Multiple Carbon Fibre Layers - how thick?

I am in the process of repairing and modifying the rudder cassette on a Farrier F-25c. I want to add an additional thickness of 6.35 mm to one section of the cassette which I will do with layers of carbon fibre and resin infusion. For various reasons I am not interested in adding CF plate and because some of the thickness will be machined out, I can't bulk the addition up with core.

The carbon fibre I am using is 6.5oz warp unidirectional CF , specified as being 0.18 mm thick which indicates 35 layers will be required to add a 6.35 mm thickness (it is only a small area). The alternating layers will be at 90 deg.

The CF specs are http://www.fiberglasssupply.com/Pro...ents/Carbon_and_Kevlar/carbon_and_kevlar.html it is C72-4109

I have cut the 35 layers of CF but even when I pinch them down it looks like their total thickness will be a lot more than the 6.35 mm.

Do I trust to the 0.18 mm/layer and the compaction ability of resin infusion or is there some other rule of thumb I should be using to calculate the end thickness?

 

I wouldn't trust it unless it says that the thickness is "X mm ,laminated".
You could use a small amount of carbon and make a test...

 

What does you trusting it have anything to do with it?

a) normally these specs assume that you are going to vaccum and infuse.
b) when you do vacuum and infuse you will have a different thickness than just holding them together "pinching them down"

Your best bet Nick is to do a test swatch , but if you are going to be milling it out anyway, why do you need it to be that precise?

 

Thanks to both of you...and yes, my precision is a bit over the top. I should have said "about 1/4 of an inch" Still, I don't really want to end up with way too much remaining thickness; though, that could also be ground down.

 

Just searched the internet and came up with this

http://www.boatdesign.net/forums/boat-design/calculating-laminate-thickness-4826.html

on this forum. Apologies for starting an unnecessary thread

 

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Just in case you haven't done it before: grinding carbon/epoxy is very, very nasty-worse itch than fiberglass and,of course, the dust is conductive so dust balls blowing around and into an electric socket or tool can be very dangerous. Good Luck!

 

Thanks Doug: I do know that CF particles were used in Kosovo to kill electricity supply...but had not thought of the other. Thanks. I had being doing some grinding and taking the necessary precautions. I am assuming that a machine shop will cut the slot for me.