9 July 2020 at 5:28 pm #574671Eric WatkinsParticipant
I have an old AE 12″ F:20/5 classical Cassegrain. I have just had the mirrors re-aluminised.
The tube is of the open skeleton type with hexagonal tube formers and several lengths of 1″ aluminum tubes forming the OTA.
These 1″ tubes are held in place within each of the aluminum hexagonal formers by small grub screws (aka set screws).
I would like to replace the 1″ aluminum tubes with carbon fiber tubes of 1.5mm wall thickness to reduce the effects of thermal changes on focus.
Does anybody have any experience with such carbon fibre tubes, I don’t? Would the carbon fibre be too brittle to be secured in this way?
Eric14 July 2020 at 1:59 pm #582819Steve KnightParticipant
I can’t give you practical advice but I have a background with lasers. Historically many used Invar (Nickel / Iron alloy) rods for laser resonators because of low thermal coefficient of expansion. Many companies switched to carbon composites as lower CTE and much lighter. Found this paper which might be of interest. https://www.researchgate.net/publication/241529927_Long-term_and_thermal_instability_of_carboncarbon_composite14 July 2020 at 5:21 pm #582822Chris DoleParticipant
I wouldn’t recommend using grub screws on a carbon tube of 1.5mm. I imagine that the screws would need to be quite tight and this could damage the tubing. A thicker wall thickness, say 3mm, would be more resilient although some marring may still occur.
Chris.16 July 2020 at 10:48 pm #582848Eric WatkinsParticipant
Thanks guys for your replies. I think what I shall do is select short lengths of various candidates and try them on one former and see how they workout. Thicker walls may be the way to go. I’m not too worried about the surfaces being cosmetically spoiled its just whether they would crack or fail.17 July 2020 at 1:54 am #582849William BristowParticipant
On the mechanical systems that I am familiar with, where hollow CF tubes are used, fixings points are reinforced by either gluing into external ferrules which are then clamped or bolted, or an internal solid plug is glued inside the tube at the fixing points that then allow for external clamping or through bolting.
Unsupported or unreinforced CF tubing is too brittle to be secured by just single or double grub screws pressing against the outside of the very ends of the tubes in a hexagonal holder.
For CF tubes in a truss OTA either reinforcement methods would be suitable though an internal glued plug of brass or aluminium is easier to build as it requires no changes to the outer holder and being hidden doesn’t need to be cosmetically perfect.
For light loading applications plugging the tubes at the clamping points with epoxy resin mixed with chopped glass fibre, or CF, extending a couple of centimetres either side of the grub-screw locations would work. If this method is used a breather hole ~0.5mm diameter, some distance along the tube from the plugs is necessary to prevent air pressure changes affecting the tube geometries once the plugs have cured and the instrument is in use.
Plugging with an epoxy CF/GF filler sounds easy but in practice is quite difficult to do successfully and in a production environment requires careful measurement of the volume of filler and spinning the filled tube on a centrifuge to ensure the filler plug sets level in the tube ends and without trapped air voids. Solid machined plugs glued into the tube ends are easier to mass produce.
How about a short piece of hard-wood doweling, epoxy glued into the ends of the tubes at the clamping points? Should be easy enough to turn to size with a bit of DIY tooling.
William.17 July 2020 at 10:40 am #582850Roy HughesParticipant
If a smaller diameter (22mm?) carbon fibre tube is rigid enough then you could try a split sleave of thin 1 inch diameter alluminium tube over it for the grub screw to bite into. This would spread the load on the carbon composite. I note (on ebay) that the clamps sold for carbon fibre tubes are not dissimilar to (posh) pipe clamps, so it might work.
Roy17 July 2020 at 2:50 pm #582851Dr Andrew SmithParticipant
When building up a carbon fibre bike I had to be very careful not to over compress the tubes. Most cf components had strict torque limits.
Regards Andrew17 July 2020 at 9:40 pm #582852Tony MorrisParticipant
Carbon composites can be tricky to mechanically fix together or join to other structures
For ideas have a look here:-
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