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Old 09-28-10, 04:56 PM   #1
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Digital caliper accuracy - or lack thereof

My eyes are not so sharp anymore, and I can't read the vernier too well on my calipers without a big magnifying glass and a bright light, so I bought myself a cheap Chinese made digital caliper from Harbor Freight today.

I immediately zeroed the and checked two alloy seat posts, which were unused and clearly stamped with their respective diameters. The smaller of the two measures 0.2mm too small, and the larger of the two, 0.2mm too large.



I checked with my old verniers and got very nearly the same results. (Any differences were minuscule).

This leads me to believe that my new digital calipers are reasonably accurate (or at least no less accurate that the results I get with my thick glasses and ancient vernier.)
  • So What do you think?
  • Are seat posts generally subject to such wide manufacturing variance?
  • Could BOTH measuring tools so innacurate?
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Old 09-28-10, 05:03 PM   #2
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Originally Posted by auchencrow View Post
My eyes are not so sharp anymore, and I can't read the vernier too well on my calipers without a big magnifying glass and a bright light, so I bought myself a cheap Chinese made digital caliper from Harbor Freight today.

I immediately zeroed the and checked two alloy seat posts, which were unused and clearly stamped with their respective diameters. The smaller of the two measures 0.2mm too small, and the larger of the two, 0.2mm too large.



I checked with my old verniers and got very nearly the same results. (Any differences were minuscule).

This leads me to believe that my new digital calipers are reasonably accurate (or at least no less accurate that the results I get with my thick glasses and ancient vernier.)
  • So What do you think?
  • Are seat posts generally subject to such wide manufacturing variance?
  • Could BOTH measuring tools so innacurate?
My Mitutoyo caliper is very accurate, but I recently purchased a cheapo one to incorporate into a cable pull measuring device. Says right on it, accurate to within +/- .2mm. That's a lot of variance when measuring seat posts.
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Old 09-28-10, 05:52 PM   #3
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Originally Posted by Dan Burkhart View Post
My Mitutoyo caliper is very accurate, but I recently purchased a cheapo one to incorporate into a cable pull measuring device. Says right on it, accurate to within +/- .2mm. That's a lot of variance when measuring seat posts.
Now that you mention it, mine say accurate to within 0.2mm also.

- But I can't fathom how one seat post measures 0.2 small and the other 0.2 large, when their nominal diameters are 26.0 and 26.8 respectively - How can I reconcile a spread of 0.4mm when there is just 0.8mm difference between the nominal diameters ?

I am beginning to think that these things should not be measured with calipers at all. I know if I was machining seat posts I would at least have certified cylindrical ring gages to check go/no-go on them.

Q - Have you tested the accuracy - or shall I say, ability to measure posts with accuracy, using your Mitutoyo calipers?
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Old 09-28-10, 06:02 PM   #4
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Originally Posted by Dan Burkhart View Post
My Mitutoyo caliper is very accurate, but I recently purchased a cheapo one to incorporate into a cable pull measuring device. Says right on it, accurate to within +/- .2mm. That's a lot of variance when measuring seat posts.
You must have bought a seriously lousy one. A digital caliper is made up of a pair of printed circuit boards which make a muliipole capacitor and some driving electronics (and a display, of course). The resolution of a standard prtinted circuit is something like 0.001 inch. Making sure the two boards move parallel, and that the jaws are square and all zero at the same point, are the limiting factors. I'd expect even a cheap one to have a accuracy of well less than 0.1 mm. Measuring seatposts (and round stock in general) can be tricky though. (I've also seen them stmaped with the wrong diameter!)
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Old 09-28-10, 07:17 PM   #5
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I've got two sets of cheap digitals and about a half dozen dial calipers. I've checked them all at some time or other against the 1 inch standard that came with my Mitutoyo 1-2 inch mic. All the digitals read exactly 1.000 and the dials read either spot on or at most a half a needle's width from true.... about 2 to 3 tenths being the guess for the worst of them. And since yours both read the same as close as you can tell I'd guess that it's the posts.

While you're at it measure around the diameter at multiple spots and at different places along the length. I'll bet they are anything BUT round and likely have fat and skinny spots. Likely the only posts you'll find that are truly accurate are the mid to higher end ones that are machined or that have better quality drawn tubing.
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Old 09-28-10, 08:21 PM   #6
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I've got two sets of cheap digitals and about a half dozen dial calipers. I've checked them all at some time or other against the 1 inch standard that came with my Mitutoyo 1-2 inch mic. All the digitals read exactly 1.000 and the dials read either spot on or at most a half a needle's width from true.... about 2 to 3 tenths being the guess for the worst of them. And since yours both read the same as close as you can tell I'd guess that it's the posts.

