As I mentioned in my Cajon project post, I had some trouble cutting a circle to PERFECTLY fit the inside diameter of the drum, so I stopped for a couple of weeks to figure out how to cut exactly the circle size I wanted, and if too big, how to cut it down. I had been using a piece of melamine coated particle board with lots of holes drilled in it for many years and it worked just fine. But you can’t drill small holes right next to each other in particle board, it falls apart. So – back to the proverbial drawing board.
I decided that having the pivot pin travel on a block attached to a running thread would give me the greatest amount of precision in zeroing in on the radius/diameter I wanted for any given circle. Since I had already decided on making a general purpose guide (rather than a fixture for a single circle), I went shopping at my favorite industrial supply house (McMaster.com of McMaster-Carr fame) and bought a couple of nylon miter gears, a 36” 3/8”-16 running thread and a crank handle. The garolite is something I also buy from them, but I had enough scrap around to make the pieces I needed. The rest of the project is made of wood and wood derivatives (like Masonite, plywood, etc.), and I added a self-adhesive steel ruler (Inches and Metric) to let me get close with the measurement. I can’t trust it exactly because different width and tooth arrangements on the bandsaw blades will also affect how the radius is cut slightly.
This started with making a plywood base and then cutting two guide bars to form a trapezoidal channel for the pin block. The block was cut from garolite in a matching taper to the trapezoid and drilled and tapped all the way through to 3/8”-16 so that it would ride smoothly.
The crank handle was ok – but clearly showed the casting marks – putting it on the drill press with a bolt to hold it steady and some wet/dry paper (wet of course, and towels to catch the escaping water) smoothed it out nicely.
The gears were drilled and tapped to be locked on to the crank handle shaft with 8-32 screws. The biggest problem was stabilizing the crank handle shaft so that the gears would stay locked to each other. That is where the extra garolite plate came into being (see notes on photo 5).
To keep the running thread from moving lengthwise along its main axis, I ground a groove into it like this:
and drilled and tapped another garolite block to keep it centered with screws going through the groove cut into the running thread. In place, it looks like the example below:
O) (O The “O” on each side of the groove is a screw holding the groove in place.
The fixture was ready for alignment, so a cross-bar which fit the miter-gauge slot was fastened to the underside of the fixture and a screw hole drilled and tapped to lock it firmly into place.
I cut a shallow dado into the top surface of the fixture to keep the measuring tape slightly below the surface, and then sanded and sealed it and sanded again to make a good solid surface for the tape to adhere to.
Since I just used nails as the pins, and I needed some variation in depth, but not sticking out, I drilled a small hole in the side of the fixture with a Forstner bit then a smaller hole further in, and glued one of those super magnets into it, so that I don’t lose the nails, but they’re not waiting to snag me either. An example is that the plywood circle to hold the Cajon in perfect roundness had a pin going well into it. But when I cut the circle for the Cajon drum heads, I wanted a very short pin – just enough to pivot in, but not make a hole in the drum head.
Hopefully that provides enough description for everyone to get the idea, but I can now cut a circle with any radius from 2” to 32+” in increments of less than a millimeter.
-- Bob www.singularengineering.com - A sideline, not how I earn a living