Diamond-Point Spade Bits: A Thinned, Wide-Angled Point Does More of the Cutting (Southall, 1975)

"As a result of the fact that the point 16 is provided with a relatively large point angle, the point 16 is caused to perform more of the cutting function as compared to that performed by the cutting surfaces 42 and 44 of the blade 14."

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 at the template's circle.

Bevel each side of the blade about 2 degrees, one side from the top face and the other from the bottom face. Bevel the two cutting edges about 10 degrees behind the edge, again opposite ways, so each cuts as the bit turns. Bevel the point's converging edges about 10 degrees the same opposite ways, giving the point its diamond section. Grind the point's top and bottom faces thinner towards the tip until they meet there at about 8 degrees. Round the two corners where the point meets the cutting edges with the edge of a needle file.

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.

**William Southall**, deceased, late of Moonah, Australia, by Dorothy I. Southall, administratrix, assignor to The Stanley Works, US 3,920,350, dated 18 November 1975, filed 11 February 1974. He claimed a point "having side edges diverging towards said side edges of said blade and a pair of faces converging to the tip", giving "a generally rhomboidal transverse cross section", with the blade's leading edges inclined towards the point and at opposite angles. His figures are for a one-inch bit and are approximate; we use his point and angles on all three sizes. His shank is round with a hexagonal end; ours is octagonal tool steel with three ground flats. He gives no heat treatment; the hardening is our house practice.