Metallurgy - What happened to this chuck jaw?

Mud · Apr 5, 2019

Odd moment of the week - This hard jaw broke while clamped on a finished part, AFTER the cycle was finished. Spindle was off, operator touched the part to inspect it and the part fell into the chip conveyor.

It was holding a 8" dia part on the OD. I don't see any signs of fretting or external cracking. It's an older jaw used on a Pratt Burnerd front acting hydraulic chuck on a Cinturn so I'm pretty sure it's a USA or UK made jaw. There aren't any ID marks on the jaws though. It tests at 51-54 Rc hardness.

So - WTH?

Mechanola · Apr 5, 2019

Didn’t get annealed after the hardening probably, or too little or too quickly. There are weakening or stress indentations and it looks to me that the rupture started at the screw bore. Maybe chips squashed between the screw head and the bore

guest · Apr 5, 2019

Mud said:
Odd moment of the week - This hard jaw broke while clamped on a finished part, AFTER the cycle was finished. Spindle was off, operator touched the part to inspect it and the part fell into the chip conveyor.

That was lucky.

It just fatigued out. Years of clamping and unclamping, vibration, out of balanced parts. Started at that bolt hole, probably a little nick- and propagated out. I'm surprised it didn't cut loose when the chuck was clamped.

Ember · Apr 5, 2019

guest said:
That was lucky.

Agreed, could have gone flying :eek:

norb · Apr 5, 2019

Ember said:
Agreed, could have gone flying

I like your avatar,made me laugh.

MichaelP · Apr 5, 2019

I told you not to hire the Superman as an operator.

jim rozen · Apr 5, 2019

Would be interesting to magnaflux the remaining jaws.

My guess from the photo of the failed one is the fracture had been
present for a while before it propogated all the way through. Note the
semi-circular line on the failed one.

thomj · Apr 5, 2019

jim rozen said:
Would be interesting to magnaflux the remaining jaws.

My guess from the photo of the failed one is the fracture had been
present for a while before it propogated all the way through. Note the
semi-circular line on the failed one.

Magnaflux or just replace whole set.

I was wondering if the right side circular patch had already been partially fractured. I am just speculating because of the color distortion. Probably been crashed a time or two...

Thom j

gbent · Apr 5, 2019

The fracture started at the edge of the jaw, probably from an impact. You can see three distinct beach or fingernail marks where the fracture progressed as a distinct event. The jaw is a thru hardening steel that favors rapid fracture propagation. The grain size from the heat treatment looks good.

For anyone in a situation where forensic examination of a fracture surface is of value, never allow the fracture surfaces to be fit together.

CountryBoy19 · Apr 5, 2019

Those are good pics of the failure. There is a lot that can be learned from them. Unfortunately, I haven't kept up on that and it's been over a decade since I learned all that stuff in college so I would have to find my text book and do some studying.

rklopp · Apr 5, 2019

Yep, looks like a fatigue crack grew from the corner. There may have been a sharp machining mark or corrosion pit there. No need for impact. I am surprised at how small the fatigue crack apparently was before the rest of the crack grew all in one go. That, plus the smooth nature of the final crack, suggest the jaw was brittle, possibly improperly heat-treated. Trash the other jaws.

Metalo · Apr 5, 2019

gbent said:
The fracture started at the edge of the jaw, probably from an impact. You can see three distinct beach or fingernail marks where the fracture progressed as a distinct event. The jaw is a thru hardening steel that favors rapid fracture propagation. The grain size from the heat treatment looks good.

For anyone in a situation where forensic examination of a fracture surface is of value, never allow the fracture surfaces to be fit together.

This is correct. The beach marks (crack arrest fronts) are where the fatigue fracture stopped for an extended period of time. In between them are striations which are the distinct events but you can't see them without a scanning electron microscope.

Cole2534 · Apr 5, 2019

gbent said:
The fracture started at the edge of the jaw, probably from an impact. You can see three distinct beach or fingernail marks where the fracture progressed as a distinct event. The jaw is a thru hardening steel that favors rapid fracture propagation. The grain size from the heat treatment looks good.

For anyone in a situation where forensic examination of a fracture surface is of value, never allow the fracture surfaces to be fit together.

Interesting, as that's the first thing I've always done...

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BoxcarPete · Apr 5, 2019

Nobody has addressed the real oddity of the situation: the fact that it died its final death after the cycle when the operator went to touch the part. Here's my WAG on that:

During the cycle, the crack propogated far enough to cause failure when the stress of full clamping pressure is applied. Being a rotating component 8" or more from the rotational axis, centrifugal force is significant and "preserved" the jaw by reducing clamp force until the spindle stopped. As the spindle slows down, clamping force is restored, the jaw cracks, but does not separate because there is no "positional" influence, just force. The operator's hand wiggled it enough to separate the two fractured halves.

Then again, maybe I'm off base here, but it sure sounds plausible to me

CountryBoy19 · Apr 5, 2019

Cole2534 said:
Interesting, as that's the first thing I've always done...

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It's basic human nature to want to try to put the pieces back together again... but you must resist that urge to preserve all of the forensic information contained in the fractured surfaces.

Hightemp · Apr 5, 2019

rklopp said:
Yep, looks like a fatigue crack grew from the corner. There may have been a sharp machining mark or corrosion pit there. No need for impact. I am surprised at how small the fatigue crack apparently was before the rest of the crack grew all in one go. That, plus the smooth nature of the final crack, suggest the jaw was brittle, possibly improperly heat-treated. Trash the other jaws.

I'm surprised by the same thing you are. Like 70% of the jaw let go in one shot!

This is a textbook example of a fatigue failure. Failure analyis people will actually go in and count the striations in the "dark" portion and they can then go back in time to see when the crack started.

BoxcarPete · Apr 5, 2019

Hightemp said:
I'm surprised by the same thing you are. Like 70% of the jaw let go in one shot!

This is a textbook example of a fatigue failure. Failure analyis people will actually go in and count the striations in the "dark" portion and they can then go back in time to see when the crack started.

Honestly I'm surprised it grew as much as it did before giving it up. By the time the fracture grew to that size the entire right half of the jaw was doing practically nothing. And, that's quite a large stress riser to be going across one entire side of the jaw. Built for rigidity rather than just to be strong enough for the job.

muckalee · Apr 5, 2019

Fatigue fracture. Fracture originated in the groove root about 2/3 of the way in from the front of the jaw. You can probably ring the other jaws to determine their status.

NRDock · Apr 5, 2019

Could be the final straw was heat from the machining operation conducted back to the grip area, adding thermal stress to the clamping load.

Spinit · Apr 5, 2019

I agree that some kind of impact is likely and just the right amount in the right place and it can happen. Thank goodness it did not fly, I notice it looks as that the break went along the upper step . I would first be curious how sharp a corner there was in that 90 degree shoulder step area. Those clearance grooves look as if the cut is sharp cornered and since it is basically cut that way likely contributed to weakness under repeated chuck pressure and use. I would want a fair radius at those points for strength. In this case a radius on that groove does not interfere with the use of the jaws.

What brand name is on the jaws or chuck and what country/state are were they made in? I would not buy the same brand for replacement if I could avoid it if this were my situation.

The most important thing is that this did not fly off and become a projectile and injure someone.

Metallurgy - What happened to this chuck jaw?

Diamond

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Aluminum

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