digital calipers

[Ady1Participant @ady1]

Got my 12incher and it's a solid lump, I don't fancy drilling those stainless jaws…

The scale was a bit jumpy though, not as good as the smaller ones so I took it to bits, yanked out the depth gauge and put it back together

Fine and steady now for 0.01 repeatability which is at least 20-50 times better than I could ever manage with the dials and backlash trubbles

If you dismantle a cheapo for the first time a bent copper strip will drop out

To reassemble find the hole at the end of the strip and fit that onto the tiny pin inside the vernier casing with the bulge part facing towards the moving slide, then re-insert the slide

edit

once I get it fitted I'll have a lathe DRO system for about 32 quid

 

Edited By Ady1 on 14/01/2015 01:30:05

[John OlsenParticipant @johnolsen79199]

One thing that the usual digital calipers really don't like is cast iron dust. I suspect it is actually the graphite component that does the mischief since it is conductive and would easily make a nice surface coating over the area where the capacitive bits live. I've wiped them down with meths to get rid of this problem.

I have one of the rack type, a Mitutoyo. My father who I inherited them from always said they were a snare and a delusion. OK for checking parts in a clean place, but not so good in a machine shop, especially machining that makes fine chips, like brass or those tiny bits that come off a milling cutter.

If you have zeroed at a point other than with the jaws closed, when you heat it that section of the bar between the zero and the fixed jaw will expand by some amount. Stainless I think has a low coefficient of expansion, but it will still expand a bit. So the zero point is now further from the fixed jaw. Also like a ruler, it is comparing the unknown to a scale which will expand with temperature, so they can only be standard at one temperature…like the rules with "standard at 25 degrees" stamped on them. Of f course, apart from that the electronics may well misbehave at higher temperatures.

I find that the ones I have a pretty stable and reliable. I do check the zero very frequently. Micrometers might well be better, but none of mine go past four inches.

John

[Neil WyattModerator @neilwyatt]

Posted by Michael Gilligan on 13/01/2015 22:32:50:

Posted by Neil Wyatt on 13/01/2015 20:36:05:

Left set at 0.01 on the bench for several hours, it crept up to 0.01mm.

.

Neil,

I'm struggling to understand that statement ^^

MichaelG.

Oops! Should have been 'left set at 0.00'. It's darn cold in here now and it reads -0.02 so there's definitely a relationship between origin reading and temperature. I tried the same experiment overnight with a pair with a flat bcell (display barely visible) left near the heater. This morning it;'s display was even fainter and jumping all over the place. A new cell and it seems to be behaving so I'll repeat the temperature cycle test on it.

I should say that the pair I tested yesterday had a new cell (about a week old) in it.

It will be interesting to see if two very different looking (electronics may be the same) show similar behaviour (origin change but still relative measurement OK) or not. Also, will they show the same amount of error for the same temperature difference – first impression is they might not!

Neil

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 14/01/2015 12:54:06:

Posted by Neil Wyatt on 13/01/2015 20:36:05:

Left set at 0.01 on the bench for several hours, it crept up to 0.01mm.

Oops! Should have been 'left set at 0.00'.

.

Thanks for the clarification, Neil

MichaelG.

[nigel jones 5Participant @nigeljones5]

I had a new 12" branded jobby for Christmas. The first few times I used it the readings were utter rubbish, then it settled down and is now stable. My staff use these but they dont trust any reading beyond .1 accuracy. Me being a saddo conducted a gauge R&R on just this, results confirmed expectation. I only use mine for roughing on the lathe/mill.

[MikeParticipant @mike89748]

I have tried two, admittedly cheap, digital calipers, and found the standard of accuracy very poor on both. I have now gone back to a spot-on accurate vernier caliper, which, at the age of 73, I can still read. Maybe I'm a luddite, but until someone can convince me of the wonders of digital calipers I'll stick with the method I was taught more than 50 years ago.

[Neil WyattModerator @neilwyatt]

The results of the second temperature test are in!

After an hour and a quarter on the windowsill for both the Proops and Workzone (Aldi) calipers they are both very warm to the touch.

As expected, the Proops caliper is reading 0.13mm. After zeroing, it reads 24.99 on a 25mm standard.

The Aldi caliper is still resolutely reading 0.00mm. On the 25mm standard it's reading either 25.01 or 25.00

My digital micrometer reads the 25mm standard as being 25.003, so some kudos to Aldi!

But where does that leave my theory? I'll try cleaning the Proops… immediate effect, the reading jumps to ~426mm.

