Countersinking Guidelines
Precision countersinking for flush-mount fastener holes in your sheet metal parts. Countersinks are integrated into your production run — no separate processing step needed.
Design Requirements
Hole-to-Edge Distance
The distance from the hole center to the nearest edge must be at least the countersink outer diameter.
Hole-to-Bend Distance
Minimum distance from a hole or countersink lip to a bend line is 2 × thickness + bend radius. The countersink's outer lip extends past the hole by (OD − hole Ø) / 2, so larger countersinks reduce the available clearance. This is enforced at quote time: catalogue options whose lip would land inside the bend deformation zone are greyed out, and designs with holes or countersinks inside that zone can't be ordered until the design or material is changed. See the materials table for values per thickness.
Hole Size & Sheet Thickness
A countersink is only orderable when the chamfer cone fits inside the material. Two limits apply: your existing hole must be narrower than the countersink's outer diameter (otherwise there's no material left to chamfer), and the sheet must be thick enough to contain the cone (otherwise it breaks through the back and widens the through-hole). The quote tool checks both for you — options that don't fit your hole and thickness are greyed out with the reason on hover. If you switch material or thickness after configuring, selections that no longer fit are removed automatically and you're notified of what changed.
Countersink Angles
82° — Imperial / SAE
ANSI/ASME B18.6.3 standard. For flat-head machine screws and general-purpose flush mounting.
90° — Metric / DIN
DIN 965 / ISO 10642 standard. For metric flat-head screws and European standard hardware.
82° Countersink Chart (Imperial)
ASME B18.6.3 flat-head screw dimensions.
| Bolt Size | Hole Diameter | Outer Diameter | Min Edge Distance |
|---|---|---|---|
| #4 | 3.28mm (0.129") | 6.48mm (0.255") | 6.48mm (0.255") |
| #6 | 3.81mm (0.150") | 7.80mm (0.307") | 7.80mm (0.307") |
| #8 | 4.50mm (0.177") | 9.12mm (0.359") | 9.12mm (0.359") |
| #10 | 5.11mm (0.201") | 10.44mm (0.411") | 10.44mm (0.411") |
| 1/4 | 6.76mm (0.266") | 13.49mm (0.531") | 13.49mm (0.531") |
| 5/16 | 8.08mm (0.318") | 16.66mm (0.656") | 16.66mm (0.656") |
| 3/8 | 10.32mm (0.406") | 19.35mm (0.762") | 19.35mm (0.762") |
| 1/2 | 14.29mm (0.563") | 22.23mm (0.875") | 22.23mm (0.875") |
90° Countersink Chart (Metric)
DIN 7991 / ISO 10642 flat-head screw dimensions.
| Bolt Size | Hole Diameter | Outer Diameter | Min Edge Distance |
|---|---|---|---|
| M2 | 2.41mm (0.095") | 4.39mm (0.173") | 4.39mm (0.173") |
| M2.5 | 2.90mm (0.114") | 5.50mm (0.217") | 5.50mm (0.217") |
| M3 | 3.40mm (0.134") | 6.73mm (0.265") | 6.73mm (0.265") |
| M4 | 4.50mm (0.177") | 8.97mm (0.353") | 8.97mm (0.353") |
| M5 | 5.51mm (0.217") | 11.20mm (0.441") | 11.20mm (0.441") |
| M6 | 6.60mm (0.260") | 13.44mm (0.529") | 13.44mm (0.529") |
| M8 | 9.00mm (0.354") | 18.25mm (0.719") | 18.25mm (0.719") |
| M10 | 11.00mm (0.433") | 22.73mm (0.895") | 22.73mm (0.895") |
| M12 | 13.50mm (0.531") | 27.21mm (1.071") | 27.21mm (1.071") |
Volume Pricing
Better per-countersink pricing with larger quantities:
| Quantity | Price per Countersink |
|---|---|
| 1+ | $0.75 |
| 10+ | $0.60 |
| 50+ | $0.50 |
| 100+ | $0.40 |
| 500+ | $0.30 |
Design Tips
CAD File Requirements
- Only include the minor (inner) hole diameter in your CAD file
- Countersinks detected from STEP files are matched automatically
- You can also configure countersinks manually in the quote tool
Best Practices
- Match countersink angle to fastener spec — 82° for ANSI, 90° for metric
- Verify outer diameter matches the fastener head for a flush fit
- Consider assembly accessibility when choosing top or bottom placement
- Adds 1 day to production lead time
Questions? Contact support@rmfg.com for help with countersink specifications or custom requirements.