I work alone, so I often have to come up with jigs as extra “hands.” If I find my cabinet-opening diagonals aren’t equal, here’s what I do to square and hold them till permanent connections are in place.
As shown in the drawing below, I measure the adjoining sides A and B. Then I square each of these numbers and add them together. The root of this total is the diagonal C.
At the C dimension, I adjust and clamp a sturdy bar gauge made from a ripped 2x4. Then I rack the cabinet box until the gauge fits into what was the lesser diagonal. Now I’m ready to make my mechanical connections.
, Langton, Ontario, ca