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Odds and Ends



Thirty odd years ago I made my tubes of aluminium. Four could be cut and rolled from a 1/16" thick, eight by four feet sheet of luminium without waste. Bands were placed around the tube to hold the edges tight together for welding. The weld was filed and sanded to give a smooth seemless finish. The internal diameter was 7-5/8", a good size for a six inch telescope.

A few years ago when going back into telescope making I was looking at all the alternatives. Noticing a cardboard mailing tube at the local Post Office I made enquiries through a Indurtry Assistance Group to find out who made the tubes and if larger sizes were available. I was put in contact with Sonoco Australia and they were able to supply 200 mm x 3 mm. tubing. This was the right size for a six inch telescope. It was inexpensive, light and rigid. I ordered several fifty inch lengths. I found out later that this was the Sono Tube used by American telescope makers.

The tubing has a spiral groove arround it and this presented a problem. I filled it with filler putty and when dry sanded it off and sprayed the tube with undercoat. It looked OK. But next morning the grouve was plain to see. So the operation was repeated, but again the following morning the groove was visible After much playing around the following procedure was developed .

(1) A frame was welded up out of 1" square steel tubing. There were two sets of trunion rollers, one fixed at one end and the other movable to accommodate different length tubes. The trunion rollers could be be adjusted in or out to accept diferent diameters. I shall call this a rotisserie. A motor with a worm reduction box and belt drive rotated the tube.

(2) A tape was wrapped around each end of the tube spaced to the length required. About 1 /8" is allowed at each end. This is a guide to drawing lines around the tube. The tube was cut to these lines. It can be cut with a coarse tooth hacksaw. I used a bandsaw.

(3) Two wooded rings that tapped neatly into the ends of the tube were cut. Close to the outside edge of one disc a 3/4" hole was made and taped over. .

(4) The tube was placed on the rollers which were adjusted to accommodate it. A polyurethane belt was placed around the tube and onto the pulley on the reduction box. The motor was switched on and speed adjusted to slowly rotate the tube. A small can of Blackboard Slating was poured through the hole in one of the end rings. The paint spread out and coated the inside of the tube. The roteressie was stopped. Belt removed, the tape removed from the hole , the tube tipped up and the paint excess poured out. The end rings were then knocked out and the tube placed back on the rollers and the belt replaced. Motor was turned on and a heat gun was directed into the tube until the paint was dry. The set-up is illustrated in (Fig 1.& Fig 2 )

(5) Next was squaring the ends of the tube. A mandrel was made out of 1" pipe and two cast iron discs were machined to fit on the mandrel and neatly inside the tube. The discs were locked on the mandrel with that extra 1/8" of tube projecting each end. The mandrel was placed in the lathe with one end in the chuck and a tailstock centre in the other. A sharp knife tool was held in the tool post and with the tube turning slowly the tool was fed in so that it just grased the side of the cast iron disc. This gives a clean cut end. The mandrel is turned around and the other end of the tube is cut. (Fig.3.)

(6) The trunion rollers were then addusted to take the mandrel. A pulley was fitted to one end or the mandrel and a belt to the pulley on the reduction box. Clear epoxy was mixed in the reccomended ratio, allowed to stand for a few minutes then poured over the tube as it was rotating. I have lost the record of how much was required. The tube has to be rotated at a speed that prevents the epoxy dripping off . If it is rotated too fast the epoxy will form in ridges and even fly off. With the right speed the epoxy spread out evenly. I do this operation late in the day and let it run overnight. (Fig.4 & Fig.5 )

(7) The next morning the mandrel is put back in the lathe. Turning at a few hundred RPM a length of coarse abrasive belt is wrapped around the tube and both ends held tight in the hands and tensioned and moved up and down the length of the tube. This is hard work. All the pimples and ridges are removed. Dimples are filled with Plastibond. When set it is sanded off again. There is no indication of the spiral grouve. (Fig.6)

(8) The mandrel it returned to the rotissorie. With the tube rotating slowly, it is sprayed with undercoat. I don't know now if this is really necessary. It is sanded off with carborundum paper. Dust is blown off. Now two pack Polyurethane paint is mixed and with the tube rotating slowly it is sprayed on. I found that about .2 litre was sufficient The painting is done late in the afternoon and the tube is left to rotate all night while the paint sets.

(9) Next morning the tube is lifted off and the mandrel and iron discs removed. I now had a tube wtth a smooth and shiny hard finish and no sign of the groove.