Vernier gauge testing.

[Dominic Collings 1Participant @dominiccollings1]

Doea anyone know of an easy way to test the accuracy of a digital vernier gauge?

[Dominic Collings 1Participant @dominiccollings1]

[roy entwistleParticipant @royentwistle24699]

Depends what other measuring equipment you have easiest way is using slip guages

incedentally I have never seen a digital vernier

Roy

[BowberParticipant @bowber]

As far as I know you can only compare it to known gauges. You can also check the faces meet and only let blue light through but I think that's about it unless you have the required test gauges or slip blocks.

Steve

[Gordon WParticipant @gordonw]

the best thing commonly available is a ball race, not perfect but will do for workshop testing.

[Dominic Collings 1Participant @dominiccollings1]

Sorry I mean a vernier calliper not a gauge but I guess testing it would be the same. What about feeler gauges? How accurate are they usually?

[Neil WyattModerator @neilwyatt]

I use the 25mm standard that came with my 25-50mm micrometer. It may not be to NPL standards but it is the most accurate thing I have, plus a test over 25mm will be more revealing than one over the thickness of a feeler gauge.

Neil

[Dominic Collings 1Participant @dominiccollings1]

I guess a slip gauge would be best. Is there anyone on this forum with a set in the Dunfermline/Rosyth area with a set, or even one for that matter I could pop round and try?

[roy entwistleParticipant @royentwistle24699]

Dominic Iv'e never seen a digital vernier calliper either ( just being pedantic )

Roy

[Dominic Collings 1Participant @dominiccollings1]

To be honest the digital one is a pain. I use it a lot and sure enough at the most critical moment the battery's flat. A dial calliper would probably be better unless they're less accurate of course.

[Peter G. ShawParticipant @peterg-shaw75338]

I bought the 25mm,50mm & 75mm Mitutoyo micrometer standards which I understand are something like 10 time more accurate than the equipment I have. With these I compared a metric & imperial micrometer, a 150mm/0.05 mm resolution vernier caliper, a 150mm/0.02m resolution dial caliper, and two 150mm/0.01mm resolution digital calipers. The 0.05mm vernier caliper appears exact, the dial caliper and one of the digital calipers appear to read slightly low, the remaining digital caliper was all over the place – I forget what the micrometers were like.

In the end, I kept the micrometers and the 0.05 vernier caliper, gave the dial caliper away, and stopped using the digital calipers due to their battery eating propensity. I then bought a 150mm/0.01 resolution Starrett dial caliper and a s/h Moore & Wright 25-50mm micrometer. I find these mechanical devices accurate enough and reliable enough for my purposes, and most importantly, they all, within reason, show the same value. Occasionally, I might use the better of the two digital calipers, but then have problems with the battery.

Incidently, never use the LR44 cells in these devices: instead use the equivalent Silver Oxide cells, SR44/357. These cells have a slightly higher voltage and seem to last longer. Unfortunately, such is the ignorance of shopkeepers that they will attempt to sell you the LR44 cell stating that they are equivalent: they are not.

For what its worth, I bought a 100mm/0.2mm resolution dial caliper for use on the lathe: being physically smaller it can often be easier to handle than the 150 mm dial caliper. Before it decided to fail, it used to read slightly low, eg 13.98mm on both the micrometer and Starrett dial caliper would read 13.96mm, but knowing this, it was usable.

Regards,

Peter G. Shaw

[Michael GilliganParticipant @michaelgilligan61133]

Dominic,

Have a look at this recent thread … and particularly my link to the Starrett test bars.

MichaelG.

