How precise are measuring instruments?

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Mr Ron
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Re: How precise are measuring instruments?

Post by Mr Ron »

thunderskunk wrote: Wed Dec 29, 2021 8:26 am
rmac wrote:I need help with a couple of your acronyms:
thunderskunk wrote: We can?t ignore GD&T because ...
What's GD&T?
thunderskunk wrote: we call that study a GRR
And what's GRR?

-- Russell Mac
GDT= Geometric Dimensioning and Tolerancing. It’s the “design intent” way to dimension a part. There’s a whole standard (ASME Y14.5) that says what means what, which is where rule 1 comes from.

If what I really care about with a bolt pattern on a plate is that it fits with another plate using bolts, linear dimensions and hole diameters don’t tell that story. It likely over-tolerances the part, which technically makes it more expensive to make. Using feature control frames, you can gain “bonus tolerances” like: if your hole diameter is bigger, you gain more tolerance on the distance between holes because the bolt has more clearance to fit between the two plates.

It can be complex, yes, but once both the design engineer and machinist have a grasp of it, it’s better for both parties.

GRR= Gage Repeatability and Reproducibility study. The result is the % error of a given gage, so if I need to measure 1.000 +/-.001, but the error in my calipers is 40% of the tolerance (a total of .002, so .0008” variation). That means a reading of 1.0005” could actually be 1.0013” which is scrap, but you wouldn’t know it. To be sure the part is in tolerance, it would have to read 1.0000”.
I'm afraid I'm lost over this discussion. I'm not a machinist, but just a wannabee machinist. If I can machine 2 parts and have them go together without forcing or too much slop, I'm happy. While working at Electric Boat back in the 90's, I had to use ANSI Y14.5 to design submarine components. I hope none of my subs sank for good.
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Bill Shields
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Re: How precise are measuring instruments?

Post by Bill Shields »

14/2=6 depending on your tolerance :mrgreen:
Too many things going on to bother listing them.
pete
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Re: How precise are measuring instruments?

Post by pete »

Very well explained Thunderskunk. Your line graphs in that link look much like what each one of my Mit. digital micrometers came with. They show exactly where there's measurement deviations along the screw and by how much. On these mics it's not a whole lot, but it clearly shows there present and where.

I haven't looked because I have the hard copy, but I've seen it mentioned on a few forums the Moore Tools book Foundations of Mechanical Accuracy is available online in a few places. For anyone wanting to learn more, it was a large jump in my own education about accuracy and even more so about the multiple directions or even variable combinations of those that can be present in any machine tool slide. Actually it was easier to find than I'd thought. https://archive.org/details/Foundations ... alAccuracy
thunderskunk
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Re: How precise are measuring instruments?

Post by thunderskunk »

I got a little red book: “Inspection and Gaging,” which I think another member here recommended. Actually has a section on squares, and had some interesting points: while you can see breaks in light between two objects to some ridiculously low number, something like .000005”, your eye can’t differentiate between that and .005”. My eyes, I’d be lucky to even see the light. It was saying to use paper shims to test gap sizes, though these days I’m finding plastic shims superior even to the brass I’ve had laying around: they don’t crumple, easier to cut to shape, and by golly they’re cheaper.
"We'll cross that bridge once we realize nobody ever built one."
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SteveR
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Re: How precise are measuring instruments?

Post by SteveR »

Just a quick addition to help clarify what GRR does: I worked at a place where we built parts to make sure our 3D printers were calibrated. One person was doing all measuring with a set of "golden" calipers. But when we started having multiple people use different sets of calibrated calipers to measure different sets of parts, we got a much wider range of measurements. Why? Because people apply different pressures, different calipers have different "feels" and so forth. The GRR measures these variations and assigns the error component to the people, the tools, the parts or just random error. This is important when your specs are as tight as your measuring tools' capability and you send parts to a customer who gets a slightly different result than you.

SteveR
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pete
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Re: How precise are measuring instruments?

Post by pete »

One of the biggest reasons I spent over a year of looking for a very good condition B & S 558 cylindrical square is there capable of providing a measurement number down into a few 10ths how far something might be out of square. It's still subjective though since you have to properly judge that light gap that can be well under what the tool itself can measure to. If the part surface is straight and highly finished enough you get what's called a black out fit where there's zero light to be seen. Luckily the human eye is pretty good at detecting variations in light gaps, we obviously can't measure it by eye, but most of us can certainly tell there is a line width variation to very fine limits. At low enough levels I've read a standard white beam of light will change color as an indicator of the available gap between two parts. So far I've not been able to observe that happening.
SteveM
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Re: How precise are measuring instruments?

