10" f/8 Beam me up [again] Potty!

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I have reconsidered the problem of primary cell torque around the beams as a result of the weight of the full thickness 40mm x 250mm primary mirror. By fixing a second channel section in front of the previous one I now have two clamping bolts to fix the cell more firmly to the beams. Unfortunately the alloy angle sections which locate the cell [pot] to the beams are now splaying. So it's back to how 3/4" / 18mm curved plywood strips might carry the torque loads into the beams to stiffen things up.

I'm still considering how best to implement the idea. Should I clamp an extended tongue between the beams for greater rigidity at the cost of slightly increased weight? It might be better than 1" / 25mm deep formers simply resting on the beams under compression from the two clamping screws.The second clamping screw is now pulling the open end of the pot cell into an egg shape.

One plywood strip with a concave arc improved matters but caused the clamping screws to fight against each other. I think it will take three strips in all. One between the clamping screws and one each just outboard of the screws. The alternative is to use a couple of pieces of studding to restrain the location angle strips from splaying. Or, use much heavier angle to avoid flexure. Or heavy angle and cross studs. Too many choices!

After repeated trials with subtly changed curved plywood spacers I gave up and fitted a single cross brace out of studding to the cell location rails. This was spaced midway between the cell clamping screws and stopped the location rails from splaying. I then tried various spacers on the forward clamping bolt and settled temporarily on a couple of large nuts. These just happened to be the correct thickness to hold the cell pot firmly without too much distortion. With me sitting, quite literally, on the twin spars/beams and trying to push the cell sideways it proved that no more needed to be done. Except to make a smarter spacer and obtain a shorter coach bolt to avoid the unnecessary spacer below the clamping wing nut.

The rearward cell clamping bolt is also far too long by several inches and I will have to obtain a shorter bolt to replace the present one. This coach bolt just happened to be the only one I had in the correct diameter in my extensive collection. The square shank under the domed heads stop these bolts from rotating when the large wing nuts are tightened. This saves me having to use a spanner/wrench anywhere near the vulnerable primary mirror inside the cell.

The images speak for themselves in showing progress so far. The three channel sections between the beams have not been cleaned up since they were sawn from the length which had been lying around for years in the garden. In retrospect I could have sawn a single length instead of three short ones but perfect hindsight is never the same as perfect foresight. At least, not in my case.

The cell pot is still distorted slightly from earlier clamping attempts. I shall remove the pot and press it perfectly round when I remove the clamping bolts to fit the new spacer. I might also curve the strip which lies under the bolt heads to provide stiffness and help to spread the loads better.

Meanwhile, I have obtained some stainless steel, M6x20mm screws to replace the previous M6 studding. The saving on unnecessary complication and slight weight loss is fair reward for the relatively low cost of the new screws. It looks neater too and the heavy channel section resists flexure. A purist would have purchased socket head [Allen] screws but I couldn't find any short ones in M6 at the DIY store. Those they had were bolts so I couldn't shorten them without losing the vital threaded portion.

So it's hex-head and washers to fit into the existing furniture screws for the moment. I shall keep an eye open for stainless steel, socket head screws on my travels. Dismantling the OTA will be much easier with screws instead of studding. It's not so much the desire to dismantle regularly. Rather, the ease of modification without the inevitable mental inertia if I should have further ideas for improvement later on.

PS: I found some socket head Allen screws online in M6x20mm and have placed an order. These will smarten up the appearance even if they are usually invisible. They will better match the socket head furniture screws which are used externally. I'll still need to obtain some stainless steel washers. Despite their large heads, the furniture screws are pulling in the very thin material of the beams when tightened well. I'd rather avoid distortion of the beams if only for appearance sake.

Click on any image for an enlargement.

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