Crown moulding procedure

This common crown moulding profile is not "isosceles" -- the ceiling leg is not equal to the wall leg. It affects the miter cuts. The correct angle can be measured graphically. Fortunately, the manufacturer displays an accurate isometric diagram. An overlay sketch shows it to have angles of about 38° and 52° (for a right triangle).

When using a radial-arm saw, two jigs are needed: a thick durable right triangle having the above two angles (I made mine from a piece of half-inch OSB); and what I'll call a bevel support bar tilted at the same angles (explained further below).
The right triangle jig is for cutting the mounting blocks. Flipping it over is easier than swinging the saw arm back and forth. Some contractors don't even bother with mounting blocks for crown moulding, but I find it to help for both precise fitting and time savings working at the ceiling. Cutting the blocks may take time. But that's again where this jig comes in handy.

Orientation of grain for such small wood pieces is important to prevent splitting, so I use 2x3 lumber and cut across the more difficult wider dimension. One good block becomes a template.Each moulding piece needs at least two mounting blocks, one at each end. But since I want to maximize the ease of point-and-shoot installation working solo with an air gun, I'm installing as many as five blocks for some walls, depending on their length. It's easy to precut all the blocks at one time.The bevel support bar jig (shown above between the stock and a 45° triangle) is simply two mounting blocks attached to some flat base. I hold the stock against it and against the 45° triangle to make the miter cut. That large 45° triangle is actually an architectural drafting tool sandwiched between pressboards for dedicated shop use. It assures a perfect 45° miter because it's much more precise and consistent than any shop tool calibration. The hand-held coping saw procedure follows as usual, resting on the same jig flipped 90°.When using jigs like these and a radial-arm saw, the best strategy is to attach the moulding from left to right in the room. The first piece should go on the longest wall and have simple square cuts. Subsequent pieces then get a miter/coped end on the left to join the previous piece (placing the stock on the left side of the saw table so the motor housing doesn't have to travel over it) -- and a square cut at the progressive end for easier fine-tune trimming. That's probably the only limitation (thus, the workaround) when using a radial-arm saw for tall-profile crown mouldings. Profiles taller than your blade's radius is obviously not possible. The last piece should then be the shortest because it requires a coping/miter cut at both ends. Measuring with a laser device helps.