Building the Octagon Pt.33 Timber pier adapter.

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I couldn't just bolt the heavy mounting to the top of the pyramid and hope it stayed up there. It needed a bigger footprint and a transition from sloping to a flat surface. So I needed a timber adapter. Adding five vertical lengths of 4"x4" in a cross form would spread the tops of the pyramid legs in both directions by 4". See diagram alongside.

Four shorter pieces of 4x4 cut precisely on the miter saw could help to lock the tops of the pyramid posts together. The image alongside shows simple off-cuts of 4x4 propped on top of the pyramid posts. The matching, compound miters have yet to be cut on the diagonals.

The five vertical posts ion the mock-up are much too tall at the moment. But they rested safely on a platform placed between the pyramid legs. This allowed me to to judge exactly what I needed without risk of the whole thing toppling over.

I can glue the five vertical, adapter posts together to form a solid, timber key. Which can be simply dropped between the tops of the spaced, pyramid legs. Or pushed up from underneath if I had that much strength. There would be five meters of 4x4 in the timber key.

This is a  closer view of the arrangement of four pyramid, five vertical posts and four shorter, mitered blocks to make a 3x3 post form. For an overall sized platform of 12" square.

The pyramid would become too tall with the mitered blocks shown in the corners. So more pyramid trimming is obviously required. Which probably means taking the pyramid posts down again to trim the legs cleanly and precisely to length with the miter saw.The alternative is to cut off the tops of the pyramid legs to make them level. Though that would rob me of the stops provided by the short blocks. The key would literally fall right through the pyramid to the ground without some form of physical stop.

A heavily built, reinforcing block will be required for the bottom of the adapter to stop it from rocking. This block can be lifted slightly with the mounting's large, azimuth pivot stud [all thread] as it is tightened. By trying to force the pyramid legs apart [against resistance] it will greatly increase the stiffness of the top of the pier.

18mm/ 3/4" plywood cladding will ensure the triangulation of the top part of the pier. It will also greatly improve the appearance of the top section of pier visible within the observatory. I don't think my worries about pier to OTA clearance due to the sloping sides are born out in practice. The angle iron, temporary pier is very similar in slope geometry.

Interestingly, the pier key offers the potential for mounting rise and fall. The problem is not being able to fit anything [like a jack] between the pier and the obs. floor. Though that does not preclude a scissors, screw jack from being inserted into the pyramid itself below the key. It just needs the support for the jack to be fixed to the pyramid instead. The pyramid would need to be modified to allow sliding motion without any play [at all.] Something better than sawn wood rubbing against sawn wood would be desirable.

A  typical car scissors jack provides about 8" of travel and the load would be relatively light compared with a car. A hand crank in place of the jack's fiddly drive rod arrangement would be beneficial for regular use. I have to brace the pyramid to stop the adapter from rocking. So I might as well consider rise and fall in the design. I have a long length of 120mm square [4.7"x4.7"] alloy tube with 5mm wall. That would be idea for the piston movement and it is quite close to the 4" timber I am using. There would be enough tube to provide a decent length for four rise and fall "pistons."

Click on any image for an enlargement.

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