Drilling hard steel

[Greenfield]

I had a stripped thread in a cast iron cylinder head so I've had the hole welded up so that I can drill and tap a new thread. Unfortunately I can't make much of an impression on the weld. I'm using a 2mm cobalt drill bit in a pillar drill. What do I need to drill this really tough steel and then my next problem will be can I cut a thread?
Hoping someone is knowledgeable to point me in the right direction.

[Kernowpete2]

What size tapping drill will you need? Is the 2mm a pilot hole? If it is go up in size, even up to the tapping drill size. Slow the drill down as slow as it will go, use cutting oil and a fair bit of pressure.

[Greenfield]

Yes the 2mm drill was a pilot. The tapping drill is 7mm for a 5/16" BSCy. I'm guessing that the combination of welding material and heat hardening has caused the problem.

[Kernowpete2]

I'd go straight in with the 7mm (as long as it's sharp), plenty of oil and pressure at low speed.

[iansoady]

I agree. I only came across cobalt drills recently and they went through a seized bolt (Yamaha) as if it was butter, unlike the HSS bits I'd been trying for ages.

[Greenfield]

Update: had a go with cobalt drills and they just blunted. Had a go at annealing by heating it up until it was red then cooled over 1 1/2 hours, but still just as hard. My engineer friend said that martensite had probably formed during welding and the way to sort it was to heat it to 800' for three days then cool it very slowly. That wasn't going to happen, so I took it round to his workshop and over an hour he gradually drilled it with some very special carbide drills. So job sorted.

[AltcarBob]

Carbide drills are very good at going through hard materials but they snap if you look at them sideways they are not for hand drilling they will just go"tink". Your engineer friend probably used a Bridgeport type vertical mill which clamps everything tight and doesn't side load the drill bit. You're lucky to have such a friend I hope you rewarded him with lots of choccy bics.

[Greenfield]

Yes he spent a fair bit of time getting things lined up and clamping it absolutely solid. He said that the drills were designed for removing broken taps. Impressive. I didn't know such things existed. It's good to keep learning.

[iansoady]

For those in the Midlands that require such services I can recommend Nametab Engineering in Redditch. Reasonable prices and excellent work.

[cornishbob]

welding cast iron? blimey!! things have come a long way