Ordering Electrodes for Twenty Frames with an 8 mm Fillet
A workshop has an order for 20 identical mild steel frames. Each frame has four fillet welds, each 750 mm long, specified on the drawing as an 8 mm leg length. The WPS specifies E7018 electrodes, 4 mm diameter, DCEP, and a deposition efficiency of 65% (this figure already includes stub, spatter and slag losses). Steel density is 7.85 g/cm³. The store has 15 kg of E7018 in stock and the supplier's minimum order is 5 kg boxes. The drawing and the WPS agree on the 8 mm size, but the production planner has written '6 mm fillet - save electrodes' on the job card. The welder asks you to calculate the electrode requirement and to confirm which size to use.
1. Calculate the deposited weld metal mass and the electrode mass required for the 20 frames at the specified 8 mm fillet size. Show each step and add 10% contingency.
Read the worked answer
Fillet cross-section area = ½ × leg × leg = ½ × 8 × 8 = 32 mm². Total weld length = 20 frames × 4 welds × 750 mm = 60,000 mm. Volume = 32 mm² × 60,000 mm = 1,920,000 mm³ = 1,920 cm³. Mass deposited = 1,920 cm³ × 7.85 g/cm³ = 15,072 g = 15.07 kg. Electrode mass = 15.07 kg ÷ 0.65 = 23.18 kg. Add 10% contingency = 23.18 × 1.10 = 25.5 kg. So about 25.5 kg of E7018 is required.
2. Calculate the electrode mass if the planner's 6 mm fillet were used instead, and state the difference.
Read the worked answer
For a 6 mm leg: area = ½ × 6 × 6 = 18 mm². Volume = 18 mm² × 60,000 mm = 1,080,000 mm³ = 1,080 cm³. Mass deposited = 1,080 × 7.85 = 8,478 g = 8.48 kg. Electrode mass = 8.48 kg ÷ 0.65 = 13.05 kg. Add 10% = 14.3 kg. Difference at 8 mm = 25.5 − 14.3 = 11.2 kg of electrodes. The 6 mm fillet also reduces the throat from 5.66 mm to 4.24 mm, about 25% less, so for the same length the strength falls by roughly 44%. That is a design change, not a saving.
3. Recommend what should be ordered and what should be done about the job card note, considering stock, minimum order and conformity.
Read the worked answer
Order the 8 mm size because the drawing and WPS agree on it. Stock is 15 kg, which is less than the required 25.5 kg, so at least 10.5 kg more is needed. With 5 kg boxes, order 15 kg (three boxes), giving 30 kg total and a sensible margin. The 6 mm note on the job card is a deviation from the drawing and WPS; it must be removed or raised as a concession approved by the responsible engineer. Do not let a planner's note override the approved weld size. E7018 is low-hydrogen and must be kept dry; check the manufacturer's baking and holding requirements and use electrodes in good condition. Record the calculation, the order and the WPS reference on the job card for traceability.