Measuring tool accuracy

[Peter G. ShawParticipant @peterg-shaw75338]

Hi folks,

 

Over the years I have acquired by various (legal) means six 6″/150mm steel rules, a metric and an imperial micrometer, a vernier caliper reading to 0.05mm, a dial caliper reading to 0.02mm, and two electronic calipers reading to 0.01mm. From which you will gather that I work in metric.

 

So, one day I did a comparison of the 150mm rules between 10mm and 150mm thus avoiding any problem with wear on the end. I was somewhat disconcerted to discover that my Rabone Chesterman Standard at 20°C rule was distinctly out when compared to the others. Repeating the comparison, but using 1″ to 6″ showed that all rules were about the same, hence it would seem to be the metric scale that is incorrect. Hence this rule is relegated to rough and ready readings.

 

I have also compared the metric micrometer and the calipers, and  discovered the following discrepancies:

 

Mic:                1.00       9.42          21.34

Vernier           1.00       9.41          21.37        30.75    45.55     58.40      103.65

Dial                0.98       9.43          21.37        30.79    45.59     58.47      103.79

Electronic1      1.01       9.41          21.33        30.71    45.51     58.36      103.66

Electronic2      0.98       9.46          21.35        30.74    45.54     58.40      103.72

 

For what it’s worth, electronic caliper No. 1 is the only device that is fully specified and gives the following:

Accuracy: 0.02mm from 0 to 100mm

Repeatability: 0.01mm

Resolution: 0.01mm.

 

It seems to me that the micrometer and electronic1 caliper are the nearest to each other, with the vernier a close second. Electronic 2 is so far aout as to be useless, whilst the dial caliper sems to be considtently reading high (apart from the first reading).

 

Thoughts and comments then?

 

Peter G. Shaw

 

p.s. All the editing was to get the figures lined up!

Edited By Peter G. Shaw on 01/07/2009 21:09:44

Edited By Peter G. Shaw on 01/07/2009 21:10:09

Edited By Peter G. Shaw on 01/07/2009 21:10:56

[Peter G. ShawParticipant @peterg-shaw75338]

[mgjParticipant @mgj]

Well – only a comment or personal opinion.

 

Rulers and calipers are all for rough and ready measurements anyway? Though accuracy of transfer on sheet metal has been improved enormously by using an optical centre punch. (About the most useful had tool there is IMO)

 

Calipers – electronic –  I can never get a good reading out of. If someone can tell me how I would be most grateful. 

 

Which leaves me with a set of electronic digital micrometers by Mr Mitutoyo. Since they are so accurate, and agree with both the Myfords dials and my cheap but large Chinaman too, I use my old calipers as bore gauges and measure across the hooks with a micrometer. Electronic calipers I use for a quick reading only, but micrometers I could not be without.  The micrometers come out spot on when used on the reference bar, to many places of decimals, and any difference is usually resolved by wiping hte anvils, because a layer of grime does affect them.

 

As a matter of interest, has anyone had any great success with accurate repeatable measurements with ordinary telescopic bore gauges in the smaller sizes.

 

I recently fitted a 6″ scale on the downfeed of the mill (awaiting saving for the 2 axis DRO) and that is spot on, and agrees with the mitutoyos perfectly. But the mills imperial dials are clearly out by .002 over 1/8 inch. That scale reads to 1/2 thou (or metric at the touch of a button of course).

 

Edited By meyrick griffith-jones on 01/07/2009 22:02:52

Edited By meyrick griffith-jones on 01/07/2009 22:06:56

[CirclipParticipant @circlip]

Peter, are you doing a lot of work for NASA or R.R. ??

 

   First point you quite rightly make, is never work to the end of a rule cos they are awfully usefull for opening tins, as a screwdriver or scraping swarf out of tee slots ??

 

  When comparing different measuring implements, are you sure you’re imparting EXACTLY the same FEEL to each?? You can give three toolmakers the same job with the same measuring bits and I’ll bet you get three different numbers despite having a ratchet thimble on the Mics.

 

  At the end of the day, what you should really do is pick your favourite one of each type of instrument and carefully put the others in a drawer for safe keeping until one of the former breaks irretrievably and needs replacing.

 

   The advent of cheap electronics has created one of the biggest dinosaurs in Muddle Ingineerin circles since Mr Whit. standardised threadforms. How many actually know what a “Thou” or a “Micron” is?? A number to be aimed at to ensure accuracy in building Toys???? What has happened to the word “Fitting”, used to be part of the formula for getting the finished effort to work.

