Digital Calipers 2

[flyfunky]

Can anyone recommend a model of digital calipers? Accuracy requried is 0.0005". I am finding online information is confusing resolution with accuracy and want to make sure I get the right tool the first time.

Thanks a million,
Rebecca

 

[diskullman]

I made the mistake of getting a solar powered caliper. I found it difficult to get measurements on dark areas of machinery, and ended up bringing a flashlight with me on service calls.

Russell Giuliano

 

[ornerynorsk]

Hi Tick, always good to banter with a fellow machinist. Your part about cross checking with another instrument (better yet a gage block) is good advice. I do have to take to issue the repeatable measurements. I have a drawer full of cheap calipers that are basically bait for the "tool borrowers" in our company, that will repeat the measurement to within a thou all day long, but it's still the wrong measurement! You can adjust to zero, but unless you wring up a stack of jo blocks that are very close to your target measurement, it ain't gonna do it! By the time you've gone to all that trouble, you could have either set up a snap gauge or just gone with a mic.

All calipers, even the Swiss ones, have an inherent parallax error because the jaw needs to slide. They contain backlash and tensioning mechanisms and all of that, but I have found that to be safe, you just can't trust them to anything finer than about 3/1000. My Browne & Sharpe and Etalon are both trustable to about a thou, but no human hands other than mine have ever touched them. (well, that are still living, anyway, LOL)

My funniest experience is when I witnessed a mechanic taking cylinder bore readings on an engine block with a dial calipers. Yes, a standard 6 inch dial caliper! I think the look on my face must have sent a chill down his spine because he immediately asked if there were a more accurate way of measuring. He now owns a nice set of telescoping gauges and the mics to go with them.

 

[benta]

0.0005"? That's around 0.01 mm.
Please check the datasheets for digital calipers, and you'll not find a manufacturer that specifies better tolerance than 0.03 mm.
That the readout shows you 0.01 mm is a different matter.

Benta.

 

[TheTick]

True, repeating doesn't guaranty accuracy. It does at least verify technique.

At my last machining job before graduating and moving into engineering, I had to watch the inspectors. They were so sloppy that good parts would get rejected just because they couldn't hold caliper properly. My "rework" consisted of a demonstration and lesson.

 

[berkshire]

The Tick,
It never changes does it? I have a friend who is a corrective action engineer for a well known aircraft company. This week that person had to deal with a bunch of rejected "good" parts because the bores had been measured with a digital caliper instead of a 3 point micrometer.
When they were measured with the right equipment, they were well within tolerance.
B.E.

 

[ajack1]

In my days on the shop floor a vernier (digital calliper) was always referred to as a very near. It will never be as accurate as a mic, slip gauges etc but it is much easier to carry around one 0-8” vernier than eight micrometers.

It all depends on how accurately you need to measure something but it is certainly a quick and easy way of getting a very near measurement, even if that is to find out if a piece around 5” needs a 4-5” mic or a 5-6” mic to measure it accurately.

 

[GregLocock]

So, be honest, who carries an appropriate measuring instrument with them most of the time?

Is that the real sign of being a crusty?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[unclesyd]

I don't know about you but I always used the universal measuring device, the 6" machinist rule. For really big jobs the 18".

That aside today's machining methods have outpaced the ability to use simple and inexpensive measuring instruments on the shop floor. One thing we used for any run of parts requiring precision measurements, little tenths' we would always try to set up a comparison gauging process in lieu of a direct measurement.

I was most fortunate in about my 5th year of employment I got to attend a ten day metrology course. 5 days with an in house instructor, 5 days with outside instruction covering some theory, and new instruments and methods. I would strongly recommend that if a course, seminar, or even a demonstration dealing with metrology comes along take it.
Again I was quite fortunate in that the company readily provided the instruments required for very accurate measurements and a relative clean room for calibration.

A bit of information gleaned from that course that has stuck with me all these years is that there are 21 geometric shapes when using a two point measurement you can get the same dimension.
I also have around 30 spring calipers of all persuasion used by my father, grandfather, and uncle for precision machining, actually comparison.

