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Accuracy of Measuring Tools

07/02/2011 11:32 AM

Which tool can be depended on day-in and day-out for accuracy and consistancy: a micrometer, vernier caliper, digital caliper or a dial caliper? Assuming all are high quality tools, like Starrett or Mititoyo.

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#1

Re: Accuracy of measuring tools

07/02/2011 11:47 AM

geometry and size of component is important in measuring device selection, But for me a one thou least count micrometer is far better then of a 10 micron least count digital caliper,

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#2

Re: Accuracy of measuring tools

07/02/2011 11:48 AM

Go laser, you don't have to account for temp changes

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#3

Re: Accuracy of Measuring Tools

07/02/2011 2:28 PM

Unless you are downloading the data to a PC, you probably won't ever know.

I think it depends more on the user than the instrument. I prefer mechanical devices myself. I grew up with vernier mics and calipers. I have digital, but if I want to get down to tenths of thousands, I get the verniers out.

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#5
In reply to #3

Re: Accuracy of Measuring Tools

07/02/2011 3:26 PM

I'm in need of an instrument that can measure between one and two inches. I don't have a 2" mike, but I do have all types of calipers that will handle that range. I only need to read to thousands of an inch. Do I really need to get a 2" mike or will a vernier be precise enough. I am an amateur machinist, not a maker of precision optics or lasers.

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#6
In reply to #5

Re: Accuracy of Measuring Tools

07/02/2011 4:28 PM

IMHO a vernier or dial caliper will be able to easily meet these requirements. The only thing I would consider you needing a micrometer then would be to validate the precision of your caliper with your micrometer by having both measure the same object that can fit in the jaws of your micrometer.

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#7
In reply to #5

Re: Accuracy of Measuring Tools

07/02/2011 4:53 PM

I'm assuming it's an outside measurement, so I don't think it matters. You should be able to get within .001". It's still a question of technique with the instrument. Consistent closing pressure (feel) is the most important thing.

Unless you "just want one", I don't think you need to buy a new mic.

Assuming you are measuring in a comfortable environment, thermal expansion should not come into play with the things you and I do.

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#8
In reply to #5

Re: Accuracy of Measuring Tools

07/02/2011 5:20 PM

I have a set of high accuracy mics (1 and 2 inches) and a number of dial calipers. My digital Mit caliper has been repeatably accurate to .001", but is rated for .005".

In reality, you should use the manufacture's rating for the instrument and that instrument should be calibrated.

However, I find you get what you pay for and my Mitutoyo calipers have been remarkably accurate beyond the rated accuracy.

Lastly, accuracy depends on your needs. If you are trying to make a slip or press fit you probably need to be with .0005". If you are fixing a wheelbarrow 1/8" is probably more accurate than you need. Your eyeball can easily do that.

Tools are something you add slowly and as you need them. I have a very large collection of tools, but I did not get them overnight. They are a collection over decades (in some cases) and with each new tool my reach is extended just a little further as is my knowledge base.

When I buy tools I usually spend the extra money (unless it is going to be a disposable tool) for quality because my tools are mine for life and I don't want to waste money having to replace them.

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#51
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Re: Accuracy of Measuring Tools

07/06/2011 1:20 PM

If the anvils of a mike (surfaces clean) are at zero, can I assume any readings will be accurate? If so, do I really need a standard gage to check at other than zero? I use the thimble friction device to keep the pressure constant at any reading.

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#53
In reply to #51

Re: Accuracy of Measuring Tools

07/06/2011 4:35 PM

For careful home use, without dropping, probably not.

For industry use, yes.

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#60
In reply to #51

Re: Accuracy of Measuring Tools

07/08/2011 8:52 AM

Micrometers do not usually get enough wear to be significantly out.

Heat from poor handling technique can be and influence, as can over tightening.

But if you think about it, checking a 1" mic against a 1" gauge block, when most of the time it's been measuring ~ 1/2", in terms of screw wear, is asinine.

