I presume it's a bad idea to modify the forks of a bike with a drill to make a screw hole for say front racks? Seems it would affect the structural integrity. Are forks with mount holes made of thicker tube than those without?

What about modifying the down tube to add more spots for water bottle cages?

Use clamps instead of drilling holes in your bike.

If bolting on something is a must, take the frame to a trusted framebuilder and have him braze in waterbottle mounts, and modify your fork. If you can live with things clamped on then I agree with Old Fat Guy, it's a much easier and cheaper way to do things

forks with rack mounts usually have brazed-on threaded bosses or a reinforcing tube through the whole blade

adding braze-ons will require brazing, although i think it's possible to use riveted bosses on thin sections of tubing (as on aluminum frames)

Agree! material is too thin, so you must add a boss of some sort (brazed or do they actually rivet?)
Actually, not sure if I would trust putting a rivet on the fork...

I've seen H20 cages added by using "nutserts". Which are basically a molly type fastener, a threaded insert that you place in a drilled hole, then using a special tool to expand the part inside the frame. If done correctly it's a decent option depending on what kind of frame you are modifying. Properly brazing would be best though.

The thickness of the tubing, and the tubing material itself would be important factors.

Also, rider weight and type of riding.

If the bike is barely strong enough to begin with, drilling holes is probably a bad idea.

If it is overbuilt, and there are margins to spare, I don't think there would be a problem.

EXCEPT: holes can create stress raisers. (It is probably a good idea to talk with a good frame builder who understands these things well.)

Guys,

If the OP is talking about drilling holes himself, I'm guessing it ain't Masi he's tapping.

In any case, I don't think the OP had brazing fittings on and repainting in mind.

I hat to see anyone drill any old frame, if they aren't a builder. I wouldn't drill a Schwinn Colliegiate, for xxxxxsakes.

There are many alternatives for fenders/bottles besides drilling holes, which is what the OP asked about in the first place!

Sorry for ranting, long day.