 

   There is NO sarchasm or synicism intended,(and no spullchuck either Dave and Richmond), but please lighten up and enjoy actually MAKING toys instead of how close to perfection it could be.

 

    And one for you Meyrick, we only used telescopic gauges for bores over 1/2″ (12MM) Dia, you could get a ball gauge from “Moores” to use up to the aforementioned or tapered bore gauges for smaller than the ball gauges could go.

 

    Regards  Ian.

[Peter G. ShawParticipant @peterg-shaw75338]

Hi circlip,

 

I wondered who would query the quantity. So here goes.

 

The rules. Some were magazine mounts. One at least is many, many years old, probably the first I ever bought. Two are ½” wide flexible ones at least one of which was bought by me specifically for it’s flexibility. The Rabone Chesterman was bought by me because it has a nice satin finish, but ain’t much use . 

 

The imperial mic was bought many, many years ago when I was rubbing down shims (for want of the correct name) for use inside the “buckets” on a Maxi 1750cc engine camshaft. The metric one because I use metric. The .05mm vernier was the first metric measurement stick and was bought when I knew even less than I do now! The dial caliper because it seemed cheap(ish) some time later.  Electronic 1 was bought because I fancied it, whilst electronic 2 was bought with the idea of turning it into some sort of vertical measuring device for use on the workshop flat plate. In total, I don’t think I’ve spent much more than £100.

 

And no, I don’t work (now) for anyone, not even “Erin” if I can get away with it. (Erin = Er Indoors). Unfortunately, other things have a nasty habit of cropping up, such as the grandson’s guinea pig palace, the bust (but now mended) pistol drill, the bicycles that need maintaining, the grass that needs cutting (and the moss/thatch that needs removing), and…… Well, I’m sure you get the picture.

 

Anyway, back to business. Yes I am aware of the differences in feel, so the readings were taken a number of times. I do use the ratchet on the mic, and approach it slowly until it clicks just once. 

 

In fact, I rather like the dial caliper, and have until recently, tended to use it for everything, but at 150mm, it’s a bit too long, so for less than 25mm dia, I’ve started using the mic, but there is this discrepancy, so I am now trying to use electronic1 (despite it’s battery eating tendency). What I would really like is a 100mm caliper, but having seen the above discrepancies, am somewhat concerned.

 

Yes, I do follow the idea of making parts to fit each other, but at the same time I am sometimes constrained by having to make pattern parts – eg the broken pistol drill mentioned above, and it is here that accurate measurement starts to be a necessity.

 

Regards, (and now back to some painting!),

 

Peter

[mgjParticipant @mgj]

Thanks for that about bore gauges. Glad I’m not alone.

 

Fitting–well its all very well scraping away, but if I want a precision running fit or whatever, I look up the fits in the Model Engineers Handbook and machine to that (or whatever the process is and appropriate clearances are) – voila, it fits and does what it’s supposed to first time (mostly) every time. If I’m planning on a bit of scraping, then I’ll machine to an allowance.

 

That’s fitting, innit?

 

Spent enough time munching metal big time on this traction engine, without spending another year or two scraping.

[CirclipParticipant @circlip]

When Fitting, don’t forget Grinding and Honing and Lapping and Reaming and —-

 

   Regards  Ian.

[RickParticipant @rick]

Peter,

 

I’ve often wondered about the accuracy as well.  I ended up taking sample measurements and then calculating the uncertainty of the measurement.  Normally I just use the mike, but if it is something needing accuracy, eg. gauges, then I apply the uncertainty to it.

 

Regards

 

Rickl

[KWILParticipant @kwil]

Peter Shaw mentions that only one of his electronic devices has a specified accuracy and that is 0.02mm.

 

Think about it, that is 0.0008″ in old money and quite a sloppy fit in some instances, give me a micrometer or proper DRO any day.

[KWILParticipant @kwil]

Peter Shaw mentions that only one of his electronic devices has a specified accuracy and that is 0.02mm.

 

Think about it, that is 0.0008″ in old money and quite a sloppy fit in some instances, give me a micrometer or proper DRO any day.

[calder percival 1Participant @calderpercival1]

in industry all equipment is checked and callibrated, if you purchase a couple of gauges and set all your measuring equipment to them then you will be fine as everything you make will match , you only need to worry about perfectly accurate calibration if you are sending work out to other people and the work you do is to fit someone elses.That brings in tolerances something that is lacking in all model engineering drawings!

