Aircraft Riveted Joints: Lap, Flush, and Joggle

• Three general types of riveted joints used in aircraft manufacture and repair: lap joints, flush joints, and joggle joints.

Lap Joints

• Skins are riveted together, creating a step or level change between them.
• Skins aren't on the same plane; they are stacked.
• This joint type is inappropriate for structures requiring aerodynamic smoothness due to airflow disruption during flight.
• Generally used on light aircraft where aerodynamic smoothness is not a concern.
• Lap patch serves as an example.

Flush Joints

• Used when aerodynamic cleanliness between two skins is required.
• Involves riveting skins to the same substructure - longerons, bulkheads, formers, etc - placing them on the same plane.
• A doubler (a separate metal piece) may be used as the substructure.
• Flush patches are an example.
• Aerodynamic smoothness is achieved by riveting a doubler under the skin and insert, tying the skin and an insert of the same thickness together.

Joggle Joints

• Combines flush and lap joint characteristics; one skin is joggled to accommodate the other, resulting in a smooth side.
• Also used when fitting doublers over internal structure, forming a joggle to allow both pieces of metal to be on the same plane instead of pulling the doubler against the skin with rivets.

Factors Governing Joint Strength

• Plate specification to withstand tensile and bearing loads.
• Rivet specification for shear loads. Lacking specification, use a rivet of the plate's material with a diameter 2.5 times the plate's thickness.
• Rivet spacing is determined to give optimum strength characteristics.

Rivet Spacing and Placement

• A rivet's sphere of influence, is about 5 times the fastener's diameter (5D), maintains close plate proximity.
• Overlapping spheres of influence when rivet pitch is 4D provides a fluid-tight joint.
• Rivet positions relative to each other and the sheet material edge must be considered.
• Excessive holes due to close rivets weaken the joint.
• Load cannot be adequately distributed if rivets are too far apart, which leads to leakage in pressurized areas.
• The edge of the sheet should not be exposed to nearby rivets, which could cause tearing when the joint is under load.
• Practical reasons modify these conditions to determine rivet spacing and pattern.
• Pitch indicates a minimum of 3D of distance between rivets measured from center in a row.
• Land refers to a minimum of 2D of distance between the rivet center and the edge of the material.
• Spacing indicates 3-4D as the minimum distance between adjacent rivet rows.
• D = rivet shank diameter.
• Pitch circle diameter determines the diameter of circular repairs according to the pitch of rivets.

Single Row Riveting

• A single-row layout, used primarily on attachment and lightly stressed joints.

Multiple Row Riveting

• Used in watertight joints and high-stress areas that require thick gauge plate.

Staggered Riveting

• Used in watertight joints, circular patches, etc, as an alternative to multiple rows.
• Staggered riveting will equal 75% of the rivet pitch.

Rivet Installation Considerations

• Shaving of the protective oxide coating may lead to buckling when the rivet is driven if the hole is too small.
• Deburring holes after drilling with specialized deburring tools eliminates sharp edges.
• An alternative to deburring tools is to use a larger diameter drill bit.
• Solid rivets can be closed manually with appropriate snaps, sets, and dollies, usually for bench work, whereas pneumatic riveting guns are commonly used for reaction riveting on aircraft structures.

Tools

• Set (Draw Tool): A hollow punch used to draw metal sheets together, bringing the rivet head against the metal surface.
• Snap: A punch with a recess shaped like the rivet's preformed head.
• Dolly: A metal block with a recess shaped like the preformed rivet head, used to support the head; should fit squarely into the dolly.
• Riveting tools come in many faces, sizes, and shapes to accommodate the various rivet sizes and head forms.

