Here is a useful tool for turners - a giant pair of compasses. When preparing large blanks ordinary commercial compasses may be too small. I made a large pair from scrap. Opened to 90 degrees, they can make a circle of 800 mm radius. You could make them any size.
The pair consists of two arms about 550 mm long, made of softwood, linked at the top by a coach bolt and wingnut. One arm has a nail inserted, the other has a simple clamp to hold a pencil.
To insert the nail, drill a small hole in the end of the arm, to stop it splitting. Cut off the head of the nail and grip it point down in a vice, then tap the arm down onto the blunt end.
To make the pencil clamp, drill a hole for the pencil first, to be a sliding fit. I made a cut-out but it is not necessary. Then drill a cross hole, very close to, but not intersecting the pencil hole. Now make a saw cut to split the pencil hole down its centre and go a bit past the cross hole so there is enough spring in the wood. Fit a small coach bolt and wingnut in the cross hole.
Smooth off any rough edges, drill a hole through both arms near the pivot so you can hang it up, sharpen your pencil and the job is finished.