THE nist gage block manual from nist

If you're interested in a short article I wrote about wringing a couple of years ago -- based on an interview with NIST's top gage-block guy -- I could send it to you. Or I could post it here, if a few others wanted to see it. I think it's only around 600 words.

Ed Huntress

Ed:

Please post it here or to the dropbox. I would be interested in having my students read it.

Errol Groff

Oh, jeez, Errol, I just remembered -- I've got that book here all wrapped for you, and I forgot to send it! Man, I'm losing my mind here. I'm awfully sorry about that. I just put it on the dining room table so I'll remember it at breakfast tomorrow and take it down to the UPS office. I hate getting old...

As for the article, it's short enough that maybe people won't mind it if I put it here. If anybody objects then they can yell at me like we yell at Gunner. This is about WHY they wring (although it doesn't answer the question very well, it appears to be as well as it can be answered), not how to wring them. I assume you teach them that:

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Machine Shop Guide March, 2001 Issue

How It's Done

By Ed Huntress, Senior Editor

Why Do Gage Blocks Wring?

Wringing gage blocks is such a commonplace that the phenomenon has lost its parlor-trick appeal, as in, "Watch as I make these simple blocks of steel defy gravity, without benefit of magnetism, adhesives, or sleight-of-hand..." It's in the low tier of tricks, a notch below gyroscopes and Jacob's Ladders. But for all science knows it could be sleight-of-hand, because the time-honored theories about why these flat blocks stick together have been debunked.

One would think that scanning electron microscopes and high-velocity particle accelerators, at least, would have cleared it up. Asked why gage blocks wring, really, physicist Ted Doiron says, "Theories are all you'll get. There probably are several phenomena going on. It's not a case of squeezing out the air, because the resulting pressure holding them together couldn't be more than atmospheric (14.7 lb./sq. in.), and we know that the force can be greater than that in practice. It isn't just moisture, because they've been wrung in low-humidity boxes with virtually no water vapor in the air. And it isn't just physical bonding, because they can be taken apart. In practice, it may be some molecular bonding, some water-vapor, who knows what-all."

One wishes for a clear, singular explanation. If there was one to be had, Doiron would know it, as the metrologist and physicist at the National Institute for Standards and Technology (NIST, Gaithersburg, MD) who's job, as he puts it, includes reading every paper ever published about gage blocks. "Every country has one person who has to read them all. I'm it for the US," he says.

If one has to spend a lot of time with gage blocks, a bit of mystery may be welcome. Much of the work Doiron's department does involves uncovering those mysteries that can be resolved. Their resolution skills are considerable. "Gage blocks don't wring consistently," says Doiron. "I've had four of my technicians wring down a set, and there was a height variation in the stacks of half of a microinch. One guy wrings them down a microinch smaller than the sum of their lengths would normally indicate."

When you work in half-microinches, your view of the world changes. "Gage blocks really aren't flat," Doiron says. "The faces of some are bell-shaped, others are wavy, others are this or that. Both ends can be out of flat by a fair amount -- as much as four microinches. Wring two together, then rotate one 180 degrees and wring them again...they'll wring differently. They can vary by a half-microinch, just by rotating them. At international metrology conferences they've had us wring blocks at opposite ends, and we get a variation of a half-microinch that way, too." We hadn't the temerity to ask if that was part of the conference proceeding or part of the entertainment.

Wringing gage blocks against an optical flat reveals variations in the gap that depend on how you wring them together. Nothing is ever perfectly clean, and it's difficult to get a strain-free wring. You can see fringes, says Doiron, that sometimes disappear if you tap the end of the block with a hammer, or even with a pencil. At this scale, everything seems a bit rubbery.

"At the microinch level, we run into limits of gage-block accuracy and we're well into the range of wringing variability. We've had numbers of cases of different gage-block companies disagreeing on the height of a given three-block stack. And they're all right. Wringing and other variables, all within accepted practice, can cause a variation of three or four microinches in that three-block stack."

When one gets serious, he uses an interferometer to measure height and he doesn't wring the stack. The methods NIST and other top-end certifying organizations use involve measuring from a point at the top of the stack to the flat surface they rest on. "I'd like to change that to point-to-point," says Doiron, "but the equipment to measure that way doesn't exist."

Doiron points out that gage blocks can be made flatter and more parallel than they are. "But who needs a microinch?," he asks. To which we respond, hardly anyone. But we're glad there are people like Ted Doiron and his department who can answer the need if it should arise.

--end--

Ed:

No problem at al about the book. Tell the truth I had forgotten about it myself. Now that you mention it though I am looking forward to looking it over.

Errol Groff Instructor, Machine Tool Department H.H. Ellis Tech

860 774 8511 x1811