Luthier Tools

Here is a short description of how I setup my neck assembly jig to cut scarf joints for a typical 14 fret to the body, acoustic guitar with a 25.4” scale. 

The GenOne Luthier Supply Blog will offer the luthier information of all products available from our store and a growing list of building tips to give you the edge in building a beautiful instrument.

Some of the first and most important specialty guitar tools or forms you will need for a successful guitar or ukulele build are the Georgia Luthier Supply Inside Form and the Workboard. Every luthier should have one set of these useful tools for each instrument shape that is built. We offer an inside form and workboard package for each guitar plan that we offer and most customers order these accessory plans to easily complete their build.

How to Make an Inside Form:



Here is a inside form that has just received a freshly bent side. Notice that the side is held in place with a clamp at the butt block area and with a spreader placed at the upper and lower bout. I don’t use any glue to hold the layers of plywood together for these forms but choose to use long screws as the only attachment.The inside form will probably be the most time intensive and one of the hardest tasks you will undertake in making of guitar tools. I say this because you really need an assortment of tools to successfully pull it off.

Recommended Tools and Materials

14” minimum band saw with 3/8’ Wood Cutting Blade
Oscillating Spindle Sander
Table Saw
Portable Hand Drill
Woodworkers Square
Screw Pilot and Countersink
Assortment of Spiral Drill Bits
Spray Adhesive
(4) Quick Release Clamps
Screw and Bolt Assortment
23/32” (19mm) Sheet of Plywood
Titebond Glue
Form Package Plans

The Initial Layout

The inside form can handle many tasks. In the photo below it acts as a holding fixture while the back of a guitar is glued in place using clamps at the head and butt blocks and rubber bands to clamp the remainder of the back to the sides. Pretty slick! No matter what the depth of guitar, I choose to make my inside forms with 5 layers of plywood. This adds up to about 3.75” to 4” depending of the actual plywood thickness. Forms can be constructed with lesser layers of plywood, but that sacrifices some of the utility of the form.

I also have all my plywood sliced into 2 pieces 24” x 96” for easiest handling in my shop. You will need to cut the plywood into 10 pieces, each measuring the length of the inside form (usually 24”) x 1/2 the width of the inside form overall width. This will vary with the guitar shape, but it is usually around 9 to 9.5 inches. All these plywood pieces should be cut very accurately as they need to be stacked on order to cut them on the band saw. When constructing an inside form for a ukulele, the pieces are much smaller and easier to handle.

Cut out the form layout from the form packing plans into to halves (this would be the plan view looking down on the top of the form). Cut the plan in half exactly down the centerline of the form. Spray the backside of the plans with 3M spray adhesive and adhere it to the plywood, with one on each half. Keep the cut line for the center of the form exactly on one edge of the plywood. Seal the plan down on the plywood and trim excess paper as necessary.

Installing a Top With The Inside Form
Next stack the plywood in 2 sandwiches, each 5 plywood layers high, with the paper layout of top of each stack. Place some quick release clamps around the sandwich. Set it on end on a flat surface and take a woodworkers square to completely square up the sandwich. Now you can secure the clamps and set the assembly flat on the clamp heads so it will be level.

Next you will want to run some long screws into each of the plywood stacks. I prefer to locate these screws outside the guitar body and located in their final locations in the form. One at each end at the should be adequate for now. Use a pilot and countersink bit to recess the screw heads below the surface of the plywood. Next drill of the holes with a bit that is slightly larger than the screw diameter for about a 3” depth. (The objective if for the screws to only grab the bottom layer of plywood and pull all the plywood layers tight. Screw length should be just short of protruding through the bottom side of the stack.

Once the screws are seated the camps can come off. Handle the stack carefully as a sharp blow can still misalign the stack.

Be sure your band saw is properly setup and has a good sharp blade loaded and tuned for re-sawing purposes. If you are unsure of your band saw setup, be sure to visit the section Band Saw Setup in our Guitar and Ukulele Handbook. Don’t use a blade wider than 3/8”. Also make sure it is a blade designed to cut wood and has between 3 and 5 teeth per inch.

