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I used an M5x0.8 tap and a 4mm drill bit. Guess a 4.5mm bit had been better?
4.2mm is the usual tap drill size for M5×0.8, using a 4mm was always going to break the tap unless you were trying to put a thread in some cheddar. It's really hard to apply pure torque to a tap by hand and any eccentric force will easily snap a small one even if the pilot hole is the right size, so a drill press with a tap follower in the chuck would have helped, but not enough to get you past the problem of trying to tap Ti (as you have discovered, and as everybody else already knew, an absolute bastard to cut) in an undersized hole. If you ever get the remains of the tap out, you might as well go to 4.5mm on the pilot, it will reduce the cutting force considerably and having the minor diameter of the finished thread oversized will still leave sufficient strength in this application.
Bruce Whitham is the god of thread repair, you might learn something useful from his Getter Out playlist https://www.youtube.com/channel/UC6HG4kXPSgb1Bhx6CY9e6iw/playlists
The chemical method seems to be Ferric Chloride, which you should be able to buy from Maplin as it is a common etchant for PCBs. It dissolves steel but not Ti
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gbj_tester
@t0-ster
Cheers for input
Haha yup i've tried drilling it out with same size bit, and much smaller bit to create a starter hole, but either the bit wont grab/cut any metal, or the bits break. Makes sense as the taps are meant to be harder than everything else in order to cut threads, right? Any specific metal drill bit i should be using? A drill press would help im sure. (ive been using a hand held)
@MCamb @Norfolkbound I used an M5x0.8 tap and a 4mm drill bit. Guess a 4.5mm bit had been better? Lubrication was used.
What are those things in the picture called, that an extractor?
@853Superfly There is no longer any tap sticking out. I tried mole grips when there was a bit (maybe about 4mm), but there was no chance. Also tried brute force. Sharpened a philips screwdriver, rested the inside of the dropout against solid metal, and hit repeatedly with a big hammer. Managed to splinter the tap a little bit, but theres still lots left. Frame is an old Ti 26" mtb. Huge sentimental value, but nothing super fancy/expensive. Pretty bunged up/diy in the first place.
Going from 4mm to m5 or 5mm to m6 hasnt been an issue before in steel and alu. Titanium is harder than i thought! Cant really go up to m6 thread either, as there is no room for a larger hole in the mudguard strut.
I thought about using a hacksaw to cut a line from the outside edge of the dropout, in towards the hole, hoping it would free some room for the tap to budge. Buuuut after a few seconds of contemplation i realized that's pretty overkill and very destructive.
The zip tie solution is actually amazingly sturdy, and the guard sits very well as is. Mostly just the thought of it that bugs me.