* NOTE: When placing the body of the guitar into the jig for cutting the mortise, be sure that the guitar sides at the heel block are pressed completely up against the underside of the top of the jig. The back of a guitar is radiused both laterally and longitudinally. This can create a gap when placed into the neck angle jig, somewhat like the colored area in the not-to-scale sketch here.
When tightening down the fixture to hold the guitar body in place, the guitar can rock back causing the face of the heel block to get out of square with the top of the fixture. The result is a mortise that is not at 90o to the soundboard.
LAYOUT AND ROUGH CARVING
First advice_ Work Scant!
Machines get over the line quickly. Any dimensions mentioned here are relative to the design of Woody Guitars. These dimensions reflect the design of the neck, width of the fretboard and dimensions of the truss rod. Your guitar may be different.
Draw the final curves and tapers of your design neck on the rough peghead, arm and heel block.
On the surface of the wood:
• Draw the Centerline on the top and back of the arm and peghead,
• Draw of the Peghead layout and thickness, (on top, back and sides of the head stock)
• Draw lines on both sides of the Arm and Peghead marking the taper of arm thickness from the Heel to the Nut,
• Draw lines on the Top and Back of the Arm marking the width of the neck from the 14th fret to the nut/peghead,
• Draw the Heel Curve and taper of the heel section from the surface of the arm to the bottom of the heel.
Woody removes the bulk of the excess wood with a 9” band saw and a small table sander (6” x 48”). You will need good ventilation on the sander. Start by thicknessing the peghead.
Final advice_ work scant.
CUTTING THE MORTISE
FFollowing video instructions for cutting the mortise, these are the router bits that Woody uses to hog out the mortise (3/8” straight bit) and, on the final pass, dovetail the walls of the mortise with a 7-degree dovetail bit. Woody uses a 5/8" OD guide bushing.
ROUTER TABLE, FENCE AND CABINET
The subtleties of using a scraper correctly are many and difficult to explain in words. Using a scraper properly is a developed skill. The end result is a finished surface that is flatter and smoother than a sanded surface. I will note here that there are probably many good videos available online regarding the use of a scraper for finishing wood. The first thing that I would look for in these videos is an explanation of exactly how to sharpen the scraper. Keep your tools and wits sharp. That is the key.
As you carve the neck, sit down with the guitar and feel the curves of the neck as you carve it. You will soon be playing it.
FIXTURE SET UP
The LMI fixture is shown here is set up on two, 2x4 arms so that there is easy access to both sides of the fixture. Woody screws the 2x4 arms to the workbench and then screws the fixture to the 2x4s. This allows the neck to extend downward beyond the bottom of the fixture.
Thickness the peghead from the back side unless you want to move the the location of the nut toward the body. Generally, you do not want to do that. Woody uses the thickness sander (very carefully) for this operation. It involves slowing the conveyor belt and lifting the sanding drum at the same time. Practice makes perfect. Thicknessing with a plane and carving tools is always an option.
Below is pictured the peghead thickness dimensions for Woody guitars with Schaller tuners. Once the peghead is the correct thickness, length, and width, start thicknessing the neck from the 10th fret to the nut.
MORTISE AND TENON VIDEO_ 38 minutes
Chapter 09_ Neck Carving and Dovetail
Alignment is the keyword here as the angle that the neck makes with the body determines the height of the bridge and saddle and that, in turn, influences playability and tone of the guitar. Likewise, we have previously discussed the importance of keeping the centerline of the guitar aligned with the centerline of the neck. All of this alignment becomes fixed once the dovetail joint is glued. The truss rod and the fretboard are installed after gluing the arm to the body.
Play Safe, stay healthy,
That's all for this month. Come back next month for a discussion of fretboards and frets...'easy money' compared to this month.
By nature, luthiers are very precise craftsmen in the cutting and joining of wood. It is only after one starts to use the jig, however, that you may realize that one millimeter, or one degree out of square will significantly impact the cut and fit of the dovetail.
The final caution with this jig and these videos is that your guitar is probably slightly different than the guitar in the video. O'Brien mentions this often. So, pay attention when he says, "this is how I do it on my guitars". Guitars can be quite different from one to the next, even from the same maker.
Bottom Line: The LMI jig is quite good but advice to practice on a few trial blocks should be heeded. The jig does have some idiosyncrasies.
