DrollTroll wrote:
"BottleBob" wrote in message

Now 4 holes each may not be statistically significant but to me
this little test leads me to believe that the difference between using
centerdrills and spot drills is so minimal as to be non existent. So
much for my "centerdrills are better than spot drills theory" but
conversely, anyone claiming that spot drills are clearly superior to
centerdrills for hole location would have to show me some hard evidence
and not just rhetoric. I WILL say that as a result of this little test
I'll be using spot drills much more often if only because of the
chamfering now that I know they don't significantly throw off the
location. I'll also be plowing into material without centerdrilling OR
spot drilling more often as well.
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Bravo! Excellent post.

DT:

Why thank you.

I guess it truly is time to recycle posts!

I don't really consider this recycling, since this is a different
newsgroup and I didn't add in amc.


A sample of 4 IS statistically significant. A sample of 3 is still
significant. Samples beyond 8 in tests like these offer little statistical
advantage over samples of 5.

But your speriment would suggest to *never* center or spot drill, if using
stub drills #1 or bigger!

Sometimes you need a chamfer on the hole edge.

60 ipm?? goodgawd.... That's like hammering in a nail!

Truthfully, I don't normally use such a high feed for drilling, but I
intentionally used that feed to simulate the worst possible condition
to see the results. That way when I used a lower feed the drill would
tend to deflect less and be more centered. Or so went my hypothesis at
the time.


The big problem with center drills is that goddamm pilot breaks off readily
in alloy steels like 4140. Very readily.

One way to get around that is to center drill ONLY with the pilot, and
use a pilot over half the drill dia.


The advantage with c-drills is that if you need a 60 deg chamfer, it's right
there.
It would probably be perty useful to have sets of spotting drills of
numerous chamfer angles, from 60 deg, to 120 deg, as my set kluged c-sinks.

Another speriment would be to vary the speed of entry of stub drills, to see
how that affects hole accuracy.
There was a thread on that in amc (by yours truly), but with no real
consensus, iirc. I do remember Gary Lucas essentially saying eff slow and
careful, just bury dat em effer right in--which is what you did at 60 ipm.

Bottle, you shoulda been a puhfessuh.... perhaps in chemistry.

An inside joke that only a handful would get, eh? LOL

--
BottleBob
http://home.earthlink.net/~bottlbob