Liutaio Mottola Stringed Instrument Design



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Carving the Neck for the Steel String
Acoustic Guitar

In the broadest sense the process of building a neck has two major operations. The first is building up a blank that is the first order approximation of what the finished neck will look like, and the second is successively taking away wood from that blank to yield the final shape of the neck.

This page discusses the latter part of the second of those operations, however it also talks about some of the steps that must be taken even before the neck blank is constructed, steps like copying the contours of an existing neck or designing the profile you want in your neck. After the plans are drawn using this data as input a suitable neck blank can be constructed and a fingerboard can also be constructed and glued to the blank. The operations discussed on this page pick up after those operations have been completed.

Neck carving is one of those things that scares and frustrates a lot of beginners. In the abstract it is a three dimensional transformation that some folks find difficult to wrap their heads around. But don't despair. The techniques described here make use of successive approximation, that is, a number of small, easy to understand and implement steps that will take you from a rectangular rough approximation of the finished neck (i.e. The neck blank) to the smooth and curvy finished neck. The approach detailed here is also useful to the more experienced builder that is trying to gain more consistency in his or her neck carving. As described, the techniques here will yield consistent necks every time.

Last updated: Tuesday, May 08, 2012

Critical Dimensions of the Neck

Although this page is really about the carving of the neck shaft and heel, there are a few issues of the overall design of the neck which must be considered way before one gets to the carving process. When the neck is designed (or copied from an existing design) the point at which the neck joins the body must be specified. This is usually at the 12th or 14th fret for acoustic guitars, with joins at the 14th fret being more common in modern instruments. Also the vertical taper of the fingerboard and the neck must be specified. This is usually done by specifying the width at the nut and the width at some other place on the fingerboard – the 12th fret, the point at which the neck joins the body, the end of the fingerboard, etc. And the horizontal taper of the neck must also be specified. This is the thickness of the neck when viewed from the side. Some necks don't taper at all, but most taper somewhat as they approach the nut.

The taper of the heel must be considered, too. Some heels taper both when viewed from the end and when viewed from the side, while others may taper just from the end or the side, while still others may have no taper.

All of these critical dimensions must be considered in the process of designing the neck as they will be important in the construction of the neck blank. There is another page on this site which details the process of building the neck blank. Another design issue is the profile of the finished neck, that is, the shape of the neck shaft. This is something you can either copy from an existing instrument that shares the same critical dimensions as the one you will be building, or you can design it yourself within the confines of the critical dimensions you have chosen. A third option is to just use the profile I'll provide here, which will yield a nice general purpose neck.

Copying the Profile of an Existing Neck,
or Designing Your Own

This is easy enough to do with a contour gage and pencil and paper. You'll want to take the contour in two places. I do this at the 1st fret and at the 12th fret. The first step is to turn the neck fingerboard side down and then carefully press the contour gage straight down onto the neck at the first point of measurement.

As you can see, I'm limiting how far down I press the gage by placing the fingers of both hands just under the surface of the fingerboard. So what I'll end up with is the contour of both the neck and the fingerboard. You can also see that I generally take the contour at both the 12th fret and the 1st fret on the same gage – mine is long enough to do that.

After the contours are taken you can transfer them to paper:

Now, draw a box around each contour that will represent the dimensions of the cross section of the neck blank at the 1st and 12th frets. Also add a line to represent the neck/fingerboard seam and a line to represent the centerline.

It should be obvious from this drawing that you don't need to copy the contours from an existing neck. Starting with boxes that represent the cross section of the neck blank at the 1st and 12th frets you can simply draw any profile you like. Note that the contours that appear on this page are all for very typical modern “flattened oval” neck profiles so if you don't want to either copy a neck or draw your own you can probably just use the profiles here.

