Copper Bend Beside a Chemical Store
You are fabricating a 22 mm copper cold-water branch in a school science laboratory. The tube must leave a wall 500 mm above the bench, run horizontally 300 mm to a corner, then turn 90° upward to a tap. Both legs are measured to the intersection of the centrelines. The available lever bender has a 90° former with a centre-line radius of 66 mm, which is 3 times the tube diameter. The wall is plasterboard on timber studs, and a stud sits 80 mm from the corner. The laboratory technician refuses any flame near stored chemicals. The copper is supplied annealed in 3 m lengths. You will cut and bend on site, then connect to a threaded wall plate elbow at the wall and a tap connector at the top. You must avoid wasting the 3 m length and produce a bend that passes a pressure test.
1. Calculate the cut length and the two bend marks. Show the formula and state your assumptions.
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For a 90° bend measured to the intersection of centrelines, the two straight legs lose one radius each and gain the arc length:
Developed length = L1 + L2 − 2R + (πR/2)
With L1 = 500 mm, L2 = 300 mm and R = 66 mm:
Developed length = 500 + 300 − 132 + (3.142 × 66 ÷ 2) = 800 − 132 + 103.7 = 771.7 mm, say 772 mm.
The tangent marks are set back one radius from the intersection on each leg:
- from the wall end: 500 − 66 = 434 mm
- from the tap end: 300 − 66 = 234 mm
Check: 434 + 234 + 103.7 = 771.7 mm. These marks assume the stated centre-line radius, no springback allowance and no extra for fitting insertion. I would trial-bend an offcut first and confirm the actual radius before cutting the final tube.
2. Select a jointing method for the wall plate elbow and tap connector, given the no-flame instruction, and describe the pressure test.
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Use compression fittings or a proprietary press-fit system. Compression is simple and does not need a flame; press-fit needs the correct calibrated tool and manufacturer's jaws. Whichever is used, the tube end must be cut square, deburred and inserted fully. PTFE tape or approved thread sealant goes on the male thread, not across the compression olive. The fitting must be held with a spanner while tightening so the pipe or wall plate is not twisted.
For the test, cap the open ends, fill with water and remove air, then pressurise to 1.5 times the maximum working pressure - for example, 6 bar if the working pressure is 4 bar - and hold for at least one hour, or as specified by the engineer. Check the gauge, all joints and the bend for weepage or distortion. A drop in pressure means a leak or trapped air; find it before boxing in or handing over.
3. The first bend shows slight flattening and does not spring back exactly to 90°. What are the likely causes and what would you do?
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Flattening usually comes from using the wrong size former, a bender with worn or excessive clearance, bending too fast, or failing to support the tube. Annealed copper should bend smoothly around the correct former; if the tube is work-hardened, it may crack or flatten. Springback is normal: the metal relaxes slightly after the load is released, so the bend must be taken a little past 90° or checked against a square while still in the bender.
I would not force the distorted bend straight. I would cut it out, anneal if appropriate, use the correct 22 mm former, keep the tube supported, bend slowly and check with a set square. If several trial bends fail, the bender or former is suspect and should be replaced before wasting more tube.