EXPLORING NECK RELIEF
By Hideo Kamimoto
The concept of incorporating neck relief whenever adjusting the action of a fretted instrument is well accepted and is a standard procedure for every builder and repairperson. Usually, adjusting neck relief is a normal part of setting up an action. Most of us, at the same time, would also like to improve the consistency and predictability of our action work so that those situations where nothing seems to improve an action occur a lot less frequently. And of course, we've all had those happy situations where the action turned out fine with no effort (and no knowledge as to why this particular one turned out so well!)
Rather than resigning ourselves to a "you win some and you lose some" attitude, or shrugging off unpredictable results to the peculiarities of the individual instrument, it is worthwhile to examine the action related areas which are under our direct control. Neck relief is one of these areas, and because it is not being used in its most refined form, a study of the concepts of neck relief will enable you to develop more consistent results and lower actions.
Neck relief is supposed to follow the curvature of a vibrating string. In its simplest form, this curve takes the form of a parabola. The center span of a suspension bridge is one example. A rope hanging between two posts, or a guitar string held loosely in your fingers is another. In guitar terms, one of the points of suspension is the nut and the other is the bridge saddle. The exact center of the span is the twelfth fret. The actual relief required is the distance between the string and a straight line drawn between the center (twelfth fret) and one of the suspension points (the nut). This distance (relief) will vary depending upon the distance from the ends. More importantly, the relief will not change at an even rate, but at a rate determined by a parabolic equation.
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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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