A month or so ago, after looking at my door shape and the quality of the cuts of the wall perimeter I decided to make a tool to help cut round/radius shapes. With a better process I think I can make better walls with more attractive doors. I posted the router compass initial design to the FB DIY Teardrop group as a possible good idea but today I actually used it to draw out the new walls and was happy that it worked every bit as well as I hoped it would. I’ll try and explain how it’s used here and then maybe post again how to make the tool yourself. Now that I have it working I consider it a “must-have” tool because it delivers accuracy close to a CNC router. (Friends have mentioned just taking it to a shop to have it all cut with CNC but I think this is actually easier because nothing ever leaves my house, and it only takes a few minutes to setup and use.)
Quick overview of tool features/usage:
- Arm rotates on center bearing attached to a plate that is screwed to the surface to be cut.
- 2 part arm comprised of a box and a tongue which rides in one direction in the box.
- Carriage bolt makes box pinch tongue when tightened and keeps the arm from moving.
- Non-pivoting end has a quick attach mount for either a router or a pencil holder.
- 2 different length tongues can be used for different ranges of radii.
Mount the pencil holder and draw the arc that will later be precision-cut.
Use a jigsaw to cut outside that line leaving only 1/16″ to 1/8″ of waste. This will be trimmed with the router. No matter how careful you are, the jigsaw cut will never be exact. Even if you sand down to the drawn line you will still see imperfections in the sanding. The jigsaw blade also flexes when cutting and you will end up with edges that are not cut exactly perpendicular to the surface. Even if you do think you could tolerate the imperfections of the jigsaw cuts you can’t really argue with the ease of a single pass with a router which gives you the freedom to be quicker with the jigsaw and not have any sanding to do.
You can see that the cut is a little sloppy. The router will make short work of turning that into a “CNC grade accuracy” perfectly cut arc. Here’s another look at the cut…
Next, I just spin off the 2 wing nuts holding the pencil holder attachment and replace with the router attachment.
I adjusted the compass arm length by loosening the arm wing nut and setting it where the router bit cutting edges just clear the edge where the cut line disappears into the edge of the plywood. When the arm length is perfect I tighten all the wing nuts.
When I made the router compass, I used the bearing from a 3/8″ flush trim bit for the pivot point. I then used the router bit without bearing to do the trimming. Set the bit so that the collet is as close to the surface as possible and that as the bit extends below the bottom of the router, the cutter part of the bit is the only part of the bit that will come into contact with the edge we’re trimming.
Make sure that the bit is clearing wood by just enough that the router won’t jump when turned on. Hold the router with one hand and turn on with the other. Plan the cut such that the rotation of the bit tends to make the router move opposite the direction of the cutting (Counter clockwise). Trimming off the ragged 1/16″ waste is no work for the router and it produces a crisp, completely perpendicular cut that absolutely conforms to the radius as if it was cut with a CNC machine.
In this example I am cutting out templates I can use in the future. I will clamp them to plywood sheets and (after rough cutting with the jigsaw) will just use a flush trim bit to produce exact duplicates of the template. You can use the pencil attachment and draw out the lines where other pieces will eventually be mounted by just adjusting the radius and drawing a concentric arc. In this example I found that the arm could not be shortened enough to accommodate the smaller radius so I mounted the pencil holder at a right angle.