Joining: mechanical fastening, soldering and brazing
This domain compares permanent and semi-permanent joining methods: bolts, screws, rivets, pins, soldering and brazing. The exam tests joint selection, capillary action, flux use, filler temperatures and rivet allowance.
Core rules
- Bolted joints allow dismantling and need correct bolt grade, washer and tightening torque. Use a washer under a nut on soft material to spread load.
- Rivets form a permanent joint. Drill the hole slightly larger than the rivet, allow a shank length equal to grip plus about 1.5D for forming the head, and space rivets evenly.
- Soft soldering uses filler melting below 450 degrees C, commonly tin-lead or lead-free alloys. Clean the joint, apply flux, heat until the filler flows by capillary action, and do not overheat.
- Brazing uses filler melting above 450 degrees C but below the base-metal melting point, for example copper-zinc or silver-based filler. Borax-type flux is common on steel.
- Flux removes oxide and promotes wetting, but residue must be cleaned because it can cause corrosion.
Worked example
Two 1.5 mm steel sheets are joined by a 4 mm diameter rivet. Grip = 1.5 + 1.5 = 3 mm. Required rivet length = grip + 1.5D = 3 + 1.5 x 4 = 3 + 6 = 9 mm. Reason: the extra 1.5D provides enough metal to form the second head without leaving a loose joint or a long, bent shank. The hole should be about 4.1 mm, and the rivet should fill the hole before the head is formed.
Traps that cost marks
- Leaving flux residue on a soldered joint.
- Overheating solder so it balls and does not flow.
- Using soft solder for a high-temperature or structural joint.
- Making rivet holes too large or spacing rivets too close to an edge.
- Joining dissimilar metals without considering galvanic corrosion.