308A Welding and Soldering for Sheet Metal 2 — Questions and Answers
Question 1: What is 'burn-through' and how is it prevented when welding sheet metal?
- A type of flame cutting
- Melting completely through thin material, prevented by using lower heat settings, faster travel speed, and proper technique (Correct answer)
- A technique for removing paint
- A method of preheating the metal
Correct answer: Melting completely through thin material, prevented by using lower heat settings, faster travel speed, and proper technique
Burn-through occurs when too much heat melts completely through the thin sheet metal, creating a hole. It is prevented by using lower amperage/voltage, faster travel speed, and intermittent (stitch) welding technique.
Question 2: What is the purpose of 'tacking' before welding a sheet metal joint?
- To test the welding machine settings
- To hold the pieces in alignment with small spot welds before completing the full weld (Correct answer)
- To clean the metal surface
- To preheat the joint area
Correct answer: To hold the pieces in alignment with small spot welds before completing the full weld
Tack welds are small spot welds placed at intervals along the joint to hold the pieces in correct alignment and prevent distortion during the full welding process. They become part of the final weld.
Question 3: What type of solder is used for potable water applications on copper sheet metal?
- 50/50 tin-lead solder
- Lead-free solder (95/5 tin-antimony or tin-silver) (Correct answer)
- Acid core solder
- Any available solder
Correct answer: Lead-free solder (95/5 tin-antimony or tin-silver)
Lead-free solder is required for any application involving potable (drinking) water. Common lead-free formulations include 95/5 tin-antimony and tin-silver alloys, as mandated by plumbing codes.
Question 4: What is the proper technique for spot welding sheet metal?
- Hold the trigger continuously while moving along the joint
- Clamp overlapping sheets between copper electrodes and apply pressure with brief electrical current to fuse the layers (Correct answer)
- Heat the metal with a torch and press together
- Use adhesive between the layers and clamp
Correct answer: Clamp overlapping sheets between copper electrodes and apply pressure with brief electrical current to fuse the layers
Resistance spot welding clamps overlapping sheets between copper alloy electrodes, applying pressure and a brief pulse of high electrical current. The resistance at the contact point generates heat that fuses the sheets together.
Question 5: What is 'heat distortion' in sheet metal welding and how is it minimized?
- Discolouration of the metal near the weld
- Warping and buckling caused by uneven thermal expansion, minimized by stitch welding, clamping, and backstep technique (Correct answer)
- The metal becoming harder near the weld
- Smoke generated during welding
Correct answer: Warping and buckling caused by uneven thermal expansion, minimized by stitch welding, clamping, and backstep technique
Heat distortion occurs when localized heating causes uneven expansion and contraction, warping the sheet metal. Techniques to minimize it include stitch (skip) welding, proper clamping, backstep welding, and heat sinks.
Question 6: What is the recommended wire diameter for MIG welding 22-gauge sheet metal?
- 0.045 inch
- 0.023 or 0.030 inch (Correct answer)
- 0.052 inch
- 1/8 inch
Correct answer: 0.023 or 0.030 inch
Thin sheet metal (22 gauge) requires small diameter wire (0.023 or 0.030 inch) to allow lower heat input. Larger wire diameters require more current, increasing the risk of burn-through on thin material.
308A Welding and Soldering for Sheet Metal 2
What is 'burn-through' and how is it prevented when welding sheet metal?