Given you are using the ratchet chances are, it's accurate - as in 'never over tightened' and has 'zero' anvil wear.

The only other 'possible inaccuracy' is from how many times it's been wound in and out (under no load) so the 'backlash' in the thread.

This its likely to be - to use the old toolmaking trade expression - 3/5ths of 5/8ths of fquc all.

And given I have been drawn out of lurking on this thread - the thumb wheel on precision/quality, calipers is designed to give the same 'consistency' as a mic ratchet.

Sure you can use the 'finger pinch' technique as raised by AH - if the object is within 'comfortable hand span' - I do it myself - especially with the plastic rubbish calipers electronics folk seem to hold dear - but it hardly applies to measurements over 200 mm.

So it's not a bad idea to learn consistent thumb-wheel technique and a feeling for squareness to the job. (squareness being what finger squeeze is about finding)

In terms of the OP - My 'carry everywhere' is a Mitutoyo 200 mm x 0.05mm dial caliper. I've had it for near on 30 years. It's still 'pointer width' accurate, ~ 0.01mm - throughout its range.

But the reality in production/manufacture, is not 'absolute dimension'; it's 'fits'.

For this you want a 'comparator' and a dial caliper is basically that tool.

If there is a rule - aside from proper cleaning and handling, it's: never lend it.

(as predecessors taught)

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#61
In reply to #60

Re: Accuracy of Measuring Tools

07/08/2011 10:05 AM

Yea

I was wondering when there would be a rebuttal to the "experts"

a practical method for perfecting your caliper feel

the reference being a measurement using the micrometer's ratchet

measure the same part with the caliper, when you are using the same "feel" the 2 measurements will match

the "squeeze the jaws" method comes up short when the measurements are inside

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#11
In reply to #5

Re: Accuracy of Measuring Tools

07/02/2011 11:48 PM

Mitutoyo and Starrett are excellent will last a lifetime and are comfortable to use. Cheap tools are a pain to use and will let you down when you need them the most costing you time and money.

-Your number one measuring tool should be a full flexible 6 inch steel rule. +/- .015"

-The 6 inch or 4 inch Mitutoyo Absolute digital vernier is a must have item for your number 2 measuring tool. Cheap verniers are not absolute and eat so many batteries within one year are more expensive than Mitutoyo or Starrett. +/-.001"

-Your number three measuring tools should be a 1", 2", 3" set of Starrett or Mitutoyo carbide tipped .0001 graduated micrometers. For high precision or measuring round objects micrometers are a must. +/-.0001

After 30 years I have my original Mitutoyo micrometers and they are still in perfect working order. I have gone through 4 digital verniers and about 20 six inch full flexible Starrett steel rulers.

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#23
In reply to #11

Re: Accuracy of Measuring Tools

07/03/2011 3:24 PM

I would also add a good magnifing glass to read the 1/64" scale.

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#24
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Re: Accuracy of Measuring Tools

07/03/2011 3:36 PM

W"hich is why I like digital at my age....

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#4

Re: Accuracy of Measuring Tools

07/02/2011 3:14 PM

The advantage of a digital readout is that the electronics does the thinking for you to generate a number for your measurement. The drawback of a digital readout is that the electronics does the thinking for you to generate a number for your measurement. Despite my background in electronics, I prefer using a caliper with a vernier scale than a digital caliper when I measure physical dimensions. Not because a vernier is better than digital but because it takes longer for me to read and understand what the vernier markings are telling me. I need to make precise mechanical measurements so infrequently now that I want the added time to just plain think about what I am doing.

In the correct hands I'm certain all of these instruments will do equivalent precise jobs (if they have the same resolution). In my hands, I'll choose a vernier caliper and the extra time it takes for me to use it.

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#9

Re: Accuracy of Measuring Tools

07/02/2011 7:15 PM

A good quality vernier, dial, or digital caliper will be adequate.

