Crown Moulding on a Radius

For crown to follow a curve is a tricky dimensional problem. Here, woodworkers discuss various approaches to creating a piece that will work. October 2, 2007

I am trying to make up a piece of crown molding that has to be put up on a 6' radius. My thought was to use Flextrim 1x5 and run it on a radius jig on my WH molder. Has anyone done this or is there another solution? The rep for Flextrim wants to have me pay him to make it up at their factory for big bucks. Any thoughts?

Forum Responses
(Architectural Woodworking Forum)
From contributor M:
It won't bed correctly if you make it flat like straight trim. Pay to have it made. It will be cheaper than doing it wrong once and paying to have it done later.

From contributor H:
That all depends on what "big bucks" is. I would pay to have him make it. If it's wrong, it's his fault. If it's right, you've made money. If there is time enough to do it twice, there is time enough to do it right.

I can't see flex trim running well on a radius jig W&H.

From contributor W:
We've tried it a few times (on a Hussey) with less than stellar results. The flex moldings tend to burn the knives pretty badly and the stuff is so squishy you tend to get chatter and it's almost impossible to sand it out. One of the companies we sell (Resinart) charges no more to pre-form their flex to your radius than for flat stock, so there's no real premium for getting what you want. If your time is worth more than peanuts, just get the right stuff.

From contributor B:
I would template the radius and run the profile on a tilting arbor shaper with a molding head on it.

From contributor L:
If I understand what you are attempting, it won't work to run it on the W&H then bend it around the curve. I only know of 3 ways to get there: CNC router, tilting head shaper, arch-molding shaper. If you don't have any of these, just have a shop make it that does it all the time. Far cheaper!

From contributor S:
There is also the old fashioned way of hand carving or making it in place out of plaster. I will also throw in the name Fypon(?). I might be wrong, but we always call flexible trim that, if they are the manufacturer or not. I think that they actually made some foam type stuff. On a side note, if you can keep any of the plastic/rubber/foam stuff from meeting up with the real wood stuff, you stand a better chance of not going insane trying to get a good joint.

From contributor P:
Just have it made up by the flex people. If it wasn't such a big radius, you could do it on a lathe, but a 12' diameter - that would be scary. I guess you could come up with a Rube Goldberg for a W&H. On that big of a radius, probably wouldn't change the angle of the knives too much. Before I did all of that, though, I would call BH Davis and have him do it.

From contributor G:
I have made crown moulding on a 12" radius with a tilting head shaper. Worked fine. 6' rad is somewhat bigger. You might need a helper or a support to run that. I am sure I was at a job a long time ago. Someone milled up straight (relatively) lengths of crown that were bent round a porch roof at about 16' rad. I thought they were laid out on a curve (across the width of the stock) using the geometry of the surface of a cone. The base of the cone was the radius of the porch and the height of the cone was derived by the 38 deg - 52 deg angle that the crown sat at when installed, i.e. the cone was point down. Can anyone verify this method, or do I have to set one up in the shop to satisfy my curiosity?

From contributor R:
Set up a radius jig, cut sections of the crown out, and glue them together on your jig.

From contributor J:
Yes to contributor G's question about cone geometry for curved crown moulding. It is possible to run the molding in flat radius pieces that will bend up properly in place. Either for inside or outside curve installation (depending on which way you shape the piece with the top of the crown detail to the convex or concave edge). This is an old milling trick that Grandpa Jake taught me thirty years ago. You are limited though to large radii only because you must bend the board.

Another trick that I have used is to rip the crown moulding into thin vertical strips and bend and laminate them back in place with glue and fine staples. Sand it smooth and you'll be amazed (little angle blocks in the back to help support the moulding and facilitate the assembly). The trick is to rip up two pieces to make one because you lose the width of the saw blade with each cut. Get it? Okay, rip one piece into strips (with the molding vertical against the fence), then move the fence over to equal the width of the saw blade and rip a second piece. Now you have a set of strips that when put together will equal the crown moulding.

You can do the whole house that way... inside and outside curves, both large and small radius, no problemo. This is a finish carpentry trick.