7" f/12 iStar refractor 16: Ringing in my ears!

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The first pair of 222mm rings I ordered were cracked across one hinge so I had to search for something which still matched my tube size of 202mm. There was an unexpected  delay of several weeks, from placing my order, for rings of the exact size from another source.

So I decided to go ahead with standard 235mm rings which is a commonly available size for 8" reflectors. These would need considerable packing to fit but I had no firm, closed cell foam which was of the correct thickness. I usually rely on packing strips cut from various camping mattresses to fill the gap between telescope tube and an over-sized tube ring.

I decided to use plywood packing rings instead of unstable foam. Once painted to match the tube ring the packing plywood rings would become invisible. Even appear as if the entire rings were made of massive, metal, cross sections in keeping with a classical refractor.

An overhead cable support is very useful to save pausing mid-cut to untangle the twisted cable. Every revolution of the router on the circle cutter adds one more twist. Left to its own devices it can be safely guaranteed that the cable will catch somewhere on the router. Usually in the gap between the fence rails.

Having routed out my first trial pair of packing rings from 12mm birch plywood I was sent a nice pair of 235mm rings by the original supplier. These proved to be far better made and finely finished than the original pair. In fact they closely matched the usual "Skywatcher" generic design in black hammered finish.

Now I had them to hand I discovered that I could cut and glue two layers of birch plywood together [12mm+18mm] to make them exactly 30mm wide to nicely match the 235mm rings. It was just a matter of cutting two more rings to the exact same radii as the first pair but out of 18mm birch plywood instead of 12mm. Fortunately I had a whole stack of 18mm disks from former projects. So there was no need to buy a whole new sheet of 18mm plywood just for two rings.

I am getting quite familiar with the requirements for making accurate plywood rings by now. Even down to using a vernier caliper to set the exact distance from the router bit to the center pin. Remembering inside or outside the router bit depending on whether an outside or inside circle was being cut.

The rings have to be cut in short steps of ever greater depth, from both sides, until the bit breaks through in the middle of the plywood. Always starting with the exterior circumference first so the center hole is still available for cutting the inner circle. Cutting the inner circle first leaves no center with which to cut the outer ring. I've made that mistake before with loudspeaker holes in very thick plywood!

One can fix the undersized circle back into the cut hole with packing and a bit of scrap, bridging board screwed on for strength, but it is not easy if one has to cut a circle from both sides! This fix will almost always mar the material outside of the circle. It is nearly always impossible to clamp when cutting circles because the router will hit the clamps. Pins or holding screws must be clear of the actual cutting circle or they will instantly destroy the router bit.

My very simple, circle cutting jig [image above] is no more than a piece of angle alloy section to bolt to the ends of the fence rails. A larger hole at 90 degrees accept the 3mm center pin. Which is on the reduced end of a sturdy piece of turned down studding. Making it this way, in the lathe, ensures a solid grip when the center pin is clamped to the angle piece via a T-nut with the gripping spikes removed from its disk. This provides a nice thin nut on the underside of the angle piece with the long threaded shank for extra strength. The 3mm center pin is a good size and ensures stability during a cut but is also long enough not to try and lift out of the center hole.

There is a lot of circling of the bench to cut four rings with several cutting steps per inner and outer circle from both sides. I would have clamped the disks at the center of the bench but no useful peg holes presented themselves for the plastic removable "jaws." Nor was I about to start boring more holes just for one job. So I had to reach right over as I walked around the far end of the bench in circles. My Bosch router has a dodgy trigger lock. Sometimes it works but mostly it doesn't. Life would be so much easier if it could be relied on.

Once all the rings were cut out and checked against each other for the best match I glued the meeting faces of each pair together with exterior, white wood glue. They were then clamped in the tube rings first before clamping the plywood rings firmly together with G-cramps. This ensured maximum concentricity of the plywood rings before they were glued irrevocably together.

Router cutting may be quite accurate but there are always slight variations in diameter. These can cause slight steps between supposedly perfectly matching rings. All similar radii ought to be cut before adjusting the router circle radius to another size. I had to do it the wrong way because I had already cut one pair of rings the day before. Hence the telescope tube rings being clamped over the plywood rings first.

I don't own a bench disk sander so must avoid too much hand sanding. Which can all too easily put a crown on the rings instead of leaving the circumference square. Sanding the edge of plywood is a slow process anyway because one is usually working on at least some of the end grain of the constituent layers.

Note that I have not split the plywood rings into two halves at this stage. I want to ensure an exact fit on the refractor main tube first while they are still in one piece. My lathe is slightly too small to be able to chuck such large rings and simply bore them to exact size. Even the face plate is rather too small to able to clamp them directly for boring.  Though the rings could be fixed to a circle of ply first and then spun on the face plate or in the chuck. Splitting the rings into two halves will allow some leeway in exact diameter [through flexure of the metal tube rings.] I certainly don't want to have to force the telescope tube into the rings. Castings should be treated with rather more respect than that. Not that a millimeter difference in diameter is much of a problem.

I haven't yet decided whether to glue the plywood packing rings to the metal tube rings or use fine screws to hold them in place. The latter would require drilling the cast metal tube rings which might weaken them slightly. I have already removed the thin, fibrous tape intended to stop paint damage to the instrument's main tube by any movement in the tube rings. Turps [oil-based paint thinners] will remove most of the sticky residue if soaked in turps first. Then scraped with a bit of hardwood to protect the paint on the rings.

This morning I smoothed and cut the plywood rings to match the hinge and latch spacing. Then I used Liquid Metal as an adhesive to fix the plywood rings to the tube rings. Finally I clamped the rings around the telescope tube to ensure the rings were compressed together. Some care was required to ensure the rings were flush with each other at the sides before final tightening. 

Click on any image for an enlargement.

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