My previous employer issued me a Mititoyo dial caliper (0"-6") +/- 0.001" which I used daily for over 17 years. No calibration required for the first 10 years. After that, our new ISO rules required annual calibration. Mine never needed any calibration adjustment. Other calipers did because people DROPPED them or used them as a PRYBAR or a HAMMER

Bought a cheap P.O.S. copy for home use after that, but returned it the next day. Mechanism felt like it was full of sand. Decided to buy a real Mititoyo and was not disappointed. My current Mititoyo digital caliper is easier to read (and converts to metric), but it requires a new battery every couple years.

Wish I could have taken that old dial caliper with me when I left

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#12
In reply to #9

Re: Accuracy of Measuring Tools

07/03/2011 5:26 AM

Mititoyo Mitutoyo... My apologies to the manufacturer!

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#59
In reply to #9

Re: Accuracy of Measuring Tools

07/08/2011 8:18 AM

I agree, a good dial caliper in good condition,when properly calibrated, can give a pretty good visual estimate of .0001 increments (better than .0005 for a digital caliper).

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#10

Re: Accuracy of Measuring Tools

07/02/2011 11:18 PM

Question: Accuracy of Measuring Tools ??

For +/- .005" or more you can use a set of calipers. I prefer "the feel" of Mitutoyo.

For less than .005" you should really use micrometers where I prefer "the feel" of a Starrett.

Notice I keep referring to "the feel"...it's all in the feel. You can use a known reference standard that's close to what your measuring to calibrate your feel. A good 2" gage block should measure 2"...if not your feel is off .......assuming you've calibrated your tool.

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#13

Re: Accuracy of Measuring Tools

07/03/2011 6:36 AM

Both are good and accurate brands, buy one which is cheaper.

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#14

Re: Accuracy of Measuring Tools

07/03/2011 7:26 AM

If you are in a home environment, you have been given enough infos already here, good infos too!

If you are in a shop environment, you need to have a method of proving that your instruments are still accurate, always. Its not my area of expertise, but something like some pieces cut to known sizes to be measured each week on the bench (daily if there is a question or requirement!).

Then have say a professional company come in once a year for calibration and adjustment, people do drop such instruments from time to time. If you have thousands of instruments, then you need your own department for that.....

I am sure your country has "rules and regs" for such control of instrument accuracy, you should make yourself aware of what is required......DIN standards are a possible way to go I would think, especially if the customer requires a world standard for any reason....

I bet there are several people here on CR4 that have done such work......hopefully they will find this blog and post.

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#30
In reply to #14

Re: Accuracy of Measuring Tools

07/04/2011 1:34 PM

I have a number of Mitutoyo 150 mm digital calipers (readable to 10 µ - accurate to 25 µ) and 50 mm digital indicators (readable to 1 µ - accurate to 2.5 µ). I send them out every year to a Class A lab (Tektronics) for certification (Accredited 17025 with uncertainties) to maintain NIST traceability. The calipers are over 30 years old, and always come back within tolerance. I also keep a bunch of gauge blocks around to check them. For anybody with questionable eyesight I recommend the LED display over the LCD types. As others have pointed out, take good care of them, don't use them as hammers, don't drop them, keep them in their cases, etc., and they will outlast you (as long as the batteries or AC/DC converters are available). Also a consistent 'touch' is critical for the calipers.

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#34
In reply to #30

Re: Accuracy of Measuring Tools

07/04/2011 3:52 PM

I agree that usage determines tool life expectancy and reliability. As a tool and die maker most of career was in production emergency repair which is brutal on rulers and verniers. But with downtime at up to $50,000 per minute tools become a casualty of war. The company would relpace damaged tools.

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#15

Re: Accuracy of Measuring Tools

07/03/2011 9:52 AM

If one really gives some thoughts on accuracy of measurements or on any available measuring tool for that matter, the accuracy will mostly depend on the very person using that chosen tool! His knowledge and ability to interpret measured parameter?

