Vernier caliper accuracy

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Hi folks,
Can anyone tell me what the typical accuracy of a decent pair of vernier calipers is (when used carefully and correctly)? I just bought a dial bore gauge and am wondering if I can reasonably set the zero point with
vernier calipers instead of having to buy a 3" to 4" micrometer. The calipers are Etalon brand, heavy and made in Switzerland. The bore gauge dial is graduated down to 0.0005". I'm not reboring an engine, I just want an idea of the wear, so I don't need the highest accuracy. From using these calipers, my own guess is that they're probably good to 0.002", but some of that might be my errors in reading them. I'd be interested to hear what other people think.
Best wishes,
Chris
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Keep them within a few degrees of some standard (68 deg. F being the most common one); keep your hands off of them and wear gloves; develop a good feel, and you should be able to measure to +/- 0.001" with them. Your Etalon probably is like my TESA Swiss vernier caliper, which is my best one.
You can easily check it with a few gage blocks. If you're going to do precision work, you need at least a few of them. A set is great but for checking mikes, calipers, height gages and so on, you only need a few in different sizes. Then you won't be chasing your tail. You'll know with good assurance how accurate your gages are. And they'll help you develop a feel for how much force to use when you're measuring. A couple of gage pins, like 1/4" and 5/8", or something like that, are also useful for that purpose. I have some precision toolmaker's buttons that serve that purpose.
--
Ed Huntress



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Ed Huntress wrote:

This is the kind I have, except mine are 15" calipers and they don't carry the P. Roch name: http://cgi.ebay.com/ETALON-P-ROCH-ROLLE-SUISSE-12-30-mm-VERNIER-CALIPER_W0QQitemZ270531036164QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item3efce80404
Why do you suggest wearing gloves? To keep the calipers cool? Right now the workshop is more like 40 deg. F.

I don't have any gauge blocks. Do you think they'd be a better investment than a 3" to 4" micrometer?
Best wishes,
Chris
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Christopher Tidy writes:

The import 1-2-3 blocks are $10 at Enco. Then you have standards for whole inches 1 through 6 to better than 0.001". Add a granite slab and you have the beginnings of some serious metrology for all of $35. Just buy $15 more worth of anything and they'll ship it free, including the 61 lb stone!
http://www.use-enco.com/CGI/INSRIT?PMAKAc0-4010 http://www.use-enco.com/CGI/INSRIT?PMAKAd0-0120
This spacer set is only $30 and does 0.050" to 4 or 5 inches in most 0.001" increments:
http://www.use-enco.com/CGI/INSRIT?PMAKAc0-4050
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Richard J Kinch wrote:

And you just answered the question I was about to ask . Thanks !
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I belive Christopher is a bit far away for free shipping.
Wes
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Wes writes:

Oh, right.
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On Sat, 20 Feb 2010 05:03:20 +0000, Christopher Tidy

No. Calipers are not as accurate as micrometers because they spring more and provide less "feel". Their value is wide range and quick operation for ballpark measurements to within a coupla thou. That's often quite sufficient, certainly in roughing or intermediate operations.
I've found the Asian import micrometers to be very good value, and some come with a reference with which to check them. Don't know about sources in the UK but an Asian 3-4" mike can be bought here for about $35. I have some good mikes (Etalon, Starrett, Fowler) and some Asian imports. They agree to well under .001", usually to better than half a thou. The better mikes do have better ratchets or friction thimbles. I prefer a friction thimble, not found on cheap mikes.
If you want to measure accurately to .0002" or better, go name brand from Switzerland, Japan or USA in that order. If .001" is close enough, about any mike with carbide faces will suffice.
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Don Foreman wrote:
Hi Don,
Sorry to everyone for the slight delay in replying.

That was about the accuracy I was guessing. As calipers go, these are about as stiff as you can get. Probably weigh between 1 and 2 lbs. But obviously there's no mechanism to ensure a constant closing force, like a micrometer thimble.
I figure that as I'm not reboring an engine (I just want to know how severe the wear on each part is, so I can decide what to replace), a measurement to a couple of thousandths is good enough for me. After all, the smallest oversize piston I've seen is +0.010". But if anyone thinks this is a bad plan, do let me know.

I've never seen a micrometer without a thimble. My 0.0001" Tesa micrometer (Swiss I think) has a friction thimble and my cheap Draper metric micrometer still has a ratchet thimble.
What do people think of micrometers with interchangeable anvils, to give several measurement ranges? So for example you have four anvils which fit a 4" micrometer frame to give measuring ranges of 0" to 1", 1" to 2", 2" to 3" and 3" to 4"? My gut reaction is that they won't be as consistent as a single range micrometer, but for brand new ones, Mitutoyo's website claims this isn't the case.
If calipers are good to a couple of thousandths, my inclination is to use them for the moment.
Best wishes,
Chris
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Interchangeable anvils are fine if you keep them very clean, and if you check them against gage blocks.

