Four dimensions decide whether a flat head sits flush or sits proud — and in sheet metal, one of them is the sheet.
Outer diameter, through-hole, and cone depth for every countersink we tool, in both the 82° inch and 90° metric series. Plus the number no fastener catalogue gives you: the minimum sheet thickness each size needs before the cone eats the material and leaves a knife edge instead of a seat. Cross-sections are drawn to scale from your gauge.
The 82° inch and 90° metric series we actually tool, with the geometry you dimension on the drawing. Minimum sheet is cone depth plus the 0.010″ of land that has to survive under it — below that number the countersink is not a seat, it is a hole with a chamfer. Cross-sections are drawn to scale against the thickness you pick.
Every other countersink dimension comes off a fastener chart. This one comes off your material, and it is the reason most countersinks that fail on a drawing fail here.
A countersink is a cone milled into the top of a through-hole. The cone has a depth set by the screw head, and it does not care how thick your sheet is. When the depth approaches the thickness, what is left under the cone thins to a razor ring — it burrs while the cutter is still in it, it deforms the moment the screw is torqued, and the head never seats square.
Here is the part that surprises people: you cannot countersink an inch flat-head screw in 16 gauge. 16 gauge steel is 0.060″, and the smallest 82° seat we tool — a #4-40 — is 0.072″ deep on its own. The cone goes clean through the sheet before it ever reaches the tabulated diameter.
In the 90° metric series the cone is shallower at the same nominal size, so it reaches thinner material: M2 needs 0.039″ and M2.5 needs 0.057″. Those two are the only seats in either series that fit 16 gauge at all.
When the land runs out, you have three real options. Go up a gauge — the fit matrix below shows exactly which one. Drop to a smaller screw, or switch the inch callout to its metric neighbour for the shallower cone. Or stop countersinking and use flush self-clinching hardware, which puts the load into a flange instead of a cone and works down to about 0.040″.
Every countersink we tool against every sheet thickness we stock. The number in each cell is the land left under the cone, in thousandths — so you can see not just whether it fits, but by how much.
A 90° screw in an 82° seat bears on a ring instead of the whole cone. An 82° screw in a 90° seat does the same. Either way the head rocks, sits proud, or chews the seat. The angle is set by the screw standard, and it is not yours to round off.
Those angles turn up two ways: as a CAD default nobody changed, and as rivet seats. 118° is a twist-drill point, not a countersink — if your model has it, it came from a hole wizard rather than a fastener. Change it to 82° or 90°, or ask before you upload and we will tell you what we can do.
Give us a through-hole, an outer diameter, an angle, and a side. That is a complete countersink. Add a driven depth on top and you have specified the same cone twice, in two ways that will not agree.
What people ask most often about flush fasteners in sheet metal, angles, and how thin you can go.
0.082″ for a #4-40, 0.105″ for a #8-32, 0.169″ for a 1/4-20, and 0.204″ for a 5/16. The 90° metric cones are shallower, so M2 needs only 0.039″ and M3 needs 0.063″. The fit matrix works it out against every gauge we stock.0.060″, and the smallest 82° seat we tool — #4-40 — is 0.072″ deep by itself, so the cone punches through before it reaches diameter. In the 90° metric series only M2 and M2.5 fit at 0.060″. For a flush fastener in thin sheet, go thicker, drop to a small metric screw, or use flush self-clinching hardware. Converting gauges to decimals? See the gauge chart.0.010″ is the floor because below it the ring is thin enough to behave like a cutting edge rather than a structure: it burrs while the cutter is still in the hole, it folds when the screw is torqued, and the head tips instead of seating. It is a machining and assembly limit, not a stress calculation, which is why it is a flat number rather than a fraction of thickness.0.193 THRU, CSK 0.359 × 82°, NEAR SIDE. Do not also drive the cone depth unless you mean it to override the table — a depth and an outer diameter that disagree cannot both be cut, and we have to pick one.0.193″ where the plain free-fit clearance is 0.177″, but a #10 countersink through-hole is 0.199″ against a 0.201″ free fit. Use the countersink value when the hole is countersunk and the clearance chart when it is not.depth = (outer − through) ÷ (2 × tan(angle / 2)) across all sixteen sizes, which is what lets the cross-sections be drawn to scale rather than sketched. Land, minimum sheet, and every fit verdict are computed from those figures and the 0.010″ floor.