While you're at it measure around the diameter at multiple spots and at different places along the length. I'll bet they are anything BUT round and likely have fat and skinny spots. Likely the only posts you'll find that are truly accurate are the mid to higher end ones that are machined or that have better quality drawn tubing.
^ This is starting to make sense - it would account for the relatively close agreement between my vernier calipers and the new digital. I'll try your suggestion and measure various spots on the posts, and see what intra-part variation I come up with.
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Old 09-29-10, 10:37 AM   #7
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I have an analog Mitutoyo caliper set that I bought from a garage sale for 5 bucks and I enjoy using it to measure all sorts of things on the bike. Really good quality instrument, but you do have to be careful when you use it to get consistent/accurate readings. It's really important to get the caliper arms to contact the item you're measuring at right angles as just a bit of angle on the caliper in relation to the item you are measuring will result in erroneous readings. Best to measure the part with it removed from the bike. as you can best control the angle of the instrument without having to do gymnastics around the frame to get at parts with the caliper.
I also notice similar innacuracies with digital scales when weighing components. Some scales will take more than a few attempts to come up with a consistent weight as there seems to always be a mechanical factor involved that will introduce friction/stiction that will result in inconsistent values.

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Old 09-29-10, 11:42 AM   #8
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Chombi, this is another reason besides batteries that always seem to be dead why I prefer my dial calipers to the digital ones. What we can do is maintain a little pressure on the moving jaw and sort of wiggle the calipers around the proper point. Look for the minimum reading that indicated where you momentarily had the jaws as perfectly aligned as practical and that's your size. Same with inside readings for tubes and such but there I wiggle them while holding light pressure while looking for the max reading.
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Old 09-29-10, 12:46 PM   #9
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yup the problem came with going Cheap. buy upmarket a ways..

http://www.mytoolstore.com/starrett/microndx.html


http://www.grainger.com/Grainger/dig...ecatalog/N-959

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Old 09-29-10, 01:21 PM   #10
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I've found that the $30-50 digital-calipers from Harbour Freight are quite good and better than the $9-12 ones. However, their accuracies were nowhere as wild as the +/- 0.2mm variations that the OP noted. It's more like +/- 0.02mm for the cheap ones versus +/- 0.01mm for the nicer ones.

I agree with BCrider that it's most likely a measurement technique issue. Measure exactly the same spot and rotation on each seatpost. And you must use a T-square on the post to ensure that the caliper jaws are at right-angles to the post and opposite each other.
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Old 09-29-10, 02:39 PM   #11
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Calipers make good precision c-clamps,good enough for a bicycle I suppose,as long as your not trying to actually make something that has to fit together.

They're OK,quick and dirty kind of thing,but there is alot of feel to get them to read consistantly,so they are on the low end of accurite measuring devices.

Last edited by Booger1; 09-29-10 at 03:04 PM.
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Old 09-29-10, 04:28 PM   #12
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Booger, calipers are a lot better than you are giving them credit for. I've used them a lot for machine work and have no trouble at all working to within .0005 tolerances with them be they dial or digital. The only time I break out the micrometers is if I'm trying to size stuff to within a tenth or two for a precision press fit or something of the like. If you can't operate calipers to within a half thou consistently then it's your technique and not the tool.

As for going upscale I've had good luck so far with all the cheapies I've bought. I got my father's 0 to 6 inch Mitutoyo micrometer set a few years back and I occasionally check all my sub $30 dial calipers against the standards that came with the mic set. All of my 4 or 5 dial calipers check out consistently to less than a half thou. And that's close enough for the sort of work I do with calipers.
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Old 09-29-10, 04:52 PM   #13
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I have to agree with the comments on technique. It's all about how you hold your mouth when you go to measure something. A vernier style caliper will never be as accurate as a micrometer, but with proper technique, it's accurate enough for anything bicycle related, and it's much more versatile. You'd need a box full of micrometers to do the job of one caliper.
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Old 09-29-10, 06:03 PM   #14
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I think that some of you missed the OP's point. He's talking about variances in seatpost diameters, not variances in caliper readings. I've noticed the same thing about seatposts.

My 99 cent Ebay Chinese digital calipers read exactly the same as the ones that I paid considerably more for, and the battery lasts longer for some reason. The expensive one rarely leaves its case anymore.
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Old 09-29-10, 07:08 PM   #15
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OK I went back and measured the posts at various points all over the place - And what I found was a few spots where deviation from the nominal ID was closer to 0.1mm instead of 0.2.
To be certain, I did a little repeatability study measuring the same point 6x and got the same measurement 6x, so its not likely the cheap calipers, or me . (I would hope not - I learned to measure working as a machinist in a pump factory when I was 17).

The posts themselves? They're crappy. Their nominal OD's are approximations.
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Old 09-29-10, 11:07 PM   #16
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So ah... what brand seatposts are these?
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