The digital caliper caper

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Greg_Lewis
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The digital caliper caper

Post by Greg_Lewis »

Digital calipers. Some folks like them, some folks don’t (I’m looking at you, Harold). But I remember as a teenager working on my first car I needed a part to be turned in a lathe, which I did not have. I had a friend who had a lathe, and the first thing he said when I presented him with the part was, “How accurate does it have to be.” I’ve always remembered that. There is no need to work to .001 when .010 is good enough. And sometimes even a pocket scale is good enough.

So I like my Mitutoyo digital calipers because they are good enough for most of what I do, and they do the math for me. I can set them for the target dimension and then they will tell me how much to remove. And it’s easy to pop back and forth between imperial and metric if needed.

But the other day I was turning some brass and I needed the piece to be good to ±.001. So I used the calipers to get close and then got out the micrometer. To my surprise there was a .007 difference in the readings. Huh? Both of these instruments are Mitutoyo and I do know how to use them.

So I fished a 1-inch micrometer standard out of the toolbox and both instruments checked out within a half a thousandth. Hm. That’s what it should be. I checked multiple times and got the same results. I put a 2-inch standard between the caliper jaws and got 1.999. Good enough. But there was still a .007 error on the part.

Now I’m thinking I must be reading the micrometer wrong. So I slow down, clear the clutter from what’s left of my mind and take a long look at the spindle of the mic. Nope. I’m not reading it wrong. There is most certainly a .007 difference between the two instruments when I measure my part.

After some thought, it occurred to me that the part I was making was in the quarter inch range, so perhaps I needed to check a different piece of material at about that point. So I took multiple measurements of the diameter of a bit of ground stock that was about .250 with both instruments and the error appeared every time. The caliper consistently read .007 more than the micrometer.

So why would I get a good reading at 1 inch and a bad reading at a quarter of an inch? The battery is good, and if it was bad that would affect all readings, not just those at one spot. Just to be sure, I cleaned the contacts inside the little battery case on the caliper slide and checked the battery with two different meters. The scale along the caliper bar looked clean but I cleaned it anyway. No change.

Now this is starting to bug me. I checked a dimension in the 3/8 diameter range and there was an error but not as much. Hm.

So I started thinking about how these calipers work. The head obviously reads by sensing the scale. And if a reading at one point is good but is bad at another, there must be something wrong with the interaction between the head and the scale.

So I decided to take the head apart and see what I could find. Under the label on the back are the four screws that hold the electronics onto the head. (I’ve had these for perhaps 30 years; you can see in the photo below that this one is serial number 91.)

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Here is what I found. Look to the right end and you’ll see little gobs of gook stuck to what appears to be a pc board. There is some to the lower left and some above that, too. Not too bad, really, for three decades of fumbling with greasy hands.

003.JPG


A little work with a cotton swab and some alcohol cleaned that off. Then I noticed the two little rubber wipers, one at each end, that obviously are supposed to keep the gook out of the insides. The tip of my screwdriver points to the one on the right, and you can see the dirt on both of the wipers.

004.JPG
Scroll down for part II.
Greg Lewis, Prop.
Eyeball Engineering — Home of the dull toolbit.
Our motto: "That looks about right."
Celebrating 35 years of turning perfectly good metal into bits of useless scrap.
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Greg_Lewis
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Re: The digital caliper caper

Post by Greg_Lewis »

The wipers pull out with tweezers and some more alcohol on a cotton swab took care of the mess.
006.JPG
007.JPG


I thought I’d also check the sliding parts of the head, so I pulled out the bronze strip that sets the tension on the head and keeps its jaw parallel with the fixed jaw of the caliper. Removing the two screws in the photo below allowed me to pull out the strip. The screws aren’t the same so if you do this, pay attention to which one goes where. It was no surprise that the bronze strip also was gooked up.
008.JPG
009.JPG


When replacing the screws I used a very small amount of the blue Loctite — barely enough to wet the end of a toothpick — and adjusted them carefully so there was just enough drag on the bronze strip to keep the sliding jaw from being tipped out of parallel with the fixed jaw. It’s an extremely small amount of turn on the screws.

I decided not to take the sliding head off the beam since there were rivets at the outboard end holding a keeper in place. I’d have to grind off those rivets and then come up with a replacement scheme, and since this is hard stainless, I didn’t want to deal with that.