WOOD WORKING TERMS
A JIG guides the cutting device so that the cut can be repeated accurately. A Jig Guides the cutter.
A FIXTURE secures the work in a fixed position so its orientation relative to the cutter does not change. A Fixture Fixes the position of the work.
CUTTING THE TENON
When the neck is placed in the tenon jig correctly it should be exactly square (left and right) to the top of the jig (i.e. the travel surface of the router). To keep the dovetail tenon square to surface of the fixture and on the center line of the neck, monitor the set up through the observation hole in the jig.
Pictured in the centering hole is a mark on the bottom, center of the hole with the truss rod slot behind. A short plug with a line marking the center has been temporarily inserted into the truss rod slot for alignment purposes.
If you’ve never done this with a compound dovetail, practice on a couple of 4x4_6” scrap mahogany or pine blocks.
LMI FIXTURE ASSEMBLY_
(harder than using the fixture)
To say that every jig and template has its idiosyncrasies would be an understatement. Robert O'Brien explains some of this in the videos produced for LMI. There is one video on how to assemble the dovetail jig and another video on how to use it. Brief notes after viewing both videos several times would include a recommendation to pay particular attention to instructions regarding square and/or parallel to the centerline when assembling the jig.
The manufactured parts of the jig are precise. They are shipped to you tightly stacked in a box. Assembly instructions rely on the video cited below. After assembling the jig, and using it for several years, Woody will tell you that the video cannot over emphasize the need for precision in the assembly of the jig.
FIXTURE ASSEMBLY VIDEO
CENTERING THE BODY IN THE DOVETAIL JIG
Pictured below is a method of setting up the jig so that the dovetail mortise will be cut square to the centerline of the guitar. The short arm of carpenter’s square is placed on the surface of the fixture with the guitar body clamped in below it. The long arm is then aligned with the center mark on the heel block and the glue joint in the center of the soundboard at the butt end of the guitar. This ensures that the mortise will be square to, and centered on, the centerline of the guitar body and neck.
When the neck is close to its final carved dimensions, especially at and around the heel, the neck is glued onto the guitar body. Gluing the dovetail and arm is a critical point in the assembly of the guitar as very little alignment of the neck and neck angle can be done after the dovetail joint is holding the neck to the body.
THE MORTISE_ Keeping it Square
The center joint of the guitar soundboard should be the center line of the guitar body. An extension of the body centerline will determine of the centerline of the neck.
When the dovetail mortise is cut, be sure to have the body of the guitar correctly placed in the fixture, i.e., with the sides at the heel pressed against the underside of the top of the jig*. Once the heel block is hard up against the underside of the jig, the critical thing is to have the centerline of the body (marked on the butt end of the soundboard) in line with a center mark on the heel block and square to the surface of the jig.
The finish carving of the neck should only require lightweight, hand tools_ files and scrapers. Woody does the final touches of this work after the arm and body have been joined, and the fretboard has been glued in place.
Pictured below are the simple tools used for the final carving of the neck. One is a gooseneck scraper which doesn't need any explanation, and the other is a lead came knife that is actually a stained-glass artisan's tool that Woody made into a scraper by squaring and rolling the edge. The came knife has a handle which makes the scraping process a lot more comfortable and controllable.
THE BOX AND THE STICK
At this point in our construction of a guitar, we have discussed building an acoustic box and a stick that we call the neck. These two geometries do not fit together naturally.
When sharpened correctly, a scraper will hold a sharp edge for quite a long time... if the tool is used exclusively in building guitars.
Woody uses the tools pictured here almost exclusively to work the neck down to its final dimensions. While doing this work, pay strict attention to the total thickness dimensions of the neck and fingerboard at the 2nd, 5th, and 10th fret. Usually, there is only a millimeter or two to remove at this point.
The finished curves and shape of neck are the result of continuously feeling the neck as wood is removed. One can also carefully monitor shadows of the curves of the neck to find flat spots. A high-intensity LED task lamp is great for this purpose. Woody keeps a finished guitar nearby. This provides a touch-and-feel as well as a visual reference.
