Yes ever tried cutting holes for speakers in cabinets, I mean takes ages.
Brian

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"Martin Brown" wrote in message
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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 can buy a jig that has a drill down the centre and three jigsaw blades
that adjust out on a spiral to about 90mm diamtere.
It works quite well and stays balanced in use.

You definitely want it balanced and on three point cutting support or it
will snatch the moment the blade tips touch the surface.

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Regards,
Martin Brown