Cracked cable brackets and bend allowance
A workshop must produce 25 identical cable brackets from 1.5 mm mild steel sheet. Each bracket is a 90° angle with outside legs of 80 mm and 50 mm, an inside bend radius of 2 mm, and a K-factor of 0.33. The first five were marked with a scriber along the bend line, cut on a guillotine, and bent in a fly press with the burr on the outside of the bend. Three cracked along the bend line. The press tool is set to an inside radius of 2 mm. The customer requires the brackets to fit a 50 mm wide cable tray, and the fixing holes must remain 20 mm from the bend. The guillotine has a 0.2 mm blade clearance and a blunt section near one end.
1. Calculate the bend allowance, bend deduction and flat length for one bracket. Show the formula and your working.
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For a 90° bend, bend allowance BA = (π/180) × angle × (R + K × T). Here angle = 90°, R = 2 mm, T = 1.5 mm and K = 0.33. The neutral-axis radius is R + K × T = 2 + 0.33 × 1.5 = 2.495 mm. BA = 1.5708 × 2.495 = 3.92 mm, to two decimal places. The bend deduction is BD = 2 × (R + T) − BA = 2 × 3.5 − 3.92 = 7.00 − 3.92 = 3.08 mm. Using outside leg dimensions, the flat length is 80 + 50 − 3.08 = 126.92 mm. The blank must therefore be cut to 126.92 mm, not 130 mm, if the outside legs are to finish at 80 mm and 50 mm. The bend line is set from the flat blank using the calculated allowance, not by scribing at the outside dimension.
2. Explain why the brackets cracked and describe the changes you would make to marking, cutting and bending.
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The crack is a stress raiser at the outside of the bend. A scriber cuts a sharp notch along the bend line, and the burr from the guillotine was placed on the outside of the bend, where the material is in tension. That combination can start a crack even when the radius is nominally acceptable. The 2 mm inside radius is only about 1.33 times the thickness, which is tight for 1.5 mm mild steel, and the blunt guillotine section may leave a ragged edge. I would mark with a fine felt-tip or a line set clear of the bend zone, deburr both edges, and place the burr on the inside of the bend if it cannot be removed. I would increase the inside radius to about 2.5–3 mm if the design allows, align the press tool carefully, and bend across the rolling direction where possible. A trial piece should be bent before the batch.
3. Describe the first-off inspection and the batch checks you would use to make sure the brackets fit.
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Bend one first-off bracket and check the outside legs against 80 mm and 50 mm, the angle with a try square or angle gauge, the inside radius with a radius gauge, and the hole position at 20 mm from the bend. Check both faces for cracks, orange-peeling or thinning at the bend. Fit the bracket to a 50 mm cable tray sample or a gauge representing the tray. If the legs are long or short, adjust the backstop by the difference; if the angle is wrong, correct the press stroke or springback allowance. Record the first-off results before running the batch. During production, check the first and last bracket and every fifth one for leg length, angle, cracks and hole position, because the blunt guillotine section can cause variation. Keep the calculated flat length and bend deduction with the job card so the next operator does not re-mark by the outside dimensions.