Miter Gauge Error

Squaring a miter gauge has become a very important ritual for me. I try to make sure that I get it as close to 90 degrees as possible. I use the same process to square a miter gauge as I do for a sled fence. Sometimes I wonder how close to 90 degrees do we really need to be? 90.0? 90.00? Some have argued that as little as 0.1 degrees off from 90 is considered significant. So how much of a problem will a miter gauge set at 89.9 degrees instead of 90.0 degrees cause? And would you be able to discern any error with a square?

In the piece of wood shown in the diagram above, angle A is the cut error caused by having a misaligned miter gauge. If A = 0.1 degrees and the adjacent side (Adjacent) is 5″ then the opposite side (Opp) will measure: 0.009″. The formula is Opp = Adjacent X Tan(A).

(edit 1/2/2012)  Use my online calculator here. (end edit)

An error of 0.009″ would be difficult to discern with a square (at least for most woodworkers). This is why some woodworkers will use something called The ‘Five Cut Method’ when squaring a miter gauge using scrap wood. I should add that when squaring a miter gauge or table saw sled using the method I describe here you don’t need to use the ‘Five Cut Method’ or scrap wood in the process.

The angle error can quickly add up when the piece is mated with other pieces in the project which also contain angle errors which could spell disaster. As the side, Adjacent, gets longer the error gets even larger. For example, an Adjacent side measuring 12″ would equal an error of 0.02″ (angle A = 0.1 degrees).

So what is the take home message from the above? When checking for square using a ‘good quality square’, it can be difficult to discern an error of 0.1 degrees, especially if you are using a small square or checking a board with a short ‘Adjacent’ side (remember the error is enhanced as the adjacent side gets longer.

What’s your take?

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