Adjustable-Knife Countersink: Three Knives Set Out Evenly by a Collar and Stopped by a Depth Pin (Logan, 1921)

"All of the cutters or knives 10 are made alike in length as well as other dimensions, so that by screwing the collar 7 into contact with the inner ends 12 of all of said cutters, the latter may be evenly adjusted to cut equally, and when their cutting edges 11 become worn down said cutters may be uniformly projected in order to provide the proper clearance."

From 30 mm steel bar turn the inner member: a 90 degree cone 3 26 mm across at its base, a short cylindrical edge under it, and a shank 2 14 mm across, threaded M14 for 20 mm below the head 6 and plain below. From 40 mm bar turn the sleeve: a ring bored 0.03 mm under the head's edge, with a flange 5 30 mm long bored 22 mm, so it leaves an annular space 13 round the shank. Heat the sleeve to 250 C in the oven and drop it over the head's edge; let it cool.

Saw and file three grooves 4 mm square, parallel to the axis, a third of the way round from each other, through the joint of head and sleeve and on into the annular space. File the cone away 14 ahead of each groove for the shavings. Cut three knives 32 mm long from 4 mm square tool steel, exactly alike, and grind each one's top end to the cone's slope with a bevelled cutting edge 11. Harden them (cherry red, water, temper to light straw). Drill and tap M4 through the sleeve over each groove for the set screws 15. Make a collar 7 with an M14 thread to screw up the shank inside the flange, and an M4 set screw 19 to lock it. Drill and tap M5 into the head's edge behind one knife for the stop pin 16, with an M4 set screw 18 to lock it.

Slide the knives into their grooves, back ends down in the annular space. Screw the collar up until all three knives' edges stand 0.3 mm out of the cone, and lock it and the knives. Set the stop pin so it touches the work when the countersink is the size wanted, and lock it. Countersink a row of screw holes in hardwood: every one should come out the same.

**Marselius J. Logan** of Springfield, Virginia, assignor of one-half to Laurence G. Alexander of Alexandria, Virginia, US 1,377,495, dated 10 May 1921, filed 14 February 1920. He claimed "a flange carried by the head and extending concentrically along the shank in spaced relation thereto, cutters mounted on the head and extending into the space between the same and the flange, and a collar adjustable on the shank within said flange for alining the cutters", and "an adjustable stop". He shows three knives; ours too. He gives no sizes; every size here is ours.