On Wed, 10 Nov 2010 00:24:41 -0600, F. George McDuffee
wrote:


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.

Thank you very much for the references.

I recall some time ago someone here mentioning spotting drills as
being distinct from center drills. For some reason I did not
investigate this further.

The forum discussion is most valuable and I bookmarked it. I read
about half of it and will finish it in peace. Then I will read it
three more times to make sure I understand it fully :-)

The drift seems to be that a) spotting drills are the correct tools
for what I have been doing, b) they can be ground from old twist
drills, c) the center drills are self-centering if used on a lathe as
intended, and d) one could therefore conclude that their behavior in a
drill press/mill such as I describe is entirely possible.

I do have a set of machine screw drills which I use exclusively on my
mill and in the Taig, one of the reasons being the space limitations.
I should add that they behaved similarly to what I described with
jobber drills.

Still, all of this is a good excuse to go through the tramming,
run-out-etc. tests again.

However, I have some broken 1/4" drills lying about so I shall have a
go at converting them into spotting drills.

Michael Koblic,
Campbell River, BC