Making a Fly Cutter from — a Fly Cutter?

The Tool

The fly cutter

A friend gave me a used fly cutter — or at least, that's what most woodworkers I know call the tool. General Tools, the manufacturer, calls it their #55 heavy-duty circle cutter. I also had a broken light-duty fly cutter, also made by General Tools, #4 4-inch-max circle cutter. These are fine tools for their intended use -- cutting circles in wood or soft metals using a drill press, and they also work using a brace and bit despite General Tools comment about their only safe use being in a drill press.

There is controversy about what this tool is properly called. The general definition of a fly cutter is a tool holder for a single-point cutting tool that sweeps the tool in a circle over the surface of a fixed workpiece. The tool flies over the work, unlike a conventional single-point lathe tool, where it is the work that moves. I've seen the term applied to single-point cutters used to cut gears, and I've seen it applied to single point cutters used for surface milling. If you ignore the center drill on the General Tools circle cutters, they certainly fit this definiton, so woodworkers aren't wrong to call their tools fly cutters. Some machinists, however, say the woodworking tool is a trepanning tool because it cuts around the perimeter of a circular hole without cutting the center plug. Indeed, you can trepan with a fly cutter, but that doesn't make it less of a fly cutter.

Regardless, I needed a fly cutter for my Taig lathe, so I modified the fly cutters to produce the result in the picture. What I did is:

• Cut off the pilot drill mount from the front of the heavy-duty circle cutter. The General Tool #55 is made of case-hardened steel, so I needed to use a carbide lathe tool to cut through the hard skin before I could use a standard Taig HSS parting tool to finish the cut.

• Taper the shank to fit the lathe's spindle taper. For rigidity and added inertia, I decided to always use the fly cutter in the three-jaw chuck, so I started the taper on a point along the shaft that would position the body of the cutter where the jaws could grab it. The rear end of the shaft is turned to be a snug fit in the throat of the spindle. As with cutting off the pilot drill holder, I had to use carbide tooling to get through the case-hardened steel.

• Drill and tap the shank to mate with the lathe drawbar. In my case, I have a spindle depth stop that uses a 1/4-20 drawbar, so I use that drawbar to mount my fly cutter. Even with the hard skin removed from the tail end of the drawbar, tapping the tough steel of this tool was not easy. What I found to work best was to alternate between a taper tap and a bottoming tap, so each could ease the work done by the other.

• Replace the long arm of the heavy-duty circle cutter with the shorter arm of the broken 4-inch one. The Taig lathe is small, so the 6-inch arm of the heavy-duty cutter was too long. The shortcoming of this is that the tool bit of the heavy-duty cutter is a 1/4" tool, while the lighter tooling on the 4-inch one is 3/16". I could have cut down the long arm instead.

• Insert a brass slug into the set-screw hole so the screw doesn't grub divots into the the radial arm. Once a set-screw digs into a particular point, it is very easy to return to that exact same adjustment, but almost impossible to ajust things slightly one way or the other away from that mark because the screw will tend to find its old divot and center itself on that. A brass button in the hole eliminates this problem.
  

Example Tool Use

Milling a notched edge

I've used my fly cutter for a number of facing jobs, converting rough hacksawed ends into fine flat surfaces, but that's just the tip of the iceberg of the kind of machining you can do with a fly cutter. The photo here shows my fly cutter in use to notch the edge of a 1/8" thick brass bar. I needed 1/16" square notches taken out of both edges of the bar so it could slide between a pair of guides that had matching notches.

The fly cutter was spinning at 880 RPM when I took the photo, so it's a little bit blurred. At that speed, it kicks little brass chips a considerable distance with each cutting pass.

Adjusting the vertical depth of cut is surprisingly straightforward:

• Start with the fly cutter mounted on the lathe, with the radial arm of the cutter horizontal and the set screw facing up.

• Set the furniture on the cross slide that you'll use to support your workpiece. I used a collection of 1/2 inch square aluminum bars in this example.

• Set a shim on the support surface that's the thickness of the part you want remaining after your cut.

• Slide the cutter to its smallest radius, rotate it so the cutting edge is directly below the lathe center line, and lower the tool so it just touches the surface of the shim, then tighten the radius set screw.

Just to be sure, turn the lathe by hand a few revolutions to make sure that the cutter just brushes the shim without even a hint of trying to dig in.

With the depth of cut adjusted, clamp the workpiece on top of whatever furniture you're using on the cross slide, and align it properly. I used a microscope for this step. I clamped the workpiece loosely, so I could slide it horizontly with medium thumb pressure, then advanced and retracted the cross slide while looking at the gap between the edge of the workpiece and the cutting edge of the tool, adjusting things until the workpiece was exactly parallel to the cross slide.

To set the horizontal depth of cut, move the carriage so the tool just kisses the face of the workpiece, lock the carriage, and then use shims to set the distance from the carriage to the carriage stop. For the 1/16" notches being cut in the photo, I cut the entire notch in two passes.

As an aside, the piece I'm cutting in the photo here is the slider for the interlocking mechanism of my push-to-stop switch. The photos in my writeup of that mechanism show the completed assembly with that slider.