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#21
In reply to #15

Re: Accuracy of Measuring Tools

07/03/2011 2:59 PM

Disagree with your ZEN outlook. Precision tools are more reliable and precise with the best quality manufacturers especially when compared to no-name imports. The ability of the precision instruments user only determines their ability to get the most out of a quality instrument. If the accuracy is not built into the instrument wishful thinking will not compensate.

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#16

Re: Accuracy of Measuring Tools

07/03/2011 10:01 AM

The micrometer can be accurate to .0001". The vernier caliper to.001" You simply can not read tenths on a caliper. I have a personal rule to not use electronic zeroing instruments in the shop. It is very easy to set the zero on a tiny piece of debris. Electronic instruments are used in inspection or in a lab, but not in the machine shop.

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#17
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Re: Accuracy of Measuring Tools

07/03/2011 10:07 AM

You are missing out on a powerful tool.

Re-zeroing a dial caliper can be used to quickly and accurately measure the difference between two objects or other measurements.

I do this all the time.

My Mit also has an absolute zero button, but I have made a general habit of always wiping the jaws before a zero.

Dial calipers have their place, too. Dials are faster to read if you are trying to move the caliper or looking for a measurement that puts the needle at a specific location. It only requires one side of the brain. Digital readouts require both side of the brain and the brain must cross-communicate to do it, so it requires more thought and time.

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#22
In reply to #16

Re: Accuracy of Measuring Tools

07/03/2011 3:06 PM

The Mitutoyo Absolute digital calipers set a permanent zero when a new battery is installed. Essentially this makes the caliper like a micrometer in use. Bonuses are floating zero set, inch and metric reading , large measuring range, and large numbers on the readout.

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#18

Re: Accuracy of Measuring Tools

07/03/2011 10:42 AM

this set as an example has the calibration blocks


I use the block to check outside accuracy on my calipers


Lock the micrometer to check inside accuracy


I have an old caliper(s) that I use for laying out, the tips of the jaws get dull & need to be resharpened & adjusted from time to time


the scratch method laying out gives far more accuracy than you can get with a tape measure & works on most materials
if I'm doing press fits on a lathe I may use a Vernier

Notice the lock & thumbwheel, which makes it possible to hold the measurement until I can get it in a position to read,

very slightly less accurate than a micometer on outside measurements, more practicle for inside on small parts

To answer the question

the stuff you use day to day is going to be the least accurate & will consequently need to be checke against the good stuff

a micrometer is going to maintain accuracy better by virtue of the larger hardened surfaces

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#20
In reply to #18

Re: Accuracy of Measuring Tools

07/03/2011 12:36 PM

You bring up a very good point here. The measuring tool used most often will not and should not be considered the most accurate measuring tool in a shop. Wear and tear from normal usage along with the undesired mishaps that happen will make the frequently used tool less accurate. While this level of quality control may not be affordable in a home shop, it is best to be aware of the principle.

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#25
In reply to #18

Re: Accuracy of Measuring Tools

07/03/2011 3:41 PM

A digital or dial caliper reading can fluctuate depending on the amount of pressure you exert on the thumbwheel. A vernier caliper always seems to have a more solid feel when taking a reading. I generally use a vernier, but when I need a quick reading, I will reach for the digital. Like any good instrument, my 12" Mitutoyo caliper is kept in it's case when not in use and handled lovingly with kid gloves. The same with my Starrett, B&S and Mitutoyo mikes. I think a 1" gage block might be a useful addition to my tool box for checking my tools.

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#26
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Re: Accuracy of Measuring Tools

07/03/2011 4:04 PM

That is because you are using them wrong. It's a common mistake.

When measuring a part outside diameter or thickness you squeeze the caliper by the jaws with your fingers, not the thumb nut on the slide.

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#49
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Re: Accuracy of Measuring Tools

07/05/2011 7:49 PM

" squeeze the caliper by the jaws with your fingers, not the thumb nut on the slide."

I didn't know that; makes sense, but why haven't I read it in a machinist's manual or as an instruction sheet for a digital or dial caliper? The Starrett catalog doesn't mention it on the page that tells you how to read a vernier.

