Bending square aluminium tubing

A secondary ring for a lightweight Newtonian truss telescope needs to be rigid and light. Harro Treur, an ATM living in Hoofddorp (close to Amsterdam), made a nice wooden jig to bend square aluminium tubing into a ring. 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. The concave part is sandwiched between two pieces of wood to prevent the bar from sliding out of the jig sideways. Difference between the radii of the two parts is the diameter of the tubing to be bend. 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. With a vice, the jigs are pressed together, the aluminium in between. Below are some photographs of a jig made by Harro.

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. The smallest radius into which a 15 mm diameter, 1.5 mm wall thickness alumium tube can be bent, is about 140-150 mm.

I used a jig with radius 145 (convex part) and 160 mm (concave part) for secondary rings for two 300 mm scopes. They came out at 327 mm inside diameter, which is 12.7% larger then the diameter of the convex part.

The jig and the aluminium tube in the vice, in starting position. The aluminium tube should be moved about 30 mm further into the jig after each press.

The ring is almost ready and comes out nicely.

Here's a secondary ring Harro has made for his lightweight 300 mm telescope. The wire spider is made of guitar string (B-string, about 0.15 mm diameter). The strings are tightened with banjo string tighteners. The focuser is made from an old 135 mm telelens. (Click here to see a larger image).

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