On 2007-12-30, komobu wrote:
Hi;

Often I need to extract broken bolts / screws and am thinking about
picking up some left handed drill bits. Usually I have to extract
broken steel bolts and screws from aluminum heads and manifolds. Most
often they are broke off clean and need to be drilled. When looking at
drill bits to purchase, I see they are made of HSS, Cobalt and
Titanium. What drill bit material is the best for staying sharp and
long lasting life?

Well ... first off -- they are *not* made of titanium, though
some may list them as so made.

What those are are (hopefully) HSS or Cobalt steel coated with
Titanium Nitride (TiN) for longer wear life.

I, personally, would go for the Cobalt steel, probably in screw
machine length, and with split points.

I don't see the TiN coating being any help in this -- it is more of a
help in production machining of specific materials. *And* -- it often
is used to try to make people think that poor steel bits are better than
they really are. You can get TiN coatings on both excellent steel and
total junk steel. (With some experience, you may become able to tell
the difference by sight in many cases.)

You'll be drilling tough bolts (thus the Cobalt steel) -- it is a bit
better at cutting tough steels than plain HSS in my experience.

You'll probably be drilling with a hand held electric drill motor, so
the split points will offer two benefits:

1) It will be easier to start the holes without the bits
walking.

2) It will reduce the force which you will need to apply
to drill through, making it easier to control the drill
and keep it properly aligned with the hole.

Now the one thing which might argue for HSS instead of Cobalt steel is
that for the smaller sizes, MSC wants to sell the bits in packages of
12, and in cobalt steel they get expensive rather rapidly. (It has been
some time since I bought left-handed bits, but I recently bought some
right-handed screw-machine length cobalts steel with split points to
replace some lost out of an index of similar bits. (I found most of
them on the floor, but about three seem to have run under the drill
press and hidden with the swarf which has made its way there. :-(
Anyway -- we were talking about $20.00 per size. -- Ouch!

Also, please comment on grabit and other extracting methods. What do
you think is the easiest way to remove a broken bolt?

No experience with the grabit. The usual broken screw
extractors work sometimes -- but are more likely to break in the screw,
leaving you with something even harder to drill out -- which will
probably need solid carbide bits. :-)

Good Luck,
DoN.

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