What to do with a half round window.

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Blog entry by rcs47 posted 07-26-2011 02:20 AM 54983 reads 10 times favorited 4 comments Add to Favorites Watch

We’ve all seen, and many have those half round windows. But what can you do? Buy a fabric, plastic or wooden fan louver? Why not build a wooden fan louver? Its really not that hard.

I start by making the outside frame to fit the window. I draw one line that will give me 1/4” clearance inside the window opening. I draw a second line the thickness of the frame marking the inside of the frame. Because this is a simple curve, I didn’t need to build a complete two part form. I just need a few clamping points along the curve. I cut a curve on the end of the clamping blocks to ensure that I was not forcing an artificial bend point into the arch, and screw them to the form along the inside frame curve line. Cover everything with packing tape, and I’m ready to go. I used 1/8” thick strips to minimize the spring back once I release the clamps.

I used Polyurethane glue (glue on one side and water on the other).

A few clamps and I have it in the forms (notice the C-clamp at the bottom. I start by clamping all the pieces together, then bend them around the form).

Because I didn’t want to take the time or material to make a two part bending form (inside & outside parts of the radius less the thickness of the material), I need to add clamps to pull the layers together. Don’t forget the packing tape on any material you use to use spread out pressure.

You can see how much the material slips as you bend it around the form in this picture.

My window has a 23-1/2” radius. I could have used thicker material than 1/8”, but with thicker material, you get more spring back when you release it from the clamps.

Now that I have the frame, it’s time to lay out the louver fans. I start by tracing the finished frame with bottom cross piece on a piece of paper.

Before I can determine the number of louvers I want, I need to figure out how much space I have to work with. I can lay a string along the curve, then measure the string, or being an engineer, use a formula. Circle circumference = 2 x radius® x Pi (3.14). Since I was working with half a circle, I only need, R x 3.14.

My inside radius is 22-1/16”
The full distance is 22-1/16×3.14 = 69.28”
I need to subtract the area covered by my bottom cross piece (set at 1-1/2”)
69.28” – 3” = 66.28” of space to fill around the top

I tried a couple different variations in number of blades (you can see them in pencil), deciding on 13 (noted in red).

66.28” / 13 = 5.098” or 5.1” at the top

I need a half circle for the bottom too and just set it at 5”.

5” x 3.14 = 15.7”
15.7” – 3” = 12.7”
12.7 / 13 = .98” / blade at the bottom
You’ll notice that I drew the red lines through the marks on both ends.

First I numbered each blade on the drawing. I lay the blade piece on top of a pair of red lines, lay a straight edge on top of the board and transferred the line to the board by eye (mark the blade number). To make sure I had coverage once I turned the blades, I used a piece of wood 1/8” thick to add width to each blade. I used on a table saw tapering jig to cut the blades along these lines.

After the sides were cut, I laid each wing back on the drawing, using the blade numbers and red lines to get it back into the correct position. I lay the frame over the blades to trace the top and bottom arcs, plus mark the dowel positions.

This is a dry fit after the final arc cuts were made. I cut away the lines thinking I needed the space to turn the blades. But after putting it together I think I could have left the lines and still been able to turn the blades.

After the dry fit, a little final tweaking, painting, and it was time to put it together for good.

The part I didn’t think about when I decided to make the louver was how to install the thing. I looked at mini blind hardware, L-brackets, but didn’t like anything. When I was looking for molding to bend into the opening I remembered how easy it was to just make something to fit. The above bending pictures are from making the molding.

Once the piece was pulled out of the bending form, I ran it through the band saw, holding the side that was against the bottom of the bending form to the fence. This gave me one relatively straight edge. Then to the table saw to cut two straight sides. Round over both inside corners, split the piece in half and I have my molding.

Quarter round molding at that (you can see the multiple laminations in this picture).

Finally, the louver mounted in the window. I designed the louver with a 1/2” space below the cross piece and space between louvers to promote air circulation.

Thanks for looking.

-- Doug - As my Dad taught me, you're not a cabinet maker until you can hide your mistakes.

4 comments so far

View Dragonsrite's profile


136 posts in 4641 days

#1 posted 07-26-2011 06:38 PM

This piece looks very nice. Thank you for describing & showing the process that you used.

-- Dragonsrite, Minnesota

View Ken90712's profile


18067 posts in 4432 days

#2 posted 07-27-2011 04:47 PM

Amazing work some might not realize how hard this could be to pull off. Well done! You saved a boat load of money if you had to buy thiat!

-- Ken, "Everyday above ground is a good day!"

View Dave's profile


38 posts in 4259 days

#3 posted 02-26-2012 12:43 PM

I priced one of these for my window and they want over $1000 for mine. Great project that I will Definetly be inspiring from. Thanks.

View Nubsnstubs's profile


1803 posts in 2974 days

#4 posted 08-24-2019 04:19 AM

So, It’s been 8 years since this was posted. Anyone do one yet??
I love the signature line. The words are different, but the meaning is the same. I say, “only a good cabinet maker can hide mistakes”.............. Jerry (in Tucson)

-- Jerry (in Tucson)

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