MITERING THE BACK and TAIL STRIPS
Remember, there are two ledges to be cut, one for the perfling and one for the binding. Cut the perfling rabbet first. Your perfling selection might have to coordinate and fit into the back strip. This will require hand work in mitering multi-line perfling to join the back and butt strips.
Once the sides are bent and cooled, they go directly into the mold as pictured here. While in the mold, the blocks and lining are glued to the sides. The shape of the guitar is now fixed. The complete assembly of the guitar body and bindings will be the subject of this chapter.
BLOCKS and LININGS
It is very important to have the guitar sides and blocks in their final shape and orientation when the blocks are glued in. This is achieved by using spreaders to hold the guitar sides in position in the mold as the blocks are glued and clamped in place.
The wood bindings (maple and rosewood) are bent on the same bending machine as the sides. In the photos below, bindings for four dreadnought guitars have been bent and placed in the free-form mold until needed.
Bending the bindings in the LMI bender begins by taping the flat and straight binding stock together, side by side on your workbench. Spritz with water and lay them in a foil envelope. Then put them into the LMI bender as if one piece. The tape will keep them from being accidentally skewed across the bending surface. They are easy to bend but, remember, maple and rosewood bend a VERY different rates. So, bend them in separate groups. Bend slowly, especially in the upper bout. Once the bindings have been bent, they are placed in the free-form fixture to keep them in shape until ready to install.
The second strips of lining will be approximately twice the length (~110mm) of the first strips that are in place up against the tail block. Glue and clamp the second strips in place remembering to keep a space for the reinforcing strip between the lining sections and glue the lining strip just proud of the edge of the side. The reinforcing strip will be installed after all of the kerfed lining is in place on soundboard side of the rim.
There are a few characteristics to keep in mind when buying a router for cutting the binding rabbet. Number one, it should be light and balanced in your hand.
It helps to have a tilt base on this router as well. A tilt base is handy on the back of the guitar, upper bout where the arch of the back is not square to the sides. Without a tilt base, you might have to fashion a custom skid plate.
The upper bouts of the back are a bit of a challenge as the back is arched and not square to the sides.
Check out the lining/reinforcing strip joints on an old Martin box, i.e., when they used wood strips for side reinforcements, as I have described here. Nowadays they use tape, which probably is just as good structurally, but it’s not wood. The wood/reinforcing strip joints of the old days are incredibly clean and tight. That’s a detail that says something about the maker.
Spool Clamp Stock
> 18' of 3/16" all thread
> 35 ea.__ nuts, washers, and wing nuts
> 6' of 1¼” hardwood dowel
> sheet of 1/8” cork board
> drill press
The lower bout rabbet then has glue applied and the remainder of the bindings on both sides are wrapped with twill tape from the waist to the butt. A scarf joint at the butt finishes off the installment of the bindings on the back side. The soundboard side of the guitar has the bindings installed and bound with twill tape by the same method. Be sure and support under the sides to accommodate the arch in the back when wrapping the soundboard binding with twill tape.
The scarf joint at the butt end is made in situ to complete the binding glue-in process. At that point, the rest of the bindings and perflings are in place with the glue drying. So, work scant and make that scarf joint as tight as possible. You might need 50' of 3/4" twill tape, but it does an excellent job of holding in the bindings as the glue dries.
When the top is centered and clamped onto the rim in the mold, the intersection of each brace with the rim is marked on the inside and the outside of the guitar. The linings are carefully marked on both sides of each brace. Then the outside of the rim is marked where the brace ends pass through.
On the inside of the guitar, the end of each brace is marked where the scallop begins to pass under the binding. The top is then removed to allow final scallop of the brace ends down to 2 mm.
A 2-mm deep notch is then cut through the rim for each brace end.
The Journal entry for December is complete. This is the 7th chapter in a comprehensive, step-by-step overview of the process of building a guitar at home. The 1st chapter was in June, 2020.
If you follow Woody until next June, you will have a complete, step-by-step description of the entire process of building an acoustic guitar in a home shop.
