Thread: tape measure accuracy

The thing about a tape measure is that it's accuracy relies on the play in the hook and it would be very difficult to get it accurate to that degree. The hook has to move by exactly the width of it's tongue and this range of movement is controlled solely by the length of the elongated rivet holes. You might get one with a fixed hook that is more accurate but then you lose the ability to do either inside or outside measurements.

Personally, I use the tape for things that don't matter, like measuring diagonals, searching for timber roughly the right length, building houses etc. I use a 1 metre or a 300mm steel rule for accurate measuring and I've got an Incra perforated rule and a vernier caliper for when I'm being really fussy.

Yeah, that totally makes sense. thanks. and i can see that as the rivet holes wear out, the accuracy would be less and less reliable. what i am talking about however is inacuracy in the tape itself. when needing a truly accurate measurement I usually "burn" 10 inches to avoid that particular problem. however, when checked against my Shinwa framing square and burning an inch all my tapes are dead on at some points, and way out (32nd) at others. I may be a picky prat, but if you want light tight furniture quality joinery on timberframe construction you have to use accurate layout tools.

light tight furniture quality joinery on timberframe construction

Cripes, how long does it take you to build a house :eek:

I guess the tapes are manufactured to lower tolerances because they are not generally thought of as a precision measuring device. Don't know much about the manufacturing side of it, I suppose the numbers are painted on to the tape and there could be a bit of slop in the process.

Originally Posted by silentC

Cripes, how long does it take you to build a house :eek:

I can usually milk the framing portion for about a year :eek: .

For your information, a local Tasmanian inventor has been building test instruments for the tape manufacturing industry for some time now. Stanley bought one and are currently testing it. The inventor made the instrument because of the very issues you have outlined.

It works with a laser and also utilises a digital PC interface. The problem will still be acceptable tolerences allowed by the manufacturer though, the measuring machine will calibrate to their standards.

Not tapes I know but I have had a variety of steel rules (12") that displayed a 1/4" variation between them, the question is of course, which one was right.
I opted for the American one (forgotten the name) over the Chinese varieties.

Ryan,

I am not sure whether there is an absolute standard for the inches and feet, but the metre is conveniently defined as "the length of the path travelled by light in an absolute vacuum during a time interval of exactly 1/299,792,458 of a second." Of course, most of us don't have a stop-watch that accurate

Seriously though, it is rather disturbing to hear that Starrett only work to a tolerance of + or - 1/16". Generally speaking, though, in woodworking we are concerned with relative distances rather than absolute ones, so that, so long as you are using the same measuring device all the time, there should not be a problem, unless the length of your inches is variable in the one tape.

Rocker

If you are using a steel tape then the +- of 1/16" is all they will be able to guarantee due to the length of the tape measure changing with the ambient temperature.
Longer measurements are likely to be less accurate than shorter measurements.

I bought an Incra 300mm rule the other day, it has slots for a 0.5mm pencil lead every 0.25mm.

This was all great until I discovered that I can't see 0.25mm increments.

Nothing wrong with the rule mind you, it's a nicely made instrument. It's my eyesight that's the problem.

Used to work for Boral Timber, very very big On Quality Assurance, every employee was issued with a good brand of tape for length. Before issuance was checked against a master rule, if out >1mm was sent back to suppliers, many many were returned. The cross section sizes were checked with quality verniers also checked against a master.
I reckon the secret is to consistantly use the same measuring device(where possible) on a project.

All you need to do for accurate measurement is apply temperature correction then corrections for tension, sag and slope.

mick

I can check them Tajima things if you want, but not for imperial measures. Only metric and Japanese scales (don't ask).

I don't want to sound callous, but the fact is that it is a tape measure, deal with it. I do mean that in the nicest possible way, but when I was on the tools for a living, tape measures were one of the disposable tools you just knew were going to be either shoddy, inaccurate or just plain junk. Great, because you assume you are going to lose it at some point.

Most of the guys who needed to rely on a tape measure, like framers, used Lufkin tapes, but even then only to a single mm, and that's it. And always allow for a little wiggle room. If it had to be truly accurate, like levels and slopes, they used lasers.

You could always get some simple molding and use it to make story sticks. It would at least be accurate to whatever you are making. And only have one tape measure on the go at any time.

I used to use stanley tape measures, I think I still have half a dozen or so back in Oz. Waaay back when there were heaps of cheapo stanley tools going for cheap. I think I bought ten or so for $2 each.

And I planted a few of them too.

BTW, Ryan. Yer reply email addy wasn't working the other week.

I've got 12 inches but I don't use it as a rule.

...and as a tape it sags........so I've heard