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Mr. Mike
Mr. Mike, Civil Engineer
Category: Home Improvement
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Experience:  Homeowner and fixit fan. What works is what I am interested in.
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How do you use a protractor (angle finder) for miter work (molding).

Customer Question

I've been trying to figure out this protractor to no avail. The guy at the hardware store said all I do is take the figure and cut one angle at that # XXXXX the other one that would add up to 90.

Example: The number the protractor gave me was 42, so I cut one piece of wood at a 42 degree angle. The other I cut at 48 as per his instructions. The fit wasn't even close.

Please tell me what I'm doing wrong.

Thx -Dan-
Submitted: 10 years ago.
Category: Home Improvement
Expert:  Mr. Mike replied 10 years ago.
The purpose of miter work in molding is to match the cut ends of molding pieces so they fit into a giver corner.

Assume for the moment that you are doing some baseboard molding. The first question is, what is the angle between the walls where they join at the baseboard. Hopefully it will be 90 degrees if the walls were built "square", but you'd better measure to be sure!

Say the angle IS 90 degrees. Take the baseboard pieces and physically lay them in place near the corner. Think about how you want them to fit together. It would be nice if the length of each cut were the same. Half the 90 degrees is 45 degrees, so that would be the angle of each cut. BECAUSE IT IS EXACTLY 45 degrees, it doesn't matter where you place the "center point" of the protractor. Draw the cut line on each piece, put them physically back in place in the corner, and get a mental picture of how they will join after the cuts are made.

Now, what if the corner ISN'T square? Say the walls join at a 94 degree angle (the corner is 4 degrees "bigger" or "wider" than a right angle). Now your cut has to be at 47 degrees (again, HALF the angle of the walls), and it will matter where you put the center of the protractor and where you measure the 47 degree angle from. Once you lay the baseboard down and look closely at the cut lines you will need, you will see where the center has to be and where to draw the 47 degree line.

The center of the protractor has to be where the tips of the cut ends meet! The angle of the cut has to be half the angle of the walls (this assumes each baseboard is the same thickness!).

This should work for you. Good luck!

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