Digital caliper accuracy

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David2011
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Re: Digital caliper accuracy

Post by David2011 »

Seems like every time someone questions the accuracy of digital calipers the conversation is redirected to micrometers. I’ve owned three dial calipers in 32 years. Maybe I’ve been lucky or maybe it’s technique. All three have given me the same results as my Starrett mics to better than .001.

The digitals have been consistently within .001” of the mics and dial calipers as long as the batteries were good. I generally use a mic on the lathe just because it’s easier to see the numbers on it compared to calipers. Digitals are great for lower precision measuring like reloading ammunition and checking the length of fired rifle cartridges prior to sizing because they’re so easy to read.

My formal machining training is in gunsmithing so even though I’ve only been machining for about 15 years the techniques are very traditional and manual.
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Harold_V
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Re: Digital caliper accuracy

Post by Harold_V »

The problem with resolution within a thou is that critical fits are more demanding of size than that. If you're trying for a snug slip fit, even a half thou is more than enough to eliminate the desired clearance, be it too much or too little. If you're trying for a proper press fit of a drill bushing, a half thou is enough to either allow the bushing to fall through the hole, or to shrink the bore so the drill won't enter, or it runs tight. Neither of those scenarios are acceptable, and calipers generally can't even provide that level of resolution.

Like I said, if you don't have a reliable reading of the present size, you accomplish your target size only by chance. Key here is to know when to use the proper tool for the intended purpose.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
Rich_Carlstedt
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Re: Digital caliper accuracy

Post by Rich_Carlstedt »

Harold_V wrote: Sun Jul 03, 2022 3:34 am............................
Like I said, if you don't have a reliable reading of the present size, you accomplish your target size only by chance.
Key here is to know when to use the proper tool for the intended purpose. H
Perfect response Harold !
Just like the old adage " You don't take a knife to a gun fight "
In machining as you go from .0" to .0000" you need to be aware of
1. what inspection tool works and what is it's-------- reliability,repeatability and accuracy
2. And what your work pieces have that give----------reliability,repeatability and accuracy

Rich are you saying that right ? Yes----- because it is not thought of by many guys wanting to get good dimensions on their parts
Just because a mike or a caliper gives you a Dim, does not mean it's right
And even with the best instruments, a part can give you wrong info... to just name four variables
you have surface finish,burrs ,tapers and temperature !

As Harold said
" if you don't have a reliable reading of the present size, you accomplish your target size only by chance"

So when you need a critical dimension, in my experience, you never trust just one measuring tool
And look at your part---carefully !

Rich
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NP317
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Re: Digital caliper accuracy

Post by NP317 »

Those are all techniques acquired with time and experience.
One's teacher makes a big difference in how quickly we learn(ed) those useful lessons.
And this forum offers extraordinary help getting us there.
RussN
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liveaboard
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Re: Digital caliper accuracy

Post by liveaboard »

I think it depends on the accuracy required for the job.
I use Google earth to measure things to within a couple of feet (fence wire, underground pipes and wires, etc.) It sure is fast, don't even need to stand up.

A tape measure will do to within a fraction of an inch.

Calipers are very useful, and then comes micrometers for the finest things I can still work on.
More accurate than a micrometer is for those with greater skill than I.
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Harold_V
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Re: Digital caliper accuracy

Post by Harold_V »

liveaboard wrote: Sun Jul 03, 2022 12:42 pm I think it depends on the accuracy required for the job.
True!
The problem is some folks get the idea that readings from a caliper are gospel, and they typically are not. There's way too much history with their use to assume otherwise. Used within their parameters, especially for a job where "close enough" is close enough, they're a good and valuable addition to anyone's toolbox. Mine happens to contain five of them. I have more micrometers than calipers, however. The background from which I came demanded greater precision and reliability.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
Lew Hartswick
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Re: Digital caliper accuracy

Post by Lew Hartswick »

<The problem is some folks get the idea that readings from a caliper are gospel> Those are the folks that think " If it's on the internet it has to be true" . :-)
...lew...
David2011
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Re: Digital caliper accuracy

Post by David2011 »

“Accuracy required for the job.”

ABSOLUTELY! I use calipers for woodworking, building RC airplanes and reloading ammunition. They’re more than accurate enough for the first two and fully adequate for the latter. I’m just a hobbyist and find that as my skills improve the tasks tend to get more demanding thus requiring greater precision and accuracy. If I live enough years maybe I’ll be lucky enough to need something better than a tenths mic. Gonna need a whole lot better lathe first.
whateg0
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Re: Digital caliper accuracy

Post by whateg0 »

BigDumbDinosaur wrote: Sat Jul 02, 2022 4:57 pm
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.
I've done it. Got tired of losing track of the turns. Was that 23 turns? Or was I starting number 23? Also, those dials may or may not be as accurate as a DRO, especially over a distance. How much wear is in the leadscrew? CNC allows mapping of the screws for just that reason.
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NP317
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Re: Digital caliper accuracy

Post by NP317 »

I learned early machining without the benefit of DROs on lathes or mills. It works.
Now, however, I am always glad I installed DROs on my machines!
Repeatability has been as required, and the programs for bolt hole positions and geometric forms are remarkable.
Also the quick switch from absolute to relative positioning definitely speeds up my work. Productivity increased.

"Glad I met that devil named DRO.
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atunguyd
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Re: Digital caliper accuracy

Post by atunguyd »

Agreed, a DRO doesn't make things more accurate but does increase speed massively

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ctwo
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Re: Digital caliper accuracy

Post by ctwo »

Mr Ron wrote: Sat Jul 02, 2022 2:25 pm That is exactly how I made the measurements. I do have a set of "JO" blocks that I can check the caliper's readings.
This is what I would do and check over the range of the instrument. IME, when high accuracy was important, calibrated standards are used as a measurement reference, and the measuring tool compares the readings. This reduces many variables. Depending on required accuracy, there may be shop standards and then inspection room standards which are traceable. I bought a gage block set from the shop where I used to work. I knew the owner and the set was used for one job that required NBS traceable measurements, so he had it sent out to be certified. It's a shop grade set (+/- 50 millionths) and it is well within spec. The parts were sent to an inspection lab to be verified.

For grins, I took out some of my favorite tools and measured my 0.500 (+14) standard.

For nostalgia, the old B&S venier set, a pair of competing B&S and Mit mics, my favorite B&S caliper, and what most amateurs probably use.

Vernier: 0.4997
B&S and Mit mics: 0.5001
B&S caliper: 0.5000 (reads to quarter thou - yes, there is a dot between each thou and it's easy to tell if the needle is on the line/dot, or in between the line and dot)
Pittsburgh: 0.5000 (reads to half a thou)

Image
Harold_V wrote: Sun Jul 03, 2022 3:05 pm I have more micrometers than calipers, however. The background from which I came demanded greater precision and reliability.

H
It takes [edit] twelve mics to equal one 6" caliper. :lol: But I probably have as many 1" mics as I do calipers.
Standards are so important that everyone must have their own...
To measure is to know - Lord Kelvin
Disclaimer: I'm just a guy with a few machines...
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