Always use a good work light and point it directly at the cutting area. Adjust the top guide of the bandsaw to just clear the sandwich. Start on one end or the other and carefully cut to follow the pattern. You will find that it is easiest to try to cut right down the centerline of the guitar form layout line. My preference though is to cut in such a way as to keep the line so I am cutting on the inside of the line.

Feed very slowly and guide the work with one of your hands and turn and feed the stack with

Clothes Pin Clamps In Action Clamping Kerfing Around a Dreadnought
Clothes Pin Clamps In Action Clamping Kerfing Around a Dreadnought
your other. Make your turns very consistent so you don’t cause flat cuts or misaligned cuts. This just creates much more work to adjust the form.

When done, do the same thing with the other stack, try to cut exactly the same way so the two half match as closely as possible. Set aside the inside portion of the cut as you can use them for form inserts if you like.

Finally cut off the outside waste portions of the form to make it more lightweight and easier to handle. I cut these on the band saw too.
Next take each stack and lay them on top of each other to compare your cuts. Don’t be surprised if they look different from each other. There can be several reasons for this:

Your cuts weren’t as good as you thought they were :-) (welcome to the club!), Also, the most common cause is the table to blade squareness is just a tiny bit off - or the band saw table is not completely flat. Or...it can be a combination of all of the above. Take a woodworkers square and check the squareness of the stack. If that is right on the money, the cutting was slightly off and this does not take much work to fix. Just a little drift to one side of the line or the other can cause a big difference.

Take the stack over to a oscillating spindle sander. Make sure that it is large enough for the spindle to reach over the top of the stack. Use at least a 2” diameter sanding tube that is fresh and has coarse sanding grid (60 to 80 grit is about right). Check the squareness of the table to the spindle.

Feed the work INTO or against the rotation of the sander. Feed slowly and with even pressure and if you have some of the line still showing, try to consistently sand to the outer edge of the line. Hold the form up to the light occasionally and check the smoothness of the curves with your hand and sand out any inconsistency. Repeat with the other half of the form.

Now set one sandwich on top of the other and mark the differences on one stack or the other. Keep touching up the stacks until they are reasonably equal. (absolute matching is not necessary as this will never be detected on an actual instrument). One alert is the be assured that the butt and neck joints match exactly, evenly, and the contours on each half of the form match each other quite closely.

Next - cut the base of the form. This needs to be the full width of the form as a single piece. Set each half of the sandwich that was just sanded on top of this base and trace the outside of the form on the base and cut off this waste on the band saw. Set each sandwich on the base and check your work. If all looks well it is time to screw the base to each half of the form stack. I like to flip the form half face down on a flat surface and set the base on the top of each stack.

To touch up the outside of the forms, I run them through a joiner, which gives them the finishing touch. You can chose to sand them or leave them rough. It depend on how fussy your are about your forms. Another great way to finish the outside surfaces of the shorter sections is by sanding with a 12" stationary disc sander. This takes a bit of practice but it sure does a nice job.

The sandwiches now are to be glued to each other to keep them from shifting from forces from the form spreaders. Remove the screws holding the sandwiches together and spread Titebond glue on all layers. Double check the squareness of the stack and install a couple of clamps. Now you are ready to re-install the screws and tighten everything up. Clean-up and glue squeeze-out and set aside to dry.

Okay we are almost done - at least we are done with the hardest tasks! Next the form needs to be restrained so when spreaders are used to push outward, the form can resist this force. This is most easily done with blocks that are placed on the neck and butt of the form. There are an assortment of methods that can be used to accomplish this using threaded inserts and bolts to thin metal bars to span the halves or aluminum bars place on the topside of the form. The choice is up to you. Also, you can choose whether or not to include a neck gate to fit the neck on the body when it is in the inside form.

The Neck Gate

The neck gate can be optional and is a handy option to skillfully fit the neck onto the body of the instrument. With classic guitars, you don’t have an option - you need the neck gate because the neck is a integral extension of the guitar sides.

