Drilling hard materials

Following last weeks post, I quizzed Brendan about their smaller drills and asked if they too were ground with the split point. He pointed out that they were too small for that.

“All drill bits from 3mm up to 13mm are 135deg split point – smaller than that are 118deg”

A big of reading of Caxton’s Engineering Workshop Practice shows that 118 is general-purpose (2×59) and 135 is used for harder materials. The shallower 135 angle does not cut so quickly and requires more power because of the longer length of the cutting edge. This can result in the drill keeping sharper for longer.

I did a bit more testing this week. I found some 304 stainless that was 6mm thick and some 316 stainless steel that was 5mm thick. I drilled these with one of the 5 mm bits. I did find that the drill slipped in the chuck slightly so I tightened that and then the rest of the drilling went smoothly. This is consistent with the notes I saw in Caxton. The drill bits handled this material well, one thing I did notice was that on breaking through the holes formed a small circular flap which needed breaking off. Not sure if this is normal for stainless but it did result in almost no need to debur the holes.

Brendan also sent over some little 2mm bits. I tested that out alongside a HSS bit on some spring steel that I’d extracted from some old windscreen wipers. This was a struggle but I did eventually breakthrough. The HSS bit on the other hand barely made a dint.

My final challenge for the little bit was to drill some holes in a PCB. The fibreglass backing on these boards is notorious for blunting drill bits. After 100 holes I found no change to the performance of the bit. I was specifically interested to check the backside of the board as that would show larger breakout if more force was necessary due to blunting. I suspect a lot more holes would be required to get a good idea of the wear and tear but so far this has shown to be a good option for this material.

In pretty much all my testing the drill bits barely got warm. This is a clear sign that they are cutting well through these tough materials.

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