Measuring devices must measure up

Even more important than knowing how to properly use and maintain your tools is the accuracy at which a tool measures, aligns or squares.
For example: Did you ever try sizing a project with more than one measuring device and then discover that you ended up with a pair of items that were supposed to be identical in size but weren't?
If you spend a few minutes making careful comparisons, you will find that there are slight differences in just about every measuring device in your workshop. Lay three rulers or measuring tapes side by side and presto surprise-o, you will discover differences between each -- sometimes slight, but often substantial.
It's OK. Don't get excited. This is a fact of life that you can easily manage.
First, choose the highest quality tape or ruler that you own; that's the one that becomes your own personal "measuring standard." From that point on, all of the measuring devices in your workshop -- rulers, squares, tapes -- need to be gauged against your chosen standard. Compare each to your standard and where there are differences you can either makes notes as to the correction needed or indicate on the tool that its measuring portion should not be used.
Keep in mind that retractable measuring tapes can be easily adjusted. The hook at the end of the tape can be bent - ever so slightly - to increase or decrease distance, thereby allowing accurate measuring.
Start at home
Although you can purchase a measuring standard (something like a block of wood or a metal plate that is "perfectly sized" to which tapes and rulers can be compared), it is really more important that all of the rulers and tapes in your own shop match each other.
In addition to overall length, one of the hardest standards to achieve is "square." One of the first things we learned in wood shop was how to "square" a block of wood -- how to create perfect 90-degree corners. Now, years later, we have figured out how to adjust a carpenter's square (a framing square) so that it renders a perfect 90-degree angle to a tolerance of within a 1,000th of an inch.
'Square' it up
UYou will need a good straight board that has been commercially edge-planed or joinered. Get this from any cabinet shop for a couple of bucks. It should be about a foot wide by two feet long. Any scrap will do.
ULay the large leg of the framing square along the long edge of the board so that you can mark a one-foot long line that cuts the board's two-foot-length in half.
UFlip the square over and try to align it with the opposite side of the pencil line that you just drew. If the square aligns perfectly (edge of square and pencil line are perfectly parallel), then you have the accuracy you want and need. If the pencil line and the edge of the square don't align, the square is not true and will need to be adjusted.
Truing a framing square is easy, but can take a bit of time. Give yourself an hour or so. You will need a hammer and a nail punch. Any size punch will do. A framing square is nothing more than an L-shaped piece of metal; it has no moving parts. To cause it to be perfectly square, you must "re-form" it. That's where the hammer and nail punch come in.
'Re-form'
UIf the line and the edge of the square diverge from the square's apex, then the legs of the square must be brought closer together and your adjusting point becomes a location near the outside corner of the square.
UIf the line and the edge of the square converge from the square's apex, then the legs of the square must be spread farther apart and your adjusting point becomes a location near the inside corner of the square.
Here's how the actual adjustment is made.
USolid concrete is best as the work surface, so be prepared to use your patio, garage floor or sidewalk to get the job done.
ULay the square down flat on the concrete.
UNext, using the rules above to determine where to make the adjustment, choose either the inside or outside corner of the square and hold the nail punch at that point and give the punch a whack with the hammer. Use a firm blow to strike the punch. The punch will leave a circular mark in the surface of the square -- the mark won't hurt a thing.
You must then re-test the square by drawing a new line. Re-test the square after each blow with the punch until the square and the line are perfectly parallel.
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