Carbon Fiber Tube By Chris Curry - ScopeX

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Carbon Fiber Tube By Chris Curry

Telescope Making > ATM Diaries 2

Carbon Fiber Tube
By Chris Curry

In search of a light, cheap and easy to make telescope tube I’ve chatted to people at telescope making and a lot of them battle to find a tube for their newly finished mirror.

I’ve also had to carry too many heavy scopes around and I know that the lightest scope is the one that gets used most but most people build telescopes that weigh a ton. Of the materials that I’ve used up until now I think that the cardboard tube has been the best but still too heavy. Even my baby 6" F4.5 with its ABS plastic tube is light as scopes go and although I can carry the assembled scope under one arm it’s still heavy.

A few years ago I attended a composites model aircraft building class and it gave me a few ideas. After a bit of web surfing and a friend in need of a light tube for a truss dob I bought the materials required and with a large role of Mylar and a case of beer I gave it a bash.

Tube 1
This was 200mm diameter and 500mm in length formed around a PVC water pipe for a 6" mirror. 3mm thick 100mm X 1000mm balsa sheets were cut in half and glued side to side with super glue to create a sheet about 500mm square.

With the grain running from top to bottom the balsa was draped over the tube. Even though the balsa had been chosen for its softness, it felt like it would snap if bent more than a little. The balsa was bent with masking tape keeping it as far as I dared bent it and then sprayed with water which caused it to bend more.

I adjusted the tape and once dry it kept that shape and the process was repeated. Eventually the balsa was the right shape. The ends were then trimmed to size so that they met nicely.

The carbon was then cut slightly bigger than required with about a 25mm overlap and about 5mm longer than needed and laid on the Mylar covered tube. After mixing up some epoxy resin I brushed it onto the carbon so that it was nicely wet.
The rule of thumb here is 100g resin to 100g carbon/glass cloth but I tended to always make more than required.

Next the balsa was wrapped around the tube and a masking tape strip stretched across the balsa at the middle of the tube to close the gap. The ends which were sticking up a bit were taped together then in between those strips more strips placed and again in between those strips until strips of tape were covering virtually the whole balsa skin obscuring the gap.

The carbon increased the diameter of the tube slightly so that the gap would not close completely but it didn’t matter as the elasticity in the masking tape exerted a squeezing force on the carbon/epoxy and compressed it so that excess resin started to drip from the ends of the tube and from the gap.

I repositioned some of the tape to keep the pressure on and as the epoxy takes around 24 hours to properly cure, left the tube to set. Once set the masking tape was removed and an epoxy filled but small gap was left protruding slightly. I sanded the gap flat and cleaned up a few drops of epoxy had run down the sides of the balsa and lay more carbon fiber over the tube. I applied more resin to the carbon and, when finished, wrapped the whole tube in electrical tape.

It took quite a bit. The tape squeezes out any excess resin and compresses the carbon fiber which is at its strongest when the fiber are compressed making a light and extremely rigid tube.

The finish is matt with a few slight ridges which easily sand out. I never weighed the result but I know it was light.

Tube 2
As it was for an 8" F4.2 mirror the diameter was 250mm and 900mm in length wrapped around 250mm PVC water pipe using carbon fiber with a Depron (extruded polystyrene) core. Depron was used as the amount of work to glue/bend the balsa was a bit much and 2mm Depron bends nicely around a 250mm former. It’s also a lot cheaper. The tube was made the same as tube 1 but without the electrical tape at the end as I wanted to try a different finish. The lack of electrical tape created a few imperfections though…
The tube is nice and rigid but can be compressed at the ends as the Depron is a little soft. The finish didn’t turn out that well as the final layer was done on a cold day and the resin went slightly milky. Live and learn. This tube I did weigh though and it came out at 1.02 kilos.

Tube 3
This was for a 6" F7 mirror and was 1000mm by 175mm wrapped around a very nice Perspex tube from a broken lamp using glass fiber and a foam core called airex C71. Airex is a commercial product used in aircraft construction and I wanted to test the compressibility of it as the Depron was a little soft. Glass fiber was used to see if carbon was really needed and I had some 163g/m2 glass cloth laying around. Once again the tube was made as in tube 1 but as the finish on tube 2 wasn’t good I thought of wrapping Mylar around it to give it a nice shine. This didn’t work at all as the Mylar bunched up and gave the tube ripples. As I had no electrical tape handy I couldn’t revert to that so had to live with a wrinkled finish. This tube came in at 600gm and felt as rigid as the carbon.

Tube 4
This I decided to build to replace the 2.5kg ABS tube of my 6" F4.5 dob. I used a glass inner, airex core and a carbon finish as I had carbon left over from tubes 1 and 2. This was wrapped with electrical tape and is easily the best so far. With a light sand and a few coats of clear aerosol paint the carbon looks cool. I didn’t weigh this tube but it’s around the 400g mark. That’s an 80% weight reduction!!!

The Cost
The resin is around R175/liter and is enough to do probably 4 or 5 tubes so let’s make it R50 per tube. The airex was R100 per meter and came in a 2.5m roll so there’s no wasting which is nice. The glass cloth I had was around R250 for 5 meters.
Tube 1 R390 – estimate 500g (1 meter of carbon (R250), 3 sheets of balsa (R75), resin (R50), electrical tape (R15))
Tube 2 R645 – 1020g (2 meters carbon (R500), 1 sheet Depron (R65) , resin (R50), 2 X electrical tape (R30))
Tube 3 R150 - 600g - (1 meter of glass cloth (R50), 500mm airex (R50), resin (R50))
Tube 4 R140 – est. 400g - (0.5 meter carbon (leftover),0.5 meter of glass cloth (R25), 500mm airex (R50), Resin (R50), electrical tape(R15))

The Time
I timed the making of tube 3 as I had a friend round and wanted to show him how easy it was. From wrapping the tube in Mylar to applying the last piece of masking tape took 1 hour and 15 minutes. The removing of the tape the next night plus the next layer of glass and Mylar took under an hour so that’s 2 hours 15 minutes to make a tube. I think the electrical tape for tubes 1 and 4 would have taken another 15 minutes. So between 2 to 3 hours max for a super light. The first tube took me an hour to remove from the former, the second was easy, the third I just cut, glued back together and wrapped green vinyl around it and the 4 th somewhere in the middle.

The End
So there you have it –they all work. The best result is tube 4 but even the worst result is stupidly light – just not so pretty. All of the tubes are at least 50% and probably more like 80% lighter than anything comparable and not that expensive to make. Now to make a lighter stand…

PS: Chris is happy to discuss via chris at barefeet.co.za

Suppliers
Aerial Concepts - Depron, super glue, balsa
AMT Composites – carbon, airex, glass cloth, resin
Mylar was used but wax paper would have worked just as well

Note:
R = ZAR = South African Rand

March 2014


 
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