The graph
illustrates the type of neck relief required in order to
follow the curve of the vibrating string. While the point
of maximum relief is at the sixth fret, the amount of
relief climbs fairly evenly on a flat curve. Beyond the
sixth fret, the relief drops down quickly as the position
nears the twelfth fret.
While the graph shows a scale
length of 24 inches, in actual fact, the scale length has
no bearing on the amount of relief. The relief required
is dependent only upon the action, which in the example,
is 4/64" at the twelfth fret. You should note, also,
that the actual clearance required at the sixth fret is
only .013" or less than a sixty fourth of an inch.
If you would like to apply the relief figures shown in
the graph to other actions, make your new relief
measurements directly proportional to the current action
of 4/64".
The most important point to
keep in mind is that on a properly relieved neck, the
amount of relief does not drop off evenly on either side
of the peak. Instead, the relief is concentrated more at
the upper frets, from the sixth to the twelfth, with a
fairly constant rate of change between the nut and the
sixth. But because necks tend to bend progressively more
toward the nut, and because careless filing of the frets
tends to accentuate the problem, you will often find that
the actual relief is exactly opposite the desired contour.
That is, there is an abrupt increase in relief between
the nut and the third or fourth fret while the relief
toward the twelfth fret changes at a constant rate. The
end result is poor action, with fret buzzes that persist,
regardless of action and relief adjustments. The string,
when fretted in the lower positions, can actually be
buzzing on the upper frets, such as the eighth or ninth,
rather than on the fret immediately In front of the
fretted position.
Determining the exact fret that
causes a buzz is very important, and can be quickly done
(when the instrument has steel strings) by clipping one
lead from your multimeter to the string you are testing
and setting the meter to read resistance. Capo the string
at the position where you get a fret buzz, play the
string and touch the other end of the test lead to each
fret. You will get an indication at the point where the
string contacts the fret. You will often find string
contact on several frets, and in fret positions which
will be surprising. If you get contact on an occasional
random fret, this would probably be only a case of uneven
frets. If you get contact on several adjacent frets, this
is probably an indication of improper neck relief.
Curing improper neck relief
contours is not easily done with just a truss rod
adjustment. You may have to selectively mill the frets in
order to change the contour or, in extreme cases, you
might even have to dress the fingerboard to the proper
contour and refret the instrument. The absolute best time
to adjust the relief contour is during fingerboard
preparation. This is the most critical stage of a fret or
refret job and if you take the time and care to properly
contour the fingerboard (taking into account the bending
of the neck under string tension) you greatly improve
your chances for a good action.
Take a critical look at your
neck and fretwork and measure for relief and for fret
buzzes carefully. The use of an accurate straightedge,
dial calipers, feeler gauges, and even electronic testing
gear will help localize problem areas. And while there
are other factors which complicate the "pure"
application of the vibrating string's parabolic curve,
such as overtones, the "end effect" of stiff
strings which do not pivot freely at the ends, and
picking position and picking technique, attention to the
principles outlined above will pay off in better and more
consistent results.
Hideo, a repair specialist is
well known for his excellent book, Complete Guitar Repair,
first published In 1976. He made the mistake of stopping
by recently when we had "catalog" on our mind.
He finally relented to write an article for us when we
promised to let him out the door. With a degree in
electronics and physical science, Hideo was well-
equipped to move into guitar repair some 30 years ago. He
is currently studying instrument acoustics using real-time
analyzers, and he spends his time repairing mostly
instruments of the violin family at Steven's Stringed
Instruments in San Jose.
Text From The Luthiers'
Mercantile Catalog - 1993
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