Carbon fiber brake duct questions

[eion]

My main winter project (i.e. repairing my car) is starting to come together - I know have all of the major parts I need, and I'm going to see the paint guy tomorrow (and I finally got my fender off!). I've chickened out of doing most of the prep work, having seen how much work it'll be .

Anyway, I bought a couple of carbon fiber brake ducts to replace my destroyed foglamp (one foglamp costs more than one pair of ducts, go figure, and I never used the foglamps anyway). As usual, though, I can't work out the simplest things on my own, so I'd like some advice from anyone with experience fitting these, or experience in working with carbon fiber, or anyone who has anything to say... my questions are as follows:

Firstly, does the CF need to be painted? I've heard about it delaminating under UV. If so, does it need any special prep? I asked my painter, he said he didn't know anything about CF.

Secondly, I think I'm going to have to drill the CF to mount it to the car (see the attached picture of the duct) - is it better to drill it from the smooth front or from the back, which is rough? My plan is to put tape on both sides before drilling and drill a small pilot hole followed by the larger actual hole, using a normal high-speed drill bit.

My mounting idea to follow...

[eion]

Here's my plan for modifying the bumper to mount the duct (see attached picture - the faded black areas denote modifications):

1 - cut a slot into the bumper for the duct's tab to fit into. I'm strongly considering the use of silicon sealant or some other gap-filling adhesive to keep it from vibrating.

2 - cut away the middle part of the bit behind the duct (otherwise I'll get no airflow, so what's the point?). I'm planning on gluing some screen-door mesh to the ducts to stop insect ingress (what glue is and isn't OK for use on CF? The back of the ducts is very rough). I'd love to use some tubing to direct air directly to the oil cooler and the AC condensor, but I haven't found anything suitable, so if anyone knows of anything that they think would work, I'd love to hear about it.

3 - the existing two bolts aren't long enough - they come up to the back of the duct when it's in position properly. I can remove the bolts by drilling the rivet to remove the plate holding them in place, but that would mean I couldn't run foglamps in the future should I change my mind. I know, I know, cutting a slot in the bumper is less changeable, so it doesn't really matter, but the bolts as they are at the moment support the duct, and don't really hurt anything. My plan is to drill into the bumper above and below the rivetted metal plate that the two existing bolts are attached to, and put some new, longer bolts in. The CF duct will be drilled in a corresponding position. To hold the duct in, I'll use a nut/washer behind the duct and a nut/washer in front.

Any comments/thoughts/ideas?
Thanks in advance!

[surflvr911sc]

Tape is a great idea when drilling composites. Either drill from the front or drill a pilot hole from the back w/ the final size hole drilled from the front. Drilling from the back often results in blowouts from too much pressure on the drill. Sometimes it’s hard to drill from the front so in that case use the pilot hole. Good luck, I would like have brake ducts on my SC but it’s hard to do and keep it looking original like I would like.

[island911]

CF is good w/ UV (Kevlar is the one that's not)

Drilling: Ryan is right about tool pressure. You don't want to use a drill-bit; that will grab too much. Some use highly specialized negative rake cutters (far from your average drill bit)
While it's not the quickest, I find using a Dremel type tungsten carbide cutters leaves a nice edge.
To minimize surface delam' (splinters) "backing up" both sides can help a lot. . .though "tape" wont do squat on CF.

EDIT: here's a link Tungsten Carbide Cutters
The ones at the top of that link are okay, but slow. If you can find the diamond shape toothed Tungsten Carbide Cutters, that would be best for the CF. Either way you should be able to see (and appreciate) just how far away you need to be from a standard Drill bit.

[5axis]

Interesting. Island, could you provide more detail of the cutter geometry? I have no immediate need, but since I have access to cutter grinders I am curious.

[island911]

here you go, 5-x . . .

the diamond shape toothed Tungsten Carbide Cutter;



and, far from your average drill bit;


'nuff detail of the cutter geometry?

I suposse, next your going to ask for speed & feed rates.

(2800rpm @ 1.5thou/rev)

[eion]

Wow, thanks Island (and Ryan)!
I've got a tungsten carbide cutter for my Dremel already, and I can pick up a couple of less cylindrical, more cone-shaped ones - they're going to be too narrow, so I'll get one with a narrow tip and one with a wider tip and drill twice... I hope that'll work.
Thanks for the advice on putting something behind it to prevent delamination, I'll give that a go.

