The calculator
Runs in your browser. Inches or millimetres at the top.
How it works
A 100° countersink is a cone. Its diameter at the surface and its depth are tied: a change in depth Δz changes the diameter by 2 × Δz × tan 50°. So Δz = (target Ø − test Ø) / (2 tan 50°), about 0.0042 in of depth for every 0.010 in of diameter.
Target. For a flush head the well diameter equals the head diameter: we use the middle of the MS20426 limits (1/8 in: 0.225 in). To leave the head a little proud, for shaving or by the drawing, the well is smaller by 2 × proud × tan 50°.
Clicks. Every cage moves the stop by a set amount per click or per turn, marked on the tool. Divide the depth change by it.
The FAA Airframe Handbook calls the microstop countersink the preferred tool and recommends testing adjustments on scrap before countersinking repair parts.
Worked example
1/8 in MS20426 rivets. The test countersink measures 0.215 in across; the cage moves 0.001 in per click.
- Target Ø = 0.225 in (flush).
- Δz = (0.225 − 0.215) / (2 × 1.192) = +0.0042 in, deeper.
- Clicks = 0.0042 / 0.001 = 4.2: set 4 clicks deeper, cut another test hole.
Four clicks deeper, then test again.
Limits — when NOT to use this
- Check the sheet can take it. A deeper countersink must still meet the limit in the countersink or dimple calculator.
- Head diameter is mid-range of the tolerance band; the test rivet in the test hole is the final judge.
- 100° cutters only. 82° and 120° countersinks have other factors.
Questions people actually ask
How do I measure the countersink diameter?
With a countersink gauge, a loupe and scale, or by fitting the rivet and measuring how far the head sits proud or below, then entering the head diameter adjusted for it.
Sources
- FAA-H-8083-31, AMT Handbook — Airframe, Volume 1, Chapter 4, Countersinking Tools — the microstop as the preferred tool; 100° as the common angle; test adjustments on scrap.
- MS20426 rivet head diameter limits, from distributor data sheets — the flush target, mid-range of 0.175–0.183, 0.221–0.229, 0.282–0.290 and 0.349–0.357 in.