Before we continue with laying out the profile for the neck it would be a good idea to discuss the process by which the neck will be carved. Once you understand this it will be obvious what the rest of the drawing operations are for. The general process used to carve the neck is one of successive approximation. The neck blank roughly approximates the ultimate shape of the neck, and each operation we perform on it will successively get it closer to that final shape. The specific process used is called faceting, and it involves cutting off the corners of the blank to add facets to it, then cutting off the corners of those facets, and then cutting those off etc. until eventually the facets are so narrow that we have a smoothly curved neck profile. The first step is cutting off the corners of the rectangular profile of the blank to form primary facets:

You can see how close to the final profile we come just by creating the primary facets. Now the secondary facets are formed by cutting off the corners of the primary facets:

For all practical purposes the secondary facets are so close to the desired shape that we really don't need to facet further. When we do the actual work of neck carving the edges of the secondary facets are just rasped and sanded into the final smooth profile. The faceted approach to neck carving is well described in the guitar construction literature. Cumpiano and Natelson outline this method in their book Guitarmaking Tradition and Technology and John Calkin wrote a very nice article on this approach (Carving Neck Facets) which appeared in American Lutherie #33.

So this is how we will proceed during the carving. Before we start carving we need to draw and accurately measure the facets on the profile drawing. These measurements will be used to mark up the blank for carving. The primary facets are drawn first. The facets are at 45° from the top or sides of the rectangular blank, and they are tangent to the curve of the ultimate profile of the neck:

Measure the distances from the edges of the blank to where the edges of the facets will be:

Now the secondary facets are drawn. These are at an angle of 22.5° to the primary facets and tangent to the ultimate profile of the neck. After they are drawn, measurements are taken from the edges of the primary facets to the edges of the secondary facets so that the secondary facets can be laid out on the neck blank after the primary facets are cut. The drawing will look like this:

You'll also need to do the same drawings and take the same measurements of the heel. On the heel, the two contours are taken at the bottom of the heel and at that point up the heel just below where it curves into the neck shaft. On my instruments I don't have to make separate drawings because I taper the heels exactly the same way as I do the shaft of the neck. So this means that the profile at the base of the heel is the same as that of the neck shaft at the first fret, and the profile of the heel just below the curve is exactly the same as that of the shaft at the 12th fret.

With the facet drawings and the measurements done, the actual carving of the neck can begin.

Carving the Neck

As mentioned earlier on this page, there is another page on this site that details the process of building the neck blank. After the neck blank is built the fingerboard is slotted and cut to its vertical profile, and then it is inlaid and bound as desired. The headplate is glued to the headstock and the headstock shape is cut out and roughly sanded. Holes for the tuning machines are drilled. The trussrod is installed, the fingerboard is glued on, and then the neck blank is cut to match the vertical profile of the fingerboard. Finally the end taper of the heel is cut on the blank and it is time to start carving. The neck blank at this point looks like this:

The first step in carving the neck is to draw the edges of the primary facets onto the neck blank, using the measurements taken earlier. Marks are made on the blank at the four places profile measurements were taken – on the shaft at the 1st and 12th frets, and on the heel at the base and just below where it curves into the shaft. These lines are connected at the heel, following the curves of the blank. Markings for the primary facets look like this at the heel:

Click to enlarge

and like this at the transition between the neck shaft and the headstock:

Note that here the two lines are extended in a curvy fashion so that they meet at the point on the bottom face of the headstock where it widens out from the neck shaft.

Now the waste wood must be removed to form the primary facets. The first step is to roughly cut out the wood on the bandsaw:

This requires you to angle the neck on the saw table and to keep an eye on both the place where the saw blade enters the wood and the place where it exits. Of course it is always prudent to keep a third eye on where your hands are while you are doing this. A mistake here can ruin the blank that you spent so much time preparing up to this point, so if you don't feel comfortable doing this, my advice is to skip this operation altogether and take the wood off in an operation that offers more control.

After the excess wood is removed with the bandsaw (or not) you can cut right down to the lines of the primary facets using a variety of tools. My personal favorite is the luthier's favorite power tool, the horizontal stationary belt sander. You can quickly create the facets on the flat parts of the neck and heel:

and if you were wise and designed the neck so that the radius of the curve between the neck shaft and heel is larger than that of the end roller on the belt sander, then you can even cut the facet on the curve with the belt sander, too:

If you don't have a horizontal belt sander then carving the primary facets can be done with hand tools. First, a rasp is used to cut the facet at the curve at the transition between the heel and the neck shaft:

and also at the transition between the neck shaft and the headstock:

In the picture I am using a round Microplane rasp. These rasps cut very fast and very cleanly, but they wear out a lot faster than they used to when they were first introduced. I'm using a standard cabinetmaker's rasp these days. After the primary facets at the transitions are rasped the facets on the neck shaft itself can be cut using either a flat rasp or a spoke shave.