I'll post back when I've done the deed, *hopefully* with the ducts drilled, fitted, and un-shattered...

[eion]

I have made some progress (sorry for the quantity/size of images if you're on a modem, I feel your pain!).

Firstly, I've cut the bumper to accept the ducts and the bolts for the ducts - see attached picture. The other side isn't quite as neat - the hole for the tab to fit into is much wider. I'm thinking that some silicone sealant (for sure on the other side) and maybe some kind of rubber sealing strip around that hole could be a good idea, too.

It actually looks better than my limited photography skills would have you believe.

[eion]

I've also done some work on the duct - here's what one of them looks like from the front (the other one looks the same, more or less):

[eion]

And here's a picture of the back of the duct. The mesh is screen door mesh, the finest that Wal-Mart had . Glued on using a hot glue gun - which worked very well indeed.

Those two blobs of glue on the other side serve a special purpose - they are my way of marking where to drill for the bolts. Set the bolts so that the ends are at the level of where the duct would be when it's in place. Apply two large blobs of glue in roughly the right place and leave them to cool down a bit, enough so they're still soft. Fit the duct, and the impressions of the ends of the bolts, in the correct position, will be left in the glue. Fantastic - hot glue guns are very good toy, I've decided.

Now there's only one thing left to do - drill the ducts. I'm really dreading this - especially because I haven't been able to find those specialised Dremel bits anywhere. However, I have what seems (to me) to be a Cunning Plan - apply a thick layer of hot glue to the both sides of the duct around where I'm going to glue. Once cooled, it'll come off easily if you pull it off, but it should hold everything together to stop it from shattering (especially if I glue some wood to the side the bit is going to emerge from).

Good idea, or bad idea? I don't know yet. I'm a little scared to try this, to be honest - I think that some Dutch courage (no offense to any Dutch people) would be useful, but probably not the best idea . I'll probably have a go tomorrow morning...

edit: speling and grammer

[eion]

I have just finished drilling the ducts - I'm just following up in case anyone cares, and for the benefit of anyone who might want to look at this in future.

Well, I used my hot glue gun method. See attached photo (sorry again for the big pictures)...

[eion]

And here is what it looks like post-drilling and after the removal of the wooden backing (that thing on the left is the glue I removed from the other duct):



edit: formatting

[smestas]

eion,

Very good looking project. Where did you purchase the Carbon Fiber Ducts from?

[eion]

And here is the finished product:



It's not bad, IMO - it looks much better in real life, and there's no significant damage/splintering on either side. I'll be using washers, which'll cover the cosmetic damage.

And now, the drill and bits I used...



Note that they're totally normal bits (well, one of them is one of those 'titanium' gold ones, if that counts, but it's very worn), and neither of the bits is in particularly good condition (the larger bit, a 3/16s, is downright blunt, hence the use of the smaller bit to drill a pilot hole). The drill is a normal (cheap 4A) Black & Decker corded hand drill. No effort was made to control feed rate or anything like that. The cosmetic damage around the hole was caused by the bolts that I'm using being 5mm, which is about .25mm wider than 3/16s. Brute force and ignorance, a very useful tool that I use almost daily, soon solved *that* problem , and caused the cosmetic damage. I don't really care, to be honest. And yes, I really ought to get some metric drill bits.

In conclusion... success . I'll post again once I've got the bumper back from the paint shop. I've decided that I won't be painting the ducts - my car is dark green and the carbon looks pretty good with it.

edit: to answer smestas' question.
I got the ducts from The Racer's Group. I would have got the Tequipment ducts from Pelican but they seem to only be available for the 993 and later, and I prefer the looks of the carbon ones; that, and the carbon fiber ones were about half the price. Go figure. Moderators, edit as appropriate if you think this is against the rules...

edit: and another thing - if you're doing what I did, make sure you leave the glue to mostly harden before drilling (it only takes a minute or two), and clean off the drill bits between drilling each hole, as the combination of glue and sawdust seems to clog drill bits rather effectively.

[johnnyo200]

A little late, but when I had to drill into CF and make it look pretty, I used a sharp bit and ran the drill in reverse (at a slower speed) . Clean and perfect everytime.