On Tuesday, November 11, 2014 8:00:34 PM UTC-8, Stuart Wheaton wrote:
Drilling into the end of a large threaded rod, centered, punched and
drilled starting at 1/8" worked up to 3/8". 3/8" is about 1/4 of the
way into the hole and quit cutting, Either I hit a hard spot in the
steel, or it got work hardened, I was using coolant, but not flood.

Swapped to a new and sharp HSS bit, no joy at all.

If the workpiece is stainless, this is a familiar problem: heating
the drillbit causes the leading edges to dull (burn away, actually)
and burnishes a work-hardened patch. It could also be
a loose inclusion, and you are just spinning a button at the
bottom of the hole, not contacting the

It MIGHT help if you center-punched the bottom of the hole before
trying to cut again. Safe bet: start another hole. This piece may be ruined.

1. Try a masonry bit, 3/8 is something I have in the shop.
2. Heat it with a plumbers torch and see if I can temper the hole.
3. Go get a TiN or Cobalt bit...

Heavy-duty cobalt bits are a good solution (they stay sharp a lot
longer in heat-buildup cuts). Better, is to cool the tool (or cut
for a few seconds, then back out and brush the tip with a
lube). It might be a good idea to drill a smaller pilot hole (so the
threaded rod doesn't deflect) before putting the fullsize drill in.
If the alloy is work-hardening, you want to keep full pressure
on the cut at all times, or the drill will polish another hard dimple
instead of steadily removing the worked material.