Coping MDF Mouldings
Carpenters don't use protractors. All squares and scale markers used in carpentry and woodworking are laid out the same. The scale is laid out for making miter cuts. A 90 degree cut is not a miter, so it has a 0 degree angle. Half of 90 degrees is 45, so the saw is turned to an angle cut of 45 degrees.
From the original questioner:
Thanks for your feedback. Carpenters (at least cabinetmakers) do, however, use protractors. Look at any angle finder; even a digital one is a protractor. Miter saw gauges do accurately indicate 45 degrees, whether measured from the board's edge or its end. There's no problem if you are mitering trim for a 90 degree corner. But if your angle finder reads 88 degrees, you have to set your saw to 46 degrees instead of a more straightforward 44 degrees. Also, compare a miter saw's gauge to a table saw's miter gauge: the angle is measured from the board's edge on the latter. I know some miter saws have dual scales, I was just curious about why the miter saw was originally designed with, in my opinion, a backward scale.
From contributor F:
Dealing with the angles is a frustrating part of woodworking sometimes. Realizing that as far as using the saw scales goes, the saw blade has already traveled 90 degrees in relation to the plane of the table in the case of a tablesaw scale, or the plane of the fence in relation to a chop saw scale when the saw scale reads zero degrees, helps you figure out the correct settings.
So, if the angle is 60 degrees, you can subtract 60 from the 90 degrees that the blade already moved (when it reads zero) and get a saw setting of 30 degrees for the whole angle. Of course, for a two piece miter, you then divide the whole angle by two for a saw setting of 15 degrees.
It also helps to learn some simple geometry when dealing with angles. If you keep in mind that there are 180 degrees in a straight line, which is the base line of a half circle protractor, you can use triangulation to work with and solve any problem related to finding the correct saw scale settings.
In the drawing below, triangulation is used to extend the lines in the drawing (red lines) and thereby expose all of the angles. Notice that if you add up all of the angles from the base line back to the base line, it always equals 180, which is the number of degrees in a straight line.
If you get an angle on a drawing that is larger than anything on your saw scale like, say, 144 degrees, you can subtract that angle from 180 and you get a saw setting of 36 degrees.
From contributor D:
180 is a line, the end of a board is a line when the second player (the edge) comes in to play, the end's relation to this edge is what you're measuring with protractors and such. Stand beside your saw facing the blade and look one time. The end is 180 parallel to your horizon. Go to the front and it's just a different view. Cut an angled block and use for a fence to get obtuse angles you normally can't set the scale to cut.
From contributor J:
Cope wood, but miter and glue MDF crown. If you cut it a hair long and glue, you have a seamless joint that's getting caulked anyway. Coping MDF crown can be problematic. I do, however, cope all base MDF and wood... if that makes any sense?
From contributor W:
I've always coped MDF but I stay away from my cutline by about 1/8". Then I use a rotary tool with a sanding drum to sneak up on the edge. It leaves it nice and smooth. This will limit chipping when cutting, but for installation, I'm just really careful. It gets caulked anyway.
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