2" shaft mounting Pt.45: Forked base.

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I have been putting off the base while I sought heavier [scrap] materials. They were not forthcoming so I am going to use what I have in 150mm wide strip x 10mm [6" x 3/8"] aluminium plate.

I shall build a simple support fork for the polar axis bearing housing. Which will pivot the housing to allow fine PA altitude adjustment via a sturdy, stainless steel turnbuckle.

I carefully marked out where the 50mm [2"] shaft resided inside the casing as well as marking the position of the heavy studs. [All threads] There seemed no particular purpose in moving the pivot any further north than half way up the housing. I may change my mind over this but at present it allows room for the RA wormwheel if I should move it to the bottom of the PA shaft.

I then drilled small holes in opposite plates and supported the polar axis on two nails. The G-cramp [C-clamp] keeps it all steady while I admire the initial layout to see if I have overlooked anything important. Which I often do these days. The worm support plate will be relocated on the side of the housing to allow a control rod to be brought back to the eyepiece.

The "nail" holes will be opened out to take the largest possible threaded rod [stud or all thread] to tie the fork tines rigidly to the PA housing. This continues the compression idea to make the housings as solid and stiff as possible.  Otherwise I could easily have used captive pivot bolts pointing outwards through the side plates. This would not have provided the desired compression as the side plates would be held only by the smaller cross studs.

I try to imagine flexure modes exaggerated into catastrophic failures. Hence the through stud rather than two loose bolts. Offsetting the pivot holes between the axis shaft and large studs offered two options. I could pivot the PA housing above or below its center line. The higher position caused clearance and appearance difficulties for the fork tines. Though the PA housing could still be flipped over if desired for a much lower housing position. But then the RA wormwheel won't fit under the housing.

Oblique view showing the fork more clearly. Once the studs are finally sawn to length below the lower flange bearing there should be room for the RA wormwheel. For the moment the studs make useful handles for carrying the hefty PA housing around.  They also provide convenient propping points for mock-ups to see how things look in practice.

The fork will be fixed down to the large 10mm [3/8"] base plate via sturdy aluminium angle both inside and out. A length of scrap angle is resting against the workbench in this image all ready to be sawn up. 

Even at 10mm [3/8"] thick the fork tines look extremely flimsy. So the sides of the fork will be further reinforced against flexure with channel section aluminium carried to full height. I have propped up a short length of channel to show the basic idea. You will have to imagine the channel rising up both fork tines and having a radius applied to the tops to match the rounded fork plates. I am considering using more furniture screws to hold the channel in place.

Perhaps I should double the fork tines to 20mm [13/16"] thickness? I still have enough 6" wide strip for this job. Epoxy between the two layers would bond the sandwich into a solid mass with far greater stiffness. I don't want to follow this route until I have finally decided on RA wormwheel position as longer fork tunes consume a lot of scarce material.

Should I decide to leave the RA wormwheel at the top of the PA axis then the fork tines could be considerably shortened. This is where a bolt-together assembly is handy for prototyping different ideas.

Click on any image for an enlargement.

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