April 16, 20233 yr I've just spent most of the weekend drilling 54 14mm holes through 5mm and 12mm thick mild steel. What I learned No-name HSS bits from Amazon do not work. At all. Screwfix erbauer coated HSS bits are pretty good but I broke a few as they tended to jam and get stuck. The gold standard for me were the cobalt HSS bits from "UK Drills". A single 7mm bit drilled about 20 holes though total of 22mm of steel in each drill (5mm walled SHS sitting on a 12mm thick UC flange). Just a 3mm pilot hole to get them started. Oil only needed when mid way through the 12mm webs. All done with a Makita cordless drill. High speed, low pressure.
April 16, 20233 yr Did more drilling this evening. The cobalt bits really are in a different league to even really good titanium coated HSS bits. Twice the price but 10 times better. My biggest issue was the cordless drill overheating, even with two of them working alternately, still had to take breaks.
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