Followup question for digital caliper

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Mr Ron
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Followup question for digital caliper

Post by Mr Ron »

My understanding is that digital calipers cannot be depended on for accuracy. Does that also apply to vernier calipers?
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Bill Shields
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Re: Followup question for digital caliper

Post by Bill Shields »

Depends on how accurately they are engraved...and how good your eyes are ..
Too many things going on to bother listing them.
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Harold_V
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Re: Followup question for digital caliper

Post by Harold_V »

What Bill said, but it's more involved than that.
One of the obvious problems with calipers (doesn't matter what kind) is that it borders on the impossible to get dead alignment. Couple that with the fact that they are easily applied under too much pressure, the readings one gets are always suspect. It doesn't help that one of the negative habits that humans have is taking measurements while watching the instrument. That should never happen, as we have a way of forcing the reading we hope to achieve.

Calipers are fine for readings that have little value, but they are worthless when the reading must be absolute. It's not that they can't provide the reading, it's just that it's always suspect. If you work with calipers, you are likely to make mistakes in your readings. If you work to improper readings, it's difficult to achieve the desired results.

That said, I hope you can see that it isn't a problem in many cases, as the tolerance that is acceptable will often allow for functional parts to be made. The problem begins when you try to work to a given dimension that has a tight tolerance (like fitting a bearing). You achieve what you assume to be the proper diameter, but the bearing either falls in the hole or is so tight that the outer race has to collapse so the bearing will go in the hole. Neither scenario is acceptable.

It is not uncommon for the jaws of calipers to yield readings that are not the same when the same diameter object is being measured, inside and outside. That means that you have great difficulty when using a caliper to take an inside measurement from which you hope to establish a fit when using the outside jaws. It's precisely the same problem when the reverse is undertaken.

Wise people don't use calipers for critical measurements. The reason should be obvious.

H
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Re: Followup question for digital caliper

Post by Rich_Carlstedt »

Just some back up data on Harolds comments
three comments :

We made dies, very precise dies and Calipers were only used for rough measurements and for confirmation sometimes.
Reading Mikes all day - you want to make sure you are not .025" off ( Seen it fellows !)
and the calipers confirm ......note , confirm ..... not determine readings!

Second ,
is the ERROR by most everyone in properly using a Caliper--DO not follow factory pics..they are wrong
You should not use the thumb wheel or push the Read Head for measuring-- you are cocking the reader head
You need to straddle the jaws with your fingers 180 degrees across or even with the diameter
Greg Lewis had it almost perfect in his picture on the other digital caliper post - he just needed two fingers pinching the rod at the measuring point to get a perfect score :D .
http://www.chaski.org/homemachinist/dow ... =89866&t=1

Last
I prefer Dial Calipers to Digital Calipers and I have both , but here is why.
Just as Greg found error with old Digitals , beware it happens with new Digitals !
Was at a tool show and even though I have Starrett Digital 722 ($$$$) ,a B & S Dial ($$$) and a Mitotoya Digital ($$) I saw a
Chinese Digital for 35 bucks at a stall, and bought it for general purpose around the shop .. Only used it for maybe two months
and never dropped it or damaged...it was cherry, so imagine my surprise when I measured a part ( example here ) and it was
.807 and I thought when I measured it earlier , it was .755 and that was weird and I wiped the jaws and did it again and it was the same
and I closed the jaws and it said + .052". But I always zero out the digital beforehand ??? So I figured I didn't do that but why such a weird number . Then two days later the same thing- the number changed between measurements.. and into the garbage can it went. ...so beware
Now back to the B & S and the Mit. but always the mikes !

