Since the bending form is going to be a circle, I will need something to draw circles on the plywood. That ought to be simple to make, so I looked around the shop for something to use and I found a used paint stirring stick. That ought to work.
I drilled a hole in one end of it that is large enough to accept a #8 wood screw. The shaft of a #8 wood screw is 11/64 of an inch across. That is the size of the shaft without the threads. The part of the shaft with the threads is about 7/64 of an inch... not counting the threads. Since I want the stick to pivot around the shaft of the screw, I need to make the hole the larger of the two sizes, so that is what I did.
By the way, I almost always use #8 wood screws for most of my projects because it is really cool to be able to remember the size of the pilot holes as "7/11". Here is a photo of the stick with a #8 screw inserted into the 11/64" hole.
3/4" Screw in a 11/64" Hole
To secure the marker to the surface I drilled a hole that is 7/64" into a two by four plank so that the threads would bite into the wood. Then I screwed it down and prepared to make a hole for a pencil.
Screw It Down to a Plank
The center of the circle will be at the center of the screw shaft, so I measure from there.
Measure From Center of Screw
Now I have to decide what size I am going to make this form. I want the banjo hoop a size that will mimic the largest gourd banjo that I have made. My largest gourd banjo has a head size of 14" across, which gives it a very deep and mellow sound. However, the length of wood needed to bend a hoop that is 14" across would be
Circumference = π X Diameter
Now, π is equal to about 3.14159265. So π X 14 is about 43.9822971". I don't think that I will be able to do the bend that close to the 48" length of wood that I have to work with. That only gives me 4" of wood on the ends for overlap. That is only 2" on each end. Instead, I decided to make a 13" diameter hoop. π X 13 is about 40.84070445". That leaves me with just over 7" of overlap or 3.5" on each end. I might be able to get away with that.
I am going to make the cylindrical form 12" in diameter. The 12" size of the bending form should give me a 13" banjo hoop because of the springback of the wood after it is bent. There will always be some springback of the wood.
The first thing I am going to make is the plate that the bending form is going to be fixed to. I am going to make that part 18" across. If I make the plate 18" across, that will give me 3" around the bending form for clamping in the vise. That ought to be plenty.
The plate is going to be 18" across, so I need to draw a circle that is 18" in diameter, which is a radius of 9".
Mark At 9 Inches
At the 9" mark I drilled a 1/16" hole through the stick to put a pencil through, and there you go.
Pencil Drawing an 18" Circle
Now I can draw a perfect an 18" circle on my plywood.
That was all I had time for today, but hey, this is looking like a pretty easy project so far.
