Digital caliper accuracy

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Mr Ron
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Digital caliper accuracy

Post by Mr Ron »

I did a little experiment with a 6" Mitutoyo digital caliper and would like an opinion. I took a small pad of paper containing 9 sheets. I measured the 1st sheet; then added an additional sheet and measured. I got the following measurements:
1st .0040 Difference between last and previous readings:
2nd .0085 .0045
3rd .0125 .0040
4th .0165 .0040
5th .0210 .0045
6th .0255 .0045
7th .0295 .0040
8th .0335 .0040
9th .0380 .0045
I tried to apply the same amount of pressure on each reading letting the last digit bounce back without applying additional pressure. I zeroed my caliper at the beginning and it maintained zero at the last reading. What can you determine from these readings, that the paper is not a uniform thickness or the caliper is at fault?
I am going to try the same experiment using a tenth's micrometer.
Mr.Ron from South Mississippi
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rmac
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Re: Digital caliper accuracy

Post by rmac »

Mr Ron wrote: What can you determine from these readings, that the paper is not a uniform thickness or the caliper is at fault?
Neither, in my opinion. Notice that the fourth digit in all your measurements is either a zero or a five. That's a very strong indication that the caliper displays its results to only the nearest half thousandth, and not to the nearest tenth. So it makes sense that your differences would bounce around by .0005, even if the sheets were all exactly the same thickness.

All that aside, it's pretty much accepted that calipers aren't reliable for sub-thousandths measurements in the first place, especially with something squishy like paper where your caliper and even your tenths micrometer are both going to compress the material by some amount as you take your measurements. I'm not sure what your goal is here, but maybe play around with aluminum foil or some actual shim stock instead of paper???

-- Russell Mac
jcfx
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Re: Digital caliper accuracy

Post by jcfx »

I'm with Russell, I'd redo your test with a material that doesn't have any give to it like
shim stock or a feeler gauge.
David2011
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Re: Digital caliper accuracy

Post by David2011 »

Like Russell said. I have far more faith in dial calipers over digitals because of the same question you raised.
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Steggy
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Re: Digital caliper accuracy

Post by Steggy »

In my corner of the universe, a caliper, whether vernier, analog (mechanical dial) or digital, is for “close enough is good enough” work. I do have an eight inch analog caliper that I use when a few thousandths either way is acceptable. It’s fast and convenient, but not very repeatable.

If precision matters...well, that’s why micrometers exist. A quality micrometer is a lifetime investment that if properly maintained and periodically calibrated, will give you an accurate measurement every time. My oldest mike is a 5 incher I bought in the 1960s for checking engine piston skirts. Other than some paint wearing off its frame, it’s as good as it was when I purchased it 55 years ago.

That said, and like the others suggested, your testing should be done with shim stock, a feeler gauge, or some other solid medium of a known thickness. Otherwise, your measurements are not useful.
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Harold_V
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Re: Digital caliper accuracy

Post by Harold_V »

BigDumbDinosaur wrote: Sat Jul 02, 2022 12:46 am Otherwise, your measurements are not useful.
That's a point I've tried to drive home on several occasions (usually in discussing calipers of any description). Sort of like the idea of garbage in, garbage out.

If one doesn't have a true reading and is attempting to make parts to exacting specifications, success is by chance rather than by design.

H
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rmac
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Re: Digital caliper accuracy

Post by rmac »

BigDumbDinosaur wrote: Sat Jul 02, 2022 12:46 am That said, and like the others suggested, your testing should be done with shim stock, a feeler gauge, or some other solid medium of a known thickness.
Unless, of course, Mr Ron's goal in the first place is to measure the thickness of the paper! At this point we don't yet really know what he's trying to do.

-- Russell Mac
Rich_Carlstedt
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Re: Digital caliper accuracy

Post by Rich_Carlstedt »

Well, The first mistake ( Sorry Ron) is measuring paper.
it is compressible and hygroscopic ( absorbs moisture) which makes it a unreliable standard for any measurement
But I also concur with your results because I did a study on paper thickness when i did my Drawings book.
20 # paper varied from .0032 to .0037, and I wound up using #28 at ~.004"-----( all over the place)

Now let me get on my soap box
I find it absolutely hilarious that guys "diss' digital calipers ---and then want digital DRO's on their mill to read .0002" accuracy !
Don't you find that funny ? I do --immensely
tell me what is the difference in the magnetic scales ? Hint, there are none ! (unless it is a real cheap scale )
The difference is in "USE" of the scale
Most machinists ( 95 % ) and even the 'caliper ads" use the instrument TOTALLY wrong !
Is it possible for a Digital Caliper to read correctly ? Absolutely !
I have a Starrett 722 that I paid over $ 200 for back in 1988 , that reads in tenths......yes, .0001" and from 1988 !
My employer made precision dies and I used the caliper when I checked dimensions that a machinist had on a die
as our tolerances were .001" on many parts. Would I say it was 100% accurate ?
No, because you never trust one source for any precision measurement, but I was always matched or within a few tenths of his measurement.