[Frank DolmanParticipant @frankdolman72357]

 

     Back in the summer, when Peter first started this post, I thought it

   would be a good idea to try out my own kit. In general, I was reassured

   but decided that I had to make a micrometer stand in case I ever did want

   a good answer.  I confess I have not yet done so.  I was impressed by

   my 1-2-3 blocks, which do seem to be 1-2-3! and horrified by my school-

   type protractor which looks nice but is desperate!  Altogether a worthwhile

   exercise

[Jens Eirik Skogstad 1Participant @jenseirikskogstad1]

Posted by calder percival 1 on 20/02/2010 23:32:36:
That brings in tolerances something that is lacking in all model engineering drawings!

 

The rule of tolerance in the machined parts are mostly used in massproduction of parts who are sold as spare parts to example.. 

 

In practise of model engineering are try and fail to perfect fit between parts are common in every home workshop. Running-in the parts who will improve the fit in moving parts, better than we can do it with tools and measure tools in the workshop.

 

I can not rely all measurements in my tools are perfect each time i am making the parts, It make me satisfied and glad when the machined parts are working very well.
 

[RickParticipant @rick]

In relation to the question of measurement, the first lesson learnt when doing metrology was “All measurements are wrong and the best that can be done is include all areas of uncertainty in the measurement”.  As model engineers, I don’t feel that it is really necessary as generally what we build is for ourselves.  As Peter Shaw showed when he started this thread was the difference between various instruments.  If an instrument you are using is out by a small bit and you use this instrument for all your measurements, it is quite OK as the error is the same each time.  It only will have an affect when matching a component from outside.  Earlier on I alluded to uncertainty of measurement.  If you are keen enough you can calculate these “errors” and work out the expanded uncertainty.  The normal practice in metrology is for 95% confidence level, so you would express your result as so many mm +/_mm. 

 

If anyone is really concerned about the accuracy of the vernier/mike, then take a gauge block or parallel and take 30 readings.  Tabulate the readings in Excel or similar spreadsheet and use the functione for Standard Deviation.  This result will give you a deviation of +/- X and you can then adjust any reading accordingly.  (this is a bit rough as you will not be taking into consideration other factors affecting the artifact you are measuring, but if will be more than enough for our purpose in the workshop.

 

Regards

Rick from sunny Sth Australia

 

 

 

[Dinosaur EngineerParticipant @dinosaurengineer]

Rick is correct in that the level accuracy is all about the certainty of measurement.  Every measuring instrument & standard has an accuracy. This accuracy can vary with the instrument displacement and also with the form / geometry of the datum and measuring surfaces / planes. Micrometers can have worn anvils to a concave form if used on predominately on round workpieces but when checked against a slip gauge they may read correctly .Slip gauges can be worn in the middle To check for these kind of errors optical flats can show the errors as fringes .Even holding almost closed anvils/jaws up against light can show quite small errors ( when the light turns blue the error is less than the wavelength of “white light” ).

To measure the accuracy of any measuring instrument requires a master of known dimension whose level of uncertainy of measurement is also known.

A rough rule we had in the toolroom was that we needed an instrument that was at least 10X more accurate than the tolerance we were trying to work to . Thus to have a high confidence level that the workpiece was correct, we need instuments that could measure to a tenth of a thou’ to produce work to a thou’ . The toolroom jig borer was kept in a controlled temperature environment .We used this room to check any measuring instrument before any long term use on a job or in a challenging tolerance job in addition to the normal periodic checks.

Chasing tenths of thous / microns can be a very expensive and time consuming business !  This why only the larger reputable instrument manufacturers specify the accuracy of their products and why others don’t.

You can’t always trust any digital readout that reads to a micron when working to 10s of microns !

Knowing the limitations of old mikes / calipers aquired over the years when trying to produce components to specific tolerances can be quite difficult !

One very important lesson I sooned learned in the toolroom was to wipe every tool/ instrument over with a moist oily clean cloth before putting it away and to clean it before use.. The foreman didn’t need your fingerprints on file to discover who had left rusty fingerprints on the height gauge !

[Gray62Participant @gray62]

Let’s face it, we are model engineers, we are chasing fit and finish not supreme accuracy.

Many years ago, when I worked in the steel industry, we chased 10 thou accuracy as acceptable for a plate to roll out of the mill. and 1 thou accuracy for a machined part.

In modern industry such as aerospace or motor racing, they aspire to 1/10 thou or greater accuracy (or the equivalent in metric!!).  Well, when you are building a traction engine or locomotive, that level of accuracy is far from achievable o necessary. In most cases. What we want to achieve is comparative accuracy, so If the bore of the cylinder is measured with a guage of similar accuracy to the tool that is used to measure the OD of the piston, then comparative accuracy is sufficient.

