On Sun, 4 Jan 2009 08:38:33 -0800 (PST), Ernst
wrote:

Hello All,

I had the occassion to drill through some very hard fasteners.
Standard HSS bits were becoming dull in a very short timespan, so I
opted for cobalt bits to continue.

Armed with a set of Craftsman bits, I went back to work. Not much
difference. Altering speeds, using light oil, nothing really worked.
Ended up using a Dremel tool with a cut-off disc to remove the
fastener heads (which presents other problems).

What gives?

Regards,

Ernst
-------------
There are cobalt bits and there are cobalt bits. From personal
experience and the many postings in these news groups it is
apparent that "Craftsman" quality has slipped in the past few
years. You may want to try some name brand cobalt drills from a
mill supply.

Another consideration is that the cobalt bits are not necessarily
"harder" than HSS (although they are more brittle) but the cobalt
provides higher heat resistance allowing higher RPM and faster
drilling, which may be important in a production environment.

If you have a very rigid setup [good fixturing, good spindle, and
a steady hand] carbide drills may be "better" as these are
definitely harder than HSS. These are also much more expensive,
difficult to resharpen, and are *VERY* brittle and prone to
breakage at the slightest provocation.
for example
http://www.use-enco.com/CGI/INSRIT?P...MPXNO=19499422
http://www.use-enco.com/CGI/INSRIT?P...PARTPG=INLMK32
There are special "die" drills for very hard workpieces.
http://www.use-enco.com/CGI/INSRIT?P...PARTPG=INLMK32

Unless speed of production is an important consideration, the
total tooling cost may well be lower with good quality HSS
drills, slow(er) speeds, and lots of HD cutting oil.
http://www.use-enco.com/CGI/INSRIT?P...PARTPG=INLMK32
http://www.use-enco.com/CGI/INSRIT?P...PARTPG=INLMK32


There are many other good brands/suppliers available, I used Enco
just as an example.

Let the group know what you settle on.


Unka' George [George McDuffee]
-------------------------------------------
He that will not apply new remedies,
must expect new evils:
for Time is the greatest innovator: and
if Time, of course, alter things to the worse,
and wisdom and counsel shall not alter them to the better,
what shall be the end?

Francis Bacon (1561-1626), English philosopher, essayist, statesman.
Essays, "Of Innovations" (1597-1625).