I was faced recently with trying to drill an 8mm diameter hole in a piece of sheet metal 2-3mm thick. The sheet was a bit from an old lawnmower blade. Sawing that off made me think this metal sheet might be particularly hard. I tried to drill a small diameter hole and gradually increase the diameter with successive drillings. I was using a tower drill and bits which had scarcely ever been used before. The drill was set at medium speed of rotation. It worked for the first 2 diameters, 3 & 5mm I think. But at 7mm the bit just refused to bite. I tried another similar sized bit from a different set, still no good. It was as if either the bits were blunt or the metal suddenly much harder. I have tried sharpening the 7mm bit and added a drop of oil to the cutting surface - less noise until the oil was gone then shrieking loadly before I stopped drilling.
Am I doing something (possibly everything) fundamentally the wrong way? Any adivice would be much appreciated.
TIA Frank Stacey