Liutaio Mottola Stringed Instrument Design



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Construction of the Tapered End Graft

Flattop acoustic guitars and some historical instruments feature a decorative panel on the tail end of the ribs. This is called the end graft. End grafts come in two general styles, a straight sided style and a tapered style which looks like the keystone of a stone arch. Instructions on this page explain how to implement the tapered end graft.

Initially appeared: December 21, 2013
Last updated: Saturday, November 25, 2017

Assembling the End graft Panel

The end graft panel is made of wood of approximately the same thickness as that of the ribs. You'll need a piece wide enough to implement the end graft for the instrument you are building. This piece should be as long as the thickness of the body at the tail end, plus another 2". In the finished end graft, the grain orientation will be vertical.

Typical tapered end graft of a steel string guitar.

Also note in the photo that the end graft often contains purfling strips on either side of the central piece of wood.

Start by marking a centerline down what will be the gluing surface of the blank. Starting about 1" up from one end, mark the bottom width of the tapered graft, centered about the centerline. Measure up from here the depth of the body at the tail end. Here, mark the width of the top of the graft. These widths will be taken from the plan you are using, or directly from an instrument if you are copying an instrument.

Connect the bottom and top width marks on either side of the centerline and also extend these lines to the top and bottom of the piece. What you want is a drawing of a graft that is about 1" longer on both top and bottom than the body is deep. This is done to aid in fitting the graft. Cut out the graft on the bandsaw and then plane the edges to the drawn lines, keeping the edges square to the faces of the piece. It helps with a small piece like this to simply clamp the plane upside down in a vise and then run the piece over the blade.

Planing the edges of the tapered end graft.

The next step is to gather up the purfling strips that will go on either side of the finished end graft. The strips should be the same width as the thickness of the graft. Cut these piece to the length of the sides of the center piece. I like to glue up the pieces on a simple jig made out of a piece of thin plastic with two small strips of the same plastic each about 1" wide and a bit longer than the graft. The glue doesn't stick to the plastic, so it makes for a good gluing fixture. Here are the pieces collected and placed within these two plastic strips.

Collected pieces of the tapered end graft.

The gluing fixture is pretty self-explanatory. Here is what it looks like in use.

The gluing fixture in use.

The plastic sheet is placed on a scrap of wood and one of the plastic strips is placed on top of the plastic sheet. The whole assembly is clamped together with two clamps. Then the other plastic strip is clamped down so that all of the pieces of the end graft can be wedged between the plastic strips for gluing. Once everything is set up, remove the pieces from the fixture, apply glue, reassemble the pieces and slide them into the fixture so the pieces are tightly wedged together. Wipe off glue squeeze out. Once the glue grabs nothing should move, but if you want you can put a clamp at the fat end of the graft to keep things from sliding out of the fixture.

After the glue is dry, remove the piece from the fixture and scrape the front and back surfaces clean.

Scraping the surfaces of the graft assembly clean.

Cutting the Mortise

If scraping during the previous step obliterated the penciled centerline on the gluing surface, redraw it. Position the graft on the tail end of the body so that centerline is aligned with that of the instrument, and so the thin end of the graft is in line with the surface of the back plate. The latter means that all of the extra length of the graft will now be above the top plate. When the graft is positioned thus, pencil the locations of the sides of the graft onto the ribs.

Marking the ribs for the mortise.

With the body in a holding fixture that presents it tail end up, place a flexible straight edge on one of the pencil lines so the straight edge is outside of the mortise. With a sharp violin maker's knife, gently scribe along the straight edge. Work from back to top. Keep the straight edge there and take another pass with the knife. Keep taking knife cuts like this, gradually deepening the cut. As the blade goes deeper the wedge shape of the blade will push the straight edge further away. At this point you don't need the straight edge to guide the knife cuts, but keep it there anyway, because is will prevent slip ups with the knife from gouging the ribs.

Cutting the walls of the mortise with a knife.

Ideally you'll make the knife cuts deep enough to go all the way through the ribs but not into the tail block. It may not be possible with really hard wood to get all the way through it, but as long as the cuts are deep enough so that when it comes time to chisel out the waste you don't end up splintering the edges of the walls of the mortise then all is fine. If there is a good difference in hardness between the wood of the ribs and that of the tail block, you'll actually be able to feel when you've gotten all the way through the ribs, because the point of the knife will stop "bouncing" over the hard grain of the wood of the ribs.

