Shop Tricks, Tips & Mini Projects
January 19, 2013
Measuring Tools Accuracy Checks
If you are looking for deals on measuring tools, it's a good idea to take along some easy to do methods for checking them out before you buy.
Here's a way to quickly check to make sure your tape, or the one you want to buy, is giving you accurate measurements. First, line the 1" mark on your tape with the edge of a piece of wood, then make a thin, precise mark at 7":
Measure over with the blade hooked onto the edge:
If the tape is of reasonably good quality and in good shape, it should be dead on the 6" mark. As shown here, my tape is very slightly off - less than 1/64" is not bad for an older tape, but it can be fixed. If the mark comes up short of the actual measurement, this indicates that the hook on the end may be bent inwards (from dropping the tape on the floor), and that's fairly easy to fix.
If it overshoots the mark, this usually means the rivets that hold the hook on are worn or the holes in the blade are enlarged. This can be fixed by bending the hook inwards slightly or by adding shim material to the hook, but if it's an older tape it might be time for a new one. If it's a new one, on the shelf, leave it there.
Next is to check the tape with the hook against something and see how close it is to the mark:
This is pretty good for an older tape that has some wear. This type of measurement should not be used much anyway, but it's always good to know just how close it is.
Are those cheap digital caliper reliable? There's something about a digital display that gives the impression of accuracy, but it's always a good idea to verify this before you buy and occasionally after you own it.
The calipers pictured here were less than $10, and that price was due to a discount when I bought a bunch of ball bearings. The regular price on these was $35.
Here I'm measuring a 1/4" hex drive which is actually very close to .25". Still, these calipers read .27".
I bought the bearings and calipers online, so I didn't get the opportunity to verify the calipers accuracy.
Here's is a fairly reliable way to judge how close they are before buying them:
With the jaws fully closed, zero the display and look at the printed scale. The "0" mark (red arrow) should line up exactly with the edge of the plastic housing:
Carefully moving it over to the 1/10" mark and the display should read very close to .100 - within 3-4 thousandths. Mine read nearly .114 - 14 thousandths over. The human eye can resolve down to a very high accuracy, and if the printed scale is correct, there should not be that much of a difference.
Here's the baffling thing on mine, at 2/10" the display is correct:
And over at 3/10" the error shows up again. At 4/10" it's "accurate" and at 5/10" it is again 13-14 thousandths off. This trend carries on over the entire operating range: even numbers ok, odd numbers off. Regardless, it's of absolutely no use as a precision measurement tool.
Matthias Wandel ran into a similar problem with what looks to be the exact same model of caliper here. I don't think it's worth my time to take this one apart looking for a solution, though, so I'll just chalk it up to experience and use these with the printed scale for "close enough" measurements.
If you are tempted by that tape measure that is on sale for 90% off, check it against a known good quality one to make sure the scale is correct. This only takes a minute in the store. For many of the cheaper items like this, accuracy is pretty much just a suggestion, and not the standard.
For example, I bought strips of 1/2" wide stick-on measuring tape that were on special. When compared to an accurate tape measure, it's not hard to see why they were on sale:
Lined up exactly at the 62" mark:
and over 14" it loses 1/32". Good for laying out accurate cuts with a chain saw.
I bought two of these stainless steel rulers for use in the house and in the shop. To be honest, I didn't expect much more than a straight edge from these, but while doing this, I thought I'd have a look at how accurate the scale is.
Lined up with the known accurate tape.
Dead even at the 1" mark:
Coming up nearly 1/32" short at 12". Quite possibly made at the same facility as the stick-on tape, using the same precision equipment.
I'm not a stickler for ultra high accuracy, but I find it absurd that measuring tools (bought from reputable sources) would have such obvious errors. It seems to me that it would take about the same amount of time and effort to manufacture things like this with acceptable precision.