything beyond quenching a hot part in water I'd love to learn more once I actually have the skills to make an accurate part in the first place....

I just finished reading this book:









						Machinery Repairman : US Navy : Free Download, Borrow, and Streaming : Internet Archive
					

Machinery repair and maintenance



					archive.org
				




Which has a pretty good practical guide on heat treatment near the end. Metal seems so solid but like everything else we touch it's just made of atoms arranged in a particular way. 

So much learning, so little time....


John


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## MrWhoopee (Feb 12, 2021)

"Something moved" is the standard excuse the first time an apprentice discovers this phenomenon. A machinist knows to expect it and when, though there can still be surprises. The classic case is removing a lot of material from one side of a piece of flat, particularly cold-rolled material, then being shocked when it curls like a potato chip. Stainless is especially bad and, in my experience, stress-relieving doesn't help.


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## mmcmdl (Feb 12, 2021)

MrWhoopee said:


> Stainless is especially bad and, in my experience, stress-relieving doesn't help.


Sure doesn't , unless it's 15-5 , 17-4 or similar . Machining stainless flat is equivalent to shooting with a bent barrel !


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## Superburban (Feb 12, 2021)

mmcmdl said:


> It actually is not out of round , but out of concentricity to other datums . Out of round would be checked with mics or snap gages .


Then how is it that the ends he turned on the center, is off.I would think turning on a center, would leave it round. Then when he uses the indicator, it moves. to me, that has to be out of round.


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## graham-xrf (Feb 12, 2021)

mmcmdl said:


> Sure doesn't , unless it's 15-5 , 17-4 or similar . Machining stainless flat is equivalent to shooting with a bent barrel !


Allow that I am ignorant of what it takes. It still begs the questions of exactly what is then done to arrive at a flat stainless, or cold rolled, or whatever. Is it a question of "compensating" (extending the "bent barrel" analogy)? Surely we are not driven to machine unwanted features on the wrong side, just to have it "bend back" to counter the first deformation?

This bending force is about the coefficient of thermal, expansion, and the way the metal gets a hold of itself when cooling from a forcibly annealed state. Not melted, but made hot enough that stresses cannot keep a grip, if cooled slowly enough, or taken through some smart temperature cycling, it ends up without internal stresses. Cool it fast enough, so don't give the adjacent metal atoms and crystal planes time to shift, then expect the force trapped to be the mighty Young's Modulus x the amount it would have contracted, but was not allowed to.

Cut a keyway in the shaft, then expect that with  structural force the keyway volume was providing now removed, the rest of the metal will win. The forces are enormous! Turning some diameter off a bar that has cooled quickly would also do it.

Unless taking the metal to a temperature most of us can't manage, stainless, and a whole lot of others actually harden when heated - like my stainless woodburner flue pipe. Others modify the surface where the cutting tool has cut - work hardening, but I am not sure that is also causing a locked in stress that would have it bend.

Can a metal ever end up "hardened" while also being free of internal stresses?

Is there a turning technique, or treatment regime that overcomes the unwanted bending?

Suppose we start with an annealed, unstressed lump, and make the part. Can it be hardened without changing it's dimensions? I do note that many are hardened before a final finish grind, but is that procedure forced?


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## Iceberg86300 (Feb 12, 2021)

Superburban said:


> Then how is it that the ends he turned on the center, is off.I would think turning on a center, would leave it round. Then when he uses the indicator, it moves. to me, that has to be out of round.


I'd probably just screw it up if I tried to explain it, so just check out this poster instead:





__





						GD&T Reference Chart ASME Y14.5-2009 | GD&T Symbols
					

IIGDT GD&T Guide. The GD&T Chart illustrates the geometric symbols referenced in the ASME Y14.5M Dimensioning and Tolerancing.



					iigdt.com
				




Basically, when it comes to metrology things are seldom what they look like on the surface.

But for a long non stepped or lightly stepped shaft, straightness is usually going to be of the greatest concern as well as easiest to visualize b/c it's just a bowed shaft. 

So, if you finish a journal at the tailstock first, then remove all kinds of material elsewhere on a shaft that started with a ton of internal stresses, the journal is still "round" b/c it hasn't been touched. But b/c the shaft is now bowed instead of straight, the journal axis no longer lines up with the axis from Chuck to tailstock & an indicator shows what appears to be runout/out of round.

Hope that helps some!

Regards,

Steve


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## Cadillac (Feb 12, 2021)

I thought it was kind of odd that when he finished turning the first end and flip the part he didn’t try and center in the spindle chuck. 
 He did take a lot of material off and I can see the part moving. He’s pretty good because he did save it.


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## SLK001 (Feb 12, 2021)

graham-xrf said:


> Is there a turning technique, or treatment regime that overcomes the unwanted bending?
> 
> Suppose we start with an annealed, unstressed lump, and make the part. Can it be hardened without changing it's dimensions? I do note that many are hardened before a final finish grind, but is that procedure forced?