Below is a list of chapters and where we stand right now. Seven Home-Shop Guitar Building chapters have now been posted in the Luthier's Journal since June, 2020. The 7th chapter is posted below. There will be one chapter each month going forward, and we will finish the complete Overview in June 2021. That's the plan anyway.
Install the reinforcing strip so that it reaches across the full width of the side, edge to edge. With a small square, lay out the reinforcing strip so that it is square to the bottom of the kerfed lining which should be parallel to the soundboard edge of the side.
Now the soundboard side of the box is lined, and the reinforcing strips are installed. The side reinforcements will protect the sides from splitting if the guitar is dropped... like this 1938 herringbone, Martin D28 that Woody put back together a few years ago. The owner had dropped it on a concrete floor. Woody had to take the back off to repair the crack from the inside.
It didn't have any side reinforcements.
The arch of the back creates an angle with the side in the upper bout. Keep the centerline of the router parallel to the side of the guitar and let the base of the router tilt.
Remember, there are two ledges to be cut, one for the purfling and one for the binding. Cut the purfling rabbet first.
Your purfling selection might have to coordinate and fit into the back strip. This will require hand work in mitering multi-line purfling to join the back and butt strips. Some nice things can also be done by incorporating continuous purfling into the heel cap.
Be sure to have a design in mind before the binding is cut. If there is no purfling (which is the easiest thing to do), the edge binding and the back strip can just butt (most manufactured guitars are built this way). .
One will need to compensate for that in the router set up in order to keep the rabbet square to the side. The Woody-made clear plexiglass skid plate pictured here is flexible enough to bend and keep the vertical axis of the router parallel to the side. The area between the heel and the waist is where the arch of the back is most pronounced. It takes practice to manually keep the yellow router vertical as you go around this curve on the upper bout. The flexible base plate helps, but it is not the answer.
The objective is to keep the back wall of the channel parallel to the side, and the width of the binding channel the same as the rest of the guitar. If the router just sits on the arch of the back as it travels, the rabbet that it makes will be tilted toward the outside of the guitar and the width of the binding will be thinner. A tilt base helps keep the centerline of the router vertical and parallel to the side of the guitar.
The angle that the back makes with the side varies, however, at every point on the curve around the upper bout. One pass of with the tilt base will keep the rabbet square, but there might still be a little handwork needed. A file, a sharp chisel, and a keen eye might be called to duty.
Chapter 07_ Body and Bindings
ROUTING for the BINDINGS
The routing of the binding channel is not a casual undertaking as many hours have been spent to this point in manufacturing and assembling the various parts of guitar box. A slip of the router in this process would be disaster.
First, make sure that the guitar body is secure and will not move. It is important to clamp the box firmly and completely while the binding rabbets are being routed. Also, make sure that the bit is tight in the router, and it will not move in or out as you proceed. This can make an irreparable mess. Finally, keep in mind that the objective is to cut the binding rabbet so that the back wall of the rabbet is parallel to the side. More on this below.
The yellow router that is pictured here is actually a laminate trimmer and weighs about 4 pounds. The power is rated at 5.6 amps. It operates at one speed; 30,000 rpm and it is top-heavy. There are many good, small routers on the market now. There weren‘t when I bought and modified this one. The website noted below provides a review of the most popular brands of routers.
LINING THE BACK
To install lining on the back side of the rim, one proceeds just as on the soundboard side of the rim. The reinforcing strip is already in place, so, the layout of each section of the kerfed lining might have to be adjusted... but not much. For instance, a strip of lining might have to end with a portion of a full block in order to be tight up against the reinforcing strip and/or square to the soundboard. This is only a cosmetic consideration, and only then, when the reinforcing strip/lining joint is visible through the sound hole. What looks worse is when the reinforcing strips are not parallel and perpendicular to the soundboard.