To install a neck gate, complete the form as above, but don’t glue the sandwiches to the form base - only use mechanical fasteners as you will want to remove the stacks. Take the completed halves of the form over the to drill press and drill the (2) holes for dowels - the size you use is subjective, but 1/2 to 5/8 or even 3/4” will work. Next cut through the sandwiches and directly through the center of the dowel holes. Next, glue the stack halves of the neck gate together. This is a bit tricky as you have a center joint do deal with. To strengthen the neck gate it is wise to actually glue it to the plywood plate that spans the joint.

The Last Steps

Finally to finish things up you may find it useful to add studs to the outside perimeter of the inside form. These aid in the fitting and attachment of the top and back plates as you can secure the body in the form, spread the sides as required to keep it in shape and apply either rubber bands or elastic strapping over the studs to quickly glue-up the back and top plates. Many luthiers prefer to glue-up the top and back plates with the guitar workboard rather than the inside form. It is simply a matter of preference.

How to Provide Hinged Access For Inside Forms

Inside Form With Hinged Access
This photo shows an inside form that utilizes a hinged access and aluminum bars to tie to the two halves together. This allows the builder to easily open up the form to both remove and insert the instrument sides after they are glued up.


One little feature that I often use on my inside forms is the hinged access option. This feature will allow the form to crack open for easy access to the sides, no matter what stage they are in. During the building process, the more complete the body of the instrument becomes, the harder it is to remove and insert into the inside form. This is usually a good thing as this is an indication that the body conforms very closely to the shape of the form.

What I often do to facilitate insertion and removal of the sides is to add what I call the hinged option. This allows the luthier to crack open the form a couple of inches and the guitar body can easily be placed into the form. To add this option you need several carrage bolts and plastic knobs. You also have to be sure not to secure the form sides to the form base - at least on one half of the form.

Making the Hinged Access

When you make your form cutout, glue the layers together, without gluing them to the form base. Check the squareness of the sides of the form so the sides of the instrument will be perfectly square to the instrument top.

Assemble the form and secure the form sides to the base with screws. Check that everything is true and square. Drill (4) 5/16” holes completely through the form sides AND the form base. Countersink the bolt heads at the underside of the form base. The bolt locations should be 1-1/2” each way (3” from center to center of the bolt holes) from the of the centerline of the form and centered on the thickness of the form sides.

To assemble, install (4) 5/16” x 18 x 5-1/2” carriage bolts through the bottom of the form. On each pair of blots, install the aluminum restraint bar, which will resist sideways pressure from the spreaders inside the instrument sides. Finally, install the plastic spinner knobs. Remove the screws that are securing the form sides to the form bottom.
Special Note: To maintain the inside form integrity. secure one half of the form sides to the form base. You can also use a few screws to secure the other half of the form sides to the form base when storing the form or when not in use.

Using The Hinged Access

It is very easy to use the hinged access. To open the form up, remove the (2) plastic knobs from the bolts at the guitar butt. Next remove the aluminum restraint bar. Remove the (2) bolts and crack open the form.

To insert a body, open up the form, insert the body and close the form as far as you can. Insert the (2) carriage bolts. If you need to squeeze the form sides just a bit to slide the restraint bar over the bolts, just use a Quik-Grip clamp on either side of the bolts and pull the form together. Insert the restraint bar and remove the clamp. Slide the restraint bar to the top of the form sides and crank down the plastic knobs and your guitar body will be held nice and tight and perfectly formed within the inside form. Now if needed you can install your spreader jacks without worry of form distortion.

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.

As the luthiers building experience grows, they will want to add additional luthier tools to their shop, which will give more versatility and focused attention to certain tasks, and at the same time giving better accuracy and repeatability to the build. The adjustable inside form is one of those tools. The adjustable inside form is a variation of our standard fixed inside forms that we offer with every ukulele drawing we have listed on our site GenOne Luthier Supply. This tool does many tasks and does them well, with minimal time and maintains a high degree of accuracy in the instrument building process.

The adjustable inside form gives you the freedom to use this tool with any size ukulele that we offer and therefore you don't need a different form for each ukulele size. The process that makes this form adjustable is the use of a series of (6) adjustable retainers at the upper bout, the lower bout, and at the waist of each instrument.

