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CNC Machining Glossary
Countersink in CNC Machining
A countersink is a cone-shaped recess at the entrance of a hole. It typically provides a seat for the tapered underside of a flat-head screw, allowing the head to sit flush with—or at a specified depth below—the surrounding surface. Picture a machine cover with screw heads that sit level with the panel instead of sticking out.
The recess is the feature; countersinking is the machining operation used to create it.
A countersink has a conical seating surface for a tapered screw head. A counterbore has cylindrical walls and a flat bottom, commonly accommodating a socket-head cap screw.
What Matters in Design and Machining
For designers, a countersink connects the choice of fastener with the finished assembly. Its angle and size must suit the screw head, not simply provide enough space to hide it. An incorrectly sized recess can leave the head protruding or sitting too deep.
For machinists, the feature requires suitable tooling, clear access, and controlled cutting depth. Moving an angled cutter deeper into the hole increases the recess’s diameter, so depth affects both its size and the screw’s seating position. Vibration during cutting can also leave an uneven seating surface.
An accessible countersink is usually straightforward to machine. However, unusual geometry, restricted access, or unnecessarily tight tolerances can add machining and inspection time.
Details Designers Can Overlook
Choose the screw first. Use the selected fastener’s specified head dimensions and angle to design the recess. Do not assume that all flat-head screws have the same head angle. The included angle is the full angle between the two sloping sides when viewed in a section through the hole’s axis—not the angle of one side alone.
Make the drawing unambiguous. Specify the hole diameter, countersink major diameter—the widest diameter at the surface—and included angle, with appropriate tolerances. Identify which face receives the countersink. Where flushness is important, state the acceptable screw-head position relative to the surface.
Check thickness and surrounding space. Evaluate the full recess, not just the smaller hole beneath it. A countersink can extend into a nearby edge or feature. In thin material, it may leave insufficient support beneath the screw head or break through the opposite face.
Control only what the function requires. Keep the feature accessible to the cutter and avoid tighter tolerances than the assembly needs. When the purpose is only to remove a sharp hole edge, specify an appropriate edge break rather than a precision screw seat.
A chamfer around a hole can resemble a small countersink. However, a chamfer intended only for edge removal is not automatically sized or controlled to seat a screw correctly.