Quick Answer: Which Angle Do I Need?
Metric flat-head screws? → 90°
Just deburring holes? → 82° or 90°
Shallow chamfer? → 120°
Understanding Countersink Angles
The countersink angle refers to the included angle of the conical cutting surface — the total angle measured across the full cone, not the half-angle from center. A flat-head screw has a conical underside that must match the countersink angle for the screw to sit flush.
If the countersink angle does not match the fastener angle, the screw head will either protrude above the surface (countersink too steep), sit below the surface with a gap (countersink too shallow), or seat on only part of the cone (creating a stress riser).
Common Mistake
The #1 mistake is using a 90° countersink for US standard flat-head screws. US flat-head screws are 82°, not 90°. Using 90° leaves the screw head slightly proud of the surface.
Countersink Angle Reference
60° Countersink
Applications
- Center holes for lathe work (60° matches standard lathe centers).
- Deburring small holes and cross-holes.
- Light chamfering where a steep chamfer is desired.
- Spotting for drill bit starting points.
Standards
60° matches the standard included angle of lathe centers (ANSI B5.4). Also used for some valve seat work.
82° Countersink — Most Common in the USA
Applications
- US standard flat-head screws — the default for all American-standard flat-head machine screws, wood screws, and sheet metal screws.
- General-purpose deburring of drilled holes.
- Chamfering for assembly and safety.
Standards
- ANSI B18.6.2 (Machine Screws)
- ANSI B18.6.1 (Wood Screws)
- ASME B18.6.3 (Tapping Screws)
90° Countersink — Most Common for Metric / International
Applications
- Metric flat-head screws (DIN/ISO standard).
- Rivet holes for standard rivets.
- General deburring and chamfering (international shops).
- European and Asian manufactured equipment using metric fasteners.
Standards
- ISO 2009 (Slotted countersunk head screws)
- ISO 7046 (Cross recessed countersunk head screws)
- ISO 10642 (Hexagon socket countersunk head cap screws)
- DIN 963, DIN 965, DIN 7991
100° Countersink — Aerospace Standard
Applications
- Aerospace fasteners (AN and MS series flat-head fasteners).
- Aircraft structural rivets requiring flush surfaces for aerodynamics.
- Military specification hardware.
- High-performance applications where the shallower angle provides more bearing area.
Standards
- NAS 523 (Countersunk holes for close tolerance fasteners)
- AN Standard (Army-Navy fasteners)
- MS Standard (Military Standard fasteners)
- ASME B18.6.5 (100° flat-head screws)
110° Countersink
Applications
- Specialized aerospace applications using 110° fasteners.
- Some European automotive flush fastener specifications.
- Shallow chamfering where a wide, gentle chamfer is needed.
110° countersinks are less common than 100° but are specified in certain aerospace and defense applications. Always check the engineering drawing before assuming 100° or 110°.
120° Countersink
Applications
- Shallow chamfering and edge-breaking on large-diameter holes.
- Deburring large holes where you want to remove the burr without creating a deep chamfer.
- Sheet metal work where a wide, flat chamfer prevents edge curling.
- O-ring groove chamfering for ease of assembly.
120° countersinks produce a very shallow, wide chamfer. They are primarily used for deburring and edge preparation rather than fastener seating.
Complete Angle Reference Table
| Angle | Primary Use | Industry Standard | Severance Product | Frequency of Use |
|---|---|---|---|---|
| 60° | Center holes, deburring, spotting | ANSI B5.4 | Chatterless, CNC-K | Common (lathe shops) |
| 82° | US flat-head screws | ANSI B18.6.2 | Chatterless, CNC-K, 3N1 | Most common (USA) |
| 90° | Metric flat-head screws, rivets | ISO 2009, DIN 963 | Chatterless, CNC-K, 3N1 | Most common (International) |
| 100° | Aerospace AN/MS fasteners | NAS 523, AN, MS | Chatterless, CNC-K | Common (aerospace) |
| 110° | Specialized aerospace | Various MIL specs | Chatterless | Specialized |
| 120° | Shallow chamfering, deburring | — | Chatterless, CNC-K | Common (deburring) |
Why Chatterless?
Conventional countersinks with evenly-spaced flutes are prone to chatter — a harmonic vibration that produces a rough, wavy finish with audible squealing. This is especially problematic on drill presses and hand-held tools where rigidity is limited.
Severance Chatterless Countersinks solve this problem with a patented 6-flute design with unequal spacing. The irregular spacing breaks up the harmonics that cause chatter, producing a smooth, clean countersink even in less-than-ideal setups.
Chatterless Benefits:
- Smooth, clean finish without chatter marks.
- Works well in drill presses, hand drills, and portable tools.
- No need for rigid CNC-level setup to get good results.
- Longer tool life because cutting forces are more evenly distributed.
- Available in all standard angles: 60°, 82°, 90°, 100°, 110°, and 120°.
For CNC Machines
If you have a rigid CNC machining center, consider the CNC-K Countersink line, which is specifically designed for programmed feeds and speeds in automated environments.
How to Determine the Correct Angle
Check the Fastener Specification
The most reliable way to determine the correct countersink angle is to check the engineering drawing or fastener specification. Look for:
- Callout on the engineering drawing (e.g., "82° CSK" or "100° CSK")
- Fastener part number (AN, MS, or NAS numbers indicate 100° aerospace)
- Thread specification: US Unified thread = likely 82°; Metric thread = likely 90°
Quick Identification Rules
- Screw head says "82°" or is a US-standard flat head → 82°
- Screw is DIN/ISO metric flat head → 90°
- Part number starts with AN or MS → 100° (aerospace)
- Drawing calls out NAS 523 → 100°
- No specification given, US shop → default to 82°
- No specification given, metric shop → default to 90°
- Just deburring, no fastener → 82° or 90° (either works)