Fellows Four-Lobed Cutter (US 579,708, Fig. 21): Curved Teeth That Cut Flat Faces

Fellows: "In Fig. 21 I show a cutter c' having curved teeth or projections shaped to form four plane surfaces c2 on a blank presented to it." The drawing shows c' as four rounded lobes meeting in cusps, turning against a square blank with its four faces marked c2. The arrows show the two turning in opposite directions, like a meshing pair: four lobes for four faces, one turn for one turn. The same rolling that generates gear teeth here generates a square.

Measured on the 300 dpi scan: the four lobes are 90 degrees apart within about 1 degree, and each is a true circular arc. The arc radius is 116.7 px, its centre 110.9 px from the axis, and the arc fits within 0.11 px. The drawn square has a side of 292.4 px. Rolling this cutter 1:1 against a round blank in a simulation tests the claim. It does cut four faces, but each dips inward slightly, by about 2% of its width, so they are nearly plane rather than exactly plane. At the drawn centre distance (381 px) the lobes cannot reach the drawn square's faces, and the result is 13% too large in area. At a centre distance 2.8% shorter (370.25 px), the cutter cuts Fellows's square to within 2.5% of its area. The drawing is close, but not exact.

Stand it on its cutting face with the shank up. The dished face is a near-flat overhang and the hollow has a 24 mm flat roof, so turn on supports from the build plate. PLA is fine for a display model. The real cutter is hardened steel.