Neil

[Ady1Participant @ady1]

That's the jaws drilled

Sharpened up an old 6mm carbide tipped masonry drill and put it into my collet chuck and it went through that stainless like butter (use intermittent stops to stop the tip overheating)

Moral of this story is the same as with milling, use a collet chuck and tough jobs are a doddle

[Robin teslarParticipant @robinteslar]

Posted by John Olsen on 14/01/2015 02:42:44:

One thing that the usual digital calipers really don't like is cast iron dust. I suspect it is actually the graphite component that does the mischief since it is conductive and would easily make a nice surface coating over the area where the capacitive bits live. I've wiped them down with meths to get rid of this problem.

I have one of the rack type, a Mitutoyo. My father who I inherited them from always said they were a snare and a delusion. OK for checking parts in a clean place, but not so good in a machine shop, especially machining that makes fine chips, like brass or those tiny bits that come off a milling cutter.

If you have zeroed at a point other than with the jaws closed, when you heat it that section of the bar between the zero and the fixed jaw will expand by some amount. Stainless I think has a low coefficient of expansion, but it will still expand a bit. So the zero point is now further from the fixed jaw. Also like a ruler, it is comparing the unknown to a scale which will expand with temperature, so they can only be standard at one temperature…like the rules with "standard at 25 degrees" stamped on them. Of f course, apart from that the electronics may well misbehave at higher temperatures.

I find that the ones I have a pretty stable and reliable. I do check the zero very frequently. Micrometers might well be better, but none of mine go past four inches.

John

Well said John

Certainly even the cheapo digital calipers have their place for say – spot measurements, having zeroed first and moved SLOWLY to the measure point, but from my experience I wouldnt trust the incremental type on say DRO applications where you could scrap your work or damage the machine.

BTW, we are addressing the linear encoder, there are rotary encoders available quite cheaply and by their nature they can be easily made as absolute position indicators (see Gray code in google). To my mind these are much more robust and foolproof – no exposed sensor elements and fundamental absolute accuracy.

Have to see what the DIY market can offer

[Anonymous]

Posted by Neil Wyatt on 14/01/2015 14:30:49:

T

The Aldi caliper is still resolutely reading 0.00mm. On the 25mm standard it's reading either 25.01 or 25.00

My digital micrometer reads the 25mm standard as being 25.003, so some kudos to Aldi!

Neil

….and if you're in the market for a cheapo caliper the Aldi Workzone caliper is on sale as one of this Thursdays' superbuys @ £8.99

[MuzzerParticipant @muzzer]

How can you use a rotary encoder to measure a linear distance with "absolute accuracy"? At some level you need to convert the linear distance into a rotary movement. We might call that a rack and pinion. Then you are back to where you started ie the rack has a temperature coefficient. Am I missing something?

Without resorting to exotic optical / laser methods, there are ways to reduce the temperature dependency. Low temp co materials and subsequent compensation to account for the temperature of the instrument for instance.

I think most of us would have the sense to realise that if we are measuring a part when it is hot, it will be smaller when it isn't. If the critical characteristic is defined at that lower temperature, we need to account for the dimensional change due to the temperature shift. I suppose you could say that IS rocket science.

Not all calipers have acceptable accuracy with temperature and that's one characteristic that clearly differentiates different brands. However, there are clearly some very cheap calipers that are actually pretty good and present a compelling performance vs cost outcome. I'd rather identify and obtain one of those than blindly cough up loads of dosh for a Mutitoyo each time I need a new measurement tool. That would be a more valuable outcome for all of us here, rather than trading opinions.

Murray

[Sam StonesParticipant @samstones42903]

Why was I told at age 15 – `Never leave a Vernier with the jaws completely closed’?

Then again, that was in the days when Verniers were just that. None of these newfangled electronic gismos.

Says Sam – Tongue in cheek.

Very cheeky

[John OlsenParticipant @johnolsen79199]

I don't know why it would matter with a Vernier, but I think you are not supposed to leave a micrometer completely closed. I think the theory is that the frame is warm from your fingers and as it cools off it will try to close the anvils, putting a stress on the frame. Maybe someone with time on their hands could calculate the forces involved……

John

[Bill PudneyParticipant @billpudney37759]

Posted by Neil Wyatt on 13/01/2015 20:36:05:

<interruption while="" one="" of="" the="" lads="" comes="" in="" and="" says="" 'have="" you="" ever="" heard="" a="" band="" called="" 'wishbone="" ash'?="">

Neil

I only have an old fashioned "unelectronic" Mitutoyo vernier, to go along with the similarly enhanced Micrometers. If I need to measure something to less than about +/-0.1mm I use a micrometer, if more the vernier gets used. If within (say) +/- 0.5mm a steel rule springs into use.