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#50
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Re: Accuracy of Measuring Tools

07/05/2011 8:29 PM

I don't know, but I just observe the pros whenever possible to learn things like that.

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#52
In reply to #49

Re: Accuracy of Measuring Tools

07/06/2011 1:38 PM

I find that gently squeezing the jaws together gives me a better feel of when the surfaces are in correct contact than pushing the thumb wheel can. My other hand does not contact caliper until after I've taken the reading.

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#27
In reply to #25

Re: Accuracy of Measuring Tools

07/03/2011 8:53 PM

Your usage of a vernier is wrong. You apply as little pressure as possible to the workpiece while making sure you are square. This is why verniers are best for measuring soft objects. Quality verniers digital or otherwise have very little deflection. The extremely tight tolerances on the sliding parts for fit, squareness, parallelism, flatness, material hardness, and the adjustable gibs make for a very rigid vernier. I have a Starrett 6 inch vernier that reads to .0001" and it is beautiful to use but very slow. What kills verniers is dropping, corrosion, and abrasion. If you have a lot of micrometers (which usually come with a standard) a gauge block set is handy to have to calibrate to an exact dimension. As a hobby machinist a really economical yet extremely handy set of gauge blocks to have is a set of .75 inch diameter round ones with 1/4" X 28tpi center hole for about $100 ( 2 sets are even handier for set ups and creating unique parallel heights).

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#31
In reply to #25

Re: Accuracy of Measuring Tools

07/04/2011 1:59 PM

I think I need to reiterate the fact that no matter how good the instrument(s) is, accuracy will most of the time will depend on the knowledge, experience and familiarity of the person using the tool!! ...If one really gives some thoughts on accuracy of measurements or on any available measuring tool for that matter, the accuracy will mostly depend on the very person using that chosen tool! His knowledge and ability to interpret measured parameter?

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#19

Re: Accuracy of Measuring Tools

07/03/2011 11:16 AM

digital caliper if have self calibration facillity.

These have 0 [zero] and some fixed length [say 25mm] where a STANDARD of that length of a material [having ZERO Temperature Coefficient] is provided in kit.

The accuracy is as required for a job. mostly well manufacturers have a good reputation for their tools.

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#28

Re: Accuracy of Measuring Tools

07/03/2011 10:27 PM

Hi Ron,

I am not a mechanical guy, but one of them left us with a digital caliper when his job ended. We had a need for it a few weeks ago. It didn't work. We tried a new battery, but that didn't help. A mechanical unit would have worked. You asked about dependability, so forget about digital.

-S

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#29
In reply to #28

Re: Accuracy of Measuring Tools

07/04/2011 12:51 PM

Digital verniers are reliable and a battery last about 2 years in a Mitutoyo Absolute or about 2 weeks in a cheap digital vernier.

The reason for leaving a digital vernier behind is because it does not work. The damage is usually from dropping or contamination with grit or corrosive substances.

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#32
In reply to #28

Re: Accuracy of Measuring Tools

07/04/2011 3:35 PM

SOME cheap digital verniers, badly stored with the battery still in. You (and many others here) need to learn "THE TRICK!"

If you don't know "THE TRICK!", post here that I should let you know.

It applies to many modern devices that use PICs or similar.......I have used it on Verniers and calculators mostly, but has also functioned on HiFi systems and radios.......

Lets see that as many as possible want to "KNOW!"

Only a few ask and I will keep it to myself!!!!

It is a fix that many will not understand, but DOES work and can recover totally "dead" devices......its easy, but only when you know how!!!

I hope this whets a few appetites!!

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#33
In reply to #32

Re: Accuracy of Measuring Tools

07/04/2011 3:51 PM

The solution:

stick things that plug into the wall or don't require power

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#35
In reply to #32

Re: Accuracy of Measuring Tools

07/04/2011 4:14 PM

Andy, I'd like to learn your trick, but while I await your response, I'll give you mine. I keep spare batteries in my refrigerator (increases their shelf life) and all electronic devices have mechanical back-ups. I may not need a height gauge for 10 years, but when I need it it either must work or I reach for its mechanical back-up.

