How to Drill a Straight Hole in a Tin Sheet

[mklotz]

I'm not sure what "neat" and "straight" mean in this context but the larger 15 mm hole is distinctly trilobal, not circular.

Use of a step drill, as suggested, is definitely preferable. Not only will you get a circular hole, but it will be much easier to place it precisely; precision locating on wood is an oxymoron.