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Ed Huntress wrote:

Sounds like I should avoid them unless I buy the gauge blocks then.
Chris
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I'd avoid them unless you really need them. Full disclosure -- Mitutoyo was my client for many years, and I wrote all of their articles during those years and some of their instructional materials. They're quite honest about what they claim. Just be aware that things like interchangeable-anvil micrometers sacrifice some assurance for the sake of convenience. They can be as accurate as any mike; you just have to be a bit more careful.
--
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Ed Huntress wrote:

Thanks. I'll remember you're the guy to ask about Mitutoyo gear! The interchangeable anvil micrometer I saw advertised doesn't include all the calibration standards Don mentioned, so I'll avoid it.
Best wishes,
Chris
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    They *should* come with standards for zeroing the micrometer after changing the anvils -- as many standards as you have anvils. These should be enough to assure accuracy at the zero point at least, and you can use gauge blocks to check for errors in mid spindle travel, but this should not be a problem with new micrometers anyway -- only after a lifetime of use by you, or someone else.
    Enjoy,         DoN.
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    Indeed -- you need to develop a feel for the sliding force on the caliper jaws once closed. (And, of course, the jaws are not carbide faced, so they will wear more rapidly than most micrometer jaws.)

    Probably adequate -- especially as you are closing the calipers to a given setting (using the adjustment screw with the extra traveling head clamped down) and then using it as a reference for the dial gauge. If you had a set of gauge blocks, you could close it to a light sliding fit on those (to free yourself from the possible errors in the vernier and in reading it) and hold it in a vise to eliminate the problems from hand heat causing expansion of the beam.

    My first one from about 1960 (long gone, and I don't remember the brand) had only a small diameter spinner, not a ratchet or friction thimble, and I developed a feel for letting my fingers slip on the knurled portion of the man thimble.
    I've got others with various designs, including a 0-6" Brown & Sharpe set which still have no torque limiting features, along with others with very good friction thimbles.

    Remember that you have to zero them against a standard each time you change the anvils -- or for that matter if you pick it up a week later and want to measure -- the room temperature might have changed between the zeroing and the time of the intended measurement.
    My main consideration of the problems with the multi-range micrometers is that you have the large C frame so there is more metal to expand as your hand warms it, thus more error from the thermal expansion.
    If you can prevent this (including hand warming of the standards you use when you zero the micrometers) you should be OK. Often a bench clamp stand for the micrometer is indicated.

    And potentially good for better than that as you will be using them in zeroing the bore gauge. If you use gauge blocks to set the calipers, you will be better than the caliper.
    But then, if you have a stack of gauge blocks, and two longer blocks at the ends, and you have a setting reference which won't need the calipers at all. There are devices designed for just this purpose -- a rectangular tube to hold the blocks, a pair of long blocks for the ends, and provisions for clamping it down firmly.
    Good Luck,         DoN.
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http://cgi.ebay.com/ETALON-P-ROCH-ROLLE-SUISSE-12-30-mm-VERNIER-CALIPER_W0QQitemZ270531036164QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item3efce80404
The reason is thermal expansion of the caliper. If you're pushing for the best accuracy, and particularly if the room is cold, holding it in your hand for a few minutes can make a difference. If you warm a three-inch-long piece of steel by 30 deg. F, from your 40 deg. shop to 70 deg., it will expand by almost 0.001". If the piece you're measuring is the same temperature as the caliper, and if they're both steel, the actual temperature matters little, but a difference in their temperatures, if it's more than a few degrees, can result in inaccurate measurement.
But it's time to back up. I suppose you realize that a slide caliper is not a high-accuracy gage. It's usually used for moderate-accuracy work. You were asking what accuracy you can achieve with it, and the answer is, using a good caliper, cotton gloves, and calibration with gage blocks, you probably can achieve +/- 0.001". But not everyone does. Some people have the touch and some don't.
You'll be able to achieve that more easily with a micrometer. With good technique and a good mike, you can cut that range in half. You won't need gloves if you just hang your pinky or two fingers over the bridge part, or if you work quickly.

