How acurate is acurate? Digital calipers verses micrometers

[Metal Mickey]

I won't take up too much of your time here. A full write up if you are interested in the detail can be found herehttp://mikes-models.com/blog/?p=218.

Basically I had reason to check some work and found a big difference between the readings of my micrometer and the digital caliper I was using at the time. This lead me to compare other measuring instruments I have. I was surprised with the variation! Perhaps I was naive in my reliance on all things digital!

All equipment was cleaned thoroughly for the results shown in the picture below. I didn't have another mic to compare but it and two other instruments were very close so I believe the difference is in the digital calipers.

What do you think?

For information the micrometer reads just over 0.6 (about 1/3rd of a degree)

 

[rake60]

I closed my cheap Harbor Freight 6" digital dial calipers and pushed
the "Zero" button. Then I check them with every standard
I have in my 0 to 6" micrometer set.

It was dead nuts on with ever one of them from 1" to 5"
Have you tried checking the calipers with standards?

Rick

 

[Mcgyver]

Mike,

in the olden days before every caliper was dial or digital, readings were done via verniers, the vernier caliper.... or as i was taught the very-near caliper. Calipers just aren't a precision measurement tool, they are quick, convenient and approximate. when I'm turning i have both on hand, the caliper to get close then the micrometer when it matters. So first rule is, calipers aren't very accurate compared to mics.

Second thing is, even the best micrometer can't trusted unless checked and if necessary adjusted.... it should be dead on new out the box but 'should' isn;t certain so a standard is supplied. If its a good quality mic, there should be a very high degree of repeatability using the ratchet or friction thimble. looked at that way, they are all comparators; if you don't start them from a good reference, you don't know how accurate the reading is, .

what would be interesting is to set them all dead on at one size, say .100", then measure say .900 gauge block and compare them.

Digital is ok, I use both, but unless there is a technological reason underpinning why the digital technology is more accurate, its not (more accurate). I sort of like digital, but it carries lots of down stroke; heavier, more awkward (especially mics) batteries to run out and in the case of calipers you lose the quick visual reference of a needle and dial - similar reasoning why a digital speedometer is terrible compared to analog. I usually grab for a dial caliper over digital when roughing i can see I'm about a turn and and quarter of needle or about .130" thou away from say a desired .5625". So I'll feed in 50 thou and think about finish cuts.... with a digital i have to actually read the numbers and think a bit more to work out .692 to .5625. small thing, yeah, but it makes life easier imo

 

[mklotz]

Most people won't trust calipers to better than 0.002". Both my Mitutoyos match all of my micrometers to better than 0.001" if used properly. The (digital) Harbor Freight calipers aren't that good. They are good for intimidating clerks at the big box stores, though.

Take a gage pin and hold it in your left hand while measuring it with the calipers in your right hand. Now, do it the right way. Close the calipers on the pin and then use your left hand to gently snug the jaws against the pin. This overcomes the lever effect of the first method and produces a more accurate reading.

If you're going to check a micrometer for accuracy, it's important to use a set of gage blocks that cause the spindle to seat at different orientations so drunken thread errors will be noticed. The preferred set for inch micrometers is: 0.105, 0.210, 0.315, 0.420, 0.500, 0.605, 0.710, 0.815, 0.920, 1.000. For metric micrometers the preferred set is: 3.1, 6.5, 9.7, 12.5, 15.8, 19.0, 21.9, 25.0.

 

[rake60]

Very well stateted by both Marv and Mcgyver!

Calipers are not, nor were they ever intended to be a
replacement for a micrometer.

It is quite easy to get an errant reading from calipers.
It is also easy to twist, squeeze and manipulate them to
cause them to show the answer you want to see.
I still use vernier calipers at work up to a 120" range.
Using those is a 3 person job. One holding the "dumb end"
one turning the thimble and a third exerting a gentle upward
pressue in the middle of the beam span to eliminate the sag
that can cause them to read up to .005" too large on the size.

Calipers are close enough for many applications.
If it's a measurement for a press fit or close running clearance
fit, grab the mics!

Rick

 

[Stan]

I can't get my nose close enough to the monitor to read the bottom mic but the the two top calipers and the top mic read within .001" which I expect is their published accuracy. The bottom caliper shows .003" difference from the top three. Follow the suggestions for zeroing and checking with a standard and if it still fails decide if it is worth the price to have repaired.

Even a .003" accuracy is good enough to use when looking in the scrap box for a suitable piece of material.

 

[Maryak]

Mike, or should it be Vern?

Here's my thoughts on it.

If you start with calipers - finish with calipers.

If you start with micrometer - finish with a micrometer.

I always seem to get things out of wack when I change from one to the other on the same bit of metal.

All the remarks about twist and sag etc. are absolutely correct.

But if the tool is in your hands you are most likely to create a consistent error with either, especially in the bigger sizes.

 

[Metal Mickey]

Interesting comments so thanks for that. As an update I did partly what Mr Mcgyver suggested and did a series of measurements against the best that I have, ground parallels. I don't have a set of gauge blocks unfortunately (I am always looking on eBay for a 'bargain') and I got repeated good results with the digital micrometer (the one at the top of the photo), one digital caliper and the micrometer.

I can't get my nose close enough to the monitor to read the bottom mic but the the two top calipers and the top mic read within .001" which I expect is their published accuracy. The bottom caliper shows .003" difference from the top three. Follow the suggestions for zeroing and checking with a standard and if it still fails decide if it is worth the price to have repaired.

Even a .003" accuracy is good enough to use when looking in the scrap box for a suitable piece of material.

The three in the photo which read from the top as 0.600, 0.6015, 0.60060 (the digital micrometer), 0.597 and finally the micrometer with 0.600 and 1/3rd of 1 thousand.

My post is one from a novice engineer. In that guise you know very little of what is acceptable until someone tells you (hence the high value of this forum). However I do realise that they need cleaning and resetting, its the fact that there is a variation in results from different digital calipers that's the surprise!

The two digital calipers that consistently give incorrect readings are one which was a free gift from a supplier if you spent more than x pounds and the other the cheapest digital caliper I could find. There must be a moral there. I thought that because electronics were the same that all digital calipers would read the same. They do not!

So I think, from what others here have said, is that novice model engineers should not believe everything they read on a readout.

On a personal level I like using the micrometer the best. However there are some things that calipers are useful for so now I will always test against a known dimension if the task warrants that level of accuracy. the other calipers will be laid to rest and if or when I replace my digital calipers I will not go cheap or free!

How this all impacts on team builds is a question I would like to know. When working alone the fit of a piston into a cylinder for example, can accommodate some variation. Sending parts to others however must be different? There have been other team builds and well known engine builds by groups so perhaps I am seeing a problem that in reality, doesn't exist.......or does it?

 

[Mcgyver]

hey Mickey, if i left the impression one needs a set of gauge blocks around the home shop, I've erred. you won't need them/use them in day to day work...but... in the context of the discussion you opened (studying/comparing accuracy) they are relevant. More practical would be a micrometer standard, kind of like a single gauge block, that often comes with the micrometer or can be bought separately - this obviously needs to be reliable so choose a quality one