On 11/12/2014 3:18 PM, whit3rd wrote:
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.

M42 grade or solid carbide should do the trick. Solid carbide will
drill when red hot. And stay sharp after cutting through tough stuff.
I cut holes in lathe tool steel to make shaping cutters. Only carbide
would do that work.

Martin