Smoothing Inside (Concave) Curves

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Curmudgeon10

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I am building an arched valance for our kitchen window in front of the sink. The bottom rail provides the arch. Simply for stability, the rails and stiles are 3/4" PVC, and the two raised panels will be MDF.

I've managed so far to layout the curved rail on the PVC and rough cut it. I've also smoothed the top side of the arch down to the layout line using primarily my belt sander but also some handwork. Now the question is how to smooth the underside of the arch, the "inside" or concave portion.

The radius of this curve is about 6'6", (the arc itself is about 5' wide), gradual, but not so gradual that I can put the belt sander to it and not risk gouging places to the rear of the belt or digging in too much in the front.

Anyone have any suggestions how to smooth this curve? It would be the same challenge if built from wood.
 
Send a pic or two ... I have an idea ... but I need to see the layout to make sure.
 
In the past I've used screen molding to bend over the bottom to cover the sawed edge. Assuming that's what your talking about.
 
Since posting this, I've made a couple of decisions: a) I'm dropping the PVC. Not because I don't like it; I've made a number of projects from it. It's too MESSY. b) I'm going to use Poplar, and I've decided to set up a jig with my router on the inside radius first, and then route the outside curve next. Setup is a pain but it should give me an acceptable result.

If routing the full thickness material is a problem, I'll use the same jig to make a template from hardboard, and then use a flush cutting bit on the router to true the poplar to the template.
 
What you needed was a drum sander, You could have set the belt sander on it's side and clamped it in place on a table big enough that you can slide the work past the sander and only use the front or back wheel of the belt sander. Best is to make a sample of half the length. When your happy with that you copy it with the router, now you have the two matching peices that you tack to your good wood and cut it out with the router.

drum sander.jpg

duplicating.jpg
 
I found the radius on my belt sander and the radius of the largest sanding drum on the drum sander to be too small to do very smooth work on this 6' - 6" radius. I'm going ahead with the router mounted on a 6'- 6" radius swing arm. Just checked with a cabinet maker who is doing my kitchen makeover, and he said that is what he does when doing "radius work."
 
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