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#36
In reply to #32

Re: Accuracy of Measuring Tools

07/04/2011 5:49 PM

I ask Andy, But I think it's: wash with HOT water, maybe a little soap, no solvents [not even alcohol], let dry a LONG time before inserting new batteries. If the display uses the "invisible" press connections on the glass all bets are off. Or are you playing with us on the 4'th, your great [great] uncles here in TX will be disappointed.

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#37
In reply to #32

Re: Accuracy of Measuring Tools

07/04/2011 10:35 PM

Yes please Andy

As long as it can measure the thickness of the saliva residue on my keyboard afterward all would be good. I hardly use it but when, I would love to see something different than a row of 8888888. And you say you have a trick? For here in the tropics?

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#38

Re: Accuracy of Measuring Tools

07/05/2011 11:31 AM

Did word search and didn't get any hits for "calibrat"

No measuring tool is worth a hill of beans, if haven't been periodically calibrated to a known standards. I am currently working in the Aircraft industry and all our measuring equipment are periodically calibrated on a six month schedule. Most of our measuring tools are from Starrett and we initially calibrate them when they come in the door.

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#40
In reply to #38

Re: Accuracy of Measuring Tools

07/05/2011 12:22 PM

Ridiculous.

No home hobby shop becomes worthless because they simply do not calibrate their tools every 6 months.

I have measuring tools that are 10 years out of calibration, but I am still able to produce precision parts out of my garage.

For the aircraft industry you must demonstrate compliance to a whole host of standards. This is because the liability the end product holds is huge and everyone agrees that a minimum standard must be assured to minimize risk to life.

Unless your measuring tools are seriously mishandled, I doubt that those tools require any adjustment every 6 months. All they are doing is certifying the tool still meets the original manufactured specifications, which helps keep your products within tolerance.

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#41
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Re: Accuracy of Measuring Tools

07/05/2011 12:55 PM

Agreed. There is no need to calibrate instruments for the home shop, unless damage has ocurred.

Coming from areospace, I remember having to pull instruments out of service, not because they were used on flight hardware, but, because they were in the same room where flight equipment was processed.

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#39

Re: Accuracy of Measuring Tools

07/05/2011 12:15 PM

The TRICK.

Its so simple, most of you will understand it instantly!!!

What happens with a lot of electronic equipment, usually with ones using batteries, particularly small cell batteries is that they get left around, not being used for months (years ?) on end.

And as we all know, almost all equipment today have keys for on/off that are not latching, eg. there a bit of electronics (that takes micro amps or less) always switched on. Its usually the "Wake-up" circuit for a small chip, a PIC or similar.

This runs the battery down until the little bit of electronics cannot run anymore, and it "stalls".

You might think that putting new batteries in will cure the fault, which it will on some devices, but as the processor is stalled and the reset on problem devices is probably not fully and correctly designed....it actually makes the problem worse!!!

The fix, if new batteries do not help, is the following THE TRICK part:-

1) Remove battery(s) and short the contacts of the device together to remove any charge built up on any caps in the device. If it is only one battery, you only have two contacts to short, if more batteries, they are usually in series, you need to make sure that you short the right contacts, check with an Ohmmeter....

This will usually revive about 50% of the devices with problems.

2) For the other ones, you not only need to short out the contacts, but you also need to leave the short in place, anything up to 7 days, till all caps are totally empty of any charge.

This will usually recover up to 30% of the rest.

All of the others are probably damaged permanently and will never work again, but a longer time shorted (it won't cost you anything!) might recover a few more......but I do believe that a cap or two is broken and not doing the job it was supposed to anymore...especially electrolytic caps.

I do think that any device which you can recover is probably badly designed ones in the first place, that is why I take the batteries out of most things if I know I won't be using it again for sometime, or that the possibility exists!!