Rich
Mr Ron
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Re: Followup question for digital caliper

Post by Mr Ron »

I am using a Mitutoyo 12" vernier caliper. I don't see the difference between a micrometer reading and a vernier caliper reading. I can understand the difference between a digital and micrometer, but not the vernier and micrometer. Both vernier and micrometer rely on the "pressure" exerted by ones fingers. Are you saying a vernier height gage is not accurate when scribing lines; how else would I do it? I also have a Starrett 12" vernier height gage. I understand that scribed lines are not precise, but must rely on machine settings for accuracy.
I could take a stack of "Jo" blocks and scribe a line, but it would not be accurate, as one must take into account the sharpness of the scriber. For a scribed line to be 100% right-on, it would not be seen, as a line has width that can amount to several thous in width.
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Re: Followup question for digital caliper

Post by Greg_Lewis »

Rich_Carlstedt wrote: Sat Apr 23, 2022 6:58 pm Greg Lewis had it almost perfect in his picture on the other digital caliper post - he just needed two fingers pinching the rod at the measuring point to get a perfect score :D .
http://www.chaski.org/homemachinist/dow ... =89866&t=1

Hey Rich: It was hard to hold the caliper and the rod with one hand and the camera with the other. If I do it over and swap out the photo would you raise my grade? :lol:
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Celebrating 35 years of turning perfectly good metal into bits of useless scrap.
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Harold_V
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Re: Followup question for digital caliper

Post by Harold_V »

Mr Ron wrote: Sun Apr 24, 2022 12:05 pm I am using a Mitutoyo 12" vernier caliper. I don't see the difference between a micrometer reading and a vernier caliper reading.
My thoughts.
That's because you have no way of knowing if the reading is correct, or not. If you assume the reading to be correct and move on, of course it will appear to be so. You could prove the error in your thinking by trying to make fits based on your readings, however. I've discussed that at great end on numerous occasions.

While a micrometer can yield an erroneous reading, it's highly unlikely unless in the hands of one who has no clue. A micrometer has the potential to make reliable readings within a tenth, while a caliper isn't capable of making RELIABLE readings within a thou. By its design, a micrometer is self leveling and has a distinct "feel" when applied. That's particularly true of small diameters. For larger diameters, a micrometer might be subject to one who is ham handed and has no clue when to stop turning the barrel, but those who are seasoned in their use tend to have developed a feel that is reliable. By sharp contrast, that isn't necessarily true of a caliper, and it doesn't matter if it's vernier or not. Experience with their use is certainly to advantage, but the reading obtained with a caliper will always be suspect because you have no clue if it has been taken properly or not. They offer a degree of error that can be seriously damaging when one is attempting to do critical work.
I can understand the difference between a digital and micrometer, but not the vernier and micrometer.
In the world of critical work, a caliper, doesn't really matter what type, isn't able to discern tenths. They're borderline incapable of determining a thou, for that matter.

A hard fact. I was assigned to precision grinding for several years. Using a common tenths reading micrometer, a feel could be easily developed whereby one could make reliable readings to .000050". That was proven time and again by having a Sheffield shadowgraph at the grinding station, where readings could be verified. There are no known calipers that have that capability. To assume that a micrometer and caliper have the same capability isn't reasonable.
Are you saying a vernier height gage is not accurate when scribing lines; how else would I do it?
Unlike a caliper, which normally relies on hand pressure to establish readings, a height gauge does not. It will be as precise as one's ability to read the display and set the scriber. As you alluded, you will still be at the mercy of the thickness of the scribed line. A dead sharp scriber can go a long way towards greater precision, as would one's ability to see the resulting line(s). More importantly would be what one does with the line, however. If your intended purpose is to center punch for location, you've already ignored the degree of accuracy you might have created. If there's anything I hold in greater contempt than a caliper, it's a center punch. Greater reliability is much more likely to be established if one chooses a wiggler instead of a punch and hammer, but that, too, is subject to error.

If one hopes to work to high precision, scribed lines are not required. The use of a DRO, or even trusting the screws, can yield a remarkable degree of precision, assuming one has a firm understanding of backlash and that locks tend to move slide locations (true of common mills, not necessarily true of a jig borer). One moves to the desired location and verifies the move by using a rule. It won't disclose an error of a couple thou, but it will readily display a turn of the handle error, or even a small fractional error. Gives one the opportunity to correct an error before scrapping the part. It slows the operation, but the time saved that would be wasted making a new part to replace one scrapped by a dimensional error can be huge.

What's important is that one understands the potential for error in any given method---and that a consistent way of applying the instruments in one's possession is exercised. Respectable accuracy can be achieved (or not) by one's work methods.

