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The calipers don't lie or do they?

Posted by DjDemonD 
The calipers don't lie or do they?
August 23, 2016 04:26PM
I'll cut a long story short, was using Lidl calipers worth about £10. Seemed reliable enough.
Then I bought some new larger 300mm ones from eBay for around £30.
.
Turns out the lidl calipers measure very small objects 2mm or so too large.
They measure mid sized objects 20mm or so too small.
They measure large objects 120mm or so, spot on.

I can't rule out that they might have been damaged at some point but I've handled them fairly carefully.

Sounds crazy but its worth checking your calipers measure accurately over a range of different object sizes before taking your printer apart to work out why its not printing objects the right size!

Does anyone know where you buy calibrated objects of known dimensions to test these things? The objects would have to be to the 1/100 of a millimetre accurate?


Simon.

[www.precisionpiezo.co.uk] Accurate, repeatable, versatile z-probe plus piezo discs, endstop cables, pt100, 50w heaters. PT1000 cartridge sensors plug straight into duet boards and others.
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Re: The calipers don't lie or do they?
August 23, 2016 04:33PM
You can buy calibration cubes but they are not cheap for a set.
You can use metric drill bits and measure them to see how close you are on small things.
Re: The calipers don't lie or do they?
August 23, 2016 05:24PM
Quote
DjDemonD
Does anyone know where you buy calibrated objects of known dimensions to test these things? The objects would have to be to the 1/100 of a millimetre accurate?

Coins? E.g a US quarter has a diameter of .955 inches (24.26 mm) and a thickness of .069 inches (1.75 mm). But it has a milled edge, so maybe a nickel or cent would be better? I did try a Google search, but couldn't find out how accurately coins are manufactured. [www.usmint.gov]

But .001mm accuracy?

Most metals have a coefficient of thermal expansion of between about 10 and 20 *1E-6/degree Kelvin. So a 10mm cube at 20C would be a 10.00015mm cube at 21C. If your ambient temperature range is more than about 10C, you're going to need your calibration cubes made of titanium or tungsten or quartz or similar.

How accurate is your printer? If you have a fairly typical 80 steps/mm, then the absolute best accuracy you can do is .0125mm. And when you factor in play, momentum, and other kinds of errors, I'd be surprised to get better than 0.1mm.
VDX
Re: The calipers don't lie or do they?
August 23, 2016 05:29PM
Quote
frankvdh
But .001mm accuracy?

... no - he wanted only 0.01mm (ten microns) winking smiley


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Re: The calipers don't lie or do they?
August 23, 2016 05:35PM
Yes 0.01mm would be enough, I'm interested to see if my calipers are accurate to that degree as advertised. Drill bits might work.

Edited 1 time(s). Last edit at 08/23/2016 05:36PM by DjDemonD.


Simon.

[www.precisionpiezo.co.uk] Accurate, repeatable, versatile z-probe plus piezo discs, endstop cables, pt100, 50w heaters. PT1000 cartridge sensors plug straight into duet boards and others.
Published:Inventions
Re: The calipers don't lie or do they?
August 23, 2016 10:06PM
Technique is important as well when striving for accuracy.

Hint: Using the thumbwheel is not the best way.

p.s. Most micrometer sets come with accurate test blocks.
Re: The calipers don't lie or do they?
August 23, 2016 10:53PM
[www.amazon.com]

as with anything else, the less you spend, the less you can trust them...


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Re: The calipers don't lie or do they?
August 25, 2016 05:20AM
I've had my share of sudo cheap calipers ($30-$60) but I finally bought a used Mitutoyo for $35. They were a bit abused (lots of grinding sparks hit the display and scale) but I took them apart, filed down any burs, cleaned them as best I could and they are great. What I love about good calipers is that I can pull it all the way out and push back in quickly (not something you should do) and yet it's still got an accurate reading not even .01mm off after doing this 5 times. If you buy some used Mitutoyo be sure they are the made in Japan ones and not the China ones.
Re: The calipers don't lie or do they?
August 25, 2016 09:38AM
If you want to check accuracy of calipers why not measure ground steel rods! Almost every printer has them, and even the really cheap and nasty ones should be ground to much less then 0.05mm tolerance.
Much better than drill bits.
Re: The calipers don't lie or do they?
August 25, 2016 01:23PM
We have a nice saying in germany "wer mit mist mist mist mist" winking smiley
Translated it means if you use garbage to measure your results will be garbage.


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Re: The calipers don't lie or do they?
August 25, 2016 04:30PM
I have a pair of cheapo calipers and a pair of Korean Made calipers that cost 3x the price (the brand escapes me at the moment) I have used the cheap calipers as a throw around tool. I do not use them where .01mm accuracy is critical but I also use them to scribe and literally hang them up on my 3d printer after use. As long as you know what your dealing with they are a fine tool that surely beats a ruler.

Here are my finding comparing the two calipers to real precision machinist instruments (starrett brand)

Cheap calipers have poor repeatability. Go slow with them and they are generally within .1mm. (I have reworked and scraped mine to increase smoothness, which also increased reputability quite a bit surprisingly, still not perfect) You also have to watch out as the battery draining causes wild readings as well as what you described above. I replace my caliper battery at least every 12 months if not sooner, I buy the batteries at thee same place I bought my import calipers 50cent a battery is not bad.

The better calipers have much better reepeatability and are usually good for .01mm (their max resolution)

starrett measuring equipment has repetability in the .001mm range if they are specked as such. My starrett paper gauge caliper (0-10mm range) is so sensitive that the needle jumps around with just my heart rate. Those can reliably do .001mm or better, want to measure a spec of dust? they can do that as well. Do you need to? Not for a 3d printer you dont. Have a jewlers lathe? Or a mill that can hold that kind of acuracy? Then these tools become not only a nice to have, but a necessity.

With all this said, the best 3d printers in the market can struggle to hold +-.1mm accuracy (try =- .2mm is very typical if not worse). This is due to minuscule filament variation which includes undersized, oversize and oval filament.


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