Custom Mouldings for Arched Window Tops

Manufactured windows may not have regular, true, or consistent dimensions. The safest bet is to make a template from the actual window. October 13, 2005

Question
I would like to find the best way to make a 31/2" MDF moulding for an arched window out of a sheet of MDF. Is it best to just find the radius and draw it out? Or should I cut it close and hold it up to the window, trace it, and then offset it by 1/8 before cutting? Any help is appreciated.

Forum Responses
(Architectural Woodworking Forum)
From contributor G:
What kind of machine are you using? MDF does make nice paint grade moulding, but it's hard on knives.

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From contributor F:
A lot of windows out there are not of true or consistent radius, especially the vinyl ones. Some molding guys will take a large piece of paper and thumb tack it to the wall so that the paper covers the window. They then take what is known as a rubbing from the window itself.

This is done by pressing the paper tight against the window outline and rubbing a lead pencil back and forth until you get the image of the part of the window that you need on the paper. You can use the rubbing as a template or as a guideline to set up a trammeled router with to cut your molding blank (CNC if you have one).

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From contributor B:
We cut either templates or mouldings for other millwork shops on a regular basis - usually an arch, which is a section of a circle and only needs to be defined by a measured radius. An ellipse or part of an ellipse tends to be more suited to a field template. We have seen the same model elliptical window, as produced by a large name manufacturer, was not consistent in shape and size from one window to the next.

For applying a moulding that was to have a consistent reveal, this would have been a very large problem had these openings not been templated and reverse digitized. As the previous post said, what machine will you be cutting the moulding with? Nothing compares to a quality CNC router for producing accurate, repeatable and fast templates and mouldings. If you do not have a CNC machine of your own - simply job it out to a shop that does.

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From contributor P:
We usually measure the radius by rise/run (height/width) then add the reveal to find the arc. Then we use a trammel with a router to cut out the blank, then run it through our moulder or shaper. I would suggest using carbide knives though..