Actually this should probably be before the previous stage, but then I’ll post all about the machining later, which should have been stage 1….

A few people have asked how I formed the parabola for the ribs. Very good question I say. “With some difficulty and skill” would be my preferred reply. Although there is some truth in this it perhaps isn’t as difficult as you might think.

First a word about aluminium. I’m no metallurgist and indeed only claim to be a hobby mechanical engineer. All the alloy I have seems to be 6063T6, however the old stock that is around 12 months old is far harder to bend. I initially thought this was due to them being different grades, but according to the invoices they are identical. This is a point of interest that I would like to investigate. There is a thing called “age hardening” but I am finding it difficult to determine if this is a thing that grows over long periods of time at ambient. To confuse matters the batches I have are from different suppliers.

The newer alloy is far easier to form into the exact shape required. It bends really easily and with less warping. I now worry that the softer ribs might deform more easily during use. As a result I have spread -bet and made 6 ribs from each material so I can alternate around the dish.

Bending the square tubing isn’t too difficult although a certain amount of “feel” is required. The tougher batch required a significant “over” bend as it was quite springy. I taped wooden blocks into the vice to prevent the aluminium getting marked by the clamping action.

Acquiring the correct shape was achieved by starting at one end, with about 100mm in the vice and bending that amount first. There will then be an almost undetectable bend 100mm in from the end and hopefully the fist 200mm will be close to correct. Offer to the template and look at where the tube starts to deviate from the parabola, this is where the next bend should occur but will probably b another 100mm along.

Over-bending can easily be corrected by a reverse bending action wherever the bend looks incorrect. Slowly work along the length until the curve is a close to the parabola as is achievable and practical. I worked to around +/2mm

My template also doubled as a jig to hold the whole rib fast whilst assembly is performed.

Cutting many of the parts in bulk and using templates save a lot of time. I only stated to do this towards the end of the construction and wished I done so earlier.

A finished rib.

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