[Dominic Collings 1Participant @dominiccollings1]

 

 

Interesting read Michael, thanks for the link. It's also interesting that you have a couple of measuring devices that read low Peter because I think that's the problem I'm having. So to put the problem in context I'll explain the issue. My son races go karts in a class with several technical rules and regulations. One such rule relates to the size of the "squish". That's the volume that remains between the top of the piston when at top dead centre and the top of the combustion chamber. This measurement cannot be less than 1.2mm. The engines are two strokes so burn oil as they run and you're left with a deposit of carbon on the top of the piston over time as a result. This lowers the squish gap and when the measurement gets down to 1.22mm it's time to take some action. This basically requires the removal of the top of the combustion chamber, the carbon is either scraped off or extra head gaskets are added to restore the squish gap. Unfortunately the engines are sealed buy a builder with a licence to work on them to prevent cheating. The squish gap is critical to the engines performance so typically engine builder will set the squish to 1.26mm so you've only got 0.04 of a mm before taking action or 0.06mm before you're failing a squish test. Since the engines are sealed to test the squish we bend a peice of 2mm solder at right angles, feed it through the spark plug hole and rotate the engine. As the piston comes up it squishes the solder and we take a measurement with a vernier to see where we're at with the squish gap. We check this measurement all the time.

That should be simple enough but I had a cheap digital vernier £35 and it always seems to read high at 1.28-1.30mm. However I noticed that when the scrutineers checked the gap with their vernier it always showed much lower. So to make sure I'm not running too tight I acquired a 150mm Mitutoyo. Since these are around £150 I figured they must be more accurate. However I sent an engine back to the builder for a squish adjustment and when it returned I checked the squish gap with the Mitutoyo to find it already reading a non legal 1.18mm. My old cheap calliper was reading 1.26mm. That's a significant difference. So on a race day I'd borrowed a few calipers (all of the cheap £35 ones) and all read between 1.24 and 1.28 so I figured it must be my Mitutoyo that was reading low. I reasoned that it was doubtful the engine builder would be using a cheap or uncalibrated vernier so we went off racing. At the end in scrutineering the guy tests the squish gap with solder and pulls out a Mitutoyo calliper and gets a reading of a barely legal 1.2mm. As a consequence I'm a bit lost now as to what to do.

Despite their price are Mitutoyo's known for reading low? Is there another brand that's renowned for accuracy?

 

 

 

 

Edited By Dominic Collings 1 on 24/05/2015 07:41:41

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Dominic Collings 1 on 24/05/2015 07:40:20:

Despite their price are Mitutoyo's known for reading low? Is there another brand that's renowned for accuracy?

.

Interesting story, Dominic … That should get a few of our brain cells going .

To the best of my knowledge, the only thing Mitutoyo is known for is excellence.

MichaelG.

.

P.S. … There are some fake Miutoyo callipers around.

[pgk pgkParticipant @pgkpgk17461]

My limited experience with digital verniers (and all things engineering ).. I wouldn't trust their accuracy at those sorts of resolutions. Where i use mine to check inside diameters that matter i usualy cross check across the inside jaws with a micrometer. Also when awkward access means i can't get my micrometer in for an outside dimension but the vernier will fit (a short stub remaining) then I again micrometer across the inside jaws to check.

Simplest suggestion is to ask the scritineers what brand they use and what they do to calibrate.

On these tolerances if you did get consistent readings on your vernier.. albeit a dubious actual number..then i'd be inclined to cross check against feeler guages o see how the vernier reads at such fine references

[Clive HartlandParticipant @clivehartland94829]

It is easy to buy a Vernier, Micrometer or a DTi with a test certificate as most large sellers like Farnell or RS will do a Calibration check. We send all our measuring devices for calibration annually and it gives peace of mind and a fall back in case of a query on measurement. These Companies will calibrate measuring devices for you as they have the equipment in their measuring Labs. If you have doubts then send it off.

Clive

[ANDY CAWLEYParticipant @andycawley24921]

Surely the simple answer to the GoCart problem is to use a proper micrometer, they don't really cost a lot of money.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by ANDY CAWLEY on 24/05/2015 10:14:20:

Surely the simple answer to the GoCart problem is to use a proper micrometer, they don't really cost a lot of money.

.

Agreed !

… and there should be a 'reference standard' slip made available somewhere "in the system".

I would suggest it needs to be in the custody of the scrutineer, but available to competitors to 'calibrate' their own measuring equipment.