Post by SteveM »

I have a set of tools for testing mics.

They provide measuring surfaces that give you different rotations of the spindle.

If you are only testing your mic closed and with a 1" block, you could be missing out on large variations in the middle.

If all your blocks are in increments of 0.025, then the spindle is at the same point in the rotation.
Mikemaster.jpg
Outrageously priced new:
https://www.penntoolco.com/mikemaster-s ... ance-kits/
but available on ebay as well, and if you had them checked by a calibration lab, you'd come in a lot lower than the new cost.

On a related note, there was a gas station that set up their pump to pump short in between gallons, but catch up and be spot on at the gallon tick. What that did was cause the pump to pass inspection, as the inspector was looking at gallon increments, while shorting anyone that was in between gallons.

Think of your micrometer doing the same thing - reading spot on at any of the 0.025 marks, but being off on the others.

Steve
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GlennW
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Re: How precise are measuring instruments?

Post by GlennW »

SteveM wrote: Mon Jan 03, 2022 11:54 pmIf all your blocks are in increments of 0.025, then the spindle is at the same point in the rotation.
The object of the game with gage blocks is to match the gage block stack to the desired dimension and then clamp the micrometer onto the stack and verify a correct reading.

My micrometers are calibrated annually by a metrology lab, but all that really means is that they were in "spec." at that particular time. Any time I need to measure something critical the gage blocks come out and the micrometer is checked at that particular dimension.
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!
SteveM
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Re: How precise are measuring instruments?

Post by SteveM »

GlennW wrote: Tue Jan 04, 2022 6:22 amAny time I need to measure something critical the gage blocks come out and the micrometer is checked at that particular dimension.
Both useful methods. Mine to be sure the mic is ok over its range and yours to be sure that it is correct for the specific measurement you are doing.

Steve
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seal killer
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Re: How precise are measuring instruments?

Post by seal killer »

All--

WOW! What a great discussion.

--Bill
You are what you write.
pete
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Re: How precise are measuring instruments?

Post by pete »

Yep I'd sure agree with you about that outrageously priced label Steve. Even more when you look at what they list for there guaranteed accuracy of +- .0001" and no manufacturer's brand name given. Maybe a brand name doesn't mean much, but why would any company prefer to hide what there making. Plus it's an additional $85 for the 1" model and $165 for the 1"-3" model to get that certification paper work. Huh ????????????? If the gauge steps are properly checked as meeting that +- .0001" guarantee it's a minute or two to fill in the paper work blank spaces for that certification. For what the extra costs are, it's almost like your paying them to sort through enough gauges to find one that will pass certification. Ok maybe I'm wrong and just don't understand enough about what's required for that certification, but that still raises some suspicion about just how good the non certified gauges are even with there price. Yes it's to a different level of precision, but I had a damaged pair of Mit. 12" dial calipers repaired by the Mit facility in Mississauga Ont. and specified I wanted them back within the standard factory certification. The repairs and that re-certification paper work cost me a bit less than $100.

Having one or those fixed gauges would probably be a quicker way of checking a mike than gauge blocks, but I'd still expect a lot better than there +- 10ths. So a used one at a sensible price might be worth while. But I'm with Glenn, decent trustworthy gauge blocks will do the same as well as double checking to the size your measuring at just as he mentioned. And yes they cost even more than those fixed gauges. They will also do a whole lot more as well. Mitutoyo and I seem to recall Starett sell a separate set of fixed gauge block sizes meant to check mikes with blocks in the correct steps to check the mikes screw in different degrees of rotation and lengths for less money the last time I checked. And with those your getting +- low millionths. I got curious and went looking, this one is $320 https://www.higherprecision.com/product ... h-parallel plus you get the optical parallel for checking the anvil flatness. But for any that don't yet know it, you'd still need a monochromatic light source to use that type of parallel.
SteveM
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Re: How precise are measuring instruments?

Post by SteveM »

pete wrote: Tue Jan 04, 2022 3:51 pmBut for any that don't yet know it, you'd still need a monochromatic light source to use that type of parallel.
I saw someone use a pen laser with a ping pong ball attached on the end as a diffuser.

Steve
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