The Three Elements That Make up This Tool:

The Form Base: The base is the 19mm piece of cabinet grade plywood where all the other adjustable elements are anchored. It consists of several curved channels which allow for complex adjustment angles of the side retainers. Also you can optionally install an option to this base so the form can easily be clamped into a woodworkers vice. This give the form incredible rigidity so major operations such as crown sanding can be completed without the concern of the form sliding around on the workbench.

The Inside Form Shape Insert: This is an additional insert made from 19mm plywood fits onto the top surface of the base. By installing  smooth-shanked pocket screws around the perimeter of each of these inserts, you can glue down top and back plates with surprising speed. We include ample pocket screws to easily go around the edges of the insert for either rubber band attachment or elastic banding.

Side Retainer Clamps: The 6 retainers are key to the working of this form. Each retainers is attached to the base with a carriage bolt/free-spinning knob assembly, giving the luthier easy adjustability for any size instrument. The retainers can easily retracted to release the  the instrument body, and again easily tightened to secure the instrument back in the form. The form is shipped with the front surface of the retainers that contact the instrument body curved to easily fit the subtle curves of the body. The retainer bases are slotted giving a lot a adjustability for different instrument body types.

If it is your intent you use this tool for attaching strapping or rubber bands to clamp

the plates of the instrument, you will need to install the included pocket screws into the Form Insert. Here is the procedure to accurately accomplish this task:

1. Install the shape insert on the form base. Next install the (6) retainer clamps onto the base and adjust to the proper setting to the inside shape insert. Mark the location of each side of each retainer clamp on the side of the insert so you know these areas cannot have the pocket screws installed that these locations.
2. Remove the shape insert and clamp in a vice. You will want to mark the locations of each pocket screw around the perimeter of the form with a pencil. I usually stay away from the line of each retainer clamp about 12mm (1/2") for the first hole. Then I take a flexible tape (like a seamstress tape) and measure between retainers and divide by about 1" (25mm). Then mark each screw location using this method completely around the shape.
3. Drill the pilot holes for each pocket screw. Use a pilot hole will allow the screw to grip, but not crack the plywood. Use a depth stop on the drill so you can drill a hole about 2/3 of the screw length. Try to drill these holes as closely to perpendicular to the edge of the form as possible. Upon completion of the side drilling, screw all the pocket screws into place so about 1/2" (12mm) of the shank is exposed.
4. Upon the installation of the instrument sides into the form, with the inside form shape insert in place, the tops of the retainer clamps may need to be trimmed for clearance needed for sanding edges of the instrument. You may want to make additional retainers if you use a wide range of instrument depths - although this really is not necessary. The retainers clamps at the lower bout of the instrument will naturally be higher than the clamps at the waist and upper bout.

 

This tool can be used for the following tasks:

• Installation of kerfed linings to the sides
• Finish sanding the sides to prepare for mounting the top and back plates
• Holding sides in place for fitting plates and cutting brace channels into the lining
• Holding sides during gluing of separate blocks for floating top installation
• Holding body during binding and purfling channel routing
• Holding body during purfling and binding installation
• Holding body for scraping, rough sanding and final sanding

Differences Between The Guitar & Ukulele Inside Form:

These forms basically operate and are setup identically. Of course the Guitar Form has a much larger footprint and the Retainer Clamps are large for more stability. The biggest difference is the Guitar Form has optional head and tail block retainers available as an additional option. Again this is for more stability for certain tasks. These instructions apply equally to both the guitar and ukulele versions of this tool.

Rough Thickness Top & Back Plates Prior To Joining

With minimal sanding to take out some light planer marks the plates should finish out about .10" or 1/10" (2.6mm to 2.8mm), which is just about right for a steel string acoustic guitar. For classical guitars take the plates down by a few more tenths of a millimeter or thickened to about 2.4mm. Ukulele tops are usually about 2mm thick. For an easy method of doing this, visit articles on plate planing on this blog. There are a lot of opinions on thickness of tonewood, so do your research. Here are some general guidelines for finished plates and sides:

The GLS Heated Side Bender one of the most valuable tools to use in producing perfectly bend sides without cracking or staining and is essential for the luthier to produce consistently precise instruments.