Finding Compound Angles for Crown Moulding

This discussion of cutting crown molding includes a set of tables for determining angle and bevel cuts when cutting flat on the saw table. April 20, 2007

I have a 10" sliding comp. dual bevel miter saw and 5 1/4 crown. Can I lay it flat and bevel it at 45 to do an inside corner? Lets say: 90 inside corner, left piece, crown on right (bottom up) right 45 to get it the same as when I use the jig (it won't fit in).

Forum Responses
(Architectural Woodworking Forum)
From contributor L:
No, you need to set the saw up so that it will mimic you cutting the crown in the upright position. Most saws will have special marks on them for cutting crown flat. Usually it is a circle or a triangle. You also need to know if the crown is a 45º-45º or a 38º-52º to use the symbols. You would set the bevel and the mitre angle to read either triangles or circles. Then cut the crown.

From contributor R:
On 38-52 crown - bevel angle is 33.9 and miter angle is 31.6. On 45-45 crown - bevel angle 30.0 and miter angle is 35.3. I have found some crowns, the face is parallel to back side and some are tilted a little in relation to back. These bevel and miter settings are correct if the back is parallel; if not you might have to make small adjustment on either bevel or miter or both to get good fit.

From contributor D:
If you have the Bosch miter saw the manual also has a chart that will give you the compound miters for cutting crown flat. I don’t know how that works if the spring angle on the crown is not standard?

From contributor F:
Sounds like you got the right info. The numbers Contributor R mentioned are the same that I have been using for ten years. I cut all of my crown molding this way and will never go back to standing it up. A simple method for not making the wrong cut is that the outside corners are always on the right side of the blade. Just move the miter angle to the opposite side and turn the molding around for the other end. Inside corners are always on the left side of the blade, same procedure. 45 degree angles are about half, a few tests are needed for safety and minimum waste.

From contributor C:
I agree with Contributor F - laying it flat equals perfect cuts and the safety issue is great. What if the angle on the OS corner was 60 degrees instead of 90? What would the flat cut angles be?

From contributor L:
What if your crown is cupped? What if the spring angle is not 38 or 45? What if the OS corner is 89 degrees? Setting the miter and bevel to cut on the flat is harder then cutting in position. Set a stop block to hold the crown at the proper angle and cut way. All you have to set is the miter angle.

From contributor R:
He has a 10" sliding compound that if like my 12" dual sliding compound Makita the fence is only 1 1/2 inch tall. It sure would be hard to use stop blocks if back side isn't tall enough.

From contributor R:

38-52 a

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38-52 b

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38-52 c

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From contributor R:

45-45 a

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45-45 b

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45-45 c

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From contributor E:
For all you out there, there was always a sure and quick way to lay out the area that you work on, especially older buildings. I start with a corner, usually inside, and mark on a scrap the inside angle with an angle divider, look at a Stanley #30, mark down my lengths, go to all corners the same way, go cut my crown, and install it.

From Gary Katz, forum technical advisor:
Opinions vary, but after installing crown for more than 25 years I found cutting on the flat is always harder and less accurate, especially if the spring angle of the crown isn't 38 or 45, which it often isn't. I frequently install one pattern that's 33 degrees. And, as Contributor L says, if the corner angles aren't 90 degrees, which they rarely are, all your cuts will be off.

I also had a Makita dual bevel for years. I had no problem attaching accessory wooden fences to that saw and used it happily, with great accuracy for a long time, cutting crown In-Position. The only time I cut on-the-flat is when the crown is too tall to cut in-position, otherwise, I can cut more than twice as fast and with much better accuracy in-position, running round 87 degree and 92 degree corners, changing spring angles in mid- stream while installing crown on kitchen cabinets where the doors vary 1/4-3/8 in. from the ceilings, etc. and you have to rock the crown a little at each corner. All of that is easy to do in position.

When you do cut on-the-flat, the back being parallel to the front doesn't matter because it's the back that determines the spring angle, not the front. Yes, if you're measuring the spring angle from the front, you could be introducing an error if the front isn't parallel with the back. As Contributor L said, if the back is cupped, even a little, it'll throw off your compound angle cuts big time. There's a reason that crown was cut in-position for so many years, before compound miter saws.

Also, when you cut on the flat, outside corners are not always on the right, inside corners are not always on the left. When compound miter saws first came out, they beveled in only one direction - just like radial arm saws (which is where the design originated). You had to flip the molding to cut the opposite angle. Flipping the molding is still the best method for cutting on the flat because each time you flip the bevel on the saw you introduce even more error: the bevel gauge on a miter saw is not nearly as accurate as the miter gauge - just look at the difference in the radius of each gauge and you'll know what I mean.

It's tough to dial in most bevel gauges to even one degree of accuracy, let alone 1/2 degree. Most of us still flip the material if the room size allows it. That means that sometimes the bottom of the molding is against the fence, sometimes the top. I always make my first cut with the bottom against the fence, if there's a right-hand inside corner or a left-hand outside corner. That way when I flip the material to make the second cut, I'm always guiding the saw blade and cutting to a measurement mark - measurement marks are always on the bottom of the molding (unless you're running crown up a sloped ceiling).

From contributor A:
I too have installed crown for 25 years. As far as the spring angle, I have installed crown where the spring angle was not determined by the back. I have also installed crown where either 38-52 or 45-45 angles were 90 degrees to each other making it impossible to get in miter box and position correctly.

They would be under cut or over-cut causing a guessing game to which side to hold flat. If under-cut you would have gap at top and bottom against fence and base. If over-cut the piece would rock up and down 1/4". You should use whatever method suites you best. To most laying crown flat only needs to be done if crown is very large or miter saw can't larger crowns in normal position.