Riveting Process

• The work must be assembled, sealant applied, and mating surfaces brought tightly together using skin grips or clips once drilling, countersinking, dimpling, deburring, cleaning, and adjusting have been completed.
• Prevent damage to the skin surface.
• Gaps between layers prevents proper rivet formation and reduces shear strength so ensure there are none.
• Riveting may stretch thin sheets slightly; don't allow this to accumulate.
• Correct closing rivet sequence can only be obtained by experience, and the order of riveting may vary.
• C-clamps or Cleco edge clamps are used to hold skins together for drilling; these are tools that clamp and hold sheets together.
• Rivet cutters can shorten longer rivets.
• Adding 1.5D to the material thickness will get the correct rivet length.
• After driving, the tail/shop head should measure 0.5D high and 1.5D wide.
• Holes must be countersunk to accommodate flush rivets.

Countersinking

• A microstop countersinking tool accurately countersinks material.
• Enlarging the hole and shearing the metal across the head instead of the shank occurs with over-countersinking.
• A countersinker may enlarge the top skin hole, which weakens the riveted joint when using thin material.
• Hand riveting is convenient if the job is small or can be done away from the aircraft.
• The preformed head should be supported in a dolly, and a hollow set or draw tool used to ensure the sheets are drawn together when closing solid rivets with a hand hammer.
• A flat snap then drives the tail to form a reaction head with a thickness of 0.5D.
• A flat snap partially forms the tail, then a suitably shaped snap finishes it for a required round or snap head.
• Rivets should never be hit directly with the hammer.
• The rivet head sits in the bucking bar recess, and a draw tool and hammer draw the rivet through the material.
• A hand set or punch then forms the shop head.
• Acceptable standards are the same for hand-driven rivets as for other methods.
• Pneumatic rivet hammers or guns drive aircraft solid rivets.
• Compressed air drives a piston back and forth, applying a hammering action to the rivet snap.
• The air pressure required to operate the hammers is around 80 lb/in².
• Rivet snaps are held in the gun with a spiral retaining spring made of heavy gauge steel wire - this allows the snap to have the full travel required without being driven out of the gun.
• Be aware of the dangers of compressed air and pneumatic tools
• Carelessly using a rivet gun can result in snaps being driven from the gun with such force that it inflicts serious injury or damages equipment.
• Retaining springs have been known to fail.

CAUTION

• Do not play with rivet guns.
• The work hardening of a rivet must be minimized without overdriving it, which leads to an unacceptable tail.
• Several types of rivet guns are available due to rivet inaccessibility
• A slow, hard-hitting, long-stroke gun is needed for larger, more accessible rivets. Some of these guns deliver only one blow each time you pull the trigger.
• A light, fast-hitting rivet gun is often most suitable for working with small rivets in thin skins.
• Installing large quantities of rivets along an easily accessible edge uses the squeeze method, which produces uniformly sized shop heads, reduces installation time and can be done with C-yoke and alligator yoke guns.
• The driving face of the bar is machined smooth and polished so that no marks are left on the rivet tail.
• A bucking bar's bucking or vibrating action forms the shop head when the rivet gun is activated - comes in many different sizes and shapes for different applications.
• The gun needs adjustment for correct pressure/hammer prior to installing rivets.
• One achieves this by actioning the gun against a piece of wood.
• Adjust the airflow to the gun so it will indent the wood but not shatter it.
• Place a rivet in the hole and hold the gun against its head with enough pressure to keep the rivet against the skin.
• Hold the bucking bar flat and square against the rivet's tail; then, pull the trigger to form the rivet.
• Minimize the number of blows to prevent work hardening, which may result in cracking.
• Select the correct snap size for the rivet.
• Using one that is too small will mark the rivet head, and one that is too big will mark the skin.
• Check closed rivets for tightness and proper formation, which should be tight and fully formed with no deformation, cracks, and the surrounding area must have no distortion or damage.
• Replace rivets that are not performing their function well, but replacing rivets that are only slightly below standard might do more harm than leaving them in position, particularly in thin materials.
• Consider the strength requirements of the joint and the effectiveness of the rivets in question before rejecting them.
• A flushness tolerance is normally checked before riveting commences if it is specified for countersunk rivets; but milling solid rivet heads may be permitted to obtain a uniform protrusion.
• Protective treatments must be re-applied after milling, which should be uniform in height with adjacent rivets, plus include a shop head concentric with a finish of illustrated size and a manufactured head flat against the metal.

