Countersink Holes for Machined Parts
The conical recess that lets a flat-head screw sit flush — standard angles (82°, 90°, 100°), countersink vs counterbore vs spotface, and how they are machined.
IsoCNC Engineering Team
CNC Machining & DFM Engineers · Updated · 7 min read
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Quick Answer
A countersink is a conical recess at the top of a hole that lets a flat-head screw sit flush with the surface. The angle must match the fastener: 82 degrees for imperial flat-head screws, 90 for metric, 100 for aerospace. It is specified by angle plus entry diameter. Countersinks are conical for flat-head screws, counterbores are flat-bottomed for socket-head screws, and spotfaces are shallow flat seats on rough surfaces. The full range of head styles and drive types behind those screws is covered in our types of fasteners overview.
The Cone That Hides the Screw Head
A flat-head screw is designed to disappear — its conical underside nests into a matching recess so the head ends up flush with or below the surface. That recess is a countersink: a conical enlargement at the opening of a hole. It is one of the most common hole modifications in machining, and one of the easiest to get wrong, because the cone angle has to match the fastener exactly. This guide covers the angles, the cousins (counterbore and spotface), and how a countersink gets machined and specified.
The Three Angles That Matter
82 degrees — the standard angle for inch/imperial flat-head screws. The default for most US-spec fasteners.
90 degrees — the metric standard. Most metric flat-head screws use this angle.
100 degrees — common for aerospace and aircraft fasteners, where the shallower cone provides more head support in thin materials.
The rule is absolute: the countersink angle must match the fastener head angle. A mismatch leaves either a proud head (angle too shallow) or an unsupported one (angle too deep), and neither seats properly. Read the fastener spec first, then cut the hole.
Countersink vs Counterbore vs Spotface
Three hole modifications, three different jobs:
| Feature | Shape | Used for |
|---|---|---|
| Countersink | Conical recess | Flat-head screws (flush seating) |
| Counterbore | Flat-bottom cylindrical recess | Socket-head screws, washers |
| Spotface | Shallow flat seating area | True seating surface on cast/rough surfaces |
The shape tells you the fastener: conical for flat-head, flat-bottomed cylindrical for socket-head, shallow flat seat for bearing on rough surfaces. They are often mixed up on drawings, but the fastener type makes the choice obvious.
How It Is Specified and Measured
A countersink is called out by its included angle plus the entry (major) diameter at the surface — for example, a 10 mm countersink at 90 degrees. Together these two numbers control the cone depth and whether the screw seats flush. The angle must match the fastener head, and the diameter must be large enough to seat the full head without the screw bottoming on the cone. Depth and diameter together are what make "flush" happen; either one alone is incomplete.
Machining the Countersink
On CNC, countersinks are cut two ways. A dedicated countersink tool with the matching included angle plunges straight in — fast for standard 82° and 90° angles. A chamfer mill interpolates the cone, which is more flexible for non-standard angles and controlled depths. Both produce the same conical seat; the choice is tooling and geometry. For the hole underneath the countersink, see our reaming guide, and for the fasteners themselves, our thread sizes chart.
FAQ
What is a countersink hole?
A countersink is a conical recess at the opening of a hole, cut so a flat-head fastener sits flush with or below the surface. Instead of the screw head sticking up, it nests into the cone. Countersinks are the standard way to flush-mount flat-head screws in sheet metal, enclosures, and machined plates where a protruding head would snag or look wrong.
What angle should a countersink be — 82, 90, or 100 degrees?
It depends on the fastener standard. 82 degrees is the usual angle for inch/imperial flat-head screws, 90 degrees is the metric standard, and 100 degrees is common for aerospace and aircraft fasteners. The countersink angle must match the fastener head angle — a mismatch leaves either a proud head or an unsupported one. When in doubt, read the fastener spec, not the hole.
What is the difference between countersink, counterbore, and spotface?
They are three different hole modifications. A countersink is conical, for flat-head screws. A counterbore is a flat-bottomed cylindrical recess, for socket-head cap screws or washers. A spotface is a shallow flat seating area machined to give a true surface for a fastener to bear on — common on cast or rough surfaces. They are often confused, but the shape and the fastener tell them apart.
How is a countersink specified on a drawing?
By the included angle plus the entry (major) diameter at the surface — for example, a callout like a 10 mm countersink at 90 degrees. The diameter and angle together control how deep the cone goes and whether the screw seats flush. The angle must match the fastener head, and the diameter must be large enough to seat the full head without the screw bottoming on the cone.
How is a countersink machined on CNC?
Two common ways: a dedicated countersink tool with the matching included angle, or a chamfer mill that interpolates the conical recess in CNC. The countersink tool is faster for standard angles; the chamfer mill is more flexible for non-standard angles and controlled depths. Both produce the same conical seat; the choice is about tooling and geometry.
When should I use a countersink vs a counterbore?
Use a countersink for flat-head screws where the head must sit flush. Use a counterbore for socket-head cap screws, where the cylindrical head needs a flat-bottomed pocket and the screw is driven from above. Our thread sizes chart covers the fasteners themselves; the hole modification follows the fastener type.
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