This is a how I cut large radiuses on my manual lathe. I have used it for both convex and concave radiuses. For making such things as press tools to smoke box doors for steam engines. I made a jig a long time ago to fit my lathe but gave it to an apprentice school as they wanted to cut large radiuses and never got it back but this simply fix works and one day I will make another jig.
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The Home Engineer