Note that I have the neck clamped down to a holding jig that was made just for this purpose. After the primary facets are cut, the neck will look like this:

Now the secondary facets are marked on the neck. The process is the same as marking the primary facets. Once marked the headstock end looks like this:

Click to enlarge

and the heel end looks like this:

Click to enlarge

The secondary facets are carved in the same manner as the primary facets were carved using whatever tools you have and are comfortable with. Now the neck looks like this:

Click to enlarge

As mentioned earlier we don't do any more formal faceting as the width of the tertiary facets would be so small as to be difficult to measure. So at this point we just take down the edges of the facets a little bit using a finer rasp (wood file). Then the file is used to round over the profile all over the neck and heel. I like to use a rather large half round file for this – the round side fits in the curves well.

Of course, if you have a belt sander and are proficient with it, you can do all of this very quickly using that tool. Try to make all the transitions as smooth as possible. It is difficult to do with the neck on the holder unless you get down on your knees to sight along the neck from various angles. I usually find that I can get close enough to smooth in this step that just a bit of final touch up is necessary. For this, the neck is taken off the holder and held in hand so that it can be eyeballed from different angles.

What you want to do is hold the neck up in front of a light surface under good light and sight over what should be the straight line of the shaft. Any bumps will be pretty obvious. When you see one, you can usually just go at it with the file while hand holding the neck.

I find that holding the file in line with my eye makes it easy to see my progress as I file the bump down. If I find any really big bumps or valleys I'll mark them with pencil and put the neck back in the holder for some heavy filing. After you flatten any bumps, rotate the neck slightly and sight and file again.

Click to enlarge

If you look carefully you can see a bump on the horizon of the shaft, just to the right of the tip of the file on the bench. This is filed down as detailed above. This process of slightly rolling the neck, looking for bumps on the horizon, and then filing them down is repeated, rolling the neck from one side of the fingerboard all the way to the other. When you've gone all the way to the other side, flip the neck around – if it is oriented as shown above so the heel end is to the left, flip it around so the heel end is to the right. Then repeat the rolling, sighting and filing process. Things look different when viewed from the other side.

Once the shaft is smoothed in this manner you can use the same method for smoothing down the heel. After the heel is smooth, the same technique is used to smooth the transitions between the heel and shaft, and the shaft and headstock. For these areas, I find it is best to hold the neck at an angle so that the angle between the heel and shaft form a V. It seems easier to eyeball bumps and flat spots when the area is sighted in this orientation. Remember to do this operation completely from one side to the other, flip the neck end for end, and then do the whole thing again. While you are doing all this filing try to stay away from the side of the fingerboard. File up to it, but if you can help it don't file the side of the fingerboard at all.

When things look pretty smooth after rasping it is time to start sanding. The neck is clamped to its holder again, and then the shaft, heel, and transitions are sanded shoeshine style with a length of 50 or 60 grit sandpaper.

Yeah, I know, this sanding is going across the grain. But these cross grain scratches will make some heretofore invisible valleys stand out so you can work them out with the file or a sanding block. Again, stay away from the sides of the fingerboard during this step if you can help it. Try to sand the ends of the shaft as much as you do the middle of the shaft, and be sure to extend this sanding right up to the base of the heel and right up to the wide part of the headstock.

At this point I like to cut out and temporarily attach the nut blank to the neck. The finished nut looks very nice if its ends are shaped to match the neck.

The next step is to refine all the curves further using the same coarse grit sandpaper on a flat and a round sanding block. I like to use a rubber block and a teardrop profile block to get into the inside curves.

This time (and forever after) the sanding is done with the grain.

And at this point you can carefully begin to include the sides of the fingerboard in the profile. Do keep your eyes open for bumps and valleys while sanding, but they may not be visible now. It is a good idea to take a break from sanding and feel the neck with your hands and fingers. You'd be amazed at what your hand can feel that your eye overlooked.

The entire neck will require sanding, down through the grits until you reach the one suitable for the type of finish you will apply. If I'm going to store the neck for any length of time (like, while I build the body) I usually put this off for a while and sand everything down for finishing at the same time. But after coarse grit smoothing, the neck looks like this:

and like this: