Vernier caliper accuracy

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

Reply to
Christopher Tidy
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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.
Reply to
Ed Huntress

This is the kind I have, except mine are 15" calipers and they don't carry the P. Roch name:

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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

Reply to
Christopher Tidy

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!

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?PMAKA=640-0120 This spacer set is only $30 and does 0.050" to 4 or 5 inches in most

0.001" increments:

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Reply to
Richard J Kinch

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.

Reply to
Don Foreman

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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.

Reply to
Ed Huntress

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You can buy a cheap set of B grade gauge blocks for about 85 bucks. They are accurate enough for setting up and calibration any gauge you will normally use. When you set your gauges make sure they are all at the same temp. Gloves will keep the metal from getting warm and also keep your fingerprints off the gauge blocks. Also if you have sine plate to set up angles, you will definitely like to have a set of gauge blocks.

John

Reply to
John

And you just answered the question I was about to ask . Thanks !

Reply to
Snag

I have good 0.0001" micrometers with a 1" setting standard for 0-1 and

1-2", cheaper second-hand ones for the larger ranges.

At home I almost never need to measure larger sizes with absolute accuracy. I have both the inside and outside pieces and only need to cut them to fit together. I use an unzeroed bore gauge to check for taper while I'm boring the cylinder, and compare that to the bore gauge's reading of the micrometer set to the piston's OD.

I've had Starrett and Phase 2 dial calipers checked for accuracy. The Starrett was considerably better than 0.001" over its range, nearly as good as I could read by interpolation. The Phase 2 barely held 0.001". When I checked the cal standards myself I noticed that the Phase 2 reading was sensitive to how I held it.

So I bought an incomplete set of gage blocks cheap at an auction and practiced on them. After a while I could get the 0.0001" mikes to read to one division but don't believe my dial and vernier calipers to better than 0.001", if that.

This Enco 1-2-3 block here shows 2.0002" at both ends, at 14C (my normal room temperature). That's good enough to set a 2-3" micrometer that reads to 0.001".

jsw

Reply to
Jim Wilkins

I would suggest that you calibrate it against a new bearing bore. The accuracy that those are ground to is amazing.

Reply to
Grumpy

I belive Christopher is a bit far away for free shipping.

Wes

Reply to
Wes

Yup, I use bearings as standards after checking them, usually for setting dial snap gauges and bore gauges. An ABEC 1 bearing can be smaller than the nominal size but if you can check it against known standards they are an inexpensive was to have a working standard for checking and setting measuring instruments.

John

Reply to
John

How new are the calipers? I would expect 0.001" accuracy, if zeroed properly, if the gibs are set right so the head does not rotate, and if you have sufficient light and magnification to read the vernier properly.

I'm assuming that you are not calling dial calipers "vernier". Just to be sure, I'll cover them too.

Digital calipers typically have a resolution of 0.0005", but check the actual accuracy against gauge blocks -- in particular at the size you want to set.

Dial calipers can have cyclic errors a function of the gear pinion which engages the rack.

Ideally, it should be set to a ring gauge, not a micrometer. These will typically be accurate to 0.000050". You really want the standard to be more accurate than what is being set.

If you use a 4-5" micrometer, you will need a 4" standard to zero it before you use it. (And I'm assuming that the micrometer has a tenths vernier around the barrel so you can set it to 0.0001".

Also -- you really don't want to hold the micrometer's C-frame, as it can expand with heat from the hand. Put it in a micrometer bench clamp so you can set it and not worry about it drifting as it cools off.

A lot of the variation in calipers comes from a loose fit of the head to the bar, allowing it to tilt when pressure is put on the jaw tips. If you have it pretty snug, you can keep this from being a problem. And the closer to the bar you do the measurement, the less the problem from a tilting head and jaws.

Enjoy, DoN.

Reply to
DoN. Nichols

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O.K. That looks like a very well made one -- except that there seems to be no provision for sliding the vernier scale to zero it as there is on some. So -- it is either quite accurate, or permanently inaccurate, and it was originally zeroed by lapping the jaws until it reached zero.

To keep the heat of your hand from changing the dimensions of the metal. Note that he suggests that you keep it within a few degrees of 68F (20C) since this is probably the temperature at which it was originally calibrated.

They would be a good investment even if you *have* a 3-4" micrometer, as it allows you to check the micrometer at several points through its range. At best, the micrometer came with a single standard rod for the zero position 3" for a 3-4" micrometer. The same advice about handling with gloves applies here, because both the gauge blocks and the micrometers also change dimensions with temperature.

Probably for your purpose, the cheap Chinese sets are good enough. They are supposedly accurate within 0.000050" (half a tenth) which is enough beyond the accuracy you expect even from the micrometer so it should be a good way to check it.

BTW For precise checking of bores, you want a three-point contact bore micrometer, which is self centering in the bore. They are *expensive* -- say about $600.00 for a 1.000" to 1.200" one which reads in divisions of 0.0002". And each pair in adjacent sizes should come with a setting ring which is at the high side of the range of one and the low side of the range of the other.

I've collected most of mine from eBay auctions over the years, and mostly have Tesa/Brown & Sharpe ones.

Good Luck, DoN.

Reply to
DoN. Nichols

Oh, right.

Reply to
Richard J Kinch

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

Reply to
Christopher Tidy

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

Reply to
Christopher Tidy

Hi Don,

Probably 30+ years old, but in very good condition. A few scratches here and there, but no noticeable wear to the jaws and no sign of having been dropped.

They're genuine verniers.

I know this is essential for a 3-point bore gauge, but is it so important for a 2-point gauge? Mine is a 2-point gauge with a centring device.

I looked at ring gauges. Any idea whey they're made in weird sizes instead of round numbers? Such as this one, 16.8349.mm:

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Good tip. But fortunately the jaws on my calipers are snug.

Best wishes,

Chris

Reply to
Christopher Tidy

Interchangeable anvils are fine if you keep them very clean, and if you check them against gage blocks.

Reply to
Ed Huntress

Hi Don,

Indeed. I searched for the type of calipers which have a vernier scale that can be moved to zero them, but I couldn't find any for sale (maybe they're an American thing - what brands have this feature?). So I bought these.

I'm specifically interested in engine cylinders at the moment. The first one I want to measure has a nominal bore of 80 mm. So that would likely be a very expensive micrometer.

Before buying the gauge, I did some research and got the impression that the standard tool for measuring engine cylinders is the dial indicator bore gauge, with two measurement points and a centring device. That way, you can tell the direction in which any ovality lies, which would be of interest to me. So that's what I bought.

Best wishes,

Chris

Reply to
Christopher Tidy

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