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How accurate are rulers?

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Deadeye

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Inspired by the thread on the 48" square.
For things that size, I'd be using Pythagoras...which relies on accurate linear measures.
I've not done it, but if I laid out a dozen different metal tape measures next to each other, how much difference do you reckon I'd see over a metre? 0.1mm? 1mm?
 

Deadeye

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For context, 1mm error over 1m (if the other arm is accurate) gives ~0.03o error; 0.1mm gives 0.003o.
 

Trevanion

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They're only as accurate as the eyes that read them :lol:

I've noticed quite large differences between brands of tape measures before(I say large, it's usually between 0.5mm and 1mm), it's best to have just one and use that so everything is accurate to that tape measure rather than a variety that give out differing numbers.
 

Rorschach

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If your tape/rule has an standard mark (eg class 1 or 2) then you have a known tolerance for the accuracy.

I checked all my tapes by measuring the longest, flat, easily measurable surface in the house, the dining table. All tapes, including the tommy walsh poundland tapes were within 1mm of each other.
 

AJB Temple

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Rulers are surely relative. if you use the same one for setting out then they are perfectly accurate. If you use one for setting out against any form of gauge that is different to the ruler then you will get a repeatable and consistent error.

In essence, a notch on a stick will give you exact and perfect repeatability. In most cases that is all that a ruler is for, especially in woodwork. If we are seeking exact repetition against some set standard, then that is a different thing, probably more applicable in science and precision engineering than woodwork.
 

Deadeye

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AJB Temple":31qn17ba said:
Rulers are surely relative. if you use the same one for setting out then they are perfectly accurate. If you use one for setting out against any form of gauge that is different to the ruler then you will get a repeatable and consistent error.

Yes... but that's no good if the ruler is inconsistent with itself - i.e. if the error is not uniformly distributed on the rule.
Also there are lots of times where dimension A is measured with one device (say a tape for length), dimension B uses another (e.g. a ruler for width) and C for a third (for example calipers for thickness)
 

MikeG.

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Deadeye":29cih57m said:
.........there are lots of times where dimension A is measured with one device (say a tape for length), dimension B uses another (e.g. a ruler for width) and C for a third (for example calipers for thickness)

Not in my workshop. That'd be nuts.
 

Suffolkboy

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Deadeye":2i93g3ep said:
AJB Temple":2i93g3ep said:
Rulers are surely relative. if you use the same one for setting out then they are perfectly accurate. If you use one for setting out against any form of gauge that is different to the ruler then you will get a repeatable and consistent error.

Yes... but that's no good if the ruler is inconsistent with itself - i.e. if the error is not uniformly distributed on the rule.
Also there are lots of times where dimension A is measured with one device (say a tape for length), dimension B uses another (e.g. a ruler for width) and C for a third (for example calipers for thickness)

Well. Not really a problem either is it?

If you use the same ruler or measuring device for all the measurements on a given project and always measure from zero with the numbers increasing toward the end of the ruler then all your measurements will be correct, relevant to each other, or will they? I've confused myself now.
 

Cheshirechappie

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We're rather in danger of sliding into pedant territory here - but it does depend a bit on what you mean by "accurate".

Rules are not really 'precision' measuring instruments. More applicable to engineering than woodworking, this, but if you want to measure 6.750" plus or minus 0.001", you won't do it with a ruler, however 'accurate' said ruler might or might not be. Thinking more like a woodworker, if you want a piece 6 3/4" long for a wooden box end or whatever, you stand more of a chance, and as stated above, how close the finished piece is to size depends as much on the user as the rule.

Worth noting that the better steel rules are made to be 'standard at 20 centigrade', and those that have that wording engraved on them are, at that temperature, as close to accurate as any rule will be. Even those could be 'off a bit' if the user regularly uses the zero end to scape glue off jobs or rubbish out of odd corners, though - which is why second-hand rules should always be treated with a degree of caution!
 

Rich C

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Suffolkboy":27i7bkhu said:
Deadeye":27i7bkhu said:
AJB Temple":27i7bkhu said:
Rulers are surely relative. if you use the same one for setting out then they are perfectly accurate. If you use one for setting out against any form of gauge that is different to the ruler then you will get a repeatable and consistent error.

Yes... but that's no good if the ruler is inconsistent with itself - i.e. if the error is not uniformly distributed on the rule.
Also there are lots of times where dimension A is measured with one device (say a tape for length), dimension B uses another (e.g. a ruler for width) and C for a third (for example calipers for thickness)

Well. Not really a problem either is it?

If you use the same ruler or measuring device for all the measurements on a given project and always measure from zero with the numbers increasing toward the end of the ruler then all your measurements will be correct, relevant to each other, or will they? I've confused myself now.
You could have a ruler that measures the first foot as 12.0", then the second foot as 12.1" as an example. In that case, two foot long pieces as measured would not be the same as a two foot piece.

If you wanted everything accurate relatively you'd need to take a measurement then scale everything off that (by halving / doubing, etc.)
 

thetyreman

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I just stick to the same rule, my tape measure is equally accurate as it's class 1 not class 2, I like my cheap 30cm rule and starrett rule equally, they are accurate enough for woodworking, what's far harder is cutting the wood to very tight tolerances consistently without taking off too much.
 

Dilbert

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One more thing to add to this: If accuracy is of great importance for a project, you will need to keep your workshop at a consistent temperature and a rule, or tape, will only meet it's standard for accuracy at the temperature specified by the standard. I'm sure, in general, such consideration is over the top for woodwork.
 
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Surely you're more likely to have more user error than anything else

- perspective between eyes? one eye shut? reading at a slight angle?
- are you holding the ruler square to the edge? and perhaps not as square the next time?
- are you pulling the tape measure as taut as you were previously? (mostly refering to the play in the internal/external measurements clip)
- wear on the metal clip for internal/external measurements
- not to mention transfering the value

all of these could easily introduce 0.5mm
 

Dilbert

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transatlantic":sqot087h said:
Surely you're more likely to have more user error than anything else

- perspective between eyes? one eye shut? reading at a slight angle?
- are you holding the ruler square to the edge? and perhaps not as square the next time?
- are you pulling the tape measure as taut as you were previously? (mostly refering to the play in the internal/external measurements clip)
- wear on the metal clip for internal/external measurements
- not to mention transfering the value

all of these could easily introduce 0.5mm
I have quite bad astigmatism and am long-sighted. With or without my glasses I would very pleased to be within 0.5mm.
 

Rich C

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transatlantic":1ijfvh7u said:
- are you pulling the tape measure as taut as you were previously? (mostly refering to the play in the internal/external measurements clip)
My tape is getting on a bit, there's about 1-2 mm of play there. Has caught me out before.
 

Trainee neophyte

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I have a cheap yellow square with measuring scale - standard Chinese tat. The printed scale one side is 3mm out compared to the other - caught me out before I realised what the problem was. Seeing that it isn't square, either, I don't use it any more.

Are squares square, is another conversation.
 

MikeG.

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Rich C":8tkdnyjh said:
transatlantic":8tkdnyjh said:
- are you pulling the tape measure as taut as you were previously? (mostly refering to the play in the internal/external measurements clip)
...... there's about 1-2 mm of play there.......

You know that there is supposed to be?
 

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