CLOSING THE BOX
NOTCHING FOR THE BRACE ENDS
In Woody’s shop, the process of closing the box begins with fitting the top (soundboard) to the rim. To do this, the rim is clamped into the mold from the back side with the soundboard side about an inch proud of the rim. Then the soundboard is centered and clamped to the heel block and the tail block on the top side of the rim to hold it stationary. The objective is to hold the soundboard in place without bending it out of shape. If anything, the sides should be re-shaped to fit the contour of the top and not vice versa. The relaxed shape of the top is acoustically important.
Installing the lining and reinforcing strips helps maintain the symmetrical shape of the guitar if the reinforcing strips and the linings are glued to the guitar sides while they are held tightly in the mold.
Manufacturers have templates and laser cutting machines that are precise down to a thousandth of an inch. Don't be surprised if you have to work with your hand-tooled lining notches to get a good fit with all of the braces at the same time.
GLUING ON THE BACK
Woody likes to glue the guitar back on first with the guitar rim held tightly in the mold by the spreaders. This ensures that the perimeter of the box will be symmetrical. Gluing the back before the soundboard also allows any visible glue to be cleaned up before the box is closed.
Clamp up the entire back-to-rim connection for a dry test before applying glue. Make sure that the rim is immobile in the mold and that the entire perimeter of the back is in contact with the lining. The scallop ends of each brace must pass cleanly through the notches. Discovering a misaligned notch in the lining after applying glue can ruin your whole day.
Apply glue to the back lining conservatively, lay on the back and clamp it down with even pressure all around. Allow to dry overnight.
The spreaders are constructed out of 1” Styrofoam, Luan, threaded knobs, threaded rods, and a little scrap plywood… all purchased from Lowe’s or Home Depot. The spreaders keep the sides tight against the mold (i.e., symmetrical) as the blocks and linings are glued into place.
THE FREE-FORM FIXTURE
Woody’s shop only has two dreadnought molds so, when bending more than two sets, improvisation is necessary. The free-form set up pictured below was generated spontaneously out of need and it turned out to be more than adequate. In fact, as one of the guitars being built is a Mini size, the sides could not be held in shape in the standard dreadnought mold. The free-form mold pictured is very simply built. It can hold the sides in any shape while they cool and set. The sides might stay in the free-form mold a few days or a few weeks depending on Woody’s informal production schedule.
FREE FORM FIXTURE DIMENSIONS
A_ Width of Base: Twice the width of the side, + 2”; (2xSide) + 2”
B_ Length of Base: Length of guitar body (Heel to Tail)
C_ Height of Hold-Down: Half the width of the guitar at the waist
D_ Fixed Clamping Caul: 1” x 2” x A _ Permanently fixed to the end of the base
E_ Slot: To receive over-length of the bent side
F_ Extender: Creates slot for over-length of bent side (1/4” wide)
G_ Coupler: Couples the Extender to the Base
HL_ Spacer: Half the width of the lower bout
HU_ Spacer: Half the width of the upper bout
J_ Hold Down: Holds the bent side down at the waist (screws into C)
K_ Clamping Caul: 1” x 2” x A, C-clamped to the fixed clamping caul
L_ 2” C clamps
Mark the perpendicular line with a light pencil mark and glue the reinforcing strip in place. Repeat for each reinforcing strip. The final result should be 6 or 7 equally spaced reinforcing strips, parallel to each other and square to the bottom of the soundboard.
Finally, the short pieces of lining (the two-kerf blocks) are glued on top of the reinforcing strip ends as seen in these photos.
As an alternative, the soundboard can be glued and clamped outside of the mold if one is confident that the rim of the box has not become asymmetrical for some reason (it happens).
If all is good, spool clamps can be used to clamp the soundboard onto the rim. In many ways these clamps are easier to work with and maintain even pressure on the glue joint.
Woody made about 35 of these spool clamps one rainy afternoon. They are very handy.
That's it for explaining the assembly of the guitar box and binding. We covered a lot of ground. Next chapter we will have a discussion of building a mini dreadnought with a cutaway.
