Drill all of the wood holes in a piece of lumber, making sure that you
dimple the frame underneath. Then slide the wood aside and drill the metal,
using lubricant andd a fair bit of pressure. You want to use enough
pressure at a moderate speed so that you get nice curls from the metal
rather than dust. Some steels do tend to work harden and that is made worse
by not drilling the hole with enough metal removal at a turn of the drill.
You also get to redrill any holes that hit welds without problems.
Redrilling the wood to a #3 drill and pounding in a 1/4" dowel will fill
those misdrills.

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Why isn't there an Ozone Hole at the NORTH Pole?