LMI now refers to neck joint device explained here as a “Neck Angle Jig” (i.e. it guides the cutter). It is also a fixture in that it holds the work in place. In deference to LMI (and Robert O’Brien) Woody will try to stick with the term “jig”. The term “fixture”, however, might be more accurate at times, and the reader should know that Woody is referring to the same LMI device, because it serves both functions.
Once you get to the final depth and have dovetailed the sides of the tenon…stop sliding the guide template forward when you think that you are 90% of the way to the finished length of the tenon, i.e., stop when the tenon is still too long. Remove the neck from the jig and test it in the mortise. It should not bottom out. It should be too wide to fit completely down into the mortise at this point.
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, and the general playability of the setup. The neck angle is very small, approximately 2.5o relative to the plane of the soundboard.
The angle is set by adjustment of the jig early in the setup procedures as explained in the first 5-6 minutes of the LMI video (URL above). In total, the video is about a 40 minutes and it explains most of what I am trying to describe here in written words.
On a 25.4” scale, 14-fret neck, the goal of the neck angle setup is to have a straight edge, when laid on the top of the arm, show a 3.5 mm clearance between the bottom of the straight edge and the soundboard at the saddle position. Assuming a 5.5 mm fretboard thickness (w/o frets), a 9.0mm bridge thickness, and a 4.0mm saddle height, the string clearance above the soundboard will be 13mm.
Explaining this final hand-work procedure to a diverse audience of various skill levels is a challenge. There is nothing like doing it yourself and everyone will have their own techniques and sensitivities in the carving of a compound dovetail joint.
Just remember that the worst thing you can do is to rout the tenon too short before you start the check-measure-and-repeat process. If the tenon is proud of the soundboard surface, you’re good. You still have wood to work with. If the tenon sinks into the mortise below the soundboard surface, you will need to shim the sides of the mortise…it happens. When it comes to the final trimming to optimize the neck alignment, the neck angle, and the tightness of the dovetail fit, the only way to know where to cut (or shim) is by feeling how the tenon rocks in the mortise.
ROUGH CARVING THE NECK
Every luthier has a different peghead design, layout, and inlays. The time is now to have a design in mind so that the peghead can be carved down to final thickness before the neck is thicknessed from the nut to the 10th fret. Be sure to consider the thickness of the peghead laminate, and the dimensions of your tuners.
LMI NECK ANGLE JIG
The joint that we will use to join the neck to the body is a compound, dovetail joint meaning that it tapers in two directions. The LMI Neck Angle jig is the fixture that Woody uses to cut both the mortise and the tenon. It is very convenient in that regard.
CHECK AND MEASURE
It should not be difficult to put the neck back into the fixture in the same place it was before. Slide the template forward one or two millimeters at a time to slowly reduce the length and width of the tenon.
Repeat checking how the tenon is seating in the mortise. When the surface of the arm stops two millimeters above the surface of the soundboard, stop and check the centerline alignment of the neck with the centerline of the body. One side of the tenon or heel might need shaving and not the other to have the centerlines to overlap. This should be done by hand. In fact, once the tenon seats in the mortise so that surface of the arm is only 1-2 mm above the soundboard, Woody likes to make the rest of the final adjustments by hand.
Likewise, with a straight edge, check the arm angle to see how close you are to being 3.5mm above the soundboard at the saddle location. Measure the distance between the top of the soundboard and the surface of the arm at the dovetail joint. Deduct that distance from the measured clearance at the saddle location.
Finally, when the neck is fully seated in the dovetail joint, the center line of the guitar body should be the same as the centerline of the neck and the neck angle should show something very close to 3.5mm clearance at the saddle location.
Before the arm and the body can be joined, most of the hand carving on the neck should be complete, especially on the heel which can be very difficult to carve once attached to the body.
Many luthiers, including Woody, find carving the neck to be one of the most satisfying tasks in the entire guitar-building process. I guess the message is, take your time and enjoy it. I can almost guarantee that you will be proud of your carving if you remember two things.
1. Leave plenty of room for the fretboard on the surface of the arm.
2. Stop carving before you get into the truss rod channel.
Both of these recommendations depend on the design of neck, width of the fretboard and dimensions of the truss rod.
ROUTING FOR THE TRUSS ROD
We need to rout the channel for the truss rod before we start with the dovetail. Some luthiers rout the truss rod channel before they even assemble the built-up neck. Woody assembles the built-up neck first because it is too easy to have the centerline of the arm end up not being the centerline of the heel (at one end) or the head (at the other). All three must have the same centerline.
