On 02/07/2020 13:41, Thomas Prufer wrote:
Given: plywood, jigsaw
wanted: circular bit of plywood, diameters 60-80 cm, small hole in the middle is
OK.

Yeah: "make a jig", i.e. an arm with the wanted radius, attached to the jigsaw
base, screw middle down and cut around.

But: I seem to recall there's a catch there somewhere, or a best way of doing
this, in positioning the radius arm with respect to the blade. Should the radius
arm attach to the jigsaw a) ahead of the blade, b) exactly off to the side of
the blade, or c) further back from the jigsaw blade?

You want the blade on the radius. I would make a ply guide say 5" wide
and as long as needed. Stick a hole through the jigsaw end with a spade
bit to allow some clearance round the blade, and then pin and glue
blocking to the ply to trap the base plate of the saw, with the blade
poking through. Then add a screw etc for the centre pin.

A quick search turns up this which looks about right:

https://www.youtube.com/watch?v=6dTzIHCPBuk

Although I would not drill the start hole such that it leaves a divot
out of the circle!



--
Cheers,

John.

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