If I want to reduce a length of carbon fibre solid rod in the middle, with a gentle taper from each end, will it affect it's strength or properties at all ? Or will it fracture in any way ?

Yes it will alter the strength and charecteristics. It will weaken slightly more than the percentage you reduce the cross section. Under terminal load it will break at the narrowest point. It will flex a little and then fracture with little warning due to the eneven load charecteristics of a bobbin tapered shaft.  C/F rod behaves much the same as hardened steel in this respect. If you do decide to taper it down be careful how you do it. Sanding it down will cause piles of potentially harmful dust. Tapering unevenly i.e. off centre will weaken it still more. Machining in a lathe is possible if you use VERY sharp tools, and a travelling steady post, similar to those used for aluminium. Wear a mask, vacuum up the dust, carbon dust is thought to cause similar lung tissue damage as coal dust, and there is no point taking a risk.  Epoxy coat the finished part. I have done this successfully for a guide roller to control the spinnaker retrieval on my sailboat. Hope this helps.

Hi Coolukey,

What ezsailor is saying seems about right; thanks for your input. If you taper the rod in the middle then you'll create an inherent point for a fracture to start and so it will have more impact on the overall strength than just being proportional to the reduction in cross section. If you engineer some 'margin' in to the rod in the first place then this doesn't necessarily have to mean that you can't do it just that the penalty for breaking the continuity of fibres will be more than you might expect. ezsailors advice for machining is helpful; you will find that turning us a really awkward way to work with a pultrusion because you're going straight across the grain making splintering/fracturing more likely.

As for the safety; follow sensible precautions always when cutting carbon fibre; wear a mask and clean up/extract the dust.

All the best, Matt

Matt Statham
Easy Composites / Carbon Mods - Technical Sales

Hi

Thank you for your comments. My main concern was, that however I reduce it, would it create a weak area as I was cutting across the fibres. The taper would not meet in the middle in the form of an angle, more of a gentle curved shape. It's use, when made, will require it to flex and return as best and repeatedly as possible. The thickness will be reduced from 6mm to about 3.5mm to 4mm. I guess I'll have to experiment somewhat. I bought a metre length of 6mm and will be cutting it down to 160/200mm pieces. I will have to reduce it by abrasion as I have no other means available.

Thanks again for your much appreciated input.

ps.   Is it possible to have a piece made to this shape? If so will it be costly?

I think experimentation is going to be your only option.

That is a unique profile rod design and I suspect you will struggle to find anyone supplying such a rod.

You are also going to find it hard to get someone to manufacture such a rod too. They are made by pultrusion which means the fibres are pulled through a die which is the profile of the rod, so a rod with a taper simply cannot be manufactured in one piece by that process.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Hi

Thanks for the response. Is there anything I could coat the reduced section with, like an epoxy or something, that would give a bit of protection to the abraided fibres and reduce the risk of fracture?

Regards.

You could coat it with epoxy resin as that would give it some wear resistance, however it would not significantly increase the rods strength nor would it stop it fracturing.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant