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Vernier gauge testing.

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Dominic Collings 123/05/2015 11:08:13
24 forum posts

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

roy entwistle23/05/2015 11:13:33
1716 forum posts

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

incedentally I have never seen a digital vernier

Roy

Bowber23/05/2015 11:16:34
169 forum posts
24 photos

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 W23/05/2015 11:36:44
2011 forum posts

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

Dominic Collings 123/05/2015 11:41:54
24 forum posts

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 Wyatt23/05/2015 11:49:05
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19226 forum posts
749 photos
86 articles

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 123/05/2015 11:54:21
24 forum posts

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 entwistle23/05/2015 12:27:58
1716 forum posts

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

Roy

Dominic Collings 123/05/2015 12:50:01
24 forum posts

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. Shaw23/05/2015 13:40:17
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1531 forum posts
44 photos

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 Gilligan23/05/2015 18:30:15
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23121 forum posts
1360 photos

Dominic,

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

MichaelG.

Dominic Collings 124/05/2015 07:40:20
24 forum posts

 

 

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 Gilligan24/05/2015 08:02:15
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23121 forum posts
1360 photos
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 pgk24/05/2015 08:07:12
2661 forum posts
294 photos

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 Hartland24/05/2015 08:09:53
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2929 forum posts
41 photos

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 CAWLEY24/05/2015 10:14:20
190 forum posts
50 photos

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

Michael Gilligan24/05/2015 10:26:02
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23121 forum posts
1360 photos
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 Gilligan24/05/2015 10:51:00
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23121 forum posts
1360 photos

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

MichaelG.

Peter G. Shaw24/05/2015 11:13:47
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1531 forum posts
44 photos

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

Bazyle24/05/2015 11:14:25
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6956 forum posts
229 photos

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?

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