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Of course; the alternative would be a simple Go/NoGo gauge.

MichaelG.

[Peter G. ShawParticipant @peterg-shaw75338]

The equipment that I bought and tested was relatively cheap. I think the most expensive was the 150m/0.02 resolution dial caliper at £32, The Starrett dial caliper was £40 but then I was prepared to pay for a recognised name.

The testing procedure was to leave all three standards, and the micrometers/calipers overnight in the same warm place – the standards are calibrated at the specific temperature of 20 DegC hence I got everything to that temperature and handled them as little as possible.

I'm not sure why I did it in the first place, possibly I noticed a discrepancy somewhere. In any event, for me, the absolute value doesn't matter that much. What is important is repeatability, and accuracy across the instruments, ie it doesn't matter if 10mm is actually 10.1mm as long as all measuring instruments show that same value, or that if they do differ, that I know what the difference is, and can account for it between instruments.

I know nothing of Mitutoyo's accuracy other than what I have read on this forum, but I would like to mention something else which perhaps shows that even high quality manufacturers can get it wrong. I have a number of 150mm rules, some were bought whilst some were magazine mounts. One of them is a Rabone Chesterman No.64R which you would expect to be quite good. (Now define "quite good"!) On comparing the metric scales, the Rabone Chesterman is noticeably different to the other un-named rules, which do compare satisfactorily between themselves. So, is the Rabone Chesterman the most accurate and all the others wrong? Or is it that I have got a duff one? I tend to think that I have a duff one, and hence it is relegated to unimportant duties. Interestingly, it's ok on the imperial scales.

There is another point regarding digital devices: they are usually specified as being accurate to such a limit plus/minus a count of 1 on the least significant digit. Therefore a value of 10.00mm could legitimately be shown as 9.99, 10.00, or 10.01mm. How accurate is that?

Regards,

Peter G. Shaw

[BazyleParticipant @bazyle]

At our club auction a couple of weeks ago two M&W micrometers went for £5 each. There were some at Sandown for that price too. With lots of people getting digtal stuff or old engineers dying there are a lot around.

If the scrutineer was using a caliper and not a mike on a critical dimension that small they are perhaps not the most well equipped. What do they use for timekeeping, an old alarm clock?

[John HaineParticipant @johnhaine32865]

If the gap is set to 1.26 mm by a licensed builder one would hope he uses something better than a vernier or digital caliper to measure it! I suggest that the OP should buy a Mitutoyo screw micrometer with calibration cert and a standard, and use that to make the measurement when the engine is collected fro the builder as a check, and also checks at the track after the scrutineer.

[Neil LickfoldParticipant @neillickfold44316]

I recently brought some insize digital calpers, a 150mm and a 200mm.

They came with individual test certificate, showing the internal error to 10mm id and the various error over 0-50 50-100,100-150, the 200 goes to 200 on the testing. When comparing to slips, their test coincided with the slips and corresponding error. The reading error over the range was as much as 0.03, but if you know the error can then compensate for it. I have never seen such a test cert with other calipers.

A ball bearing is a good test,as the od and the id of bearings are well within 0.01mm which is usually the resolution of the caliper.

Neil

[Anonymous]

I concur with John; bin the calipers and buy a 0-25mm micrometer. All my micrometers are secondhand and I've only had one duffer. With the money saved buying secondhand you can buy a couple of slip gauges around the sizes you need to measure. Total should be well under £150.

Andrew

Postscript: The duff micrometer was 4"-5" bought on Ebay and consistently over-read by 15-20 thou. I think the frame had been distorted. I put it back on Ebay, stated that it was mis-reading, and it still sold.

[Jesse Hancock 1Participant @jessehancock1]

Before binning any micrometer GOOGLE (micrometer adjustment) and follow the instructions. You may save yourself pounds.

On the other hand I found that most inexperienced people over tighten micrometers. There's a knack or a feel to using one properly and usually a ratchet which saves over tightening.

[Author]

Posts