7.3 Countersinking, Dimpling Techniques & Rivet Installation

7.3 Countersinking, Dimpling Techniques & Rivet Installation

Approved-Data Control

The figures and hardware examples in this section illustrate principles. For an actual aircraft or component, current approved maintenance data, product instructions, organisation procedures, and applicable law control the material, limit, interval, sequence, tooling, PPE, and acceptance decision.

The physical installation of aircraft solid rivets requires precision manual skill and strict adherence to structural engineering physics. An improperly drilled hole, a chamfered edge, an over-countersunk sheet, or an unaligned bucking bar will compromise joint fatigue life and induce premature airframe failure. EASA Part-66 Module 7 demands comprehensive practical and theoretical understanding of hole drilling tolerances, deburring protocols, machine countersinking limits, cold and hot dimpling methodologies, pneumatic rivet gun dynamics, bucking bar inertia, and the precise geometric proportions of standard shop heads.

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Precision Hole Preparation, Drill Sizing & Deburring

Rivet holes must be drilled round, smooth, perpendicular to the sheet surface, and sized within tight diametral tolerances to ensure 100% radial expansion of the rivet shank during driving.

Numbered Drill Bit Sizing for Aircraft Rivets

Aircraft solid rivets are never drilled using nominal fractional drills matching their exact shank size. Because a solid rivet must fit freely into its hole before upsetting, holes are drilled slightly oversize using standard numbered and lettered twist drills:

• 3/32" Rivet (0.0938"): Drill with #40 bit (0.0980" / 2.49 mm)
• 1/8" Rivet (0.1250"): Drill with #30 bit (0.1285" / 3.26 mm)
• 5/32" Rivet (0.1562"): Drill with #21 bit (0.1590" / 4.04 mm)
• 3/16" Rivet (0.1875"): Drill with #10 bit (0.1935" / 4.91 mm)
• 1/4" Rivet (0.2500"): Drill with 1/4" or Letter 'F' bit (0.2570" / 6.53 mm)

Step-Drilling and Pilot Holes

When drilling holes 5/32" and larger through structural stacks, technicians must always drill a pilot hole (typically using a #40 or #30 drill bit) before opening the hole to final diameter. Pilot drilling prevents the drill point from wandering, eliminates hole ovalization, and ensures hole perpendicularity.

Swarf Removal and the Critical Deburring Rule

Drilling through stacked aluminium sheets pushes metal chips (swarf) and creates ragged burrs between the faying surfaces. Prior to riveting:

1. Clamps and Cleco fasteners must be removed.
2. Sheets must be separated to clear away all trapped swarf and cutting fluid.
3. Hole edges must be deburred using a specialized zero-flute or curved deburring tool rotated lightly by hand.

• THE CRITICAL DEBURRING RULE: NEVER CHAMFER THE HOLE! Deburring must strictly remove raised burrs. Technicians must never chamfer, countersink, or bevel the hole edge. Chamfering reduces the cylindrical hole bearing surface and creates an intense geometric stress riser where the rivet shank transitions into the head, precipitating fatigue failure under cyclic airframe loads.

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Machine Countersinking & The Knife-Edge Hazard

Flush riveting requires creating a conical recess to accept the 100° head of an MS20426 rivet. Machine countersinking cuts away parent sheet metal using a rotating multi-flute cutting tool.

Microstop Countersink Cages

Machine countersinking must never be performed using freehand drills. Technicians utilize a microstop countersink cage:

• The cage features a hardened non-marring nylon or phenolic foot that rests flat and perpendicular against the skin surface.
• A micrometer sleeve adjusts cutter depth in increments of 0.001 inch (0.025 mm).
• An internal ball thrust bearing and positive stop arrest cutter travel when the calibrated depth is reached, ensuring identical depth across hundreds of holes.

