Counterbore (ザグリ) & Countersink (皿)
The chart has two sections: counterbores for recessed socket-head screws and 90° countersinks for flat-head screws. Select the screw type and thread size, then read the clearance hole, recess diameter and depth from the matching row.
Counterbore for socket-head cap screws (JIS B 1001)| Thread | Clearance hole d' (mm) | Counterbore D' (mm) | Depth ≈ (head) (mm) |
|---|
| M3 | 3.4 | 6.5 | 3 |
| M4 | 4.5 | 8.0 | 4 |
| M5 | 5.5 | 9.5 | 5 |
| M6 | 6.6 | 11.0 | 6 |
| M8 | 9.0 | 14.0 | 8 |
| M10 | 11.0 | 17.5 | 10 |
| M12 | 14.0 | 20.0 | 12 |
| M16 | 18.0 | 26.0 | 16 |
| M20 | 22.0 | 32.0 | 20 |
90° countersink for flat-head screws| Thread | Cone angle | Head dk (≈ flush mouth) (mm) |
|---|
| M3 | 90° | 6 |
| M4 | 90° | 8 |
| M5 | 90° | 10 |
| M6 | 90° | 12 |
| M8 | 90° | 16 |
Counterbore (JIS B 1001): d' is the through clearance hole for the shank (grade 2 is common); D' lets the socket-head cap screw head sink; make the depth ≥ the head height (≈ the nominal diameter) so the head sits at or below the surface. Countersink (皿ザグリ) for 90° flat-head screws (JIS): a mouth diameter ≈ the screw head diameter dk (JIS B 1111) leaves the head flush; a larger diameter sinks it deeper — the depth from the surface = (D − d_hole) ÷ 2 because of the 90° cone. The countersink form follows JIS B 1017; exact figures still come from the drawing and the screw used.
Frequently asked questions
What is the difference between a counterbore and a countersink?
A counterbore creates a flat-bottom recess for a socket head, while a countersink creates a 90° cone for a flat head. Use the chart section matching the screw head.
How do I choose a counterbore diameter for a bolt size?
Find the thread size in the JIS B 1001 section and read clearance hole d', counterbore D' and depth across the same row. The exact values are listed in the chart.
How is the depth of a 90° countersink determined?
Depth depends on the mouth and clearance-hole diameters; for a 90° cone the chart note gives (D − d_hole) ÷ 2. Use the listed head diameter, then verify against the drawing and actual screw.
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