On Tue, 9 Nov 2010 18:27:53 -0800, "Michael Koblic"
wrote:

The purpose of center drilling is to start the hole exactly where intended
without the drill point wandering all over the place, yes? This is then
normally followed by a twist drill of the desired size etc. From this
concept I would assume that the axes of the drills are concentric, or in
other words the hole drilled by the twist drill is exactly concentric with
the hole started by the center drill.

snip
First -- center drill and spotting drills are like
everything else, you can get cheap ones.

While center drills are commonly used for this purpose, they
are intended to drill holes for lathe centers for turning
between centers. While 60 degree is the most common because
most lathe centers are 60 degrees, 82 degrees and 90 degrees
are also available. the 60 degree center drill is the least
expensive because it is the most common.
for examples see
http://www.google.com/images?q=%22tu...w=1336&bih=701
http://www.use-enco.com/CGI/INSRIT?P...PARTPG=INLMK32
http://www.use-enco.com/CGI/INSRIT?P...PARTPG=INLMK32

The more correct tool is what is called a spotting drill.
These come in several included point angles and should be
matched to your drill bit geometry, i.e. 135 degree or 118
degree point. These come in several sizes and if you use
one slightly larger than the drill or limit the depth to
limit the size of the taper, you will get a chamfered edge.
These come in 90 degree, 120 degree and 150 degree included
angle.
for examples see
http://www.use-enco.com/CGI/INSRIT?P...PARTPG=INLMK32
http://www.use-enco.com/CGI/INSRIT?P...PMPXNO=3007493
These drill tend to be stiffer and last longer as these do
not have the small "tit" like a center drill that isprone to
drift or break off.

for a discussion see
http://bbs.homeshopmachinist.net/showthread.php?t=38729

Depending what you are doing, the standard jobber length
drill may be introducing run out and is more flexible than a
short drill like a screw machine length. I suggest you get
one or a few good screw machine length drills with 135
degree split points and give these a try with the spotting
drill. Most likely you wont need a complete set, just the
sizes you use for you usual tap drills.
for examples see
http://www.use-enco.com/CGI/INSRIT?P...MPXNO=19503626

The sites are examples only and most any mill supply should
stock.

Let the group know what you discover for your shop.



-- Unka George (George McDuffee)
...............................
The past is a foreign country;
they do things differently there.
L. P. Hartley (1895-1972), British author.
The Go-Between, Prologue (1953).