John B. Shadle, CMC
I've made many escape wheels for clocks in many ways, but I wanted a more scientific way to produce them.
I use a CAD program for the best results. I draft the escape wheel with the proper angles and dimensions, then use the CAD drawing to dimension a cutter which will produce the chosen tooth profile.
This one, made for a deadbeat escapement, has 30 teeth, with a 10-degree slant on the front of the tooth and a flat bottom at the root of the tooth.
You can't cut the front slant with a slanted cutter -- the cutter will just remove the slant. The only way to produce a slant is to offset the cutter a measured distance from the center of the escape wheel, shown as dimension b in the drawing.
Lines a and b are shown on the right, after the offset is made. Line a is now a vertical line, producing the 10-degree slant on front of the tooth. The exact dimension of the offset depends on the angle of the slant, the size of the wheel, and the number of teeth in the wheel.
When the cutter is offset the proper amount, the space cut between the teeth will be correct. The angle on the back of the tooth isn't as critical. I usually just pick an angle that looks right, in this case about 30 degrees.
Notice that, because of the offset, the part of the fly cutter which makes the flat bottom at the roots of the teeth isn't flat, but angled a small amount, 8 degrees in this case. This angle is determined by simply drawing a line between the roots of the teeth, then measuring it with the CAD program's angle-measuring function.
The flycutter itself is made from 1/8" thick carbon steel, known in the trade as "gauge plate". When finished, the cutter's tip is hardened and tempered to straw. The cutter is mounted in a homemade holder which will fit on the spindle of my nice little Taig mill.
The width of the fly-cutter isn't critical, as long as it's wider than the finished cut in the wheel. When cutting the wheel, I first cautiously lower the cutter in the mill, then cut two teeth. There should be a land on the end of the tooth between the two cuts. I lower little-by-little, until the land is only a few thousandths of an inch wide. Darkening the rim of the wheel blank with Dykem makes it easier to see the land.
The holder is made from Taig blank arbors. They are economical to buy, and very versatile. They are internally threaded with 3/4" x 16 threads. (They will fit on a Sherline mill, as well as a Taig.) Shown on the left are two blank arbors, showing top and bottom. I milled an 1/8" groove in the end to hold the flycutter, which is secured in the holder with 10-24 set screws.
The angles are ground on a Harbor Freight tool grinder, which has a protractor fence and a table which can be tilted in measured degrees. Shown here grinding the 8-degree angle on the bottom of the fly cutter. A more simple tool, a bench disk sander, would do as well, as long as it has a protractor fence and a table which can be tilted.
In addition to the angles on the fly cutter which will produce the tooth profile, there also must be some relief, about 8 degrees, behind the cutting surfaces, on all three cutting surfaces, shown here as a, b, and c. These edges are relieved in the same way as you relieve the edges of a lathe tool bit.
The relief is provided by tilting grinder table downward, a feature built into the grinder.
Also, a slight slant on the edge a will provide side relief as well -- shown much exaggerated in the drawing on the right.
Shown with the escape wheel in an indexing fixture. There are always some burrs on the back of an escape wheel cut with a fly cutter. They can be downsized by backing the escape wheel blank with a disk of free-cutting aluminum or another disk of brass. Remember than not all aluminum is free-cutting, nor is all brass. Run the fly-cutter as fast as your mill will go.
A well-formed flycutter will make a buzzing sound when cutting, not a loud rattle or clatter. If you hear a clatter, the cutter is not sharp or it does not have the proper relief behind the edges.
Also, it will dull quickly if it is not properly hardened and tempered. Bad burrs or bent teeth are also a tipoff that all is not well with your cutter.
This is the escape wheel for a new clock. It is a 90-tooth crown wheel destined to drive deabeat pallets.
This wheel was cut with a profiled flycutter ground from a strip of high-speed steel (originally, an old lathe cut-off blade).