Atten, all noobs locked-in stress in metals

One of the most exasperating problems in machine work is locked-in stress. Steels aluminum alloys, copper alloys, etc distort in differing amounts in different materials depending on how the stuff was processed at the mill, (rolled, forged, cast) and its shape (round square, plate, casting, forging) and where you remove the material. Machine a surface and the material distorts, sometimes by surprising amounts.

In this video

Stress in machining - YouTube

the presenter demonstrates part distortion and how he deals with a specific part and material up to 14:00 minutes. Then goes on to a pen-on-paper discussion of locked-in stress in metals and how it's manifested as part distortion, a by-product of machining.

Those of you who are newcomers to machine and metal work need to know how seemingly solid and immobile metal items can twist and turn as work is performed. Many otherwise good parts, products of fine craftsmanship, have fallen victim to distortion from locked-in stress. The linked video shows the basics of what to look for and how locked in stress affects your work. Older guys can tell you stories and job planners, estimaters, and industrial bean counters can probably lay out budget figures allowing for that particular line item of damage and waste.

By the way, what's commonly called "stress" or "part stress" in many of the posts addressing the topic of part distortion on this forum is what I'm calling "locked-in stress" a legacy term of my early mentors. I like "locked-in stress" as it's a vivid and specific descriptor of a metalworking phenomena plaguing several trades. "Stress" by itself might be confused with the result of mis-clamping, problems with temperature, gravity, personal response to events of the days etc. So I vote for "locked-in stress" when describing part distortion as a material's response to machining.

Also BTW, that's inconel the presenter is machining; maybe 150 lb. A very expensive material. Anyone care to guess how much a hunk of inconel bar stock 6" dia x 20" or so long would cost?

BTW again, notice in the video how the dovetail "death grip" machined in the soft jaws is interlocked with a complementary feature in the work. For all practical purposes, chuck and work are a single piece. A wreck sufficient to dislodge the workpiece will certainly damage the soft jaws and possibly the chuck. There is a hazard lurking here: an imperfect grip allows slippage that acts as a mechanical fuse, If you traverse the tool into the work, there will be a hellova bang but the only thing damaged is the tool the work piece, and the operator's clean underwear put on that morning. The slip protected the chuck and the tool slide. Call it "failing gracefully."

Forrest Addy said:

Also BTW, that's inconel the presenter is machining; maybe 150 lb. A very expensive material. Anyone care to guess how much a hunk of inconel bar stock 6"dia x 20" or so long would cost?

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We get around $10 / lb for our chips. So I would guess around $10k for the blank.

Larry Dickman said:

We get around $10 / lb for our chips. So I would guess around $10k for the blank.

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I believe he said it was about $5k in a different video. And due to his contract he gets to buy his mistakes!!

Forrest Addy said:

"Stress" by itself might be confused with the result of mis-clamping, problems with temperature, gravity, personal response to events of the days etc. So I vote for "locked-in stress" when describing part distortion as a material's response to machining.

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I just think of it as a heavier grade of wood. Seems to work

The stress is referred to as "residual Stress," in text books. Each part will have residual compressive and tensile stresses, in balance. Every time energy is applied or removed, the balance of residual stress is impacted and the part may change its geometry when the stress system balances.

I machined a lot of Inconel,its the worst for internal stress. We would take a 2.25 x 30" long bar and turn down 28" to 1.25 for valve stems,you could actually see the center starting walk around as youre roughing it. Had to at times put in a press and tweak it back.
Also used to get stainless cast butterfly bodies we found that if you didnt rough them then let rest for a day or so that they would be out of tolerance.
We had drawings that had a "free state tolerance" meaning that when removed from machine it could be out of flat,round a certain amount.
These were 20 - 30 dia .4 or so thick stainless plates that required alot machining. Used pie jaws but impossible to achieve perfection.

For the sake of analysis, I like the idea of calling it either "locked-in stress" or "residual stress". Either one works for me, if there are already text books using "residual" as appropriate terminology we should stick with that. But I agree with Forrest, in that using the correct term. would reduce the amount of blather to read to get to the point.

R