Fellows Gear Shaper

Fellows claims a machine that cuts a gear by acting on the blank at many points as it rolls, not by a cutter shaped like a single tooth gap. That rolling is "generation".

Make a cardboard rack — a strip with straight-sided, angled teeth. Straight sides are easy to draw and cut precisely.

Form a flat disc of modelling clay as the "blank". It must be soft enough to take an impression from the rack teeth.

Press a single rack tooth into the clay without rolling. You get a straight-sided notch — a copy of the tool. Note that shape.

Roll the rack in a straight line while the clay disc turns in step, like a gear on a track. Let each tooth press as it passes. Do not copy — roll.

The disc's teeth are now curved, not straight — even though every cut was made by a straight edge. The rolling motion generated a curve the tool never had. That curve is the involute.

With a protractor, measure the slope of the rack's straight flank. That single angle — the pressure angle — sets the whole tooth shape. Standard gears use 20°.

Roll the same rack against a larger disc turning more slowly. It gets more teeth, differently curved — yet made by the same tool. One cutter, every gear size.

Roll the two clay gears together. They mesh smoothly, because both were generated by the same rack — the guarantee of interchangeability.

Fellows's real tool is not a rack but a small gear-shaped cutter that reciprocates up and down. Model this by rolling a rigid pinion template against the clay instead of the rack.

Roll the pinion cutter against the inside of a clay ring. It cuts internal teeth — the thing a big external rotary cutter physically cannot reach. This is Fellows's decisive advantage.

Because the shaper cuts on a down-stroke and lifts on the return, it can run teeth close against a raised shoulder. Mark how near the tool can finish — another job a rotary cutter cannot do.

The patent. US 579,708, "Gear-shaping machine", granted 30 March 1897 to Edwin R. Fellows of Springfield, Vermont — a nine-sheet drawing for a machine of real subtlety. Fellows had worked for the Jones & Lamson machine-tool works and built his shaper into one of the great names in gear-cutting.

The idea is generation, and it is worth stating plainly. A gear tooth's working face is an involute — the curve traced by the end of a string unwound from the base circle — and its exact shape depends on how many teeth the gear has. You cannot keep a drawer of form-cutters, one per tooth count, and get true gears; the economics and the accuracy both fail. Fellows's insight, building on the earlier generating work of Joseph Saxton and others, is that if you roll a cutter against the blank as though the two were already a meshing gear pair, the cutter automatically removes exactly the metal that stops them meshing — and what is left is the correct involute, whatever the tooth count. Steps 4 to 6 are the whole idea in clay: a straight tool, rolled, leaves a curve.

Why the cutter is a gear, not a rack. A rack (steps 2 to 9) demonstrates generation beautifully but is awkward to build into a machine — it would have to travel a long straight path. Fellows made the cutter a pinion: a hardened gear-shaped tool that reciprocates up and down to do the cutting while it and the blank rotate together a hair at a time. A rotating cutter that is itself a gear can do what no large disc-shaped cutter can — reach inside a ring to cut internal gears (step 11), and finish teeth hard against a shoulder or between the gears of a cluster (step 12). Those two capabilities are why the gear shaper never went away.

The pressure angle is the quiet standard underneath it all. The straight flank of the rack has one slope — the pressure angle — and that single number (today almost always 20°) fixes the tooth shape for an entire interchangeable system. Two gears of different sizes made to the same pressure angle will mesh; that is the whole basis of buying a gear from a catalogue. Step 7 measures it.

What it enabled. Interchangeable, quiet, load-bearing gears in quantity are a precondition for the automobile gearbox, the aircraft engine, the machine tool itself — every geared machine of the twentieth century. Hobbing (a rack-like generating cut with a rotating worm-shaped tool) does the same for external gears at higher speed, but for internal gears, cluster gears and gears against a shoulder, the Fellows shaper's rolling pinion remains the answer. The tool writes the curve, and the curve runs the century.