I managed to get some shop time on Saturday. The weather was nice, but it was very cold. I believe the temperature was in the lower 30s. So before I even stepped into the garage, I drove up to the local Ace hardware store & bought an oil filled 1500 Watt electric heater. No, this won't heat the room quickly like a gas burning unit would, bit it only cost about $65 and it will get the chill out of the room.
When I got it home, I plugged in the heater & gave it a half hour or so to raise the temperature in the garage a degree or two. The room never got to be so warm that I'd have to break out my shorts & tank top, but by the end of the day, the temperature in there was a lot more bearable.
As you can see, the arch is about 17 1/8" long and 1 1/4" high at its highest point, in the middle. The drawing shows a circular arch, but I wanted to actually use a fair curve. I drew a circular arch in SketchUp because I didn't know how to draw a fair curve. A fair curve is actually a parabolic shaped curve and generally is considered to be more aesthetically pleasing to the eye than a circle.
To make the curve, I decided to make a full size pattern using a scrap of plywood. I would lay out the curve, cut it out roughly on my band saw, sand it smooth & to the line. Once the pattern was done, I would then use it to lay out the curve on the work piece, rough cut it on the band saw, then use the pattern and a pattern bit in my router table to cut the arch on the work piece.
The picture at the right shows me measuring the length of the 3/4" plywood scrap that will become the pattern. I measured it because I wanted to center the arch in the pattern, as it will be centered on the workpiece.
The pattern piece is 33 1/4" long, which means that the center of it would be at 16 5/8".
I measured 16 5/8" from one end and made a mark. Then I used my combo square to draw a perpendic-ular line at that point. The photo above shows me drawing the center line. Then I measured 8 5/16" in each direction from the center and drew two more square lines to establish the end points of the arch.
Next, I set my combo square to 1 1/4" and drew a line parallel to the bottom edge of the pattern piece. This established the area within which the arch will be cut. Strictly speaking, drawing the parallel line isn't necessary, but I didn't realize that until later. All I needed was to measure up 1 1/4" at the center and draw a mark where the highest point of the arch will be.
To actually draw the curve, I placed a 1/8" thick strip of poplar, which is a cut off from another project that I have kept around exactly for situations like this, on top of the parallel line I drew. I drove a brad nail (by hand, not with a pneumatic nailer) into the plywood just deep enough so it wouldn't move easily. The nail was placed on the center line and so that it would keep the edge of the strip closest to me even with the parallel line.
Next, I aligned the strip with one of the arch's end points. I drove another brad into the plywood behind the strip so that the strip would intersec the end line & the edge of the board. I then drove a third brad in at the other end, placed similarly.
Now that the curve was laid out, I ripped the pattern to 3" wide, which is the finished width of the molding I was working on. I then cut out the curve on my band saw, trying to stay on the outside of the line, but as close as I could without destroying it. The closer I could keep to the line, the less sanding I would have to do in the next steps.
I now took a piece of 1/8" hardboard and prepared a sanding pad. The pad was sized to be be about 1/3 the size of a sheet of sandpaper. The hardboard is flexible enough to conform to the curve, but stiff enough that it won't dip into any irregular areas where I had removed more material than in neighboring areas. Basically, it acted kind of like a plane. Using spray adhesive, I glued 1/3 of a sheet of 80 grit paper to the hardboard.
I clamped the pattern into my Workmate (I do not have a workbench) and started sanding to the line. The important thing is to get the surface smooth and consistent. That means you have to sand any high points down to the level of the lowest low spot and remove all traces of any saw marks. You don't need to go to higher grits; you just need it smooth. Remember, every imperfection larger than a sanding scratch will get transferred to your workpiece when the bearing on the bit rolls over it.
The photo at the left shows the finished pattern with the curve sanded smooth to 80 grit. I had to change the paper on my sanding pad twice to get through all of the sanding, and I was done with it in less than 30 minutes.
At this point, I carefully traced the curve on to the workpiece and cut it out using my band saw. The key here is to make sure you leave the line. Most articles I've read recommend that you stay about 1/16" to 1/8" away from the line, and this is a good margin to shoot for. But if you just leave the line untouched by the blade, you'll still have enough material to remove the saw marks with the router bit.
I have to admit that I usually do a lousy job of staying outside the line while band sawing. Getting a good cut when free handing curves on the band saw requires a good amount of eye-hand coordination, and this is something that I really need to practice more. As it turns out, though, I didn't ruin the workpiece on the first try like I usually do (thank God). This is good because I didn't have enough stock to remake the part.
After cutting out most of the waste, I attached the pattern to the workpiece using double sided carpet tape. Next, I mounted my Bosch 1617 router in my home made router table. This table mounts in my Workmate. I have a Woodpecker aluminum router plate that fits into the table top. I chucked a 3/4" flush trim bit in the router and I routed the workpiece. The photo above shows the finished workpiece & the pattern.
All that was left to do is some hand sanding to remove some burn marks from the workpiece, and to fit and miter everything
Next: Miter, Miter! Toil and Trouble!