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Countersinking Bits

Countersinking Bits

Countersinking Bits

Countersinking Bits (also known as Countersink, Hex Quick Change Countersink, 3 Flute Countersink or 90 Degree Countersink) are 90-degree HSS Cobalt 3-flute countersinks with a 1/4-inch hex quick-change shank, cutting the recessed cone for a flush-fitting countersunk fastener. BNA stocks 28+ of these ex stock from our Brisbane warehouse, covering the full range of diameters, lengths and shank styles that engineers, tradies, manufacturers and DIY users need for everyday drilling tasks. Same-day dispatch on online orders placed before 1pm.

Also known as: Countersink, Hex Quick Change Countersink, 3 Flute Countersink, 90 Degree Countersink.

Drill Bit Construction & Materials

Alpha HSS Cobalt 3-Flute M35 grade HSS with 5% cobalt. The cobalt content lifts operating temperature to ~700 degrees C, which is exactly what's needed for countersinking stainless and hardened plate without dulling. 3-flute geometry balances cut speed and chatter.

Quick-Change Hex Shank 1/4-inch hex shank with a precision detent groove. Locks into impact drivers and quick-change holders without slipping. Allows on-the-fly bit swaps without re-chucking.

Drive & Use

Drill the clearance hole first with a twist drill, then run the countersink in the same spot until the cone diameter matches the bolt-head OD. Slightly deeper than flush is a workshop finish, slightly proud is a tradie finish.

Always use 90-degree countersinks for metric countersunk fasteners (the BNA standard). 82-degree countersinks are for imperial UNC/UNF under American standards. Match the countersink angle to the fastener head angle or the head sits proud or below flush.

Why 3 flute and not 1 or 5: 1-flute (cross-hole style) is fastest but chatters on hard plate. 5-flute is smooth but slow and clogs in timber. 3-flute is the everyday compromise — controlled chatter, good cut speed, easy chip clearance.

Stainless without burning: run slower, use cutting fluid, don't push hard. The 3-flute geometry is suited for stainless when speed and pressure are controlled. Stainless work-hardens if the cutting edge dwells, so keep the bit moving forward.

Depth control: count seconds, count revolutions, or use a depth-stop collar on the bit. A pilot ring on the bit gives consistent depth across a batch. Test on scrap first.

Sizing & Measurements

Diameter range 13mm to 20.5mm cone diameter (which sets the maximum bolt-head OD that will sit flush). Steps roughly every 1mm-1.5mm through the range.

Length range Overall bit length 50mm-65mm typical. The cone itself is roughly 6mm-9mm deep depending on size.

Shank 1/4-inch hex quick-change. Fits Festool, Milwaukee, Makita, DeWalt, Bosch and Ryobi quick-change collets. Also fits any 1/4-inch hex impact driver chuck.

Cone diameter must match the bolt-head OD. Typical head ODs: M4 CSK = 8mm, M5 = 10mm, M6 = 12mm, M8 = 16mm, M10 = 20mm. Match the countersink size to the head, not the bolt thread.

If the cone is too small, the head sits proud. If the cone is too deep, the head sinks below the surface and the joint loses clamp load. Aim for the rim of the cone to match the head OD exactly.

Combination drill + countersink bits exist for repetitive work like deck screws or cabinet hinges. Drill the clearance hole separately if you need flexibility on bolt sizing.

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