Some do believe that device caps, especially polarized ones, live longer with power....I do not subscribe to that!!!

Perhaps the devices should be powered on at least once every three months of so, to fit in with both ideas......

I hope the Guy with the dead Vernier can test the TRICK out!!!

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#42
In reply to #39

Re: Accuracy of Measuring Tools

07/05/2011 1:44 PM

Technically, a Vernier caliper never uses a battery in the first place. Vernier is a mechanical means that uses two unequal sliding scales to resolve a measurement to a degree greater than you would with a simple pointer or mark sliding by a scale.

Note that the upper and lower scales are not the same in incremental length and no batteries are required since the device is purely mechanical.

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#43
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Re: Accuracy of Measuring Tools

07/05/2011 3:42 PM

What about the LED so you can make measurements in the dark?

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#45
In reply to #42

Re: Accuracy of Measuring Tools

07/05/2011 4:37 PM

Look in catalogues and you will see callipers as digital, vernier, dial etc.

What is the correct vernier reading for the measurement below?

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#46
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Re: Accuracy of Measuring Tools

07/05/2011 4:48 PM

What is the correct vernier reading for the measurement below?

You mean the correct technique?

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#47
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Re: Accuracy of Measuring Tools

07/05/2011 5:21 PM

0.0985 cubits, but it is hard to read the scale from here.

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#44
In reply to #39

Re: Accuracy of Measuring Tools

07/05/2011 3:51 PM

I'll try it today, thanks.

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#48
In reply to #39

Re: Accuracy of Measuring Tools

07/05/2011 5:30 PM

anything up to 7 days

I'll try that now but will not hold my breath.

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#54
In reply to #39

Re: Accuracy of Measuring Tools

07/07/2011 11:06 AM

Hi Andy,

I have one of those digital angle devices that is used to set up blade angles on table saws. It uses a disc battery and it lasts only a few months. I checked with the company that makes it and they told me the battery maintains a memory even when it is turned off. My solution was to remove the battery and reinstall it when needed, which is infrequently. I wonder if digital calipers use power when shut off?

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#56
In reply to #54

Re: Accuracy of Measuring Tools

07/07/2011 7:01 PM

I believe they do, if I wasn't so lazy, I would measure for certain....

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#55

Re: Accuracy of Measuring Tools

07/07/2011 3:20 PM

Ronseto

Many good answers here. May I just add: While a micrometer will usually maintain better long term accuracy, periodic calibration checks for all is necessary. Also for general accuracy of technique for your intended use, measure the same part (or a standard) 10-20 times and if the total variability (6 sigma) does not exceed 20% of the mean measured value you are reading it is statistically acceptable for use.

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#57
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Re: Accuracy of Measuring Tools

07/07/2011 7:24 PM

"measure the same part (or a standard) 10-20 times" Variations in readings will always fall within ±.0005", so you cannot get an average reading using a .001" tool.

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#58
In reply to #57

Re: Accuracy of Measuring Tools

07/08/2011 8:05 AM

Depending on what feature you want to measure, the technique or "feel" as previously mentioned (how you hold it, the incident angle, applied force, etc.), can introduce variability to your measurement result. So can variations in straightness, parallelism and circularity. (burrs, nodules, etc. too) Actually I stated the statistical rule incorrectly: The total variation of your repeated measurement sample should not exceed 20% of the total tolerance of the feature to be measured. (sorry!)

What you are referring to is the measuring device capability. For example a digital caliper where the fourth place is a step of .0005, when you zero it the zero could actually be -.00049 to +.00049 due to the physical condition of the caliper blades (including wear, damage and contamination). So if you wanted to accurately measure a feature of .001, the caliper would not be the proper choice of a measuring device. (unless +/- .00049 is acceptable. Also if you wanted to validate any feature with an allowable tolerance of +/-.0005, this caliper would not be an appropriate tool Unless you can accept that uncertainty). You should probably use a stand mounted "super micrometer" that measures in "tenths" (.0001) with the fifth place in steps of .00005.

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