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Clamp-On Countersink: Two Blocks Screwed Round Any Drill, Twin Cutters Countersink the Hole as It Is Bored (Brackin, 1903)
George W. Brackin · Public domain
Forge

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Forge

13. November 2026NO
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Clamp-On Countersink: Two Blocks Screwed Round Any Drill, Twin Cutters Countersink the Hole as It Is Bored (Brackin, 1903)

George W. Brackin of Philadelphia received US 727,580, dated 12 May 1903, for a countersink that clamps onto a drill or auger bit. Two blocks are screwed together across the bit at any point along it. A twin cutter is screwed into an inclined seat in each block, its point reaching into the bit's flutes. As the bit bores, the cutters arrive and cut the cone for the screw head without taking the bit out. A slotted stop sets how deep it goes, so it gauges the hole too. We make three for 4, 5 and 6 mm drills. The patent gives no sizes; every size here is ours.
Intermediate
About 1 day

Instructions

1

Setting it on the bit

Loading Jupyter Notebook...
2

Read US 727,580

"From Figs. 2 and 3 it will be observed that the lower ends of the inclined cutters project slightly into the twists or helical channels of the drill, and this fact permits the cutters to enter the hole bored and then commence cutting away the edge of the hole as the tool is rotated."
3

Clamping blocks 2 and 3

For each attachment cut two blocks 40 x 12 x 10 mm from square bar. In each, 6 mm from one end drill a 4.5 mm plain hole and 6 mm from the other end drill 3.3 mm and tap M4, through the 12 mm face, so the two blocks screw together with the plain hole of one over the tapped hole of the other. Across the middle of each block's outer face file a seat 6 mm wide, sloping down and in at 45 degrees, and drill and tap M4 in it for the cutter's screw.

Materials for this step:

Mild Steel Square BarMild Steel Square Bar1 piece

Tools needed:

HacksawHacksaw
Bench ViseBench Vise
ScriberScriber
Center PunchCenter Punch
Drill PressDrill Press
Drill Bit SetDrill Bit Set
Tap and Die SetTap and Die Set
Tap WrenchTap Wrench
File SetFile Set
ProtractorProtractor
Digital Caliper 6-InchDigital Caliper 6-Inch
4

Twin cutters 8 and 9

Cut four cutters 22 mm long from 3 x 6 mm carbon steel strip. Drill each 4.5 mm for its screw at the top, and grind the lower end to a point with a straight cutting edge 13 on its leading side and a rounded back 15, as his Fig. 4. Harden: heat to a cherry red, until a magnet no longer sticks, quench in water, rub bright, and draw to a light straw colour.

Materials for this step:

Carbon Steel StripCarbon Steel Strip1 piece

Tools needed:

HacksawHacksaw
Drill PressDrill Press
Drill Bit SetDrill Bit Set
Bench GrinderBench Grinder
ForgeForge
Forge TongsForge Tongs
Small MagnetSmall Magnet
Quench BucketQuench Bucket
WaterWater
Sharpening StoneSharpening Stone
Welding GlovesWelding Gloves
Safety GlassesSafety Glasses
5

Stop 17 and assembly

Bend a strip of 3 mm flat bar into an L, its upright 30 mm long with a 4.5 mm slot 18 15 mm long, its foot 20 mm. Screw a cutter into each block's seat. Put the blocks either side of the drill, the stop's slot under one clamping screw, and screw them together until they grip the drill, the cutters' points just in the flutes and their edges leading as the drill turns.

Materials for this step:

Mild Steel Flat BarMild Steel Flat Bar1 piece
Machine ScrewsMachine Screws4 pieces

Tools needed:

HacksawHacksaw
Bench ViseBench Vise
Drill PressDrill Press
Drill Bit SetDrill Bit Set
File SetFile Set
Screwdriver SetScrewdriver Set
6

Bore and countersink

Set the drill's point the hole's depth below the stop's foot and the cutters for the screw head's size (the notebook gives both). Drill into hardwood until the stop touches: the hole is at depth and its mouth countersunk. Check a screw sits flush.

Tools needed:

Cordless DrillCordless Drill
Drill Bit SetDrill Bit Set
Hardwood BoardHardwood Board
Digital Caliper 6-InchDigital Caliper 6-Inch
Safety GlassesSafety Glasses
7

Troubleshooting

Clamp-on countersink troubleshooting.

Flow

Loading...
8

History and honest limits

**George W. Brackin** of Philadelphia, US 727,580, dated 12 May 1903, filed 18 June 1902. He claimed "the rectangular clamping-blocks arranged parallel to each other, clamping-screws passing through the said blocks near their ends", each block "provided exteriorly at its middle point with a cutter-seat inclined downwardly and inwardly", and "duplicate and interchangeable cutters" screwed into them. His was for wood, with the note that the cutters could be hardened for metal; ours are hardened. He gives no sizes or angle; every size here is ours.

Materials

4

Tools Required

23

This blueprint makes

Thumbnail
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Countersink Drill Bit Set

Free

325
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