Made the base plate for a Pat Warner style circle jig today and used it to cut the radii for a router template, for making the hand hold cutout in the ships ladder stringers. I printed and pasted a full size pattern for the base plate on to a piece of 1/2" MDF and band sawed the outside and sanded to the lines on a disc sander. Then, drilled a hole for inserting a scroll saw blade and cut out the center hole, then sanded it to the line on a spindle sander. Milled the slots on my vertical mill with DRO, but I don't have the right metal stock for the pivot arm, so did not make that much of the jig today. Instead, for the immediate need, I drilled a 1/4" pivot hole at the right distance from the center of the jig, which can be seen just inboard from the 1/4" wide clamping slot.
Here is a test run in a scrap piece of MDF.
Rats, I was going for 7.040". Missed it by a smidge
Cutting the radii on the production template. I need to work on dust collection. Dust went everywhere and these were just 1/2" deep cuts. The shopvac hose didn't do a thing, and I even had the shopvac turned on. Chips will come off of the stringer lumber rather than dust as fine as what came off of the MDF, but I still need dust collection very close to the cutter. A dust hood came with the edge guide, but I haven't figured a way to mount it on the router bases yet.
Straight edges were milled out on my vertical mill. I used a laser center finder to set the stock up, splitting the beam over the edge of the radius and then shifting over about 0.150" to rough it out. Then a final cleanup pass at 0.125".
The completed cutout. I had backed the template up from the mill table with 1/4" Baltic birch plywood scrap, which was screwed to the center section for the second cutout, so the middle part would not be loose as the cut was complete.
There are some small rises at the transitions from the straight edge to the radii. I'll blend those tomorrow with a spindle sander and then it should be ready to try out.
This template is for creating cutouts for the handholds. The cutouts will be started by boring holes on each end with a large diameter forstner bit and the sides with be cut with guided, circular saw plunge cuts. These are going into 12 ft long stringers of 2x12 fir boards. The stringers will be on sawhorses. I have another template for guiding the circular saw cuts and will get close to the final dimensions. Final size will be done with the router/template and a 1/2" dia. straight router bit.
If there is a better way to make the straight cuts in these boards, I'm eager to hear about it. I'm not thrilled about making the plunge cuts in the 2X boards, but that is probably because I haven't done it much. I'll be testing it all out on scrap before going to the stringers.
The hand holds will be finished with a 1/2" radius roundover bit.