Microstop Countersink Tool Architecture:

         [ Drill Motor Drive Shank ]
                    |
        +-----------v-----------+
        | Micrometer Sleeve     |  Adjustable in 0.001" increments
        | (Depth Calibration)   |
        +-----------+-----------+
        | Internal Ball Stop    |
        +-----------+-----------+
        | Spring-Loaded Housing |
        +-----------+-----------+
        | Non-Marring Foot      |  Rests flush against aircraft skin
        +-----------+-----------+
                    |
             [Conical Cutter] (100°)

The Sheet Thickness vs. Head Depth Rule

Machine countersinking is permissible ONLY IF the thickness of the top sheet ($T$) is greater than the height ($h$) of the countersunk rivet head:

$$T_{\text{sheet}} > h_{\text{rivet head}}$$

As a standard rule of thumb, machine countersinking requires that the sheet thickness must be at least 1.5 times the depth of the countersink.

The Knife-Edge Condition

If machine countersinking is performed on thin sheet metal where $T \le h_{\text{head}}$, the conical cutter cuts completely through the sheet thickness, eliminating the straight cylindrical bearing wall. This creates a razor-sharp perimeter known as a knife edge.

• Under cyclic aerodynamic and pressurization loads, the knife edge acts as an acute stress riser, causing severe localized fatigue cracks to radiate outward from the hole.
• The thin knife edge provides zero bearing resistance, allowing the rivet head to tilt, loosen, and pull through the skin.
• Mandatory Policy: When the top sheet is thinner than the rivet head height, machine countersinking is strictly prohibited. The sheet must be dimpled.

Machine Countersinking vs. The Knife-Edge Hazard:

        Correct Machine Countersink                     The "Knife-Edge" Defect
        (Thick Sheet: T > Head Height)                (Thin Sheet: T < Head Height)

         100° Angle                                    100° Angle
        \          /                                  \          /
         \________/ <--- Flat Head                     \________/ <--- Flat Head
        |          |                                    \      /  <--- NO CYLINDRICAL WALL!
        |          | Cylindrical Bearing Wall            \====/   <--- Knife-Edge Stress Riser
        +----------+ (Absorbs Shear Loads)              +------+  (Cracks and Pulls Through!)

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Dimpling Techniques: Coin Dimpling vs. Radius Dimpling & Hot Dimpling

Dimpling forms a recessed conical nest for a countersunk head without removing any metal. It is achieved by pressing the sheet between precision male and female dies.

1. Coin Dimpling vs. Radius Dimpling

• Radius Dimpling: Uses simple male and female dies to bend the sheet downward. However, bending introduces spring-back and causes the surrounding sheet metal to distort and warp.
• Coin Dimpling: The standard aerospace dimpling method. The female die incorporates a spring-loaded or hydraulic coining ram (pressure pad). When the male punch forces the sheet into the die cavity, the coining ram exerts extreme compressive force directly around the hole perimeter. This compressive stress exceeds the material's yield strength, "coining" the metal into intimate die contact, eliminating sheet warpage, and producing sharp, crack-free dimple contours.

2. Process-Controlled Hot Dimpling

High-strength aluminium alloys—specifically 7075-T6 and 2024-T3—possess high yield strength and limited room-temperature ductility. Attempting to cold-dimple these alloys induces severe tensile hoop stresses, causing radial, circumferential, and peripheral cracks around the hole perimeter.

• Hot Dimpling Protocol: Where an approved process calls for hot dimpling, its equipment and controls may include electrically heated dies. The listed range is an example, not a universal instruction:
  • Operating temperature: Regulated between 260°C and 315°C (500°F to 600°F).
  • Dwell time: Pre-set between 1.5 and 3.0 seconds before full forging squeeze pressure is applied.
  • Metallurgical Rationale: Controlled heat temporarily elevates the alloy's ductility, allowing crisp plastic deformation without cracking, while the short dwell time ensures that the T3 or T6 heat-treat temper of the surrounding parent sheet is not compromised.

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Pneumatic Rivet Driving Tools & Bucking Technique

Solid rivets are driven by pairing a pneumatic rivet gun on the manufactured head with a handheld bucking bar on the shank end.

