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So much for precision in measurements

RScott

Well Known Member
I liked the way those electronic calipers switched between metric and inches with the push of a button, so when the HF ad that came yesterday had ads for a couple models of electronic calipers, I bought one.

Hmmm., its Harbor Freight, so I better check it for accuracy. First I opened up my old mechanical calipers and tightened it at .310 and measured that with the HF unit. Got .314. Which one is off? So I measured my .026 leaf on my feeler gauge and both calipers showed about .028. Or .029, depending on where on the feeler gauge I measured. And I was being as consistent and as square to the feeler gauge as I could be.

Hmmm, again. So I opened up my micrometers to 1.000 each and got an error of about .003 or .004 on both calipers, but the calipers gave the same reading for each micrometer, so at least the micrometers agreed.

So take it back? Are calipers just not really accurate? Am I expecting too much? I can see the way they slide along the blade they could have some error.
 
You get what you pay for when it comes to precision measuring tools. If you really need to measure something with close tolerance like engine parts, you should use something other than calipers. Calipers are not intended for precise measurements, even if they are "electronic".

It is handy to have a beater pair of digital calipers around might as well keep them.
 
I have a couple of those digital calipers around the shop and I love them - but then I use them to measure things for which I already "Know" the dimensions. "Huh?" you ask, "Why the heck would you measure what you already know?!" Well, what I generally use them for is to make sure I have the right thickness of aluminum, or the right size bolt for a job. Since I know the thicknesses of aluminum that might be in the parts (or scrap) bin, it is easy to use the digital caliper to find the right thing. I know that the stock is probably 0.016, 0.020, 0.025, 0.040, 0.063, etc....Aluminum angles com in thicknesses of 0.063, 0.125, etc. If the caliper gets me within .02 or so, I know what I have - I don't need it to be perfectly accurate (although the pair I bought six years or so ago at Oshkosh is amazingly good!).

So they do have uses - if you already know what it is you're looking for.

Paul
 
The one I have is very precise but it requires a good touch to use it. The slide is fit to the bar with a spring mechanism that keeps enough tension on it to hold it steady but not so much it can't slide. A side effect is that the caliper can wiggle so you can't push up tight to an object and expect a good reading. A light touch and a steady hand help. Also, because the caliper blades have thickness, being a little off square can affect the reading. They're great for measuring round stock, tubing, bolts, screws, etc. but bar stock or sheet presents a conundrum: it's hard to keep the tool from moving in relation to the material. I often see readings vary a few thousandths while I'm trying to hold a large piece in one hand and the calipers in the other. But it's still handy for checking if I've got the right thickness.

For measuring hole locations and such, I usually depend on the Mark I eyeball and a steel rule but a draftsman's compass/caliper set can also be handy. You can also use the same for point to point measurement. Since I'm not machining, that's good enough for my purposes. I'll bet the machinists amongst us can provide more insight, though.
 
calipers are hard to repeat exact measurements at the thousandth level. but my mitutoyo comes pretty close to doing it regularly. If you really want to get exact you use a micrometer anyway.

the battery did finally go dead, i think it was over 5 years old. My dial calipers never run out of battery power and seem faster sometimes!

very few things on a vans project are exact within a thousandth, I can only think of the close tolerance bolts for the wing unless you start getting inside an engine. I don't think you need it for 99.99% of the airframe


edit- do you zero them for every use, and wipe the blades free so nothing interferes with your zero?
 
And international too

The good thing about my cheap electronic caliper is that it has a button called mm/in which is quite useful on this side of the Atlantic.
 
Calipers are not as accurate as a micrometer, .003 or .004 at an inch is pretty good with a caliper. Try cleaning off the blades and the piece you are measuring, it makes a huge difference.
 
I agree with the others, if you are looking for a repeatability to a couple of thousands, you are using the wrong tool. You need to go with a micrometer if you want to be closer than .002 or .003".
 
touch and technique. Someone who uses a good caliper on a daily basis can get very accurate and consistent results. When I was a machinist at Livermore Labs, I only broke out the micrometer if I needed in the ten-thousandths range.

Then again, I've seen guys who think a micrometer is part C-clamp and couldn't get repeatable numbers, if their life depended on it.
 
As a professional tool maker, I have used micrometers and calipers through the years and all my high grade calipers get consistent results.

I bought a HF caliper for $10 and was curious as to if the measurements were accurate. They were not as they were off three to four thousands per inch.

But for what I use these for, like others, measuring bolts and whatnot.. great.

Would I rely on a measurement from the HF calipers to be accurate, NEVER!

Close is as good as it gets with these, but for ten bucks I got my moneys worth.
 
make rules

An acceptable method/standard I use when measuring material: on parts with plus or minus .005thousanths tolerance, calipers are a safe method of measurement (first be sure that your calipers, "margin of error" falls within that type of tolerance). On anything that requires closer tolerances you will want to find a better method of measurement, like a micrometer. Like i said this is just a general rule I use you can do as you like. !cheap tools are great!
 
Kind of funny

I agree with Paul on this one. For the purposes of building a RV, the calipers are confirming what you probably already know.

On the comical side, unless you are building a missile or Widget1900, the calipers won't do you much good in the construction of the plane:D:D

It cracks me up that the plans have measurements such as 38 and 17/32". Yeah right!!!!
 
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