To precisely guage accuracy of a measuring instrument, it needs to be compared to a precision standard which is held at a constant temperature.

We are not working to these levels of accuracy and have no need to. We need to compare and be aware of the degree of accuracy required to achieve the end result – a model that runs, is as efficient as it can be and represents the design from which it is scaled.

 

Supreme accuracy is extremely difficult to achieve!!

 

Graeme

 

 

 

 

 

 

 

 

 

 

 

[Sub MandrelParticipant @submandrel]

OH DEAR!

 

I have just carried out my own set of tests, and my conclusion has to be that Peter is spending too much money… 

 

I have a 25mm test bar that came with a Chester 1-2″ digital mike. I tested all my 1″ gear with it. i used the thimble click on the mike, and held the two blades of the calipers tight against the standard (rather than using the roller). All was very clean and slides were adjusted firmly to avoid play.

 

Chester 1″ digital mike    25.007mm (electronic) 25.01mm (from the vernier).

 

Halfords vernier caliper 25.02mm (from the vernier, subtracting a zero reading of 0.1mm)

 

Draper pressed steel vernier caliper 25.0mm

Cheap plastic(!) digital vernier 25.0mm (I use these two as my ‘hack’ verniers, especially as the Draper one reads in 128ths of an inch)

 

Proops Digital Vernier   25.00mm (occasional flicker to 24.99, but battery is getting flat)

Aldi Digital Vernier   25.00mm

Woolworths Digital vernier – has been hidden by a little man.

The dodgy device was the Halfords mechanical vernier – the scale is suspect, and I think it needs the holes opening up a little and re-aligning.

I think the readings with the mike would have been more accurate if I had used feel, rather than the ratchet; I have struggled to get consistent results with the ratchet before.

All the verniers gave slightly odd readings until the blades were scrupulously clean – this affected both zero adjustment and the actual measurement. I wonder if Peter’s had slightly contaminated blades/anvils.

All in all, although my experinec of measuring ground stock/bearings etc. has suggested the digital devices are accurate, I was stunned that all the cheap digital verniers read dead on within their resolution, suggesting they are both precise and accurate. I think I need to practice with the micrometer though to see if I can get more consistent results.

Neil

 

[Sub MandrelParticipant @submandrel]

For what it’s worth the Woolies digital vernier reads 25.00 as well, although I had a little trouble with getting a consistent zero – it was reading 0.00  mostly, with the odd -0.01. A couple of resetes and wipe cleans got it conasistent and it then read 25.00 dead for the length of the guage. Either everything is out or this says something about the quality of the gauge bars that come with the chester mikes as well!

 

Neil

[RickParticipant @rick]

For anyone who would like to be able work out the uncertainty for their micrometer go to the following site.  You will find can download for free the program Uncal3.2. 

 

http://www.ecalibration.com/Resources/17025d.htm

 

 Regards

 

Rick

[John OlsenParticipant @johnolsen79199]

Hi Guys,

I suppose we all know that the digital calipers, or any of that style, eg dial or Vernier, have inherent limitations due to the force not necessarily being in line with the measurement. Eg if you just slide the slide along using the grippy bit on the plastic, or the little wheels if fitted, then the force is being applied about an inch and a half away from the point being measured. This can cause an error. For instance, I have an M30 nut lying around on the desk here which measures 1.799″ across one pair of flats if I apply the force out on the end of the anvils, so that it is directly in line with the measurement point. If I just push the slide along while holding the instrument in my hand the measurement can go as low as 1.795″ with what seems like reasonable pressure. If I push harder it will go a thou or two lower. This sort of error is inherent to the design, and can be avoided for outside measurements by applying the force directly. It is not so easy to do that for inside measurements, which is one reason they tend to be a bit harder to get consistent.

Watch out when working with cast iron, as the measurement face, which is the plastic area with the numbers printed on it, can get contaminated with graphite. Since the measurement is done by capacitance, this can cause everything to go a bit haywire. I have found that wiping them with meths is a good cure, it doesn’t seem to attack the plastic.

There are several sorts of errors with digital instruments in general:

Linearity error in the actual scale

Absolute error

temperature dependent error

Inherent digital uncertainty of plus or minus 1 count.

The first two are pretty much taken care of by the manufacturing process, and will depend on the accuracy with which the pattern is printed on the little printed circuit board behind the plastic face. Since this sort of stuff is done on a vast scale with high quality machines, there is little excuse for much error, and it seems that even the cheap ones are actually quite good. Temperature dependent errors will depend on the material that the scale is made from, and I suspect here that the stainless steel ones are probably more stable than the plastic ones.

