Quill stem bolt hard case.

[gbb]

In my experience,starting small and progressively using slightly larger drills each time doesn't work well. It means the actual cutting is focused on only the extreme edge of the drill bit, which soon overheats.
Slow, lots of lube, quality drillbits (Dormer certainly used to be good quality) and the correct amount of force.

 

[midlife]

Other option is to use one of those drills that act as an apple corer and remove the alloy collar holding the bolt head?

 

[SkipdiverJohn]

Small sized drill bits are easier to keep correctly centred though, and once you have made a pilot hole through whatever you want to drill, the larger size ones to follow don't need anything like as much pressure applied to them to make them cut. I always pilot drill any but the smallest of holes, and if I'm using a hand held drill of limited torque output I will often follow the pilot with an intermediate size, then a final finish size last. I want to keep the loading down to minimise the chance of the drill stalling. Preventing overheating is a matter of adequate lubrication combined with an appropriate speed for the size and metal spec of drill bit being used. HSS will tolerate higher speeds than plain carbon steel.

 

[curzons246]

Other option is to use one of those drills that act as an apple corer and remove the alloy collar holding the bolt head?

The bolt head is 7cm below the surface might be tricky but I like your thinking

 

[curzons246]

In my experience,starting small and progressively using slightly larger drills each time doesn't work well. It means the actual cutting is focused on only the extreme edge of the drill bit, which soon overheats.
Slow, lots of lube, quality drillbits (Dormer certainly used to be good quality) and the correct amount of force.

Thank you for sharing your wisdom, what you say makes a lot of sense.

 

[curzons246]

Small sized drill bits are easier to keep correctly centred though, and once you have made a pilot hole through whatever you want to drill, the larger size ones to follow don't need anything like as much pressure applied to them to make them cut. I always pilot drill any but the smallest of holes, and if I'm using a hand held drill of limited torque output I will often follow the pilot with an intermediate size, then a final finish size last. I want to keep the loading down to minimise the chance of the drill stalling. Preventing overheating is a matter of adequate lubrication combined with an appropriate speed for the size and metal spec of drill bit being used. HSS will tolerate higher speeds than plain carbon steel.

Thanks John that's really comprehensive