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The Polypropylene cutting boards from the supermarket were never meant to be other than bird baths. They are far too curved to be cutting boards let alone precision mating surfaces for optical components.
I have recently discovered two potential sources of aluminium. Where I can hopefully obtain enough for the two, matching bayonet plates and the backplate. I had better check the plates are flat if I am offered anything. Aluminium is three times more dense than PP so there will be a weight penalty. Thankfully a large circle is cut out of both objective supporting plates. This will reduce the weight considerably. The backplate will only have a 55mm hole for the focused light beam to reach the focuser. Perhaps I can get away with only 3mm aluminium for the backplate? It will be supported on all four sides by the Porsa tubing flange.
I have recently discovered two potential sources of aluminium. Where I can hopefully obtain enough for the two, matching bayonet plates and the backplate. I had better check the plates are flat if I am offered anything. Aluminium is three times more dense than PP so there will be a weight penalty. Thankfully a large circle is cut out of both objective supporting plates. This will reduce the weight considerably. The backplate will only have a 55mm hole for the focused light beam to reach the focuser. Perhaps I can get away with only 3mm aluminium for the backplate? It will be supported on all four sides by the Porsa tubing flange.
I may be able to lessen the overall load by employing a suggestion from a fellow ATM. He too has folded an 8" f/9 objective using a tube from a Fullerscopes reflector. He helpfully suggested that I cut down the baking tin, mirror shells to only the depth required to house the mirror blanks and a backing disk of plywood. I can potentially save up to 40mm or 1.5" per cell. By 're-tuning' the folding the overall length of the OTA framework could be shortened by as much as 3" or 75mm. Thereby saving even more weight. The baking tin, mirror shells are not particularly heavy. Though the rolled rim is arguably the heaviest of what remains after cutting out the bases to leave only a narrow, mirror retaining lip.
The extra joints were dispatched yesterday by Porsa so hopefully I will see them today. Though it is even colder now with the day starting at -6C, 21F, albeit in bright sunshine. Not ideal for handling bare metal and tools. My feet felt the cold yesterday while working outside at -1C. I shall have to wear better footwear if I do any ATM today.
After a 12 mile ride I was redirected to another firm several miles further on which had loads of aluminium in stock. I was going to get the sheet over-sized and saw it up myself but they had a huge guillotine with digital readouts. So they cut it to size on the spot for a very reasonable sum. It's good to know they have all that stock and friendly service if I do need more aluminium. The extra Porsa corner joints have arrived in the post.
I spent the afternoon outdoors cutting out the 20cm disks from the 'bayonet' and OTA front plates. It was lucky I tried the first hole on the objective because it was rather loose. So I scribed the next one 2mm smaller. Then spent literally ages filing the hole to fit the rear of the objective. The close fitting one will be used as the collimation and bayonet plate. While the other can become the matching, OTA support plate. With the disks removed the surrounding plates are very much lighter.
24cm x 28cm = 672 cm^2. Disk = Pi x r^2 = [3.142 x 100] = 314 cm^2. 672 - 314 = 358 cm^2.
So, very nearly half of the area [and weight] was in the unwanted disks.
After a short chain drill to make a starting hole for the jig saw I found that ordinary bicycle oil made sawing almost effortless. Run dry, there was a lot of resistance as the blade teeth immediately became coated in melted swarf. I drew an arc of oil on the surface of the plate and that was enough to keep the saw blade nicely lubricated at modest sawing speeds. The results of my labours can be seen in the image above. The plates themselves will be smoothed and lightly grained with abrasive paper on a sanding block. Then they might be painted matt black.
After a 12 mile ride I was redirected to another firm several miles further on which had loads of aluminium in stock. I was going to get the sheet over-sized and saw it up myself but they had a huge guillotine with digital readouts. So they cut it to size on the spot for a very reasonable sum. It's good to know they have all that stock and friendly service if I do need more aluminium. The extra Porsa corner joints have arrived in the post.
I spent the afternoon outdoors cutting out the 20cm disks from the 'bayonet' and OTA front plates. It was lucky I tried the first hole on the objective because it was rather loose. So I scribed the next one 2mm smaller. Then spent literally ages filing the hole to fit the rear of the objective. The close fitting one will be used as the collimation and bayonet plate. While the other can become the matching, OTA support plate. With the disks removed the surrounding plates are very much lighter.
24cm x 28cm = 672 cm^2. Disk = Pi x r^2 = [3.142 x 100] = 314 cm^2. 672 - 314 = 358 cm^2.
So, very nearly half of the area [and weight] was in the unwanted disks.
After a short chain drill to make a starting hole for the jig saw I found that ordinary bicycle oil made sawing almost effortless. Run dry, there was a lot of resistance as the blade teeth immediately became coated in melted swarf. I drew an arc of oil on the surface of the plate and that was enough to keep the saw blade nicely lubricated at modest sawing speeds. The results of my labours can be seen in the image above. The plates themselves will be smoothed and lightly grained with abrasive paper on a sanding block. Then they might be painted matt black.
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