There are two things that affect how you make this seat scooping jig. The first is the size (thickness and diameter) of the seat blank. Then there's the question of how deep you want the "scoop" to be.
I built the jig shown here for the Shop Stool. The seat is 14" in diameter and the scoop Vs" deep at its centerpoint. Also, I wanted to leave a 1 W'-wide flat rim around the top of the seat.
The jig consists of a base with a pivot pin that holds the workpiece. and a pair of curved rails for the router to ride on. Then, to steady the router as it slides along the rails, a plywood base is attached to the router. (All the plywood parts are W-thick.)
curved rails. Start the jig by making a pair of side rails. To do this, first cut two rectangular pieces of plywood to the same size, see Fig. 1. Now draw an arc with a radius of 42" on one of the rail blanks.
Next, temporarily sandwich the blanks together (I used double-sided carpet tape) and cut the arc on both pieces at the same time. Then lay out and cut the openings on the bottom of the taped-together blanks for the workpiece to fit in. see Fig. la.
base. Once the rails are cut out, the next step is to make the base that the curved rails are attached to. To do this, first rip a piece of plywood to width, see Fig. 2. Then cut this to the same length as the curved rails. Now the rails can be glued and screwed to the base.
pivot pin. After the rails are screwed down, bore a hole for a removable pivot pin in the base of the jig, see Fig. 2. To make it easier to insert and remove this pin, counter-bore a 1" hole on the bottom side of the base, see Fig. 2a.
rolter base. To make a base for the router, first cut a piece of plywood to the same length as the jig base is wide (9V&"). Now drill a l"-dia. hole centered on the plywood for the bit.
Next, remove the plastic base from your router and use it as a template to mark the position of the counterbored screw holes for mounting the base to your router.
Complete the router base by cutting rabbets on its bottom lace, S6C Fig. 3a The rabbets prevent side-to-side movement of the router on the rails.
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