The unwanted bending is just that - "unwanted" and you can't control it fully.  As for hardening, it is a process that locks IN the stresses.  The final result of the stresses are not totally predictable, but can be somewhat controlled.  For instance, quenching a alloy piece of round stock by dipping it vertically in the quench tank will result in radial stresses across the entire piece.  This piece can be turned will little distortion.  However, quench that alloy piece by dipping it into the quench tank horizontally will result in a horrible stress pattern across the entire piece.  Now this piece will distort quite a bit from turning.  I suspect that the piece of prehardened that the guy in the video received was horizontally dipped.


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## mmcmdl (Feb 12, 2021)




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## kb58 (Feb 12, 2021)

In telescope mirror making, sometimes a large mirror is damaged to the point that it can no longer be used. To recoup some of the expense, sometimes it will be cut into fourths and sold off as "off-axis" mirrors. Here's the catch: no matter how perfect the original mirror was, once it's cut, the pieces change shape slightly.

Somewhat related, tempered glass is manufactured by rolling semi-melted glass through a chiller that "freezes" the outside surface much faster than the inside. What this does is create very high stress (think of it as wearing a jacket that suddenly shrinks on you). The shrunk outer layer of glass puts enormous stress on the inside, but since its fully encapsulated, it remains intact. But if something breaks that outer "coat", it relieves the stress to the point that the entire volume suddenly shatters.

In both cases, if the material experienced dissimilar cooling, it's going to put bending forces on the material as a whole, but if it's machined or formed to finished shape before you get it, it's not a problem. BUT, if you alter the material by cutting, grinding, or welding, it shifts the internal forces, causing the material to change shape to reduce the stress. Annealing and heat treating, when done right, relieve the internal stresses. It's a big reason why heat treating takes many hours - the point is to have the material have the same temperature even all the way through, and that can only happen if temperature is changed veeeery slowly.


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## Scra99tch (Feb 12, 2021)

Superburban said:


> I guess I am just not grasping it totally. Does this warping/ stress releasing happen while turning, or hours later?


Cast Iron to be seasoned correctly back in the old days, went through hot cold cycles for almost a year.  

Or so I've read on the internet.


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## Ischgl99 (Feb 12, 2021)

Just like any other product, there are good steel producers and there are bad ones.  Kurtis made a comment in that video that he does not buy material from the supplier his customer got that from, and I think we saw why.  

If you watch any videos of Chris Maj, he turns a lot of long slender shafts and will rough an end, rotate, rough the other end, rotate and then sometimes do another roughing cycle before finishing to minimize any stresses in the material from warping the piece after machining.  Some of the shafts he turns takes several days to finish, so the heating and cooling cycles from the machining process might help relieve the internal stresses before he gets to the finishing stage.  

A centrifuge manufacturer I used to work for had all the main rotating components centrifugally cast and then sat outside in the yard for at least a year so they would go through multiple heating and cooling cycles to relieve stresses induced during casting.  After the parts were rough machined, I believe they sat them outside again while the metallurgy department did their tests.  Since these parts spun at 5,000+ rpm, they had to be sure nothing would change after final machining.


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## Mitch Alsup (Feb 12, 2021)

Ischgl99 said:


> If you watch any videos of Chris Maj, he turns a lot of long slender shafts and will rough an end, rotate, rough the other end, rotate and then sometimes do another roughing cycle before finishing to minimize any stresses in the material from warping the piece after machining.  Some of the shafts he turns takes several days to finish, so the heating and cooling cycles from the machining process might help relieve the internal stresses before he gets to the finishing stage.



It generally takes 600ºF+ to get to the point you are doing anything to the crystaline structure.
Machining does not put the part anywhere near these temperatures.



> A centrifuge manufacturer I used to work for had all the main rotating components centrifugally cast and then sat outside in the yard for at least a year so they would go through multiple heating and cooling cycles to relieve stresses induced during casting.  After the parts were rough machined, I believe they sat them outside again while the metallurgy department did their tests.  Since these parts spun at 5,000+ rpm, they had to be sure nothing would change after final machining.



The engine builder for Richard Petty used to take new engine blocks, dig a hole in the back yard of the shop, and put the blocks underground for a few years so the stresses would relieve themselves.

On the other hand, F1 engines are machined out of a forged aluminum billet directly to the finished product. Perhaps the forging process either adds so much stress, or makes it so uniform there is little differential stress ??


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## eugene13 (Feb 12, 2021)

Not to hijack the thread, but, has anyone used cryogenic stress relieving?  I have on a few race motors.


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## Ischgl99 (Feb 12, 2021)

Mitch Alsup said:


> It generally takes 600ºF+ to get to the point you are doing anything to the crystaline structure.
> Machining does not put the part anywhere near these temperatures.
> 
> 
> ...


Aging a part helps with stress relieving, so I thought there might be some benefit giving the part some rest between machine operations in addition to letting it deform from the stresses.  At the centrifuge company I worked at, they called the step between rough and final machining stress relieving, but that could have been a poor translation on my part, as well as a failing memory, my training at the factory was almost 30 years ago now.  My knowledge of metallurgy is limited to what I learned in college and bits and pieces learned during my career, so not much lol.