H
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Re: Followup question for digital caliper

Post by Mr Ron »

Thank you, Harold, for your as usual professional response to a "hacks" question. I can never achieve the understanding about accuracy and precision that you have collected over the years. As I am just an amateur machinist, I have and always will never attain that level. I'm happy just to get things I make work. At 87, I don't think things will change. I can usually work to a thousand, but not without a lot of effort and remake of parts. A file is my best friend.
Mr.Ron from South Mississippi
Lew Hartswick
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Re: Followup question for digital caliper

Post by Lew Hartswick »

As Rich_Carlstedt said in ;
<Second ,
is the ERROR by most everyone in properly using a Caliper--DO not follow factory pics..they are wrong
You should not use the thumb wheel or push the Read Head for measuring-- you are cocking the reader head
You need to straddle the jaws with your fingers 180 degrees across or even with the diameter
Greg Lewis had it almost perfect in his picture on the other digital caliper post - he just needed two fingers pinching the rod at the measuring point to get a perfect score :D .>

I have been trying for 20 years to convince hundreds of students and a few teachers to do that with mixed success . :-)
...lew...
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Re: Followup question for digital caliper

Post by Rich_Carlstedt »

]
Harold_V wrote: Sun Apr 24, 2022 5:26 pm .............................................................................................
While a micrometer can yield an erroneous reading, it's highly unlikely unless in the hands of one who has no clue. A micrometer has the potential to make reliable readings within a tenth, while a caliper isn't capable of making RELIABLE readings within a thou.
..............................................................................................
What's important is that one understands the potential for error in any given method---and that a consistent way of applying the instruments in one's possession is exercised. Respectable accuracy can be achieved (or not) by one's work methods.

H
Harold, I think your post is dead on in intent and instruction. I vouch for most all of what you said, but have some additional experiences that can add to your comments .
Having worked in a precision Die Shop for many years , there are rules on using Calipers as well to get good reading accuracy and foremost was my comment of maybe 5 posts ago , using the tool knowledgeably is similar to your comment about micrometer use. It is absolutely amazing how skilled machinists in all fields do not know this simple rule !
I have one correction about your remarks on caliper tenth reading, A Starrett 722 ($$) could read tenths, I know because I carried one for years to double check my die makers' work. And I still have it !
Do not be mistaken , whenever precision work is being measured, using one inspection tool is a no-no. AT least in die shops and my shop , that is the rule.
The 722 had a short life in the market--- due to battery usage and price ( $200 /1986) ..which is too bad as I had a modification that made battery life comparable to ordinary digital calipers. I should have told Starrett back then about a simple mod.
Recently there was a very long post on the PM website and all the readers there ( but one) say they have never seen a .0001" reading caliper and that such an animal did not exist ! Too bad they never crossed paths with a Starrett 722
It shows that not everyone knows all in this hobby or trade ...lucky we have this website to share information !
Rich
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Harold_V
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Re: Followup question for digital caliper

Post by Harold_V »

I have no qualms about the Starrett instrument resolving tenths, Rich. Any maker can provide such an instrument if that be their choice. My qualm is in the value of the reading. In spite of one's best attempt, regardless of what the display may say, the reading is suspect when relied upon to three places. Four places? Not worth reading, and that's what's wrong with calipers in general. They simply can NOT provide a reliable reading. The size you read is rarely the true size. How does one do fine work when the readings are suspect?

The first caliper I purchased was a Helios, way back in 1958. I still have it. I still use it, in spite of the fact that it looks like a thousand miles of bad road. It is not capable of resolving even a thou, but it's great for measuring small bits of scrap material when I'm searching for a piece to fulfill a need for a project. I even use it when roughing a part, but never when the dimension is critical. If I happen to drop it, no harm done. It's already no good. A guy needs a tool that is disposable.

H
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Re: Followup question for digital caliper

Post by Steggy »

Digital calipers are a classic example of the adage “new technology isn't always good technology.” I'd trust a vernier caliper before a digital one, which isn’t saying a whole lot.
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