Next, the notches for the back braces are laid out and cut in a manner similar to the top braces. These notches are easier to layout and cut because, unlike the top, the back braces are all square to the centerline of the back.
Proceed around the soundboard edge of the rim with the same, repeating steps as above. When the heel block is reached, the number of blocks in the last strip will probably differ from the others. There is nothing sacred about the spacing between each reinforcing strip, but it should be the same on each side of the rim, square to the soundboard edge of the rim, and parallel to each other. One should end up with 6 or 7 reinforcing strips on each side of the rim (depending on the length of the rim, of course).
GLUING the BINDINGS
Woody's method is to lay in the perflings first with a minimum amount of Titebond glue and blue masking tape.
After only a half hour the already-bent bindings can be laid in. Before the bindings are laid in on top of the perflings, however, one scarf joint is cut to join the treble and bass side bindings at the heel. Glue is applied to the rabbet on the upper bouts, then, the bindings of the upper bout are laid in and wrapped with twill tape.
Also, some nice things can be done by incorporating continuous perfling into the heel cap. Be sure to have a design in mind before the binding is cut. If there is no perfling (which is the easiest thing to do), the edge binding and the back strip can just butt (most manufactured guitars are built this way). Take a look at some of your favorite guitars the next time you are in a music store. There is no harm in copying, or modifying a design that you like.
Play Safe, stay healthy,
Come back next month.
After the blocks are installed, the spreaders are removed and the kerfed linings and spruce reinforcement (rebar) strips are installed.
Adjust the position of the rim in the mold so that one inch of the soundboard-side edge is visible outside of the mold. This will allow easy clamping of the kerfed lining. Clamp the rim in place from the back side of the mold.
Working on the soundboard side of the rim at the butt end, begin by cutting two strips of kerfed lining with an equal number of blocks. (7 or 8 blocks, ~55mm) The number of blocks varies with the length and shape of the rim. The important thing is that these two strips of kerfed lining must be equal length. Glue these lining strips on both the treble and the bass side of the tail block, just one mm proud of the edge of the rim.
Woody's back braces are all nearly the same height and width, therefore, they are often cut to lenght, sanded to shape and scalloped on the ends by a table sander before being glued in place.
Once all 12 lining notches under the soundboard are at the correct depth and angle, (7 notches on the bass side and 5 on the treble side) the soundboard should fit down tightly to the binding around the entire perimeter of the rim. This can be tedious as each scalloped end passes through the rim at a different angle. Clamp the soundboard down lightly for a dry test.
The equal number of lining blocks per section (~15) should have kept the reinforcing strip locations at equal intervals as you move around the side. The rim is now ready to have the reinforcing strip installed.
When building an instrument without a mold, which is how I learned to build guitars...(thank you Charles Fox, 1976), symmetry is always a challenge. This free-form set up allows adjustment to the shape of the sides as they are drying and cooling right after bending. One can see the shape of both sides and adjust to match if needed.
Next, cut a two-block section of kerfed-lining stock (~15mm total width). This will be the width of your reinforcing strip. Use the two-block lining section as a spacer to mark where the next lining strip will begin.
GLUING ON THE SOUNDBOAD
It is a very satisfying day when the soundboard is finally glued to the sides, thus, closing the box. Before the top is glued to the rim, however, put the rim in the mold and using a dowel for a spreader at the waist, make sure that the sides are tight up against the mold. Check also that the lining notches are still aligned with the scallop ends of the braces. When satisfied with the fit of the top braces in the notches, the top can be glued and clamped to the rim. Make sure that the spreader can be taken out of the box through the sound hole after the soundboard is glued on.
The box is now closed. The last major effort on the box is to rout channels for the purfling and bindings. The first thing that you will need is a set of roller guide bearings. These will allow you to control the depth and width of the rabbet. They take a little practice to get used to, so, always check your depth and the roller set up on a piece of scrape wood first. Adjust to keep the rabbet very slightly smaller than your binding/purfling plan. That is, the bindings and purflings should be just proud of their respective rabbets.