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November 08, 2020
Note: This Blog article is an excerpt from our book: Guitar and Ukulele Handbook.
It is essential to pre-bend the wood binding strips to conform to the shape of the instrument body. The best tool to use for this is the Heated Side Bender Machine. By utilizing the Heated Bender, you can bend several sets of binding at one time, which will result in a lot of preparation and saved time for additional instruments of the same shape and similar wood species.
If you plan or storing the binding strips for usage at a later date, it is best to place them in a squeeze jig. This is an easy-to-make jig from a couple pieces of plywood, cut to the shape of the instrument they will be used with. Much like 1/2 of an inside form, but with only 2 layers of plywood and no floor is required. I like to make the squeeze jigs with a couple of feet so they will stand up on the work bench. To complete the jig, add some studs at strategic areas around the jig so you can secure the bindings with rubber bands. This will keep the bindings tight against the form and keep them in shape.
Also, it is a good idea to place shims at the ends of the binding to keep the bindings in an “overbent” state, which will make them much easier to glue in place when it is time to use them.
In order to bend the wood binding there are some steps to prepare them prior to inserting them into the heated binder. This is especially true if they have wood veneer feature strips glued to the binding.
First sort the bindings and rough sand them. Then check that they are all the same thickness, height and length.
Mark where the center of the waist falls on the binding with a pencil line. Next place each pair of bindings together with the veneer feature strips facing each other. Tightly wrap between 4 and 5 areas with masking tape, pressing quite firmly around the pair so they are securely fastened together. Put one of the tape winding around the center-mark you made for the waist. Do this with the other pairs as well. Now mark the center of the waist on the face and edge of the tape winding with a waterproof dry marker so you will have a registration mark to work with.
Also, since a guitar only needs 2 sets of bindings to completely wrap around the perimeter of the top and 2 sets for the back of the guitar, that can be the minimum that you bend. I prefer to bend one additional pair just in case I break one of them or I find a defect that would cause concern. You will find that the heat from the heated bender will really dry out the wood and make it very brittle, so it is wise to be a bit cautious and have a few extra on hand.
To create the steam we will need for the bending process, the binding should to be soaked thoroughly with water. This can be done with a spray bottle and when you spray make sure all sides of each binding pair is very wet with water.
Another option to wet the wood binding is to build a small water tank from PVC piping with a cap on the end. This can be as small as 1” pipe. Glue the cap with the PVC glue to make it watertight. I soak my binding for a minimum of 20 or 30 minutes.
In some instances, it will be necessary to partially bend the strips, then remove them, re-wet the strips and finish the bending. This is especially true with such difficult to bend wood such as Rosewood and curly grained wood, or sharp bends for some guitar shapes.
If you are using the light bulbs for the heating source for the bender, rather than a silicone heating blanket, you should be pre-heating the side bender by this time to bring it up to temperature. Also you will need to have stainless steel slats to perform this task properly. If you are using a heating blanket and thermostat system, that will not be necessary and it comes up to temperature very quickly.
Place the silicone heating blanket on the bottom stainless steel slat and carefully line it up with the slat. Place the strips on the inside face of the first stainless steel slat and heating blanket, being careful to line the waist up with the waist reference line you have placed on the slat. As with the side wood, it is easiest to place one of the binding tape strips near the waist centerline and mark the centerline of the waist on the edge of the tape with a medium Sharpie dry-marker.
The first pair you place on the slat/blanket should have the reference line on the side of the masking tape facing toward you. If you have (3) or more pairs, space them out across the sandwich evenly. If you will be bending several more strips of binding, try to but the side together so they will remain straight when placed in the bender. Finally place the upper or top stainless steel slat on the top and place a spring clamp at each end to hold the sandwich in place.
Load the preferred form insert and lock into place with the bender guide bar. Open the waist retainer all the way, remove one spring clamp if necessary to slide the sandwich into the bender and place the clamp back on the end of the sandwich afterwards. Carefully slide the sandwich over the form (Note you may find it easier to load the sandwich on the form insert first and then load it into the bender).
Turn the heating blanket on and set to about 140 degrees Celsius. Place the thermocouple sensor between the blanket and the wood binding, and you should see the temperature run up to set point very quickly.
Before you start to bend around the upper and lower bout, it is a good idea to lift up the top stainless steel slat and take a spray bottle filled with water and soak the exposed portions of the strips while you have access to them. As the bender is rapidly heating up, be sure to wear heavy leather gloves prior to touching the stainless steel.
Double check that the waist of the binding is centered on the guide bar slot by sighting through the slot before you hook up the guide bar springs. Once satisfied, hook up the waist springs and bend the waist down to about 1” to 1/2” of the form insert and many time you can bend the waist with the waist retainer screw until the guide bar just begins to seat in the form insert. Make any adjustments as deemed necessary. Check the marking on the side of the masking tape one last time to make sure the sandwich didn’t slide out of position.
Now start to bend the lower bout and with your hands (while wearing leather gloves), bend the slats around the curvature. Take the front retaining block and slide it around the slats smoothly, again, while listening carefully as to what is going on. All the while the slats are heating, keep on working the slats around the curvature, stopping to let the sandwich heat more if needed. If you see quite a bit of steam rising from the slats it is likely the optimum time to be bending. Continue the bending until the end retainer is at the end of the form. Check that the wood is conforming well to the form insert and adjust by pressing with your hand (with the leather glove)
Complete the same procedure with the upper bout and secure the retaining blocks at the very ends of the slats on each end of the guitar bender. Since the upper bout is quite a bit harder to bend without cracking, it may be necessary to stop bending about half-way around the upper bout and let the binding steam a bit more. If you hear cracking or popping it usually means the fibers of the wood are not bending, but fracturing. It this is minor cracking, it won’t be much of an issue, but sever cracking will mean the strips will be ruined.
Now it is time to make the final cranks on the hand screw for the waist and crank it down securely until it is tight. Don’t force the hand crank as more pressure cannot only damage the wood strips by compressing them, this excessive pressure can cause damage to the bender.
You will find that most wood strips will not need any additional heating once bent. You do risk burning of the wood if you let the wood cook too long. I usually just unplug the bender once the bending is complete. You will have to experiment a bit to find out what works the best for your wood and bending methods. Total time for the heating to be applied is usually between 10 to 12 minutes total. After that amount of time, unplug the bender to avoid burning the wood.
I find that if you do have one of the more stubborn woods it works to heat the bender up to temperature with the wood still clamped in the form insert and then unplug the machine upon reaching the set point again.
Leave the strips in the bender as long as you can. If you need the bender for other operations, consider making a binding jig retainer where you can secure the bindings into a guitar shaped mold (even the inside mold works for this). I usually place blocks at the head and tail and a shaped block at the waist to over-bend the bindings slightly. The strips can remain in the mold until you need them
You are likely to experience some spring-back, but don’t be too concerned about it yet. I find that with the heating blanket and the temperature controller you have a good deal of control over the bending process and I rarely have any spring-back at all.
GenOne Luthier Supply Tip #1: You can make the binding squeeze jigs from left-over pieces of wood that you used to make the form insert for the guitar shape you are using. Each 3/4″ piece of plywood will handle 2 bindings, so two of them secured together will do an entire guitar. Refer to the photo of the Squeeze Jig that I use for my bindings.
GenOne Luthier Supply Tip #2: Don’t have a Guitar Side Bender? You can make your own. We have a great set of plans and instructions for you to make this essential guitar making tool.
GenOne Luthier Supply Tip #3: You can also use an instrument bending iron for bending the binding strips. It works great and with some practice. You should still tape the piece together if you wish to keep the veneers in place while bending.
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