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have a look at this and the next page, I drew one out in CADhttps://www.modelenginemaker.com/index.php/topic,7162.195.html
This one https://www.modelenginemaker.com/index.php/topic,7162.msg153641.html#msg153641 Its the land width that should be kept a constant Yes the groove is not a true semi-circle but elliptical which can also be achieved by cutting on the side of the cutter like Chris mentioned Jo