 

[drawoh]

unclesyd,

I took Metrology in college. It has proved useful.

I keep a metric/English tape measure in my briefcase. I keep losing my 6" pocket scales, but I have one at the moment. The 6" vernier, my 0-1" micrometer (with vernier scale) and my 0-25mm micrometer (also with vernier) live in my desk drawer. Since they are vernier rather than digital, they are in my drawer. Perhaps some of you work in places where people put stuff back when they are finished with them.

JHG

 

[unclesyd]

I forgot to add that the only geometric shape you can measure with a fair degree of accuracy with a 2 point gauge is a sphere.
In my previous post the 21 geometric shapes pertain to bores.

drawoh/b],
Our two big shops used the brass token approach to getting anything checked out of tool crib back. As far as scales go I usually could sneak in an order for dozen at a time. I was real bad about cutting them down to rough align a Faulk coupling.

It just dawned on me that years ago when a new batch of machinists in training were given there first it would be to make a C Clap with a 1" throat. I guess that was to replace the 1" micrometer.

 

[KENAT]

I used to have a nice 8" steel rule that I did carry almost everywhere and even got callibrated yearly. However, that vanished at some point and I never got round to replacing it. I have a cheap set of 6" digital callipers that are mine, and I have a slightly better work pair (shared in my dept, which is now just me and a colleague and he I think has his own, so they're basically mine). However, I don't routinely carry them with me like I did my rule.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Griz42]

Not to be the wet blanket... but I will.

Given todays competitive environment, should you be loaning out poor measurement tools to people in your own company?

 

[eadwine]

The accuracy in any measurement lies in the proper training and use of the metering device. Also; accuracy, repeatability and reliability lies in the requisite regular testing and calibration of the instruments in use.

Check the ISO 9000 std.

 

[MikeHalloran]

ISTR that if you work for an ISO company, you should leave your own measuring tools at home, and use the controlled and calibrated ones provided by the company.



Mike Halloran
Pembroke Pines, FL, USA

 

[TheTick]

The ISO companies I worked for paid to calibrate personal instruments at work. On calibration day, bring every caliper, rule, and tape measure you own!

 

[unclesyd]

Speaking of measuring I just got a call asking where i bought some Hastelloy C Cylinders (5" dia x 24" lg with 1.0625 wall) I purchased in he early 80's and the company is no long in business.
It seems that someone just cut them down to a new length, the only problem being they are now 1" too short.
Supposedly this was a digital readout that was in error.

TheTick,
I have a ten foot tape measure that is all white no numbers, how would it fit in the calibration scheme.

 

[btrueblood]

Simple, syd, I'd calibrate to measure 10 feet plus or minus 10 feet.

Better, a sticker we once put on a broken pressure gage said "calibrated to measure: nothing, +/- infinity".

 

[gwolf2]

As a slight diversion, never underestimate the usefulness of cheap measuring devices. Some very large firms which I have worked for (design and stress depts) won't provide a measuring device unless it's properly calibrated - these cost money, therefore they don't have anything and trying to weigh something has to be done with a job number somewhere off site.

This is all fair enough unless you just want to know the weight of a 125g object +/- 5g, in less than 2 weeks! I just brought my digital kitchen scales in and calibrated them myself with an old 1lb stamped weight. What do you know, the $20 scales were there within 2g!

Metal verniers for me every time and yes I do keep them near all the time and was trained how to use them.

A slightly inaccurate answer is often 1000% better than no answer at all.

 

[gwolf2]

By 'stamped' I mean that the lead plug in the weight had a customs and excise seal imprint.

 

[drawoh]

unclesyd,

I built a shed in my backyward last fall. I had problems with my tape measure which measures inches, feet and inches and millimeters. The feet and inches text is on the opposite side of the tick from the inch dimension, which caused me to measure a half in inch off, repeatedly.

And I was unable to find my lumber stretcher.

If I were serious about framing, I would get a tape measure calibrated only in feet and inches.

JHG