Tap tools

[coreyaugustus]

Didn't see this in a search, so perhaps worth a new post. A first frame-related question.

Looking for a recommendation for a specific tap: I need to drill and thread a hole in the bottom bracket to mount a cable guide. Anyone got recommendations on what tools they use for the job? Drill bit size and thread metrics?

Thanks!
-Corey

[false_aesthetic]

IIRC they're M5 8mm or 10mm length.

M5 = 4.2mm drill bit (~halfway between 5/32 and 11/64)

[coreyaugustus]

Thanks for the input! It's not something I'll use often, but always welcome an excuse to buy new tools.

[Gattonero]

An M5 tap is pretty much essential for any mechanic, the vast majority of sundries & threaded bosses will be M5.

In case you didn't know, the size of the hole to be tapped makes for the amount of thread engagement. The smaller the hole, the more engagement the thread will have, but also the chances of breaking the tap will increase. It depends a lot on the material you are going to drill & tap through.
For an M5 thread on steel, a 4.2mm drill bit ought to give about 80% of thread engagement, which is overkill, and you're very likely to overheat the tap and break it.
An average 70-75% of thread engagement is the best compromise, and a lot easier to work with, since you will have to drill 4.40-4.35mm.

Now, a BB cable guide will get very little stress, so don't need to go mad, but since you're buying the tools just do it right in the first place: get yourself two or three sizes of drill bits per every thread size you're going to work with, and keep those drill bits aside and use them only for drilling holes to be tapped.
For M3x0.50: 2.70 for 60%, 2.62 for 70%
For M4x0.70: 3.55mm for 60%, 3.45mm for 75%
For M5x0.80: 4.45mm for 65%, 4.35mm for 75%
For M6x1.0: 5.30 for 65%, 5.20 for 75%
For M8x1.0: 7.30 for 65%, 7.20 for 75%

[Andrew R Stewart]

There are many tap drill charts available for the viewing on line. I generally use a #19 bit for a 5x.8 thread pilot. Andy

[Gattonero]

Now this is exaggerated, but it shows what happens when the size of a tap cannot cope with the ID that's dealing with.
Taps do not just "cut" a thread, they actually extrude the crests of the thread (and this is the reason why you cannot put back in the formed thread the pilot drill bit you've used!) so it is important to choose the size of the drill bit according to the material: if the tap is getting too much resistance it will overheat and snap.
this was easy to get out, but an M4 or worst an M3 cracked in a dropout are a real pain to remove.

Long story short is: pratice on a similar piece of steel (material and thickness) before you actually go and tap the frame