Routing the assembled built-up neck allows the builder to align the truss-rod channel so that it will actually create the centerline of the heel, arm and head going forward. Using a fence and a router table, one might have to lightly plane the side of the arm to ensure that it is parallel to the desired position of truss-rod. When using a table router, one side of the arm will ride the fence as a guide to rout the truss rod channel.
The website cited below offers an alternative ‘simple-jig’ for routing the truss rod. An advantage to this jig is that the centerline of the arm can be easily adjusted to lie in the center of the router track from end to end. It requires, however, that the head and heel are not attached to the arm when routing.
“SIMPLE JIG” 9-minute video
This chapter contains a discussion of the building, carving, and joining of the neck to the guitar body. The technique demonstrated here uses the LMI Neck Angle Jig to cut the dovetail mortise and tenon, and to set the neck angle.
Whether one is interested in using the LMI jig or not, it would be useful to watch the LMI video as there are many common variables in the construction of a dovetail joint that one should understand regardless of the fixture used.
Discussed here are the tools, machines, jigs, and process requirements of most any dovetail fixture. For discussion purposes, the LMI neck-angle video will serve as a point of common understanding. Woody has incorporated photos and text explanations for the correct use of a dovetail fixture, as well as certain 'cautions' when cutting and joining a compound dovetail joint.
BEGIN WITH THE END IN MIND… Here’s Where We Are Going
Cutting the dovetail tenon will set the angles at which the neck joins the body both left and right and up and down, i.e., both are critical variables. Proceed carefully and check your work often when cutting the tenon.
Use the router bits and the bearing guides recommended in the video. Proceed slowly and monitor each pass to be sure that the emerging tenon is centered and the same depth on both sides. If there is a problem with centering or depth, something has moved. If not too drastic, it can usually be corrected, and you can proceed again.
Observe that the 5/8" dovetail bit is as wide at the end of the cutter as the guide bushing outside diameter. One must be sure to keep the bit extended so as not to crash the bit into the bushing wall. Do not allow the bit to withdraw fully into the router. Set up a stop to avoid this.
If you look carefully in the photo below you can see a nick on the bit where I learned this lesson. Fortunately, the guide bushing is aluminum, so, no great harm done.
THE DOVETAIL JOINT
The joint that holds the neck onto the body of the guitar is probably the most critical joint in constructing a guitar that will be playable, stay in tune and not require major repair down the road.
Even major guitar manufacturers differ in how they join the neck to the body. For the longest time, hardware was considered a no-no even though many guitars needed to have the dovetail neck joint disassembled and modified after only a few years of ordinary wear and tear. This repair is called a neck re-set and every acoustic guitar repairman worth his salt has had to learn how to unglue a neck from a guitar, shave or add a millimeter or two in the right places and rejoin the neck to the body.
Nowadays, with international travel and instruments manufactured offshore to begin with, neck resets are often required even on new instruments due to changes in climate and humidity. As one might imagine, resets on dovetail instruments are an expensive repair. A solution to that problem has been the acceptance of hardware-assisted neck joints. Much credit needs to be given to Taylor guitars for persevering in the development and acceptance of bolt-on necks.
All this being said, Woody guitars (and Martin, and most manufacturers) still use the traditional dovetail joint to secure the neck to the body. The jig that Woody uses to cut the dovetail mortise and tenon was purchased from Luthier's Mercantile International (LMI) and it works quite well... up to a point.
Place a straight edge on the surface of the arm while attached, but not glued, to the guitar body. One last time, check the arm angle as indicated by the clearance of the straight edge above the spruce top at the saddle position. On my guitars this should be about 3.5mm as previously discussed.
Also, with a piece of black thread, check that the centerline of the neck is in line with the centerline of the guitar. By tweaking the dovetail ever so slightly, minor adjustments can be made to the final fitting. Then, when satisfied, glue the arm of the neck onto the body. This is a significant milestone in the creation of your guitar. You have built a guitar from what was previously just a box and a stick.
For more information, see this LMI video on routing for and installing the truss rod with a table router.
ROUTING FOR THE TRUSS ROD _ 9-minute video