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???
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
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
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...
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