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9/8/98 AC 43.13-1B curing process. Each type of adhesive requires a specific temperature during the cure cycle, and the manufacturer’s recommendations should be followed. 1-10. CLAMPING PRESSURE. a. Use the recommended pressure to squeeze adhesive out into a thin, continuous film between the wood layers. This forces air from the joint and brings the wood surfaces into intimate contact. Pressure should be ap- plied to the joint before the adhesive becomes too thick to flow and is accomplished by means of clamps, presses, or other mechanical devices. b. Nonuniform clamping pressure commonly results in weak and strong areas in the same joint. The amount of pressure re- quired to produce strong joints in aircraft as- sembly operations varies with the type of ad- hesive used and the type of wood to be bonded. Typical pressures when using resor- cinol may vary from 125 to 150 pounds per square inch for softwoods and 150 to 200 pounds per square inch for hardwoods. Insufficient pressure or poorly machined wood surfaces usually result in thick bond lines, which indicate a weak joint, and should be carefully guarded against. Some epoxy adhe- sives require much less clamping pressure to produce acceptable joint strength. Be sure to read and follow the manufacturer’s instruc- tions in all cases. 1-11. METHOD OF APPLYING PRES- SURE. The methods of applying pressure to joints in aircraft bonding operations range from the use of brads, nails, small screws, and clamps; to the use of hydraulic and electrical power presses. The selection of appropriate clamping means is important to achieving sound bond joints. a. Hand nailing is used rather extensively in the bonding of ribs and in the application of plywood skins to the wing, control surfaces, and fuselage frames. Small brass screws may also be used advantageously when the par- ticular parts to be bonded are relatively small and do not allow application of pressure by means of clamps. Both nails and screws pro- duce adverse after effects. There is consider- able risk of splitting small parts when install- ing nails or screws. Metal fasteners also pro- vide vulnerable points for moisture to enter during service. b. On small joints using thin plywood for gussets or where plywood is used as an outer skin, the pressure is usually applied by nailing or stapling. Thin plywood nailing strips are often used to spread the nailing pressure over a larger area and to facilitate removal of the nails after the adhesive has cured. c. The size of the nails must vary with the size of the members. If multiple rows of nails are required, the nails should be 1 inch apart in rows spaced 1/2 inch apart. The nails in adjacent rows should be staggered. In no case should the nails in adjacent rows be more than 3/4 inch from the nearest nail. The length of the nails should be such that they penetrate the wood below the joint at least 3/8 inch. In the case of small members, the end of the nail should not protrude through the member below the joint. Hit the nails with several light strokes, just seating the head into the surface of the gusset. Be careful not to crush the wood with a heavy hammer blow. d. In some cases the nails are removed after adhesive cure, while in others the nails are left in place. The nails are employed for clamping pressure during adhesive cure and must not be expected to hold members to- gether in service. In deciding whether to re Par 1-9 Page 1-7PDF Image | AFS-640
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