Wishbone Ash, first saw them on OGWT in the early 70s. Got their first few albums somewhere. A bit later I was astonished to find that my girlfriend was sharing a flat with a sister of a band member. Small world init

cheers

Bill

Edited By Bill Pudney on 03/02/2015 05:47:05

[Brian O’ConnorParticipant @brianoconnor49474]

Anyone contemplating buying a Mitutoyo caliper should be aware that there are many fakes out there. If you Google 'fake Mitutoyo calipers' you will fiind several sites describing the differences between genuine and fake calipers. Although there are several differences the easiest way of telling them apart is to look at the digits on the inch scale. The genuine one has four full size digits after the decimal point whereas the fake one has a half size last digit.

Brian

[Michael GilliganParticipant @michaelgilligan61133]

Thanks for that, Brian … I followed your suggested search.

… Here is quite a helpful illustration.

MichaelG.

[Neil WyattModerator @neilwyatt]

I'm wondering if it would be worth making a few standard bars of roughly 1, 2 and 3" long, and light enough to go in an envelope.

We could post these round from person to person, asking them to send me their results for three measurements with each of their calipers/micrometers. The exercise to be done at as near to 20 degrees as reasonable, and blind with an oath not to reveal the measurements to others.

I would then collate the results.

Because no-one would know the actual length of the bars, this would prevent user bias.

We could finish up by asking the SMEE or someone with the right gear to measure them to better than a tenth to complete the exercise – or ideally before and afterwards to see if the change length!

Any takers, and how do we source suitable test bars without any length markings on them – could someone with a surface grinder oblige?

Neil

[Martin KyteParticipant @martinkyte99762]

Why do the test bars need to be exact lengths?

Are you not trying to compare measuring devices rather than the length of the test bar.

I am assuming you are not talking about making multiple bars ?

Martin

[Martin KyteParticipant @martinkyte99762]

True enough John but you could look at it the other way. Instead of thinking of it as a way of comparing the possible accuracy of each measuring device it would be a comparison of the whole process each person used.

This might enable people to get a handle on how inaccurate we are as a statistical spread.

Most measurements are comparisons for our uses, stuff just has to fit.

regards Martin

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 03/02/2015 08:59:13:

I'm wondering if it would be worth making a few standard bars of roughly 1, 2 and 3" long, and light enough to go in an envelope.

…

Any takers, and how do we source suitable test bars without any length markings on them – could someone with a surface grinder oblige?

.

Neil.

Nice idea, BUT

Suitable Test Bars look like this

Note the radiused ends. [and the bargain price]

MichaelG.

[Robin teslarParticipant @robinteslar]

Hi All

I think we may be straying into the realms of esoteric measurements more applicable to aerospace toolrooms than home modellers. My OP highlighted a serious flaw with using the cheapo calipers (called incremental types rather than absolute). FYI the Mitotuyo uses incremental ladder scale together with a reference track that checks with the datum point – clever stuff I dont really quite understand but its termed and absolute vernier and you have to move the jaws a little to reset zero.

It wasnt so much a concern over temperature expansion etc. I observed that my caliper would inexplicably jump to a completely different reading of several mm out from true zero. I found I had to be sure to zero each time and move the jaws slowly. Its a time wasting operation you dont get with mechanical micros and verniers

I wouldnt trust digitals for critical machining measurements. I wonder how people fare with domestic machines using DRO (as opposed to industrial quality machines)

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 03/02/2015 08:59:13:

… how do we source suitable test bars without any length markings on them …

.

Neil,

Given Robin's fair comment … I will send you an eMail

MichaelG.

[Neil WyattModerator @neilwyatt]

> Suitable Test Bars look like this

One like that came with my 1" micrometer.

Thanks for the email, Michael.

This would be a test to see how things measure up in the 'real world'. I've also been advised that too much shouldn't be put on the performance of different brands, as the actual electronics inside them (and other parts) can vary between batches so that supermarket cheapo may have the same board as a high-end device – or it may not.

What it could show is how realistic it is to work to various levels of accuracy with caliper and micrometer in a home workshop*.

Neil

*On user bias, I am reminded that for my A-level Chemistry Practical we were told how the examiners realised that variables in things like beaker size etc. could affect the results. Absolute accuracy wasn't required, they would be looking for us to be getting good repeatable results within +/- 1 graduation from our titration.

I suspect lots of people's results came out as two runs the same, one out by one graduation…

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