Combination Square Buyer's Guide

The combination square is the most-checked, most-trusted measuring tool in a furniture shop. Layout, machine setup, joint fitting, glue-up checking — all of it relies on a square that is actually square. Most squares sold at home centers are not. WHAT MAKES A SQUARE ACCURATE Two things. First, the blade is straight and the edges parallel. Second, the head is machined perpendicular to the blade. Cheap squares cut corners on both. The blade may be slightly bowed. The head may be slightly out of square (often by 0.5° or more, enough to cause a visible gap on a 12" measurement). Both errors compound when you transfer the layout to a workpiece. THE STARRETT STANDARD L.S. Starrett combination squares are the standard against which everything else is measured. The blade is hardened, ground flat, and the head is machined to better than 0.001" perpendicularity. A 12" Starrett 4R-blade combination square is about $120 new. It will be accurate when you buy it. It will still be accurate in 50 years if you do not drop it. THE PEC ALTERNATIVE Precision Engineering Co. (PEC) makes US-made squares at about half the Starrett price. The blade is similar quality. The head is similar accuracy in the squares I have tested, though I have heard of one bad one. At $50-70 for a 12" PEC, it is the value sweet spot for someone who does not need the Starrett name. THE iGAGING / EMPIRE TIER iGaging and Empire make $25-40 squares that look almost identical to PEC and Starrett. The accuracy is hit-or-miss — some are fine, some are not. If you buy one, check it the first day you own it (see below). HOW TO CHECK A SQUARE Set the square against the planed edge of a long board with the blade overhanging. Mark a line along the blade. Flip the head to the other side, set the blade against the same edge in the same spot, and mark a second line. If the lines are parallel, the square is square. If they form a V or wedge, the square is out by half the angle of the wedge. This works for any square — combination, try, engineer's. Do it once when you buy a new one. BLADE LENGTHS A 6" and a 12" cover most furniture work. The 6" lives in your apron and gets used dozens of times a day; the 12" is for benchwork. A 4" miniature combination square is useful for inside corners and joinery layout where a 6" head is too big. Not essential. THE HEAD'S OTHER FEATURES Most combination squares include a 45° face and a built-in level. The 45° face is useful for miter checks; the level is small enough to be approximate. Some include a scriber in the head — handy for marking pins and quick reference lines. WHAT TO ACTUALLY DO If you can spend it: a 12" Starrett 4R-graduated combination square ($120). One tool, lifetime accurate, never replaced. If you cannot: a PEC 12" combination square ($60). Same accuracy in practice, US-made. If you bought a cheap one and want to know if it is OK: do the test above. If it passes, use it. If not, return it and buy a PEC.