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Select a reliable tool with these simple tests.
I I can’t think of a project I’ve made that didn’t involve using one or more of my combined squares. I use them to arrange joints and cuts and to check cuts to confirm they are square (a test I run on a scrap piece each time before using our shop’s bench saw on real pieces).
So when it comes to must-have measuring tools, a reliable 12 “combo team is at the top of my list and a 6” version is pretty close. And while their scales are unlikely to disagree, they could, then, if possible, buy them at the same time. Or, when you get a new one, compare and confirm that they match. (It’s also a good idea to show the scales on all of your measuring devices to each other – ideally, they’ll all be exactly the same.)
If you are buying these tools yourself, bring a mechanical pencil and a piece of wood at least 6 “wide x 6” long, with a jointed edge, with you to the store. If you’re shopping through a catalog or online, take a critical look at your new tool or tools as soon as you open the box.
And maybe it should go without saying, but buy the best combo squares you can afford (and don’t even consider for a second one with a plastic head). These are tools that are used a lot; the good ones will last and be worth the cost.
First, consider the rule
Of course, it’s crucial that a combination of squares be, well, square. If rule and head do not meet at 90 °, the tool is useless. (Yes, you can sometimes fix an out-of-square square, but you shouldn’t, particularly on a new instrument.) But you also need to be able to actually read the thing.
So, before testing the square, make sure the rule itself is up to scratch. Are the numbers legible? (Engraved numbers and lines, rather than stamped, are usually better.) Are there denotations in 8, 16, and 32? (If you’re an engineer, you might work up to 64th; I don’t.)
If the marks are fine, make sure both long edges are straight. To do this, remove the rule from the head, then press one edge of the rule against the hinged edge of the board. If the rule is straight, you won’t be able to see any light sticking out between the ruler and the wood. Now check the other long side.
Now try for Square
Reassemble the square and register the head firmly against the straight edge of the board. Hold the tool tight to the chalkboard, then draw a thin, consistent pencil line against the rule on the chalkboard face.
Now turn the square over and “show the squared line” – that is, with the head again registered on the hinged edge of the board, line up the other side of the tape measure with the pencil line. If the rule and your line are parallel, the square is effectively square. If it’s off, try again to make sure no user error was involved. Still out? Passage.
Once you are satisfied that the rule is good and that it meets the head at 90 °, loosen the nut under the ruler, remove the rule, then reinsert it. On most combined squares, the locking nut mechanism has a spring inside it; pushing up the lower part of the nut releases the tension of the mechanism and brings it into position to facilitate re-insertion of the rule. The rule should be easy to remove and replace.
Tighten the nut and roll the rule; it shouldn’t move.
Some squares include a spirit level and a removable metal tip that must function as a scribe or awl. The scribe is likely to get lost and the spirit level is quite useless for furniture. So I wouldn’t worry much about these.
If you buy good combined squares and treat them well, they will last beyond your life. (At home, I use a 6-inch Starrett that my grandfather acquired before my mother was born.)
Try your best to never drop a combined square, but if you do, immediately try it for the square. (And if it’s kaput, you can at least keep the rule.) Don’t unnecessarily slide the rule back and forth in your head; wear the surfaces where the rule and the head meet.
And make a rule to test your squares every now and then. If you find that after a few months or a few years they fail, save for a better brand.
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