When laying out a sheet metal bend, what is the bend allowance?
ACS code: AM.II.A
Correct answer: The length of material in the curved section of the bend
Rationale: Bend allowance is the length of material actually consumed in the curved portion of a bend. It must be added to the flat dimensions when computing the total developed (flat-pattern) length of a part, because the metal in the radius is neither a straight flange nor part of the setback. Setback is a separate value (the distance from the bend tangent line to the mold-point intersection), and the bend radius is the inside curvature, so those describe different quantities.
The neutral axis (neutral line) of a piece of sheet metal that is being bent is the location where the metal
ACS code: AM.II.A
Correct answer: is neither compressed nor stretched
Rationale: During bending, metal on the inside of the radius is compressed and metal on the outside is stretched. Between these zones lies the neutral axis, where the material is neither compressed nor stretched and its length does not change. Bend-allowance formulas are based on the length of this neutral line, which for practical purposes lies near 0.445 of the material thickness from the inside surface.
What is the primary purpose of an aircraft sheet metal stop drill at the end of a stress crack?
ACS code: AM.II.A
Correct answer: To prevent the crack from spreading farther
Rationale: A crack acts as a stress concentrator with a very sharp tip. Drilling a small hole precisely at the end (tip) of the crack blunts that sharp point and spreads the load over the larger radius of the hole, which arrests the crack's progress. It is a temporary measure to stop further growth until a proper repair is made; it does not relieve panel-wide stress or serve as a rivet pilot.
The diameter of a rivet shank should generally be about
ACS code: AM.II.A
Correct answer: three times the thickness of the thickest sheet being joined
Rationale: A common rule for selecting rivet diameter is to use a rivet whose shank diameter is approximately three times the thickness of the thickest sheet in the joint. A rivet that is too small will not carry the shear load; one that is too large can cause excessive bearing stress and crack the sheet. This three-times rule produces a rivet sized to develop adequate strength in the joint.
After a solid rivet is properly driven, the formed (bucked) head should have a height of approximately
ACS code: AM.II.A
Correct answer: one-half the shank diameter
Rationale: Standards for an acceptable driven rivet specify that the formed head should be about one-half the shank diameter in height and about one and one-half times the shank diameter across its width (diameter). These proportions ensure the rivet has been upset enough to fill the hole and clamp the sheets without being over- or under-driven. A formed head that is too thin or too small indicates insufficient driving.
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