The ring is almost ready and comes out nicely. The aluminium tube should be moved about 30 mm further into the jig after each press. The jig and the aluminium tube in the vice, in starting position. They came out at 327 mm inside diameter, which is 12.7% larger then the diameter of the convex part. I used a jig with radius 145 (convex part) and 160 mm (concave part) for secondary rings for two 300 mm scopes. The smallest radius into which a 15 mm diameter, 1.5 mm wall thickness alumium tube can be bent, is about 140-150 mm. Here's a picture of a jig made for bending a 15 mm diameter, 1.5 mm wall thickness aluminium tube into a ring with inside radius 225 and outside radius 240 millimeter. Below are some photographs of a jig made by Harro. With a vice, the jigs are pressed together, the aluminium in between.
The diameter of the resulting ring will be 11-15 percent larger then the diameter as defined by the jig, which needs to be accounted for while making the jig. Difference between the radii of the two parts is the diameter of the tubing to be bend. The concave part is sandwiched between two pieces of wood to prevent the bar from sliding out of the jig sideways.
Essentially, the ring consists of two parts: a convex part defining the inside diameter of the ring and a concave part defining the outer diameter. Harro Treur, an ATM living in Hoofddorp (close to Amsterdam), made a nice wooden jig to bend square aluminium tubing into a ring. Jan van Gastel: bending aluminium Bending square aluminium tubingĪ secondary ring for a lightweight Newtonian truss telescope needs to be rigid and light.
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