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One of the more accurate approaches to checking a case or another assembly for the square is to compare the internal diagonal measurements for the equidistance. The easiest way to do this is to use grip rods – a pair of sticks attached together with a collar to create the appropriate length for measurement. To use the grip rods, simply slide them until the ends slide into two opposite corners of the case, then lock them together to control opposite diagonals. I make pinch rods of various lengths to fit different sized projects. For rods longer than 24 ″, I use two collars, attaching one to each end of the overlapping rods.
I make my pinch rods by tearing them 3⁄16“Removes from 3⁄4“- thick hard wood, beveling one end of each about 30 °. I make the collar by gluing together 1⁄2“- thick hardwood pieces as shown. The collar is 1 3⁄4“-Wide of 1 3⁄4“-Long, with a 1⁄2“X 2 5⁄32“-Large tunnel for the rods. The 1⁄2”Size accommodates the two rods plus the head of a T-nut that holds a 5⁄16Locking thumbscrew “-diameter x 1” -long. I install the barbless T-nut before mounting the collar, epoxidizing it in place from the inside face of the collar. After gluing the collar together, I roll up the edges for convenience, using a 3⁄8“-Roundover bit in my router table. (For safety, hold the piece with a wooden screw clamp when milling.) –Pete Dulak
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