Door Casing for Arches

[Key Bored]

Hi All,

I need to curve some door trim and molding to fit an opening with an arched top. The arch is the eyebrow type and has a radius of about 20". The finish door opening is 32" wide. The molding is the standard colonial design, about 2-1/4" wide by 3/4" thick. The trim for the door opening is

4-1/2" wide by 3/4" thick, and the door stop is 1-1/4" wide by 1/4" thick. I want to use MDF stock.

What are some methods to fit the door frame and casing to the arch? Should I make a series of cross cuts (but not all the way through) so that the stock will bend, and then spackle and sand? Or should I rip the stock into thin strips, bend them on a form, and glue together? Other ideas???

Thanx, Key Bored

[marks542004]

If you cut most of the way through solid timber it can be bent so that the cuts open and you get a solid timber surface on the inside edge of the arch. The trim should cover the edge so you do not need to fill. I have had no luck trying to bend MDF by this method. Plywood works well if you get the surface ply running along the arch.

For the trim the only way I have seen that seemed to get a good result was to cut a solid piece to the shape of the arch and route the profile in it.

[Mike O.]

I've never tried to make an arched jam from MDF. When using hardwood I always rip thin strips and bend it on a form. You can either make a form of the entire radius or use L shaped brackets made from plywood for the form and screw them down to a line with your radius. Then glue and clamp it up. If your MDF won't bend without breaking you could make the arch out of something else maybe poplar if it paints. When I make casing for arches I normally cut it out from pieces of solid wood. In your case I guess MDF. I cut a pattern from 1/4" plywood or hardboard with the router (and homemade circle jig) then rough cut the stock and use the pattern with a pattern bit to make the casing. When using stock for the casing I would have to cut this radius from two pieces but with the MDF you wouldn't have to do that.

When we have the millwork make arches (for custom casing), they actually rip thin strips of hardwood to the width of the casing and bend the casing too. I think this helps with grain direction when cutting the detail with their machines. They always come out in one piece.

Mike O.

[bridger]

the header jamb you will probably want to laminate from 3 or so layers of bending ply. it'll take a form to get it right, but that form can (sometimes) be the wall framing. you might get decent results with 3 or 4 layers of masonite, but I doubt that kerfing 3/4" mdf is going to get you a smooth curve at 20" radius.

for the arched head casing you'll probably want to strike the arc with a router and trammel base. profiling that piece will be a hairy operation unless you have access to a moulder. it can be done safely on a router table. fortunately 3/4" mdf is cheap...