Originally Posted by powers2b
High speed is your friend.
Cuts the fibers and seals the resin.
Use a diamond blade (tile saw) or cutoff wheel (dremel).
NEVER A HACKSAW
Use a jig to guide the cut.
Sandpaper the edges and you are done.
The cutting of carbon fiber composites is more of a grinding process then a cutting process; essentially, you want to grind away the fiber and resin slowly to make the cut.
This actually isn't as tricky as the above implies. Cutting with a fine tooth hacksaw works perfectly fine, at least for me and pros who have told me to do it this way - including the Park Tool website:
The cut with a fine tooth hacksaw is very smooth without any fiber breakage or raggedness.
This job is actually quite simple, but the old saying of "measure twice, cut once" should be followed, even if you're a very confident person!! Also, since you have so much to work with (1 1/2"), I might suggest that you do a practice cut, maybe take 3/4" off it, just to see how your technique works.
I've cut a couple using my miter box and hack saw. The miter box isn't an optimal way to guide the saw for this though and I might try some other sort of guide next time. However, it worked fine in the end.
With my miter box, I wasn't able to get it perfectly square cut, and had to finish the job with a large flat file to get it squared up. It was a little fussy to carefully square up the cut. I found the high spot using a carpenter's square, sighting towards a window to see the light peeking through the lower area. I marked the high area with a pencil, and then carefully flat-filed to bring down that area. Re-sight, repeat, etc. Took about 5-10 minutes (easy does it!). You could do the same thing with a bench belt or wheel sander - ASSUMING you know how to use it, select the correct grit and are very, very careful!!!
I'll probably do it the same way next time since it's not that tough to square it off with a file, and I feel comfortable doing it, but really, it would be much easier to either buy a dedicated cutting guide that are available for cutting steer tubes, or I've read that two hose clamps can be used as a guide.
I also lightly dressed the edges - inside and out - with a file (flat file for outside, round/rat tail file for inside). This was just a tiny, tiny bit to take the sharp edge off. Not really noticable visually, but you can feel it. You could do the same thing with sand paper.
Finally, if you're at all uncomfortable, take the bike - as is - to whatever bike shop you have the best luck with and tell them to cut the steer tube and install the stem in exactly the same position you currently have it. It should be a minimum shop charge for that (whatever their minimum is). I'm thinking $25 or so would be reasonable.