Please do tell us how exactly you compensated for the indexing error
in your X/Y spindle position to claim base accuracy to a couple 10's
in your hole positions.....
JR
Dweller in the cellar

On Thu, 28 Aug 2008 12:19:58 -0700, BottleBob
wrote:

To All:

This is from a post I made some years ago it amc. I thought the
readers in here might enjoy a break from all the political trash
talking. g

Now let me just add that before my test I was of the opinion that
centerdrilling was more accurate than spotdrilling.

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I did a little test today on my day off. In a piece of 6061 I
center drilled 4 holes with a .125 piloted center drill .1 deep at 2000
rpm and 5 IPM. Then I spot drilled another row of 4 holes .125 deep at
2000 rpm and 5 IPM. I then peck drilled eight .234 dia. holes 1" deep
at 5000 rpm and 40 IPM. As a side test I peck drilled 4 more holes
with a #1 (.228) cobalt 135 degree split point stub drill without
spotting or center drilling at 6000 rpm and 60 IPM. I then took a
tenth indicator and checked each hole for location going the exact same
amount into each hole. The holes checked as follows:


First row of center drilled holes.

X Y
#1 +.0003 0
#2 +.001 -.0001
#3 -.0007 -.0002
#4 +.0003 +.0005


Largest amount out = .001 Average amount out = .00039


Second row of spot drilled holes.

X Y
#1 -.0012 0
#2 -.0002 -.0002
#3 -.0005 -.0006
#4 +.0002 -.0002


Largest amount out = .0012 Average amount out = .00039


Third row of hole made with NO spotting or centerdrilling.

X Y
#1 -.0011 +.0002
#2 -.001 -.0006
#3 -.0005 +.0004
#4 -.0007 -.0007


Largest amount out = .0011 Average amount out = .00065


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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