What makes a DRO more reliable ?
well, it is setup on a non-binding ( free floating ie) installation : Note, they want +/- .001 for 30 inches on the install !
What about the caliper ?
Well you grab the thumb wheel and squeeze the part and the slop in your "head guides" twists and cocks the scale and you get ---Bogus numbers.

What is the proper use of a caliper for accuracy ?
Adjust the head so it just moves.. not loose.
Clean the jaws and check for parallelism and burrs and then zero.
Never use the thumb wheel for measuring !!!!!
Put your finger and thumb on the outside of the jaws DIRECTLY in line with the part at it's touching point and let the jaws tell you that you are square, then read the dimension.. simple and effective , but done wrong 99 % of the time
Off soap box

Rich

Starrett only made the 722 for a few years and then dropped it because battery life was very short ( it never shut off)
--too bad as I made a simple modification that gave it 6 months minimum or longer
to the case that eliminated the
Mr Ron
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Re: Digital caliper accuracy

Post by Mr Ron »

Rich_Carlstedt wrote: Sat Jul 02, 2022 11:02 am .
What is the proper use of a caliper for accuracy ?
Adjust the head so it just moves.. not loose.
Clean the jaws and check for parallelism and burrs and then zero.
Never use the thumb wheel for measuring !!!!!
Put your finger and thumb on the outside of the jaws DIRECTLY in line with the part at it's touching point and let the jaws tell you that you are square, then read the dimension.. simple and effective , but done wrong 99 % of the time
Off soap box

Rich
That is exactly how I made the measurements. I do have a set of "JO" blocks that I can check the caliper's readings.
Mr.Ron from South Mississippi
Mr Ron
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Re: Digital caliper accuracy

Post by Mr Ron »

rmac wrote: Sat Jul 02, 2022 6:21 am
BigDumbDinosaur wrote: Sat Jul 02, 2022 12:46 am That said, and like the others suggested, your testing should be done with shim stock, a feeler gauge, or some other solid medium of a known thickness.
Unless, of course, Mr Ron's goal in the first place is to measure the thickness of the paper! At this point we don't yet really know what he's trying to do.

-- Russell Mac
I just chose paper as it was at hand. If I used a feeler gauge, I would have to get the 12" long strips and cut it up into repeatable lengths. I build model trains (HO), so this is not a highly precise venture. I work principally with wood and some metals and plastics. ± 0.001" is accurate enough for me
Mr.Ron from South Mississippi
Rich_Carlstedt
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Re: Digital caliper accuracy

Post by Rich_Carlstedt »

Ok MrRon , I think you are dealing with rounding factors in your digital readings as
outlined by other posters.

I live in Green Bay WI, The Toilet paper capitol of the world. We have paper mills all over town and they make
other paper as well. So I have half a dozen friends who work in paper mill maintenance and one of the big
issues is a constant thickness and while they do a good job, if you were to really analyze paper, it looks like a plowed field because the roils are never exactly true when rolling and the rolls them selves may not be round even though they try like heck to make them perfect, but the result is ripples .. sort of like corrugated cardboard with one side ripped off. ..and it has grain as well - cross ridges
So if you want to do your test, measure each piece individually and mark the direction of the measurement
then you may get a better test
What you are looking at is called
"Stack Up Error" in Machine shops where you add pieces and get a different total dimension than design allows

rich
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Steggy
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Re: Digital caliper accuracy

Post by Steggy »

Rich_Carlstedt wrote: Sat Jul 02, 2022 11:02 amI find it absolutely hilarious that guys "diss' digital calipers ---and then want digital DRO's on their mill to read .0002" accuracy !

Dunno about the others, but I use the calibrated collars on the feed controls to get positions. That’s how I learned to do it over 60 years ago—well before DROs existed. It worked then and works now. Unless you are prepared to spend a lot of money, you will never get a DRO that can be trusted to be accurate and repeatable.
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