I have a Real Bull mini-mill that I converted to belt drive (which had the unfortunate, undesired side-effect of actually LOWERING the torque), and I have no trouble drilling deep 1/4 inch holes in mild steel. I keep my bits sharp (at the risk of starting a debate, thank you, Drill Doctor!), and clear the chips from the hole every so often. I use cutting fluid while drilling (I use Re-Li-On for this). I have cut 1/2 inch holes, though I definitely step up to this size, probably drilling 3/8 inch before drilling the 1/2 inch. I think there have been a number of excellent suggestions above and won't repeat them here. The one that I will add is that a boring head would be a useful way to make larger holes with minimum machine load, though the increased number of passes needed means it will take longer. Good luck, and let us know how you make out!