That's a good question. If you need a mike, you'll want both. There will be differences of opinion about this but my vote, if you already have a caliper, would be for the gage blocks first.
But it depends on whether you're measuring *relative* dimensions or *absolute* dimensions. If it's the former, you don't need a well-calibrated gage. If it's the latter, you'll never know for sure how accurately your gages measure unless you check them from time to time against a gage block (or a stack) that's somewhere in the middle of the gage's range, and another one near its largest opening. For a 3" - 4" mike, I'd want at least a 3" block (or a stack to make that dimension) and a 4" block or stack. I'd like to have a 1/2" block, too, to wring with the 3" block to test the midrange.
If any of this is unclear, ask, and I or someone else will explain. FWIW, most home-shop work doesn't require accurate measurement of absolute dimensions. Usually we're trying to make two things fit together, and what you need to know for that is their *relative* dimensions. If that's the case, forget the gage blocks for now.
--
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Ed Huntress wrote:
Hi Ed,

Right. I rarely wear gloves except for particularly dirty jobs. I prefer barrier cream.

Thanks. That's the figure I wanted. These are good calipers and I think my touch is fair, but I'm not being careful about temperature and the scale on the calipers cannot be moved to calibrate them. Sounds like my guess of +/- 0.002" is probably fair.

What's a stack?

In this case, I think it's the absolute dimensions. The engine's piston wobbles noticeably in the bore. It's an aluminium piston in a cast iron bore, so I suspect there could be much more wear on the piston than the bore (the bore looks good, without a ridge at the top). So I want to know how well a brand new piston will fit in the existing bore, before I buy one.
Best wishes,
Chris
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Multiple gage blocks "wrung" together, which add up to their combined dimension. If you wring three 1" gage blocks together, they'll equal a single, 3" block. The wringing technique excludes air between them, and adds less than 2 millionths of an inch to the stack height.
This only works with good-quality gage blocks. Some of the other setting tools that have been recommended in this thread can't be stacked up with that kind of accuracy.
The ideal accuracy of dimensional standards, such as gage blocks, is ten times or more the accuracy of what you're trying to measure. In other words, if you're trying to determine the accuracy of your micrometer to +/- 0.0001 in., you ideally should have gage blocks that are accurate to +/- 0.000,01 in. That's a workshop-quality gage block, grade A or a good quality grade B, which today is called a "grade 3" (or AS-1) in new blocks.
If you're going metric, the picture is a little different.
Having said all that, a cheap workshop-grade gage block will be more accurate than you need for most shop work. But the troubles with cheap sets are twofold: they have parallelism problems, as well as looseness in absolute dimensions; and they often won't wring.
Back to Earth: For what you want, almost anything will do. <g> Any decent gage standard is a whole lot better than nothing. But you have at least one mike, and you might want to use it for more demanding work at some time. For that, get a few gage blocks.

It would really help to have a good engine man chime in here. There are several around; try a new thread with a title like "Measuring engine bores" or something like that. You may drag one up.
Many of us can help you with handling gages but engine cylinders have other issues, like taper and ovality, etc. If it's an old engine, the pistons may be cylindrical. If it's a newer one, they're probably elliptical ("oval"). And they're tapered along their lengths, too.
In engine work, you have to know not only how to measure, but what it is you're really measuring. It's not difficult but you do have to know what you're doing. You want to ask someone who really knows his stuff.
Good luck!
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Ed Huntress wrote:

Might do that, but I'm going to try taking the measurements first. I have the urge to get into the workshop :-).

Interesting. Why are pistons intentionally made elliptical? I've heard of pistons where the top land is of a smaller diameter (apparently it reduces wear because the lubrication is poorer at the top), but I've not heard of an elliptical piston. I'm surprised it doesn't increase the wear on the sides due to the reduced surface area, and I'd have thought it could allow the piston to vibrate in an angular sense about the gudgeon pin. Any more information, Ed?
Best wishes,
Chris
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Because the thicker sections, which are the boss areas for the wrist pins (piston pins) expand with much more force than the thin sections. So the pistons have a smaller diameter across the boss area.

Almost all production automobile pistons made today are elliptical. In fact, they're often elliptical with the major axis in one direction at the top of the piston, and in the other direction at the bottom. The bottom ellipse is for better sealing, to meet emission requirements. It has to do with differential friction and heating between the neutral axis, which is parallel to the crankshaft, versus the other axis.
When I was at Wasino we had some drawings from Ford that actually had three different ellipses along their length, from top to bottom, and they had to blend into each other.

If you don't get an expert to chime in here, I'll see what I can dig up for you. There is one guy who stops in here from time to time who is an engineer for one of the world's top piston manufacturers; you won't get any better info than that from him.
If you're eager to search on it yourself, try both "elliptical piston" and "oval piston." They're often, incorrectly, called "oval pistons" in the trade.
--
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