Pneumatic Rivet Gun Types

• Slow-Hitting Gun: Delivers 1,000 to 2,500 blows per minute with a heavy, controlled stroke. This is the standard tool for structural aircraft riveting. The slower, heavier impacts upset the rivet shank quickly without work-hardening the metal prematurely, giving the bucking partner excellent control.
• Fast-Hitting Gun: Delivers 2,500 to 5,000 blows per minute with rapid, light vibrations. It tends to work-harden rivets before the shop head forms and is restricted to small-diameter or soft aluminium rivets.
• One-Shot Gun: Drives the fastener with a single massive impact. Used in high-production specialized jigs.
• Corner Gun & Squeeze Riveter: Corner guns operate in confined spaces; pneumatic squeeze riveters deliver uniform hydraulic/pneumatic compression without impact, producing flawless shop heads where open C-yoke access is available.

Rivet Sets & Bucking Bar Dynamics

• Rivet Sets: Must match the manufactured head. Universal rivets (MS20470) require a cupped set matching the exact head radius; using a flat set flattens and destroys the universal crown. Flush rivets (MS20426) require a polished flat set fitted with a soft rubber retaining sleeve that prevents the tool from skidding across the skin.
• Bucking Bar Selection & Mass: A bucking bar is a block of hardened, mirror-polished alloy steel. Its mass provides the reaction inertia that upsets the rivet shank. As an engineering rule, the bucking bar mass must be approximately 3 to 5 times the mass of the rivet assembly. A bar that is too light bounces uncontrollably; a bar that is too heavy stretches the sheet metal.
• Bucking Bar Alignment: The bar face must be held strictly perpendicular (square) to the rivet shank axis. Tilting the bar produces a tipped or clenched shop head.

Pneumatic Rivet Driving Configuration:

    [PNEUMATIC RIVET GUN] 
              | Forward Pressure
              v
       [ Rivet Set ] (Cupped or Flush)
       +-------------+
       | RIVET HEAD  |
    ===|=============|=== Aircraft Skin Panels
       | RIVET SHANK |
       +-------------+
       |  Shop Head  | <--- Formed against polished face
       +-------------+
              ^ Forward Inertia (Firm Resistance)
              |
       [ BUCKING BAR ] (Hardened Steel, Mass = 3-5x Rivet Mass, Perpendicular!)

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Formed Shop Head Dimensions & Tolerances

A properly formed shop head (driven head) must meet strict geometric dimensions to ensure optimum clamping force and shear capability:

• Formed Diameter (Width): Must equal 1.5 times the rivet shank diameter ($1.5D$).
• Formed Height (Thickness): Must equal 0.5 times the rivet shank diameter ($0.5D$).

Standard Formed Shop Head Dimensions:

                 |<- - - - - Width = 1.5D - - - - ->|
                 .----------------------------------.
                /                                    \   ^
               |                                      |  | Height = 0.5D
    ===========+--------------------------------------+==v==========
                               |      |
                               |< D ->| Original Shank Diameter

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Realistic Maintenance Scenario & Common Exam Traps

Realistic Maintenance Scenario

A certifying technician is repairing an external skin panel on an ATR-72. The skin is 0.032-inch 2024-T3 aluminium, and flush MS20426AD4 rivets are specified. The head height of an MS20426AD4 rivet is 0.042 inches. The technician recognizes that the skin thickness (0.032") is less than the head depth (0.042"); machine countersinking is strictly forbidden as it would create a catastrophic knife edge.

Furthermore, because 2024-T3 is prone to cracking if cold dimpled, the technician selects a portable hot dimpler, sets the die temperature to 280°C, and performs hot coin dimpling. Before driving, the technician adjusts the slow-hitting pneumatic gun to 90 psi and uses a 1.5 kg polished bucking bar held square to the shank. Inspection with a Go/No-Go gauge confirms the shop heads measure exactly 0.188" wide and 0.063" high.

Common Exam Traps

• Trap 1: Chamfering hole edges during deburring. Deburring must strictly remove burrs without beveling or chamfering the hole. Chamfering removes bearing wall area and creates severe stress risers.
• Trap 2: Forgetting the shop head dimension ratio. Exam questions frequently invert the ratios: remember that Width = $1.5D$ (larger) and Height = $0.5D$ (smaller). Never mix these up.
• Trap 3: Machine countersinking thin sheet. If sheet thickness is less than head height, machine countersinking creates a knife edge that fails rapidly under fatigue; the sheet MUST be dimpled.