14.10.2020 Making a new backplate for the FTF2535 focuser.

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I made a start on a new back plate to fit the new Feather Touch 2.5" x 3.5" for when I finally receive the base adapter. If the focuser is to rotate on this base adapter then the rack will rotate with it. Which means I need a hole with enough clearance for the rack to lie in any orientation. 75mm Ø will be quite close. So a couple of extra mm will avoid any risk of contact. 

The first images shows the 77mm Ø clearance hole for the drawtube. The blank has also been faced, while in the 4-jaw chuck to make it tidy and true. It is important not to remove partially finished items from the chuck if concentricity and truth are important.

Here the blank has been reversed in the 3-jaw chuck with expanded internal jaws. So that it can be stepped at the rim to fit  snugly inside the main tube. It will not look quite as massive once completed and fitted. The weight should "tip" the OTA to a more aesthetic position. At present it is nose heavy with a very long "tail" end to balance.  

I fitted the new backplate in the tube and marked the screw fixing holes. Then I drilled and tapped radial, M4 threaded holes in the step surface. These are to hold the backplate securely in the main tube. I turned a tapered shoulder to make it easier to fit into the springy main tube.

Having been turned from the same blank thickness the new backplate is very similar to the previous one. Only it has a much bigger central hole. I did not want to spoil the old backplate in case I needed one with a 2" bore one day. I had a good length of 180mm, 7"Ø aluminium bar form a flea market for small change. It has lasted me for years and I still have some left. A local engineering company slice it to order on one of their band saws free of charge.

The FTF flat baseplate still needs to be marked and drilled on the new backplate. This flat base plate has levelling screws but I need to be absolutely sure it is central on the backplate. I will probably wait for the arrival of the missing, FTF base adapter just in case it affects centralisation. I can place packing around the drawtube of the completed focuser to ensure perfect concentricity. It shouldn't take more than a couple of minutes to mark and bore the necessary fixing holes. The weight of the complete set-up probably means I'll need my block and tackle to lift it all into place. 😉

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