How to make a circular window moulding using straight moulding pieces

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Forum topic by RookieAtThis posted 07-20-2010 04:46 AM 24256 views 1 time favorited 8 replies Add to Favorites Watch
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5 posts in 3073 days

07-20-2010 04:46 AM

Topic tags/keywords: question

I need your assistance in doing this:

I am in the midst of replacing a window that has a circular half moon at the top of a rectangular window. The installer asked me if I wanted a custom moulding trim piece made for the curved section to match the straight colonial trim that he was going to use. I researched this and the typical job is taking the knifes that were used to create the straight piece and use them on a piece of MDF. This would then create the 1 pieced trim for the curved window. That’s all fine and dandy but where would the fun be with that? I noticed that the current trim (made by the builder’s trade) is pieced together using just pieces of straight moulding. They then sanded this into a more please circular shape. I envisioned that this was the way trades people did these “odd” shapes on site.

Besiding templating the curved shape and using trial and error to calculate the the angles of the cut, does anyone know of a mathematical calculation (solution)?

For example, my current window diameter (without the 1/8 reveal on either side)) is 72 inches witha radius of 35.6 (close enough to be a circle). The tradeperson made the curved moulding by piecing 26 segments together. I figure it I doubled the pieces to make the curve even smoother.

I figure I can hand sand the piece smooth. I also has access to a press drill so I can sand the inner and outer edges smooth if necessary.

Splitting, laminating and bending pieces of the straight moulding is not an option for me.

8 replies so far

View ksSlim's profile


1290 posts in 3095 days

#1 posted 07-20-2010 05:35 AM

If the original craftsman used 26 segments for a semicircle, 180/26=6.92 degrees of circumfrence.
Joint angle should be (6.92/2) or about 86.5 cut. segments about 4.347” ?
More segments would mean shorter pieces and more joints.

-- Sawdust and shavings are therapeutic

View childress's profile


841 posts in 3747 days

#2 posted 07-21-2010 05:26 PM

Or you can do it the easy way and by a piece of flexible molding made by someone like flex trim. They even have a radius calculator to help you out. Your installer should know about this type of product…

-- Childress Woodworks

View a1Jim's profile


117342 posts in 3782 days

#3 posted 07-21-2010 05:40 PM

I agree with childress. using 26 segments has been done by someone that was improvising . otherwise you need to make a pattern and glue up some stock ,band saw it to shape and then use a router or shaper to mill your profile also using a pattern.

-- wood crafting & woodworking classes

View Nomad62's profile


726 posts in 3163 days

#4 posted 07-21-2010 10:33 PM

A good friend of mine does this for a living, and does a fantastic job of it. He creates an internal-dimensioned form for the curved piece, then using the same wood as was used for the straight pieces he bandsaws it into about 1/8” strips; glues the faces of said strips and clamps them to his form with a clamp about every 6 inches. Sometimes that is dozens of clamps. He then runs the whole thing thru his shaper (a special tool, to say the least) to match. I know this isn’t an option for you, but it’s the way it’s done by him anyway. Really nice.

-- Power tools put us ahead of the monkeys

View RookieAtThis's profile


5 posts in 3073 days

#5 posted 07-22-2010 09:32 PM

How did you come up with this number?

86.5 cut. segments about 4.347” ?

View Dennisgrosen's profile


10880 posts in 3320 days

#6 posted 07-23-2010 12:43 AM

eight straight pieces thats weiter than the original
joint together with 12½ degrees angle with a spline
(if its a fuld half cirkel you want) otherweise you have to correct the angles
and scrip two aches , cut them out and make the muolding
with either a router or by cheisels and a few carving cheisels
shuold do the job


View millworkman's profile


2 posts in 3064 days

#7 posted 07-28-2010 11:26 PM

I agree with Childress, why do all the labor with wood? The simplest solution for your project is with flexible moulding. There are many casing profiles to choose from. You may also want to take a look at DuraFlex. It’s environmentally friendly, and it can be stained to match your wood or painted.

View RookieAtThis's profile


5 posts in 3073 days

#8 posted 08-06-2010 05:55 AM

Besides saving money ( I need to do 2 arches); it’ll give me in my opinion a good hands on project. If it turns out well, I can proudly say to the Mrs. ... look a job well done! She will probably reply … and it only took you X months to do it.

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