stanley tape measure accuracy

[EricU]

I'm a part-time metrologist, and I am still confused about the difference between precision and accuracy.  I can work up an error budget if someone is willing to pay, then they can decide how good my measurements are. Usually, I work in  nanometers and sometimes in hundreds of picometers, so hopefully the accuracy and precision are in the same ballpark.  I inherited a scale from my mom, it's in inches on a French curve. It's on my desk at work, never did figure out how to use it.

I think most of the decent tape measures are within an 1/8" or so.  I have never tried to figure out how good my Starrett tape is, hopefully it's better than that. When I'm using a tape measure, I use the same one for everything, so errors should cancel out for the most part. It's like using some other unit of measure other than inches.  Plus I'm an engineer, we aren't supposed to use tape measures (or other tools for that matter).

[Mike Sieweke]

I tested (visually) 3 Stanley tape measures (16',16',30'), a 12" Bridge City rule, a Lufkin 6' folding rule, and 6" dial calipers.    To the best of my ability, I found they all read the same except the Lufkin rule.  The Lufkin is 3/32 short at 6', compared to the tape measures.  The Lufkin is visibly off by roughly 1/64" from 0-12" and from 12-24", which matches the overall error.

The tapes are identical at 1' and 6'.  The tapes also read identically for inside and outside measurements, so the sliding tabs work as designed.

I think it's a good idea to know whether your measuring tools match.  But I don't do much measuring after roughing out the stock.  I generally mark parts off each other or off the openings where they need to fit.

[AHill]

I'm a part-time metrologist, and I am still confused about the difference between precision and accuracy.  

In my world (military aircraft), precision means the bomb hits the same spot every time.  Accuracy means it hits where I aimed.

[Handplanesandmore]

In my world (military aircraft), precision means the bomb hits the same spot every time.  Accuracy means it hits where I aimed.

That analogy works well for woodworking, too, and another good way to explain the difference between accuracy and precision.

Simon

[Jim Waldron]

Jim, Using your drawer construction as an example, this is how I see it.

For the drawer to function properly, you must first decide what its final dimension should be (by measurement or by butting the stock against the opening). As long as the dimension arrived at is the same or smaller than the opening then it will work physically. Say, the width for the drawer front should be 21".

Accuracy: You cut the front to spec. accurately at 21", using the measuring device as the guide (the same tape, stick, whatever that was used to arrive at the 21" figure in the first place). The drawer now goes in as it is designed to.

Precision: You cut the front precisely to an inaccurate width (21" + 1/64"). The drawer can't go in. So, you re-cut the front precisely, this time, also to another inaccurate width (21" - 1/64"). The reveal is now 1/64" larger than what you sought to attain. Is it fatal? It depends on your goal. For a shop build, no one would care, but if you were supposed to deliver an air-tight (piston-fit) construction to your client, you could be in trouble. Or every drawer has an even reveal except this one, the odd man out that everyone can see, including yourself.

When you say 20" or 21" and who cares, it means you are the boss and you can change the design or dimensions on the fly (most of us often do that either to accommodate the stock or to cover a mistake we make). That however is different from the question of accuracy vs precision.

If you trust your tape to measure to an accuracy tolerance of 1/64th, you're welcome to it.  If I want a piston fit drawer front, I'm certainly not going to rely on any tape measure to get me to it.  I'll keep using my story stick when I need an accurate fit.  And when I make a set of drawer fronts, I shoot the ends with a stop block so they all very precisely match the length tested in situ.  And my precision and accuracy are both a good deal better than 1/64th in that specific case. Other measures are looser, of course, around that 1/64th.

A tape would be an unacceptable source of error for me, both per se and by measuring and marking technique errors as I go from board to board.  So I just don't do it.

Please note that I have never suggested that accuracy is unimportant, as you seem to suggest.  What I have said is that the accuracy of a tape measure is not ever assured; it's okay for loose tolerances, as they generally are within a reasonable tolerance. A tape's precision is generally pretty good except for the user errors that are so easy with tapes.  As already said, okay for construction, not so good for furniture making.

Do you measure the length of your drawer fronts, one by one, with a tape measure?

[bandit571]

I use the tapes to get things CLOSE to whatever sizes I need....then I mark as needed for final size.  

There IS a reason with tape measure do NOT have marking below 1/8", after all.  Too hard to read, anyway.
 

Same as with a square.....I use the same square throughout a project.    I have a 12" steel ruler I can use, IF I think there is a need.  

Horse has been buried...and still people are beating on the ground it was buried in....

I use what WORKS for me..YMMV

[Greg Jones]

There IS a reason with tape measure do NOT have marking below 1/8", after all.  Too hard to read, anyway.

I've never seen an imperial tape measure that didn't have at least 1/16" markings, and one of mine has one line at 1/16" and the other line at 1/32". The 1/32" is useless to me, but I wouldn't want a tape measure that only went to 1/8".

[Roly]

Here is a LINK to verified accuracy tapes.    If you notice the prices that is why we don't have them.   Roly

[Handplanesandmore]

Here is a LINK to verified accuracy tapes.    If you notice the prices that is why we don't have them.   Roly

Apart from the prices, a typical woodworker may find it difficult to maintain the integrity (or accuracy) of the tape if it is used on a day to day basis in the shop:

"These tape measures have been carefully selected and verified for end hook accuracy at the 6" calibration lines. End hook accuracy +/-.005"..."

Does that mean each time the tape needs to be retracted manually by hand? Definitely the case for Paul Sellers if he owned one, because he is seen all the time letting his tape recoil rapidly and you can actually hear the bang when the hook reaches the end. After a few Sellers' kind of recoils, I wonder how the end hook accuracy would hold up.

Simon

[hbmcc]

Here is a LINK to verified accuracy tapes.    If you notice the prices that is why we don't have them.   Roly

It's a scam.