GLS Inside Forms

May 11, 2020

GLS Inside Forms

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. 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.

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