Working with Carbon Fiber and Aluminum

[Swissyhawk]

I received a 38 mm CF Mongoose for Christmas. Most of my rockets are FG. I've never built a CF rocket, so I was wondering about a couple of things.
1) Do I still need to sand it with 60-80 grit paper wherever I'm going to apply epoxy?
2) The fins aren't beveled. I've always used a little sanding drum on my Dremel tool to bevel the fins. Will this work with CF? I don't want the carbon layers in the fin to start splitting apart or to have a bunch of loose fibers.
3) Are there any tricks to drilling it or is it pretty much like FG? I always try to put something behind the hole so it doesn't split out too much.
4) Anything else I should know?

Also, it came with 2 Aluminum bulk plates for the AV bay. Each plate has three holes - probably 2 for the all-thread and one in the center for the eye-bolt. How do I pass the wire for my ejection charges through the bulk plate? Do I have to drill another hole in it? Can I drill aluminum? Do I need special bits?

Thanks for the help.

 

[Ravenex]

1) You should sand any composites you plan to bond, CF sands easier and faster than FG. Also 60-80 grit seems to course. I defer to the stickied composites thread: https://www.rocketryforum.com/showt...what-your-government-doesn-t-want-you-to-know

2) See above, also be sure to wear a mask and gloves, possible long sleeve. CF dust is really irritating to your skin and bad for your lungs, I usually immediately throw my clothes in the wash and shower after working with it. Carbon splinters are sharp as heck, worse than fiberglass.

3) I drill a lot of CF for my shroud products. I use a special type of brad point drill made for carbon that cuts the threads and doesn't pull them up. They can be found at mcmaster carr: https://www.mcmaster.com/#composite-material-cutting-drill-bits/ Also drill at high rpm with very little pressure.

As for the aluminum it can be drilled with a standard drill bit and drill. A drill press will make it easier but isn't needed. You will want to use a spotting punch to create an indent in the material for the drill to start on, otherwise it will walk across the surface before cutting and your hole won't be where you wanted it. Make sure you have a sharp drill and use a coolant or cutting fluid to aid in cutting and prevent the chips from melting to the bit. There are lots of proper cutting fluids that are great but dawn dish liquid also works very well with aluminum and can be washed off with water.

 

[manixFan]

Brian has great suggestions but unfortunately in the thread he linked to the images are no longer available. I agree that 80 is too coarse and I've found that for CF 120 - 200 grit works well to create a bit of tooth for the epoxy. Not only are the CF dust and fibers being bad for us, but they can also bad for electronics. The particles can be conductive so care should be taken to keep them out of electric motors, off circuit boards, etc. Using a wet/dry vac with some water in the reservoir can help trap the dust.

I like the idea of using Dawn as a cutting fluid for aluminum. Already have it on hand and can use it to clean up afterwards. Great tip.


Tony

 

[mpitfield]

One thing to keep in mind when deciding how to wire your charges through the bulkplate is that carbon fiber is conductive. I have successfully used the two piece Delrin mini rail buttons to insulate using a 316 stainless machine screw as the terminal. The standoff part of the rail button becomes the insulator and the washers insulate from the stainless wahers and nuts on both sides.

Sent from my ONEPLUS A3000 using Rocketry Forum mobile app

 

[MClark]

Aluminum and carbon fiber can have galvanic corrosion issues.
A rocket may not be around long enough for it to be a problem.

 

[Swissyhawk]

Thanks for the replies.

Yesterday I beveled the fins using my dremel tool. I'm happy with the way they came out. I wore a mask, gloves and long sleeve shirt. All my clothes went in the washer when I was done. Now I'm tacking the fins on. I sanded the body tube where the fins attach with 100 grit sandpaper. It seemed to work pretty well.

mpitfield - Thanks for the info about CF being conductive. However, the bulkplates are Aluminum, so I won't have that concern. My plan is to run the ematch from a BP vial through the bulkplate and wire it directly to the altimeter. The bulkplate came pre-drilled except for a hole for the wire, so I'll have to drill that. I've also thought about just borrowing the FG bulkplates from my mini-screech - which has already been built and flown.

I also ordered a 3D printed 38 mm nosecone AV bay sled for an eggfinder. I figure tracking is going to be a must have with this rocket.

 

[mpitfield]

mpitfield - Thanks for the info about CF being conductive. However, the bulkplates are Aluminum, so I won't have that concern.

Of course had I paid attention I would have realized this, :wink:.

Are you going with surface mounted fins with fillets or are you planning on going down the T2T road?

BTW I have the original Mongoose 38 that has the hand-laid CF nosecone, which I did not use. On Mongoose I opted for a single layer 3K 2x2 twill T2T layer. To flame proof this, I am not suggesting this is necessary on this build, because I believe that you can easily get away with not doing it. However in my case, and this is something you may want to take into consideration. I knew I would be aiming for builds that would push the limits of composite components and I wanted to gain some hands on experience before I attempt it for the first time on a build that really needs it. This just seems pragmatic to me.

The point being, if you think this build may lead to going further down the rabbit hole then you could use this as an opportunity gain some good hands on experience with building techniques and knowledge. I say this not knowing your objectives for this build but keep in mind adding more, within reason, is not necessarily a negative, it just takes more resources and effort. BTW my first T2T was also a single layer 3K 2x2 twill layup on a 54 mm carbon fiber Tomach, which was my first MD build.