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Miter Angles and Miter Saws – Toolbox

Miter saw gauges confuse a lot of finish carpenters for one simple reason—they aren’t designed for finish carpentry, they’re designed for framing and stairs. Let me show you what I mean.

When a framer builds a roof, they first establish the PITCH of the roof—4/12 or 6/12. That pitch is the angle the rafters follow. All of the cuts made to that rafter—the ridge cut, the plumb cut, and the birdsmouth are all measured off the BACK of the rafter—off 90 degrees to the angle of the roof.

(Note: Click any image to enlarge.)

That’s why most miter saw gauges are set up off 90 degrees to the back of the fence! In fact, some miter saws even include roof pitch angles on the miter saw gauge.

But those angles just confuse finish carpenters. Finish carpenters are always bisecting corner angles—which is easy to do with a protractor.

A 135 degrees angle looks like this:

A 135 degree angle is an OBTUSE angle. The miter for this corner is 67 1/2 degrees. That’s easy to cut on your miter saw. Just set the saw at 22 1/2!

But the angles on a miter saw gauge are off 90 degrees to the angles on a protractor, which causes a lot of confusion. Some carpenters opt to use a Starrett Protractor. But I prefer to have the guys on my crew use a standard protractor, so they’ll know at a glance the difference between an acute angle and an obtuse angle. That means when they visualize the miter, they’ll be starting off on the right foot!

A 22 1/2 degrees angle is an ACUTE angle. You can’t cut that miter on a miter saw without an acute angle jig.

Miter saws didn’t always come with miter saw gauges that were set 90 degrees off from protractors. Back before framers used miter saws, finish carpenters could use a protractor to read corner angles without any confusion.

Miter saw gauges like this one (see photo, right) were easy for finish carpenters to use, and didn’t cause another problem: Let’s say you’re installing base molding at a corner that measures 86 degrees. You first bisect the angle and determine that the miter should be 43 degrees. But when you set your saw at 43 degrees and cut the piece, the miter is NOWHERE near close! That’s because 43 degrees on a standard power miter saw gauge is really 47 degrees.

The easiest way to solve this whole problem is to use a Sharpie to mark your miter saw gauge with protractor numbers.

In fact, some manufacturers include protractor angles on power miter saws:

I just wish they all did!!!


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