The two methods
| Machine countersink | Dimple | |
|---|---|---|
| What it does | Cuts a conical recess for the head | Presses the sheet into a cone; removes no metal |
| When | Sheet thick enough for the head | Sheet thinner than the minimum for countersinking |
| Depth limit | Some manufacturers: no more than 2/3 of the outer sheet, or down to 0.020" minimum shank depth, whichever is greater | Not a cutting limit; depends on alloy, temper and rivet size |
| Strength | Removing metal means more fasteners for the same load | "Gives a stronger joint than the non-flush type" |
| Tools | Microstop cage and 100° piloted cutter | Male and female die pair, in a squeezer or under a hammer |
| Watch for | Knife edge, chatter | Cracks in hard alloys; work hardening |
All rows from FAA-H-8083-31 Vol. 1, pp. 4-41 to 4-43.
The thickness rule, and why it changed
The old rule was that the rivet head had to be contained within the outer sheet. The Handbook says that rule "has been reevaluated in recent years because countersunk holes have been responsible for fatigue cracks in aircraft pressurized skin": a countersink cut too deep leaves a knife edge, and a knife edge under pressurization cycles cracks.
What it reports now: "In primary structure and pressurized skin repairs, some manufacturers are currently recommending the countersink depth be no more than 2/3 the outer sheet thickness or down to 0.020-inch minimum fastener shank depth, whichever is greater. Dimple the skin if it is too thin for machine countersinking."
Note the wording: some manufacturers. The limit for a given aircraft is in its SRM. The Handbook also notes that countersunk fasteners need more edge distance than universal heads: see the edge distance calculator.
Which alloys can be dimpled cold
2024-T3 "can be satisfactorily dimpled either hot or cold, but may crack in the vicinity of the dimple after cold dimpling because of hard spots in the metal".
7075-T6 "alloys are always hot dimpled". Magnesium and titanium too. Hot dimpling needs heated dies and is usually stationary shop equipment: the hand dies in this guide are for cold dimpling.
For any alloy: "keep hammer blows or dimpling pressures to a minimum so that no undue work hardening occurs". Alloy properties: 2024-T3.
The tools, as listed
| Tool | For | Listed spec | Where |
|---|---|---|---|
| Dimple die set 3/32" | Dimpling | 100°, male + female, 3/16" shaft, spring-back | US · Yardstore |
| Dimple die set 1/8" | Dimpling | 100°, male + female, 3/16" shaft, spring-back | US · Yardstore |
| Spring back dimple dies | Dimpling | 100°; #8, #10, 1/8", 5/32"; .187" shank | UK/EU · LAS |
| Texas hand squeeze | Driving the dies | 3" yoke, adjustable ram | US · Yardstore |
| ATS Pro hand squeezer | Driving the dies | 1-1/2" or 3" yoke | UK/EU · LAS |
| Wescon 596-S microstop | Countersinking | 0.0005" depth steps, 1/4"-28 cutters to 3/4" | US · Yardstore |
| Microstop + 8 cutters kit | Countersinking | 100°, pilots #40–#10 and 1/8"–1/4" | US · Yardstore |
| Cobalt countersink set | Countersinking | 100°, 3 flutes, pilots #40–1/4" | US · Yardstore |
| 100° pilot cutters | Countersinking | 100°, 3 flutes, HSS, fixed pilot | UK/EU · LAS |
| Extended microstop | Countersinking in channels | 3/8" cutters, 3,000 RPM | UK/EU · LAS |
As the sellers list them. Each name links to the page with its source.
What to look for when buying
A die pair per rivet size. "A separate set of dies is necessary for each size of rivet and shape of rivet head." The male die's guide is the size of the rivet hole, so a 1/8" die does not do a 3/32" rivet.
Angle: 100°. The Handbook calls 100° the standard countersink angle; 82° and 120° are sometimes used. Every die and cutter in the table is 100°.
A microstop, not freehand. "The microstop countersink is the preferred countersinking tool": the cage limits depth and holds the cutter square. Freehand countersinking is for where a cage cannot fit.
Pilot about 0.002" under the hole. That is the Handbook's figure for keeping the cutter centered without binding. Match the pilot to the hole on the drill chart.
Flutes. The Handbook says "a six-fluted countersink works best in aluminum" and that four- and three-fluted cutters "are harder to control from a chatter standpoint". The cutters listed here are three-fluted, so expect to manage chatter: sharp tooling, slow speed, steady pressure, backing behind thin sheet.
Dies in a squeezer must be set to the sheet. "They must be adjusted accurately to the thickness of the sheet being dimpled": an adjustable ram or set holder is what makes that possible.
Questions people actually ask
Should I countersink or dimple?
Countersink if the sheet is thick enough that the recess does not leave a knife edge; dimple if it is thinner. The Handbook reports that some manufacturers limit countersink depth to two-thirds of the outer sheet in primary structure and pressurized skin. The aircraft's SRM sets the real limit.
Is a dimpled joint weaker than a countersunk one?
No. The Handbook says dimpling removes no metal and, through the nesting of the sheets, "gives a stronger joint than the non-flush type", shifting load from the rivet to the sheets. Countersinking removes material and calls for more fasteners.
Can 7075-T6 be dimpled?
Only hot. The Handbook states that 7075-T6 alloys "are always hot dimpled" because of their low formability. Hand dies in a squeezer are cold dimpling.
What angle are aircraft rivet countersinks?
100° is the standard. 82° and 120° cutters also exist, so check the cutter before using it on an AN426 hole.
Can I dimple the skin and countersink the structure under it?
Yes, the Handbook describes it as the combination predimple and countersink method, used where a thin sheet is joined to a thicker one: the thick part is countersunk to receive the dimple.
Limits of this guide
Flush riveting is where repairs are most often rejected.
- The SRM decides. The two-thirds figure is what "some manufacturers" recommend, per the Handbook. The aircraft's approved data sets the minimum thickness and the method.
- No tool was tested. The table gives listed specifications only.
- Hot dimpling is outside this guide. It needs heated dies and controlled temperature and dwell.
- Practice on scrap. The Handbook recommends testing microstop adjustments on scrap before cutting the part.
Sources
- FAA-H-8083-31, AMT Handbook — Airframe, Volume 1, Chapter 4 — pp. 4-37 and 4-41 to 4-43: countersinking and its revised depth rule, microstop countersinks, flute count and chatter, dimpling, hot dimpling by alloy, and the predimple and countersink method.
- Seller listings — tool specifications in the table, via the linked product pages at Yardstore and LAS Aerospace.