I have a need to drill 4 x 12mm holes and 2 x 17mm holes in some 6mm mild steel but the biggest drill bit I have is 10mm.

To buy 12 and 17mm bits is going to cost a bit – are these cone drills any good or tat?

Look for blacksmiths drill bits, 12mm is £1.86, 17mm is £3,36 on eBay!

6mm might be a bit thick for cone drills too – they might start drilling the next size up before the hole has bottomed out at the right size.

Another recommendation for blacksmith drills.

they might start drilling the next size up before the hole has bottomed out at the right size

That’s what I thought. I’ll go with a couple of blacksmiths bits if they’re that cheap!

edit: less than £5 for the two delivered!

Cutting oil is your friend. I’ve managed to drill fairly large holes using cheap hss drill and a low speed pillar drill.

Use cutting oil & don’t spin them too fast.
12mm…800rpm
17mm…560rpm
Once they are cutting give them a nice steady pressure.

A pillar drill, even a basic one, is your friend.

Cobalt bits.

Complete manfail in this thread.

Pillar Drill with morse taper shank together with your random collection of MT Imperial bits in nowhere near the size you actually want is the answer.