Fluted-Point Spade Bits: A Groove Down Each Face of the Point Sharpens Its Cutting Edge (Robinson, 1957)

"I prefer to forge the bits of this invention from solid or integral pieces of steel, with heat treatment all over, and have discovered that, despite the provision of the flutes or grooves, which are preferably milled or hollow ground, the bits are sturdy and will not bend."

Cut three 170 mm lengths of 10 mm octagonal tool steel. Heat 50 mm of one end to a bright orange in the forge and flatten it on the anvil to about 3 mm thick, centred on the bar. Hammer across it with the peen to spread it wider than the bit, about 2 mm over its size: 18, 22 and 27 mm. Draw the neck down smoothly where the blade meets the bar and keep the blade in line with it. Reheat as soon as the colour fades to red. Finally heat each one to a cherry red and let it cool slowly in the air, to soften it for filing.

Glue each bit's outline from the template on its blade, centred on the bar. Grind to the lines on the bench grinder, dipping the steel in water often: the two sides parallel, the shoulders square across, the point in the middle. Grind the blade's two faces flat and parallel to 2.5 mm. Grind three flats on the end of the shank, a third of the way round from each other, 20 mm long, for the chuck's jaws. Drill a 3 mm hole through the blade where the template's circle is, to hang the bit on a nail.

Hold the bit flat in the vice by its shank. With the 3 mm round needle file, file a groove 0.3 mm deep along the point's left edge on the face towards you, from the apex to just below the shoulders, leaving a thin sharp edge where it meets the bevel. Turn the bit over and file the other face the same way: again along the left edge, which is the point's other edge. Both flutes face the way the bit turns.

Heat the blade and the neck evenly to a cherry red, until a magnet no longer sticks to it, and quench it straight down in water, moving it up and down. Rub the faces bright with a stone. Heat the shank gently 30 mm behind the blade and watch the colours run down; quench again as soon as a dark straw colour reaches the shoulders and the point. Do the other two.

Hone the cutting edges on the shoulders and the point with the stone, keeping their angles, and take the burr off. Chuck a bit by its flats and bore a scrap of hardwood: it should start on the point without wandering, cut two clean shavings from the shoulders and leave a round hole of its own size; check it with the caliper.

**Clarence J. Robinson** of Wilmington, Ohio, assignor to the Irwin Auger Bit Company, US 2,782,824, dated 26 February 1957, filed 27 April 1954. He claimed a spade type wood boring bit whose point has "a pair of substantially parallel edges" from its apex towards the blade, "the juncture of the other of said narrow bevelled edges and one side face of said point body portion providing a cutting edge". He shows four flute shapes; we file the arcuate one of his Figs. 1 and 2, where he preferred to mill or hollow grind them. His shank is round with flats on a reduced end; ours is octagonal tool steel with three ground flats. The patent gives no sizes or heat treatment; every size here, and the house hardening, are ours.