Once the cut is deep enough, do the same thing on the other side of the mortise. Remember, the straight edge goes outside of the mortise, and make the cuts from the back end to the top end. When you are done the tail end will look like this:

Knife cut wall of the mortise for the tapered end graft.

Now the waste wood of the ribs inside the knife cuts must be chiseled out. But before starting it is extremely important for the waste wood of the top and back plates to be cut away separately. If you don't do this first it is very likely that you'll splinter the edge of the top while chiseling the mortise, and that would involve a really tedious repair.

Place the body on the bench with the back facing up and the tail end facing you. You will see the ends of the knife cuts in the wood of the back plate. Take a piece of cutoff from the ribs and use it to mark the thickness of the ribs on the back plate. The pencil line should connect the two knife cuts. If the knife cuts are not deep enough, carefully extend them so they reach the pencil line. Be aware that you are cutting into the end grain here, so be careful not to split the wood.

Now place the straight edge on this pencil line and scribe the back plate, connecting the two knife cuts. Continue this scribing to deepen this cut.

Trimming the end of the mortise on the back plate.

Continue the knife cuts until you've cut all the way through the back plate and you can remove a nice rectangular chunk of the plate at the end of the mortise. Clean the cuts up with a small chisel if needed. Then repeat this process, cutting out the other end of the mortise on the top plate. When you are done, the trimming will look like this:

Trimmed end of the mortise on the top plate.

Now you can excavate the waste from the mortise. Place the body in a holding fixture so the tail end is oriented up. Using paring chisels, chisel away waste, moving the chisel from the back to the top. Take care when chiseling next to the knife cut walls of the mortise, to avoid marring the surface of the ribs or the walls of the mortise.

Chiseling the waste wood from the mortise.

If the wall knife cuts are not deep enough, deepen them at this point until they reach the tail block. Continue clearing waste right down to the surface of the tail block. Do not cut into the tail block. When you get down to the block, change to a small scraper to remove the remaining bits of the rib wood and glue from the mortise. Take particular care to clean up the inside edges between the walls and floor of the mortise.

Waste is cleared from the mortise.

Fitting and Gluing the End graft

A trial fitting of the end graft can be done now. Before doing this, chamfer the back edges of the graft with a scraper:

Chamfering the back edges of the graft.

This will make for a better fit. Now test fit the graft. It is rarely the case that it will fit perfectly the first time. The things that must be examined and possibly corrected are the fit between the walls of the mortise and the sides of the graft; the fit between the floor of the mortise and the back of the graft; and the depth of the mortise. These should be examined and dealt with in that order.

The walls of the mortise may need to be straightened, and the taper may need to be adjusted as well. Corrections are performed on the walls of the mortise rather than on the graft, because of the thin purfling strips on the sides of the graft. The best tool to use is a flat rectangular cross section file that is longer than the length of the mortise and has grooves so it can cut on the side.

Straightening the walls of the mortise.

Dry fit the graft, check out where and how much the walls of the mortise need to be cut, and then remove the graft and file the walls. Check your progress often and be sure to work both sides of the mortise at the same time so that you home in on a good fit. Here is a picture of the graft dry fitted into the mortise:

Dry fitting the graft.

Notice that, even though the width of the mortise was originally marked to the width of the short end of the graft, that now, what with all that adjustment to the walls of the mortise, the graft now slides deeper into the mortise. This is why the graft is made over long and why we start with a narrow mortise width. We need some room for these adjustments.

After the lateral fit is worked out, check out how well the back of the graft fits the floor of the mortise. It is often the case that the floor is humped up in the middle and needs to be flattened. If this is the case, use chisel and/or scraper to flatten it. Once you've got a good fit here, check to see that the edges of the graft and those of the walls of the mortise are at the same height. If the graft is too high, the mortise will have to be deepened. If the graft sits too low in the mortise, the floor of the mortise will have to be shimmed up with a thin piece of veneer.

When all is ready to go, remove the graft, apply glue to back and side edges, and reinstall in the mortise. Once it is wedged in and the glue grabs it is not likely to work its way out. Wipe off excess glue, put a piece of wax paper over it, and put a weight on top while the glue dries. Here I'm using a sharpening stone:

Gluing the graft.

Let the glue dry, then trim the graft flush with the top and back plates using a flush cut saw. I'm using a flexible Japanese flush cut saw in this photo:

Trimming the graft.

After trimming, use a scraper to clean up the surfaces and to blend the curve of the ribs into the end graft. Fitting the end graft is usually the last construction step on the guitar body before starting in on the binding and purfling.

The fitting of the end graft is complete.