Fig 10 shows a portable workbench designed for use on building sites. (a) The frame corners are joined using MIG welding. (i) Describe the MIG welding proce...

(a)(i) Description of MIG welding for joining the steel tubes at each corner: [4]

1. MIG (Metal Inert Gas) welding uses a continuously fed consumable wire electrode from a spool. The wire simultaneously serves as the filler material, so no separate filler rod is needed. [1]
2. An electric arc forms between the tip of the wire and the mild steel workpiece, generating intense heat that melts both the wire and the parent metal to create a molten weld pool where the tube surfaces fuse. [1]
3. An inert shielding gas (argon or an argon/CO₂ mixture) is fed through the torch nozzle. This gas envelope surrounds the arc and weld pool, preventing atmospheric oxygen and nitrogen from contaminating the molten metal, which would otherwise cause porosity and weak joints. [1]
4. The welder guides the torch steadily along the joint line at a controlled speed. As the torch advances, the weld pool solidifies behind it, leaving a continuous bead that permanently fuses the two tubes at the corner. [1]

(a)(ii) Two advantages of MIG welding over brazing for these steel tubes: [2]

1. MIG welding is faster with a higher deposition rate than brazing, making it more efficient for joining multiple corners on a production workbench. [1]
2. MIG welding produces a stronger joint because the parent metals are fused together into one continuous piece, whereas brazing relies on a filler metal with a lower melting point that bonds by capillary action without melting the parent steel. [1]

(b)(i) The clamping system uses a first-class lever, where the fulcrum (pivot) is positioned between the effort (applied by the user's hand) and the load (the clamping force on the workpiece).

The sketch shows: the fulcrum/pivot positioned between the effort and the load [1]; effort applied by the user's hand at one end of the lever arm [1]; the clamping load acting on the jaw and workpiece at the other end [1]. Because the effort arm (fulcrum to hand) is longer than the load arm (fulcrum to jaw), the force applied to the workpiece is multiplied. This is the principle of mechanical advantage: a small effort over a large distance produces a large force over a small distance. [1]

(b)(ii) Increasing the length of the lever arm increases the distance from the point where the user applies effort to the fulcrum. [1] Since mechanical advantage equals the effort arm length divided by the load arm length (\( MA = \frac{\text{effort arm}}{\text{load arm}} \)), a longer effort arm means a greater mechanical advantage. The user can therefore apply a larger clamping force to the workpiece for the same physical effort, making clamping easier. [1]

(c)(i) A suitable permanent mechanical joint is a bolt (or clevis pin) acting as a pivot. A bolt passes through aligned holes in a bracket welded to the frame and a lug attached to the leg. [1] This allows the leg to rotate freely about the bolt axis while keeping the leg permanently attached to the frame. The bolt is secured with a nut or split pin to prevent it from working loose. [1]

(c)(ii) A locking device to hold each leg rigidly in the open position:

The sketch shows: a toggle clamp / over-centre latch mechanism that engages when the leg reaches its fully open position [1]; a stop plate or shoulder on the frame that prevents the leg from rotating past 90 degrees [1]; both the locked position (latch engaged, leg rigid) and the released position (latch lifted, leg free to fold) are indicated [1].

(d)(i) Two methods of protecting the steel frame from corrosion: [2]

1. Galvanising (hot-dip zinc coating) [1]
2. Powder coating (or paint / spray finish) [1]

(d)(ii) Galvanising is particularly suitable for a workbench used on building sites because the zinc layer provides sacrificial protection. [1] Even if the surface is scratched, chipped, or dented during rough handling on site, the zinc corrodes preferentially to the steel underneath (because zinc is more reactive), continuing to protect the exposed steel from rusting. This self-healing protection is critical for equipment subjected to heavy wear. [1]

(e) Advantages and disadvantages of aluminium alloy compared to steel: [4]