There are times when a pin router would be handy, and a radial arm saw provides a handy platform with its arm adjustable for height and the motor carriage that can be locked at any position along the arm. And, if necessary, the motor can be tilted for very special needs.
Some radial arm saws, like Sawsmith and Montgomery Ward's Powr-Kraft, came from the factory with a high-speed spindle on the right end of the motor for a router collet chuck. The Sears radial arm saws have a spindle with 1/2 inch thread for a Jacob's Chuck. The Sears spindle runs too slowly for use with a router bit.
Step 1: Mounting Holes
This Sears radial arm saw came with two threaded holes on the bottom of the motor. The holes were used to secure the motor in the crate for shipping. The holes are 5/16 inch by 18 threads per inch.
Step 2: Attaching Angle Iron
I used a piece of 1 inch angle iron 11 inches long. I drilled two 5/16 inch holes in the angle iron to match the holes on the bottom of the motor. I used two 5/16 inch bolts about an inch long each and put a nut onto each before threading them into the holes. The nuts are used to snug the angle iron against the bottom of the saw.
Step 3: Upright Frame Member
I had some 3/4 inch birch leftover from another project. I cut a piece 4 1/8 x 5 5/8 inch for the vertical piece you see here attached to the end of the angle iron.
The four nuts and washers seen in the corners defining a rectangle are 1/4 inch threaded rods 7 1/8 inch long each. They are part of the clamping system to hold the router body. The center bolt at the bottom fastens the end of the angle iron to the vertical frame member and is a bevel head screw long enough to pass through the lower part of the router body clamp. That will become clearer in another step. One longer piece of angle iron would have made this extra bolt unnecessary, but my angle iron was too short.
Step 4: Secure the Upper Portion of the Frame
A 5/16 inch eyebolt fits closely around the saw's motor shaft. You may have to open the eye just a little to make it slide onto the shaft. Notice the washers used as spacers between the eye and the back of the upright frame member. Between this and the angle iron, the router is mounted very securely. Naturally, the saw's motor does not run when the pin router is in use.
Step 5: Holding the Router Body
My router has a round body with no rack and pinion adjuster, but rather a large threaded ring that fits around the whole body and acts on threads around the body of the router. I made two squares from the 3/4 inch birch that were 5 5/8 inch on each side. I mounted them on a lathe faceplate and turned out the center sections to leave an open circle the same size as my router body, which is 3 13/16 inch. (The image is from Google Sketch-Up.)
Step 6: Make Halves of the Squares
I cut out a portion of the middle of each square to leave two halves. I drilled a 1/4 inch hole through each of two sides as shown by the red arrows and the red circles. These will be for the 1/4 inch threaded rods.
Step 7: Assemble
The sets of halves are bolted to the upright frame piece. On the bottom set you can see the head of a 1/4 inch bolt mentioned earlier. It is recessed and attaches the end of the angle iron. I placed the threaded rods through holes in the halves of the squares. Each half that attaches directly to the upright frame piece has a nut and washer to hold it firmly in place. The outer halves are secured by wing-nuts.
Step 8: Insert the Router
Turn the router's threaded adjusting ring as far to the top of the router body as it can go. Insert the router bit you wish to use. Slide the router down into the hole made for it. Tighten the wing-nuts. You are ready to do some routing.
Step 9: Pin Routing
A pin router allows you to make a pattern from 1/4 inch plywood or Masonite and reproduce that pattern on multiple copies. What you see is an example of a possible pattern. Small brads are usually used to fasten it to the bottom of a piece of wood to be routed with the pattern.
Step 10: The Pin
The guide pin would be the same diameter as the router bit. A dowel can suffice. Mount it in a piece of plywood and align the pin with the router bit so the bit is directly above the pin. Fasten the plywood to the saw table so it does not move. Raise the saw arm with the crank under the saw's table. Place the pattern over the pin. Turn the router "on" and lower the arm so the bit cuts in the work piece at the desired depth. Follow the pattern below until it is reproduced in the work piece.
I do not use a pin router often, but this is an economical alternative to a commercially produced pin router and works very well.