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  The Bosch Angle Finder  
I used to dread cutting crown on the flat, especially for odd corners and oddly angled crown (not all crown molding has a spring line of 45 or 38 degrees, but more on that in a moment). It took me forever to figure out the right miter and bevel angle, and then it seemed like each corner was different—even on the standard 90’s, and especially the 22 1/2’s, the joints just wouldn’t close up perfectly tight. I always had to trim a little off the long point or a little off the short point of one or both pieces to get a tight fit.
But I don’t have that problem anymore. I use a Bosch Angle Finder (BAF) and read the angle of every corner before I cut even one piece of crown.
Crown Molding & Spring Angles  
Before you can use a BAF, you must know the spring angle of the crown molding you’re working with. You can use the BAF to determine the spring angle of the crown molding, too (the protractor technique in the following paragraph is more accurate).
Start by holding the molding against a wall with the bottom edge down, just as it will mount to the ceiling. Then place one arm of the BAF against the wall beneath the crown molding.  Next open the other arm until it’s parallel with the face of the molding. The spring angle is the compliment of the angle on the LED display, so subtract the number on the LED display from 180 to arrive at the spring angle.
For greater accuracy, use a protractor or angle finder to determine the spring angle of the crown molding. Hold the crown against a wall or flat surface, with the bottom edge down in the same position it will be installed at the ceiling. Then spread the protractor open between the wall and the back of the crown. Using this method, there’s no need to subtract 90 degrees because the angle on the protractor is the spring angle of the crown.
Using The Bosch Angle Finder  

Start by spreading the arms on the BAF until the LED reads the exact spring angle of the crown molding you’re installing, then press the black “BV/MT” button. The spring angle will be entered into memory--confirmed by the appearance of “SPR” in a black box at the lower left corner.

Next, place the Angle Finder in the corner of the walls and open it until both arms are flat against each wall, then press the same black Bevel/Miter button again. The abbreviation “CNR” will next appear in the LED, confirming that the corner angle has now been entered.
Press the BV/MT button once more and the miter angle will come up in the display, along with the abbreviation MTR.
Finally, press the BV/MT button one last time and the bevel angle will be displayed, along with the abbreviation BVL. It’s that simple.
Sure, I have a few complaints about this ‘perfect’ tool, too: The BAF won’t save the spring angle—you have to enter it every time you use the tool (too bad the tool isn't equipped with a key pad so you could just punch in the spring angle). Also, the BAF won’t read acute angles—like the forty-five degree triangles I run into on many coffered ceilings. On those rare occasions, I grunt a little and use Joe Fusco's Crown Molding Chart. But I hear that Bosch will soon be improving the angle finder...
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