Speaking of shoemaking tools and the lathe...
It is common practice to use a pair of dividers to mark seam allowances on patterns...making lines parallel to an edge. That or a special set of "fork" shaped tools set for specific distances. And either way works well.
But if you want to add a margin to a pattern whether in the making of a larger "cutting" pattern or to a piece of leather, there is another way that I borrowed from woodworking. And it's slicker than a slug in a bucket of butter.
Essentially what you need are a set of discs with a hole in the center. If you want to add a 5mm turning allowance, for instance, the disc would be 10mm in diameter. If the discs are made correctly--completely flat on the bottom and with a sharp even edge and the hole as small as necessary, this idea works like a charm.
If you have a metal lathe these could be made of brass; if you have a wood lathe, or know someone who does, any fine grained, hard wood will work.
Mine are made of either boxwood or osage orange, IIRC.... (click on photo to see larger version)
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