Faults in Riveting

• Cracks in the rivet head.
• Several faults can occur if snap heads are formed on the tail of the rivet, including flash around the rivet head if the shank was too long and the head was small, possibly with snap marks on the skin if the shank was too short.
• Other faults:
  • Countersinking too deep – incorrectly set countersink tool.
  • Holes out of line – skins incorrectly drilled.
  • High shop head – rivet too long or rivet not driven enough.
  • Clinched shop head – bucking bar held at angle.
  • Flattened shop head – rivet too short or hit too hard.
  • Uneven shop head – bucking bar held on angle.
  • Skin bulged - holes not lined up or rivet hammered too hard for thin skin.
  • Swelling between sheets – parts not properly held together, or chips between sheets.
  • Open head – rivet snap not held straight.
  • Burr under head – not deburred correctly.
  • Cut shop head – dolly did not cover entire end of rivet.
  • Cracked shop head – hard rivet or hit too long.
  • Skin marked by rivet set – set head is too big for rivet.
  • Skin marked by rivet set – set not centered on head of rivet.
• A flush shop head may be required to install a rivet due to interference from adjoining parts or airflow disruption.
• The NACA (National Advisory Committee for Aeronautics) flush riveting technique, where the skin is countersunk and the rivet is installed from inside the structure.
• A micro-shaver mills excess material following rivet driving, after which it fills the countersink.
• Use feeler gauges to check for gaps under the rivet head after installation and replace the rivet if necessary.
• Similar procedures are needed if there's a gap under an old rivet's head, where the rivet must be replaced, as it may be cracked or separated.

Team Riveting

• Hearing protection is essential. Communication is essential. The person on the bucking bar is in control.
• The person on the bucking bar informs the gun operator when to rivet, if the result is good or bad, and whether the rivet needs more hammering.
• The gun operator places the rivet straight and firmly into the hole, and then acts on the dolly operator's instructions.
• The riveting team may devise a way of communicating non-verbally, such as:
  • One tap with dolly – rivet
  • Two taps with dolly – good rivet
  • Three taps with dolly – bad rivet

Dimpling

• Coin dimpling
• Radius dimpling
• Hot dimpling

Coin Dimpling

• Pressure is applied with a male die through a hole, which forces a coining ram in the female die on the underside, contouring the skin to the coin’s shape.
• This offers the following advantages:
  • It gives the hole sharply defined edges
  • Both the top and bottom of the dimple are formed at 100°, so multiple sheets can be stacked and dimpled.
  • It provides a tighter fit but leaves a sharper bend around the rivet head, making the skin prone to cracking.

Radius Dimpling

• A cone-shaped male die is forced into a female die recess with either a hammer or a rivet gun.
• A flush rivet may be used as a die. In this case, a sharp blow hits the male die, forcing the metal into the female die and forming a dimple.
• A dimple formed in this way does not have parallel sides; its lower side angle is greater than 100°, making it unsuitable for stacking.
• It is chosen because the equipment is smaller than for coin dimpling.

Hot Dimpling

• Magnesium and harder aluminum alloys need to be heated before dimpling; otherwise, they are brittle and will crack.
• The equipment needed is similar to that for coin dimpling, except that electric current heats the dies, then the metal is positioned.
• Dies should only touch the metal, allowing sufficient dwell time for heat to soften the metal before dimpling.
• Adjustability and automated handling of dwell time is available to avoid destroying the metal's temper
• Rivet removal is a five-step process, as follows:

1. Center punch the manufactured head.
2. Drill through the head.
3. Knock the head off with a cape chisel.
4. Break the head off using a center punch and a tipping motion.
5. Back-up the sheet and punch the rivet out with a pin punch.

• The collar of a Hi-Lok rivet is installed onto a pin with a spanner/socket while an Allen key prevents the pin from moving.
• The wrenching part of the collar snaps off when the rivet reaches the torque.
• Removal procedures:
  • Remove the collar with lock pliers and tap the pin out.
  • Split the collar with a small chisel and tap the pin out.
  • Drill off the pin head and tap the pin out.