The digital uncertainty you just have to live with, or else buy a more expensive instrument with more digits. But really these devices have enough digits for the purpose, due to the inherent limitations of the caliper design. Plus their versatility makes them so useful. A micrometer will do one thing extremely well, over a limited range. A digital caliper will do inside an outside measurements, steps, and hole depth, will work in metric or imperial, and will zero to a set point so you can work down to size. For this sort of veratility I think we can forgive a slight loss in accuracy, especially since they are generally good enough for much of what we do.

Incidently I have a very handy little four inch or 100mm digital caliper which is really useful in confined spaces, like under the milling head or checking a job in the lathe. Highly recommended if you see them about. The local importer was sent a batch by accident, put them out on special, and was surprised at how the word got around. they sold out in no time at all.

regards
John

[TerrydParticipant @terryd72465]

To add my penn’orth.  When training as apprentices,  we learned that no measuring instrument is accurate, and that perfect measurement is impossible, however as Graeme points out we are chasing fit and not Absolute. 

 

The system of limits and fits was devised to overcome these problems in volume production systems to improve interchangeability and not ensure absolute accuracy.  If I was making nuts and bolts it would concern me, but I am not.  This system recognises the fact that it is impossible to achieve perfection.

 

Our own equipment should be considered not as measuring devices, but as  comparators.  This enables us to make parts that fit in a one off, craftsman production system.  We don’t need to worry about absolute accuracy, merely adequate fit.  If any of our models outlast us to become valuable antiques, when they need repair no-one will expect buy a part ‘off the shelf’ to fit as if they were repairing a car or washing machine  They will make a part to fit, just as the engineers in the past did.  If my vernier is 0.1 mm out, it doesn’t really matter as long as it is consistent and I use the same vernier for all my components.

 

Terry

Edited By Terryd on 04/07/2010 07:31:39

[Peter G. ShawParticipant @peterg-shaw75338]

Well, I started this thread just over 12 months ago, and never thought it would engender (now that’s a word I’ve never used before, and don’t really know where it came from!) such a correspondance.

 

Since then I’ve bought the 25mm, 50mm & 75mm micrometer standards from Mitutoyo and repeated the tests a number of times. Same conclusions as before which suggests that my measurement technique must be about right.

 

I’ve also come to the conclusion that digital verniers are a p-in-the-b in respect of batteries because I discovered somewhere that despite the On/Off button, they don’t actually switch off: they only blank the display. Which means that as long as the battery is in them, they are consuming power. Same source did state that the Mitutoyo devices have the lowest current usage, but one can buy a lot of batteries for the price of a Mitutoyo!

 

Staying with digitals and batteries, I also discovered that SR44 batteries are a slightly different composition to LR44 batteries, hence have a slightly higher voltage, and hence last longer before the display starts blinking. Interestingly, the more consistent of the two electronic verniers is specified for use with SR44 batteries whilst the other one is specified with LR44! Which means that when I stop using the digital vernier, I end up taking out the battery.

 

However, I’ve also come to realise that the micrometer is much easier to use on external surfaces due to it’s wider measuring surfaces. Also that I think that dial verniers are possibly slightly easier to use than the digitals. Certainly there is less confusion caused by the uncertainty of the last digit. Against that the digitals are more flexible with their resettable zero.

 

So, I’ve spent yet more money! And ok, as someone said, maybe I do have too much money, but so what, I can’t take it with me. Anyway, I’ve now bought a S/H Starrett 25-50 micrometer, cleaned it, lubricated it (drop of 3-in-1) reassembled, readjusted and tested it. Accurate at 25mm. About ½div (0.005mm) high at 50mm. Also bought a Starrett 150mm dial vernier with a .01mm resolution. As far as I can tell seems to be accurate.

 

If nothing else, at least the vernier and the mic’s are all to the same resolution.

 

I’ve decided to keep the good electronic vernier, & sell the poor one, but at £7.99 new, it isn’t exactly going to break the bank if no-one buys it. I’m going to experiment with the old dial vernier to see if I can get the depth guage part of it to match the jaws: there’s about 0.3mm difference, and if I can, then present it to my grandson who has expressed a liking for it. I’m also going to keep the slide vernier (0.05mm resolution) as for a lot of purposes it’s more than adequate, and is unlikely to go faulty: the only problem is that of reading the scale – magnifying glass here I come.

 

So there we are, the end of my quest for a better comparable readings in my instruments. Does that make sense? What I mean is that I want them to all read the same, and I think I’ve now achieved that.

 

Regards,

 

Peter G. Shaw

[Author]

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