Setting up, step by step
| Step | What the Handbook says | Source |
|---|---|---|
| 1. Cutter angle | Match the rivet head. 100° is the standard; 82° and 120° are sometimes used | Handbook pp. 4-37, 4-41 |
| 2. Pilot | About 0.002" smaller than the hole, to keep the cutter centered | Handbook p. 4-42 |
| 3. Depth | Only deep enough for the head to sit flush; some cages adjust in 0.001" steps | Handbook pp. 4-37, 4-41 |
| 4. Depth limit | In primary structure and pressurized skin, some manufacturers say no more than 2/3 of the outer sheet, or to 0.020" minimum shank depth, whichever is greater | Handbook p. 4-41 |
| 5. Test | "Test adjustments on a piece of scrap material" before the part | Handbook p. 4-42 |
| 6. Cut | Slow speed, steady firm pressure, cage square to the work | Handbook p. 4-42 |
How the cage works
The cage is "an adjustable-sleeve cage that functions as a limit stop and holds the revolving countersink in a vertical position". The cutter threads into the shaft; its pilot rides in the hole. The Handbook's Figure 4-85 names the parts: pilot, cutter, micro-sleeve, skirt, locking ring.
Cutters "may have either a removable or an integral pilot". LAS's pilot cutters are integral, one cutter per hole size. Too thin a sheet for the cut, and the Handbook says to dimple instead; the rule is set out in the dimple guide.
Chatter, and the cutter
"Chattering is the most common problem encountered when countersinking." The Handbook's list: sharp tooling; slow speed and steady firm pressure; a pilot about 0.002" under the hole; backing material behind thin sheet to hold the pilot steady; a cutter with a different number of flutes; or pilot drill undersize, countersink, then enlarge the hole to final size.
Flutes matter. "A six-fluted countersink works best in aluminum"; three- and four-fluted ones "are harder to control from a chatter standpoint"; single-flute works best in corrosion-resistant steel. The cutters listed here with a flute count are three-fluted.
Choosing a cage
Thread. Every cage and cutter listed here uses 1/4"-28. Check it before mixing brands.
Cutter size. The Wescon 596-S takes cutters up to 3/4". Standard cutters have a 1/2" body; LAS's extended microstop needs 3/8" cutters instead.
Adjustment step. The Handbook mentions 0.001" increments; Wescon lists 0.0005". The kit cage's step is not listed.
Speed. Wescon lists 10,000 RPM maximum; the extended microstop, 3,000 RPM. The Handbook asks for slow speed when countersinking, and gives 3,000 rpm as the usual rating of sheet metal drill motors.
Reach. The extended microstop is for channels such as seat tracks, per LAS. Where no cage fits, the Handbook allows freehand countersinking, practiced on scrap first.
Check pilot against hole. Yardstore's kit and cobalt set include a #10 pilot. #10 is 0.1935"; the 3/16" rivet hole on our drill chart is a #11, 0.1910". A #10 pilot will not enter a #11 hole.
Cages and cutters, as listed
| Item | Type | Listed spec | Where |
|---|---|---|---|
| Wescon 596-S | Cage | 1/4"-28; 0.0005" steps; cutters to 3/4"; 10,000 RPM max | US · Yardstore |
| Microstop kit, 9 pieces | Cage + 8 cutters | 100°, 1/2" body; pilots #40, #30, #21, #10 and 1/8" to 1/4" | US · Yardstore |
| Cobalt countersink set | 5 cutters | 100°, 3 flutes, 1/4"-28; pilots #40 to 1/4" | US · Yardstore |
| Extended microstop 133-NX | Cage | 3/8" cutters; 1/4" shaft travel; 3,000 RPM max | UK/EU · LAS |
| Pilot cutters | Cutters | 100°, 3 flutes, HSS, fixed pilot; #40 to #12 | UK/EU · LAS |
As the sellers list them. Each name links to the page with its source.
Questions people actually ask
How do you use a microstop countersink cage?
Fit a cutter of the head's angle with a pilot about 0.002" under the hole, set the depth, test on scrap, then cut at slow speed with steady pressure, cage square to the work. The steps and their sources are in the table at the top.
How deep should a countersink be?
Deep enough for the rivet head to sit flush, no deeper. In primary structure and pressurized skin the Handbook reports some manufacturers' limit of 2/3 of the outer sheet, or 0.020" minimum shank depth, whichever is greater. The repair data for the aircraft governs.
Why does my countersink chatter?
The Handbook's fixes: sharp tooling, slow speed, steady pressure, a well-fitted pilot, backing behind thin sheet, a cutter with a different flute count, or countersinking an undersize hole and enlarging it afterwards.
Can I countersink without a microstop?
Where a cage cannot fit, yes: the Handbook allows freehand countersinking, practiced on scrap first, with the drill held steady and perpendicular.
Do all microstop cutters fit all cages?
The listed ones share a 1/4"-28 thread, but cutter diameter differs: the extended microstop needs 3/8" cutters, not the 1/2" standard body.
Limits of this guide
What the sources support, and no further.
- The depth limit is not a universal rule. The Handbook attributes it to some manufacturers; the aircraft's data governs.
- No six-flute cutter is listed here. The Handbook's preference for aluminum is not yet in the catalog.
- Pilot diameters are not listed. Sellers name pilots by hole size; the 0.002" clearance cannot be checked from a listing.
- No cage was tested.
Sources
- FAA-H-8083-31, AMT Handbook — Airframe, Volume 1, Chapter 4 — p. 4-37, "Countersinking Tool" and Figure 4-85; p. 4-41, countersinking, the revised depth rule, flute count; p. 4-42, the microstop as preferred tool, pilot clearance, scrap testing, freehand countersinking, chatter.
- Seller listings — cage and cutter specifications, via the linked product pages at Yardstore and LAS Aerospace.