This question is from Volume 6, Issue #4 of RodMaker
Magazine:
I've been using the Common Cents method on all the fly rod
blanks that pass through my shop now. But I'm having a
problem getting the same readings twice. I had a GUSA blank
that I tested and recorded the stats on. A week later I was
showing a customer how I rated thses blanks and used the same
GUSA blank for the purpose of illustrating everything to him.
I happened to look at the ERN rating I got that day and compared
it to the ERN rating I got the week before and they were quite
a bit different! How can this be? I compared a few other
rods and found the same thing. My ERN ratings are changing
from week to week. Any help is really appreciated. Mike, Greenville, SC
If you're using the exact same blank, then we have
to assume the inconsistency you're seeing is due
to some sort of inconsistency in how you're implementing
the system. There are two main areas that simply must
be carefully controlled, if you going to get consistent
results.
The first regards the use of the Cents you're using
to take your measurements with. The Common Cents System
dictates that you must use pennies minted during or after
1996. Pennies minted prior to 1996 have a different weight
than those minted during and after the 1996 year. So if you
just use a random handful of pennies for taking your measurements,
you're not really using the same constant to get the required
deflection.
How much difference could having a few pennies minted prior
to 1996 have on your results? Quite a bit, actually. Let's
say your first rating required 40 pennies to get the required
deflection. And in those 40 pennies you had 15 that were minted
prior to 1996. Now let's say you repeated the measurement and this
time you had 20 pennies minted prior to 1996 in the amount needed
to attain the required deflection. Because of their greater
individual weight, this time you might only need 36 pennies to
get your deflection. Suddenly the same blank has changed its
ERN by nearly a full point! The Common Cents System is sensitive
enough, has enough resolution, that not using the prescribed
weight constant will have a measurable effect on your results.
Use only clean pennies minted after 1995.
The other variable regards the orientation of your blank during
the deflection phase. Most rod blanks varying in stiffness or
power depending on the axis you're working against. Remember
that the wall thickness on most blanks is not consistent. This
is one of the manufacturing anomalies that contributes to what
we callthe spine effect and it plays a role in the power or
stiffness that a blank has on any given axis. While the
difference is usually slight, the Common Cents System is
sensitive enough to easily discern differences in power on any
axis of the blank.
Thus, if you take your first reading with the spine up, then
all future readings need to be taken with the spine up as well.
If you take your reading with the blank's natural curve in a
particular position, then all future readings must be taken
with that curve in the same position. This is the only way
to get consistent results. In the Volume 6 #3 issue of
RodMaker, we covered how to "zero" the tip and the
importance of using the same blank orientation for each
measurement. For future measurements, I'd suggest you mark
the blank butt in such a manner so that any future readings
can be made with the blank in the same orientation.
When you become consistent in how you implement the Common
Cents System, you'll be able to get far more consistent results.
~ Tom Kirkman
Publishers note:
If you have any tips or techniques, send them
along! Help out your fellow rodmakers!
~ Publisher, FAOL
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