Copied from the Seaton Chest book, a router plane with the iron from a wooden plow plane.
Drilling the holes. I completely forgot to take more pictures from the build, but it is quite straightforward. Opening the holes with chisels, bedding the iron, making a wedge, moulding the front with moulding planes etc.
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Here she is in action. Nothing special, just flattening the bottom of a groove. Bedding angle of the iron is 65 degrees.
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And the underside. Showing why it's called an old wifes tooth.
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