Blank Size and Leak-Proof Corners for a 200 × 150 × 40 mm GI Tray
A sheet metal shop must make 10 open rectangular trays, inside size 200 mm × 150 mm × 40 mm, from 0.8 mm galvanised iron sheet. The corners are to be notched and folded, and the vertical corner joints must be leak-proof because the trays will hold a water-based cleaning solution. A trainee cuts the first blank 280 mm × 230 mm, notches 40 mm × 40 mm squares at each corner, and folds all four sides over the same edge of a folding bar. The finished tray measures about 196 mm × 146 mm inside and the corner joints show gaps of 1–2 mm; when filled, it leaks. The bend radius on the shop's bar is about 1 mm. The trainee proposes to close the gaps by heavy hammering and to paint over the seams.
1. Calculate the correct flat blank size and corner notch, using a bend allowance of about 2.2 mm per 90° bend. Explain why the trainee's 280 mm × 230 mm blank produced an undersized tray.
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For a 90° bend, bend allowance BA = (π/2)(R + t/2) = 1.571 × (1 + 0.4) = 2.2 mm per bend.
Across the length there are two bends (one at each side wall), so blank length = 200 + 2(40) + 2(2.2) = 284.4 mm. Across the width, blank width = 150 + 2(40) + 2(2.2) = 234.4 mm. The corner notches are 40 mm × 40 mm, set out from the bend lines, with a small radius allowance at the inner corner.
The trainee used 200 + 2(40) = 280 mm and 150 + 2(40) = 230 mm, which ignores bend allowance. Each side loses about 2.2 mm, a total of 4.4 mm per direction, so the tray finishes about 4 mm undersize - matching the measured 196 mm × 146 mm. The 1–2 mm corner gaps are a separate joint problem, not a blank-size error.
2. Recommend a corner joint and seam method that will hold water, and criticise the proposal to hammer the gaps shut and paint over them.
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Hammering closes the gap only by stretching and work-hardening the GI sheet; it can crack the zinc coating and leaves a flexible, mechanically open joint. Paint will not bridge a moving seam and will fail on a wet tray.
Use a soldered corner seam: clean the mating surfaces, apply non-corrosive flux, tin both faces, clamp, and solder the corner. Alternatively, use a riveted lap corner with a sealant, or spot-weld and seal. For a side seam, a double grooved seam is stronger and more leak-proof than a simple lap. After forming, water-test one tray before making the batch of 10. The corner is a joint-design problem; it cannot be solved by cosmetic painting.
3. Describe the folding sequence and spring-back control you would use for the 10 trays.
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Mark all bend lines from the base and cut the four corner notches first. Fold the two long sides first, then the two short sides, so that the short sides overlap the long-side corners in a consistent order. Use a folding bar and a mallet, not a hammer, and keep the same edge of the bar for every bend so the radius stays at about 1 mm.
Spring-back in 0.8 mm GI is small but present; overbend by about 2–3° and check the first tray with a try square and by measuring the inside 200 mm × 150 mm. Make one sample, solder the corners, water-test it, and only then repeat the sequence for the remaining nine. Keep a wooden template for the corner angle and check each tray before soldering.