So the final result is that the cleaned calipers measure to within .0005 at one inch, and to the same at a quarter inch. Better than they have been in a long time and good enough for me.
010.JPG
011.JPG
Greg Lewis, Prop.
Eyeball Engineering — Home of the dull toolbit.
Our motto: "That looks about right."
Celebrating 35 years of turning perfectly good metal into bits of useless scrap.
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NP317
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Re: The digital caliper caper

Post by NP317 »

Thanks for that lesson.
I will be checking my Mitutoyo calipers tomorrow. Also several decades old.
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Harold_V
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Re: The digital caliper caper

Post by Harold_V »

Greg_Lewis wrote: Fri Apr 22, 2022 9:11 pm Digital calipers. Some folks like them, some folks don’t (I’m looking at you, Harold).
Chuckle!

Well, truth be told, it isn't really about digital, but calipers in general. They simply aren't up to the task of precision readings, as you discovered.

That was a nice report on the issue and its resolution. Great investigating! :wink:

As much as I don't like calipers, I own several of them. I rarely rely on them (for reasons mentioned above), but they get use almost daily. In most instances all I want to know is an approximate reading. When it matters, out come the micrometers.

H
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earlgo
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Re: The digital caliper caper

Post by earlgo »

Years ago I took Harold's advice and use calipers for rough and dig out the mics for finish work. I have digital calipers, but depend mostly on the good old Helios vernier type that I have had since the '60s. Must be good because it says on the tail end "Hardened Throughout Germany. Doesn't sound cost effective to me, but that is what it says. For my "reverse engineering" tasks I use a KURT set that has a dial caliper and vernier micrometer.
Thank you, Mr. Lewis for the info.
--earlgo
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Richard_W
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Re: The digital caliper caper

Post by Richard_W »

I use 8" digital calipers daily at work. Its the first tool out of the box along with a 6" scale. I use them for most every thing, except close tolerance work. Then out come the micrometers. I get a new one every few years when the jaws wear a tapered. We are doing more and more metric work and the instant conversion from inch to metric is really handy. I have one of the inexpensive Harbor Freight 4" digital calipers. They come in real handy reaching into tight spaces and larger bores to measure groove width and location. One tool I bought about 10 years ago is the digital double hook caliper. That is one I pull out just about every day. Faster and easier to use than a depth micrometer in the lathe. Although it doesn't replace a depth micrometer. Also for checking groove widths and location in bores. I use an 8 inch and would like to have a 12 inch. But at 68 I don't see myself working much longer in the trade.

https://www.ebay.com/itm/265576345763?h ... SwS1ZiIEjM

I have done the cleaning on several calipers over the years and found much of what you posted. Good job and thanks for sharing!

Richard W.
RSG
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Re: The digital caliper caper

Post by RSG »

Great read and good investigative work! I try to keep my calipers as clean as possible at all time and if I'm turning with coolant I use my coolant proof set.
Vision is not seeing things as they are, but as they will be.
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Steggy
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Re: The digital caliper caper

Post by Steggy »

Harold_V wrote: Sat Apr 23, 2022 2:00 am
Greg_Lewis wrote: Fri Apr 22, 2022 9:11 pm Digital calipers. Some folks like them, some folks don’t (I’m looking at you, Harold).
Chuckle!

Well, truth be told, it isn't really about digital, but calipers in general. They simply aren't up to the task of precision readings, as you discovered.

Yep! I look at a caliper in the same vein as a steel rule. Good enough for “close enough” but not for thousandth-of-an-inch work. The latter is micrometer territory.
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Mr Ron
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Re: The digital caliper caper

Post by Mr Ron »

BigDumbDinosaur wrote: Sun Apr 24, 2022 9:42 am
Harold_V wrote: Sat Apr 23, 2022 2:00 am
Greg_Lewis wrote: Fri Apr 22, 2022 9:11 pm Digital calipers. Some folks like them, some folks don’t (I’m looking at you, Harold).
Chuckle!

Well, truth be told, it isn't really about digital, but calipers in general. They simply aren't up to the task of precision readings, as you discovered.

Yep! I look at a caliper in the same vein as a steel rule. Good enough for “close enough” but not for thousandth-of-an-inch work. The latter is micrometer territory.
See my follow-up post .
Mr.Ron from South Mississippi
Conrad_R_Hoffman
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Re: The digital caliper caper

Post by Conrad_R_Hoffman »

I have those same mics as in the second post and they're extremely good. They'll split tenths with a bit of practice. We use Mitutoyo Digimatic calipers here at work and trust them to about a thou for outside measurements only. Since they don't get used near coolant, they don't seem to get dirty. Inside measurements are another story. The jaws have a flat and the reading is a bit dependent on hole size, plus the agreement with the other jaws is never perfect. I have a Starrett 120 dial set and those are about as good. Now, if you're fitting a bearing and accumulate 0.002" of error, you'll probably notice it. :shock:
Conrad

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RONALD
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Re: The digital caliper caper

Post by RONALD »

Even Robots prefer Vernier over Digital!
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