That's why the saddle is laid into the bridge on an angle... so that all of the strings will have the same vibrating length (dashed line in the graphic above). String Length Compensation is essential for an instrument with a fixed scale length and fixed fret spacing, but different string thickness (stiffness).
On a 25.4" scale (645 mm) Woody compensates the treble- side e string 2mm and the bass-side E string 5mm. Thus, the saddle provides compensation for all of the strings by being placed into the bridge at an angle.
The graphic below shows that the B string is compensated more than the G string. This is necessary because the B string is plain steel while the G string has a core of steel but windings of brass or bronze. This makes the outside diameter of the G string greater but does not add to the stiffness. It is the core diameter that provides the stiffness and creates the dead length. The B string 'core' is stiffer than the G string core.
One can buy a pre-slotted nut from LMI or StewMac (or others). They are not expensive and they are very accurate. But they are not made by the same person that made the rest of your guitar and it will show... a perfectly machined and polished nut will (IMO) shout out at the end of a completely hand-made fretboard. Maybe this is just one of those things that luthiers see but others do not.
On your first guitar maybe one should buy a pre-slotted nut. String spacing and slotting is a bit tedious and if one is not completely focused on the little piece of bone in the vise, it might take several bone blanks to carve it satisfactorily. As luthiers, focus and tedious are what we do.
The first assist that one can have is a small vise that is set up directly under a task light.
Setting the Neck Angle_
The neck angle is the angle at which the plane of the neck diverges from the plane of the guitar top. This angle will determine the thickness of the bridge, and the height of the strings above the soundboard. These dimensions are critical to the action, tone and 'playability' of the guitar.
For a 14-fret neck and 25.4" scale, the measurable dimension of the neck angle set up is to have ~3.5 mm clearance between a straight edge laid on the arm of the neck and the soundboard at the location of the saddle. (The arm is the neck without the fretboard.)
The reasoning behind this dimension is to achieve a ~12.5 mm clearance of the strings above the top at the bridge_ (9.0 mm bridge thickness plus 3.5 mm saddle height, on a standard dreadnought).
A way to test for neck relief is to string up the guitar to pitch. Then slide a straight edge up the neck toward the nut, beginning at the 5th fret. At the 4th or 3rd fret you should feel a little bump as the corner of the straight edge hits the top of the fret. Sliding the straight edge further, one should feel that same little bump at each fret as the straight edge climbs the arc of the neck toward the nut. This arc is the neck relief.
The easiest way to put relief into a neck that is too straight is to crank a two-way truss rod counterclockwise in very small increments, testing with a straight edge after each 1/4 turn. This should be done by an experienced repairman. If you don't have a two-way truss rod, or don't know, then putting relief into the neck should definitely be done by experienced repairman.
Make a pencil-line layout of these dimensions on the nut blank. Then begin by kerfing each slot line with a narrow-kerf fret saw right on top of the line. These initial kerfs are then widened and deepened with the appropriate file until it reaches the curved radius-plus-0.060" arc drawn on the blank.
Woody uses the LMI neck-joint fixture pictured above. On this fixture both the heel-block mortise, and the neck tenon can be cut. Together, the angles on the mortise and tenon determine the neck angle. The LMI YouTube video explains the use of this fixture better than I can. Copy the link below and paste into your browser.
It has been difficult to stay focused on guitar building these days... especially without March Madness and Major League Baseball to take my mind of certain things. I have resorted to learning to play fiddle as my challenge of the day... every day. I'm taking online classes from Michael Ismerio. He teaches Old Time fiddling out of his home in Ashville, NC.
This month, Woody has a few tips on building and maintaining the proper set up of an acoustic guitar.
? For next month Woody had intended to present a beginners lesson in carving a violin neck. ? Now, that doesn't seem practical. Instead, we're working on a presentation on carving a guitar bridge by hand versus on the CNC.
Some topics to be covered this month are:
Carving the Nut
and more... when I think of it,
Be Safe, Be Healthy,
The top of the nut is filed off until the string slots are roughly the the depth of half the string diameter. The plain steel string slots (the 1st and 2nd strings usually) might be left a bit deeper.
To understand guitar string compensation some basic components of the vibrating string need to be noted. Essential among these is the Vibrating Length of the string which, without string compensation, is not the same for each string. The Total Length of the string, between nut and saddle, is made up of the Vibrating Length plus a Dead Length at each end. These terms are explained in the graphic below.
Hey, we covered a lot of ground this month. I am sure that some of this stuff you knew already. Hopefully, we all learned something that we did not know before.
I hope to see you back here next month.
Play Safe, stay healthy,
Finally, the slots need to be sloped back at the same angle as the peghead (15 degrees).
More than a couple of things to remember:
First, fit the nut blank into the slot where it will be seated between the end of the peghead veneer and the end of the fretboard. Leave it there as you layout the string slots as described below.
With this gauge Woody measures the string height above the fretboard at the nut (target 0.060") and the string height above the 12th fret (target 0.100"). Depending on the style of the instrument, the attack of the player, and the proper neck relief, these numbers will get you into the park on any acoustic guitar.
These days there must be some expensive electronic device that will do the same job almost as good. Woody has many little DIY tools like this around the shop which, when used many times over the years, improve the luthier's hand and eye coordination. After a while the luthier will see things that others do not, just as an experienced musician can hear things that the untrained ear does not. I guess Woody's message here is that the key to using simple tools is to train and trust your hand and eye.
The width of the nut is, of course, the same width as the end of the fretboard which should have been planned for long ago. Manufacturers use different fretboard widths at the nut for different models of guitar. For dreadnoughts, a common nut width is 43 mm (1-11/16").
To layout the string slots, divide the total width by six. You will have 5 spaces at this (divide-by-6) width. And you will have two spaces that are half this width. One of these half-widths is on each end of the nut.
String Length Compensation
Remember Also: While concentrating on the neck angle, one must also keep the guitar body and neck on the same centerline.
A word of caution: The LMI video advises that the neck angle bar on the jig should be adjusted to 3.5 mm off the top of the guitar. This assumes that the top and the sides are at a perfect 90 degree angle at the neck block. Woody has found that is not always the case in home-shop guitar construction.
So, the first thing to do is to make sure that the soundboard and the sides form a true 90-degree angle at the neck block when constructing the guitar body. If that angle is more or less than 90 degrees, more or less adjustment will be needed to correct the neck angle.
This little feeler gauge was made from a chunk of brass way back when Woody was doing a lot of repair and set ups (1970's). It was specifically made to check string clearance above the fretboard or, as the case may be, above the fret.
Briefly explained, neck relief is needed to eliminate string buzz and keep a low action in the first 3 or 4 frets of the fretboard. If the neck is perfectly straight, it will not be able to accommodate the natural arc of a vibrating string.
Also pictured are saws and files of various kerfs and widths specifically made for the task of making six evenly spaced slots of varying widths. These tools were purchased from the usual suspects.