Forming, Bending and Surface Finishing
What is examined
This domain covers changing the shape of metal without cracking it, and producing a specified finish. Expect questions on rolling, wire drawing, bending, doming, raising, annealing between operations, abrasive sequences, polishing mops and decorative finishes.
Core rules
- Rolling: reduce thickness by roughly 10–20% per pass, keep the rollers clean, and anneal when the metal resists or starts to crack. Roll in one direction and maintain a square edge.
- Wire drawing: use a draw bench and draw plate with lubricant; reduce the diameter gradually and anneal between passes. Skipping annealing causes breaks and surface cracks.
- Bending: anneal first if the metal is work-hardened; use round-nose or flat pliers with jaw protection; bend over a mandrel for controlled curves; allow for springback. Avoid sharp internal corners that concentrate stress.
- Doming and raising: use a dapping block and punches or a sandbag and hammer. Work progressively, anneal often, and avoid creases. Raising thins the walls, so check thickness as you go.
- Abrasive sequence: file, then emery or abrasive paper in steps such as 240, 400, 600, 800 and 1200. Remove the previous grit's scratches before moving on.
- Polishing: tripoli on a stitched mop for cutting, then rouge on a soft mop for colour. Keep mops separate to avoid cross-contamination. Clean ultrasonically or with detergent and rinse before drying.
- Finishes include high polish, satin, matte, brushed, hammered, sandblasted and patinated. Rhodium plating is a finish, not a repair for poor polishing.
Worked example
Finish a sterling silver ring after soldering. File the joint and edges, then use emery 400, 600 and 1200 in sequence, rotating the ring so scratches run in one direction and are removed by the next grit. Charge a stitched mop with tripoli and polish, cleaning the ring before moving to a soft mop with rouge. If a matte finish is specified, stop at 600 grit or use a abrasive blast, and do not rouge-polish the surface afterwards. The reasoning is that each stage must remove the previous stage's marks; polishing over a scratch only makes it shiny.
Traps
Rolling too heavily without annealing and splitting the sheet; drawing wire without lubricant; bending work-hardened metal and cracking it; skipping abrasive grits and polishing in scratches; using a rouge mop contaminated with tripoli; over-polishing and rounding crisp edges; overheating thin sheet while polishing; plating over a poorly prepared surface.