I would suggest a 1 x 10 pine board,,you will need all the plunge you can get from your router,,it's true you can use some plywood but you will need to rip some 1st..pine board that you can pickup from the lumber yard.the pine board you can drill a 3/4" on one end and put a wood screw on the other end so it turns free and use your brass guide,then just drop in the hole and cut the circle..
I made 63" arcs for this atlas stand. http://www.routerforums.com/axlmyks-stuff/4637-atlas-stand.html
You can lay the wood on the shop floor, and use a tape measure to draw the arc. A piece of string works also. The easy way is put the measure or string at one end of the arc, and a center point. Just move the center point till you get the arc where you want it.
Nice Stand Mike! But drawing the radius is not the problem. Looks like i need to go the plywood/1xpine route. Cutting 5/4 (actually 1 1/16 after i planed it) would take multiple passes. I'm thinking of making one pass with the 13' compass then using a flush trim bit to finish it off.
IIRC I cut the bigger two radii on my Orion speakers (design by Siegfried Linkwitz) at around 5' and 12'. Essentially no sanding was needed where the convex and concave arcs or convex arc and straight line connected.
Lengthwise slices off 1/4" plywood worked fine for the circle jig and allow plenty of plunge depth. I joined the pieces with counter sunk wood screws and 1x lumber scrap.
Use scrap of whatever you're cutting plus whatever you're using to backup the cut to hold the pivot pin. Bisect the chord using a long compass which can be one of the pieces that makes your circle jig with a couple of holes in it. Measure along that to find your center. Either stick everything to the floor with double sided tape or form a triangle with 2x lumber between chord ends and pivot.
A pattern is a fine idea even for a single piece. It'll be cheap if your pivot moves and you can just copy what you have and resume where you left off.
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