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Arch Mold Tutorial

GoBoard Arch MoldNearly every guitar, both acoustic and classical will have an arch in the back plate. The reason for this arch is two-fold.

First, and most importantly it is for structural integrity of the back of the guitar. As the guitar goes though environmental changes, the arch relieves much of the stresses in the wood. Let’s say the guitar is constructed in a relative humidity level of 45 percent. At some point the guitar will be subjected to humidity levels as low as 20 or 25% for extended periods of time.

Because of these changes in humidity, you will notice the arch in the back, depending on the amount of arch, begin to flatten out. In the most extreme cases, I have even seen them go completely flat or even go convex in shape. This is a very serious situation and it is likely the guitar braces will separate from the back the the center joint of the guitar will open up.

The arch allows the wood to move, and more or less relax, as it shrinks. If the guitar remains in a very dry environment over long periods of time the back will split, the center joint will split, and you will also have other associated problems. See the article on Guitar Humidity for more information on this.

Top Plywood Held In Place While Securing To Struts
Second, the back is arched to reflect back the tone toward and out the sound hole, much like an arched violin back, mandolin back, or banjo resonator.

All of the plans we sell at Ultimate Guitar OnLine incorporate a back arch, for both acoustic and classical guitars. Your plan should as well. If not visit our Guitar Plan Store and pick one up today.

Constructing the Arch Mold

Most guitar arching uses the same arch radius over the entire length of the back plate. On many of our guitars,  there is also an arch from front to back or from head block to tail block. This type of arch does not affect your bracing arch though. You can make this arch by manipulating the taper on the sides.

Making the arch mold is almost like making a reverse back plate. After all, it needs to exactly mirror the arch of the back and provide support for the back while the arched bottom braces are glued in place.

Recommend Tools and Materials
  • Band Saw

  • Table Saw

  • Spindle Sander

  • Portable Hand Drill

  • Assorted Twist Drills

  • 1/8” to 1/4” Plywood or Hardboard

  • Hardwood or Softwood Lumber
Titebond Glue

Start by cutting the plywood to size. I usually make the arch mold sized to fit the movable platform of the Go-Board so I don’t have any registration problems of the arch within the gluing jig itself.

Square Up Plywood or Hardwood StrutsNext, cut (5) pieces of plywood about 3” wide for the full width of the mold. Stack the pieces and use a woodworkers square to make sure they are perfectly square

Now use a screw pilot and countersink and drill down a couple of places to secure the plywood pieces together. It is best to clamp the plywood together prior to doing this. Use screws long enough to reach into the bottom layer of plywood.

Cut Struts From Plywood Using Arch Template

Glue a copy of the back brace arch from a print of the UGOL Template set of your guitar. Use 3M spray adhesive and make sure it is square and centered on the top layer of wood.Cut the support arches to width using a band saw. Set the  plywood base of the mold on the workbench and mark the location of the arch supports.
Glue each arch support into place using Quick-Grip clamps on the end and deep reach clamps at the enter of each support. and screw them in place when the glue is dry. Be sure to countersink the screw heads so the mold will sit flush within the GoBoard Jig.

Glue Struts To Bottom Of Arch Mold

Next set the top plywood or hardboard on top of the base and mark the centerline of each support on this platform. Drill the support screws and countersink the screw heads. Glue the support plate into place and screw it down.

Finish up with the end closure if you like and you have a completed arch mold ready to glue your back arches quickly and accurately!

To Learn more about this and much more about guitar and ukulele construction, take a look at our brand new eBook Guitar and Ukulele Construction Handbook