Forgings can control internal stresses much better than with a casting through the design of the part, and use it to their benefit, but there is still a danger of the part deforming after machining if not done right.  The owner of one of the centrifuge companies we competed against said he would forge everything instead of casting if he was able to, the material properties are so much better, and they could forge it closer to final shape than they could with a casting significantly reducing machining time.


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## Flyinfool (Feb 12, 2021)

This is why the best race motors use a block from a contractors Pickup truck that is worn out and in the bone yard. That block has bee abused every which way and probably overheated a few times and just generally abused. Any warping it is gonna do has already happened.


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## brino (Feb 12, 2021)

kb58 said:


> Annealing and heat treating, when done right, relieve the internal stresses. It's a big reason why heat treating takes many hours - the point is to have the material have the same temperature even all the way through, and that can only happen if temperature is changed veeeery slowly.



Excellent, thanks for posting!
-brino


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## brino (Feb 12, 2021)

Mitch Alsup said:


> It generally takes 600ºF+ to get to the point you are doing anything to the crystaline structure.
> Machining does not put the part anywhere near these temperatures.





Mitch Alsup said:


> The engine builder for Richard Petty used to take new engine blocks, dig a hole in the back yard of the shop, and put the blocks underground for a few years so the stresses would relieve themselves.
> 
> On the other hand, F1 engines are machined out of a forged aluminum billet directly to the finished product. Perhaps the forging process either adds so much stress, or makes it so uniform there is little differential stress ??




So you kinda agree that you do NOT need to hit the high temperatures to have some stress relief?
.......at least if you agree with Richard Petty.

Aluminum would be a totally different beast, I think.....it's melting temp is so much lower, it's atomic structure is so different....

brino


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## brino (Feb 12, 2021)

Ischgl99 said:


> centrifuge manufacturer I used to work for had all the main rotating components centrifugally cast and then sat outside in the yard for at least a year so they would go through multiple heating and cooling cycles to relieve stresses induced during casting. After the parts were rough machined, I believe they sat them outside again while the metallurgy department did their tests. Since these parts spun at 5,000+ rpm, they had to be sure nothing would change after final machining.



Great info, thanks for posting!

-brino


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## Weldingrod1 (Feb 12, 2021)

For steel you need to get really hot for true stress relief.
You may be able to reduce motion a tiny with a run up to as hot as you can manage in your home oven.

One way to think about what's going on is to imagine coating something rubber with epoxy, stretching a balloon over it, and letting the glue set. If you cut the balloon off one side, the tension in the other side will drag it out of straight.

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## hman (Feb 12, 2021)

Ischgl99 said:


> A centrifuge manufacturer I used to work for had all the main rotating components centrifugally cast and then sat outside in the yard for at least a year so they would go through multiple heating and cooling cycles to relieve stresses induced during casting.  After the parts were rough machined, I believe they sat them outside again while the metallurgy department did their tests.  Since these parts spun at 5,000+ rpm, they had to be sure nothing would change after final machining.


Slightly off topic ... usedta was that, among manufacturers of mechanical vacuum pumps, Welch had the very best reputation ... because they let the castings "age" in their yard for a year before machining.  It's just too bad that nowadays, time and unfinished inventory costs prohibit such a practice


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## Ischgl99 (Feb 12, 2021)

hman said:


> Slightly off topic ... usedta was that, among manufacturers of mechanical vacuum pumps, Welch had the very best reputation ... because they let the castings "age" in their yard for a year before machining.  It's just too bad that nowadays, time and unfinished inventory costs prohibit such a practice


It's been almost 20 years since I worked for them, so not sure if they still do that or not.  I just looked at a satellite map of their factory and where they used to age them is now a production building.


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## mmcmdl (Feb 12, 2021)

We did the folding wheels for Maryland Cup years back . Very large round aluminum castings with flats milled around the entire surface . .001 tolerance . These were machined and inspected on the Devleigs before removing . The process was to rough them to size , then send them out for relieving which was a vibratory type operation . I guess they were basicly shook to death . Then were finish machined . We had quite a few come back after final inspection from MC because they relieved during the delivery ride to their factory .


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## Flyinfool (Feb 14, 2021)

Vibration can be ued for stress relieving, When I worked at a heavy fabricator they made things that there was no oven big enough to hold. They had a company come in and mount transducers (?) all over the place and shake the stress out of it before final machining. The whole city block shook when they were doing this.


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## Weldingrod1 (Mar 2, 2021)

Fun demo! I'm cutting a 45 degree bevel on 7/4" x 4" cold rolled steel, the poster child for locking in stresses. I'm about 18" into a 4' cut here. That gap is nearly 3/4" now!
	

	
	
		
		

		
			








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## Weldingrod1 (Mar 3, 2021)

Ended up with a 1.5" gap right before I cut it off! Amazing motion!

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## Superburban (Mar 3, 2021)

That is surprising.


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