Universal Wood-Bending Machine For Guitar Builders

A UNIVERSAL WOOD-BENDING MACHINE FOR GUITAR BUILDERS

By Charles Fox

Bending wood by hand to the gentle curves of a guitar silhouette is an unusual skill, one that the guitar builder shares with few other woodworkers. Doing it well involves the builder in a particularly intimate, interactive relationship with his or her materials. Many builders so enjoy the process that they would use this method to bend sides, regardless of the alternatives - it's that challenging and very satisfying an activity.

Other builders, less charmed by the process, would gladly bend sides and bindings some other way and forego the stress of risking fractured, scorched and/or irregularly shaped work. Even among builders who have mastered the skills and minimized the risks, many would prefer an alternative to such time and energy invested In each instrument.

Production shops turning out a number of identically shaped guitars can justify investing in a mechanical bending device. This might be an elaborate industrial style press with heated forms between which moistened wood is held under pneumatic pressure while it sets to the shape of the forms. Or, it might be a single form over which the wood is bent with clamps holding it down at the ends and a "shoe" pressing it into the form at the waist.

What about the custom builder though? it doesn't make sense to create such tooling just to shape the sides and bindings for a single one-of-a-kind guitar. The creative builder who constantly experiments and refines designs has had no choice but to bend wood by hand - risk, stress and all. An attractive alternative for many such builders would be a device that offered the advantages of commercial wood bending machines without the limitation of a single fixed shape.

This article describes just such a device. With it, the builder can bend sides and bindings - at the same time - for any size or shape guitar, and enjoy both the ease and efficiency that production shops take for granted and the flexibility that custom building requires. The machine's materials are economical and basic enough that it can be built in any woodworking shop. Its operation is quite straightforward once it is practiced and understood. In many years of use by hundreds of students at the School of the Guitar Research and Design Center, the machine has never fractured, burned, or distorted a set of sides and bindings when used properly.

The key to the machine's flexibility is its interchangeable forms. The forms are simply two pieces of 1/2" or 3/4" plywood, cut to the half shape of the guitar being built, and drilled through at their edges to support ten 1/2"x 6" rods which span the width between the forms as they sit in the press. Each new set of forms adds to a library of guitar shapes, which can be reproduced by just slipping the appropriate set of forms into the press.

The shoe that presses the sides and bindings down into the waist of the forms is adjustable, able to conform to the waist shape of each new set of forms. The shoe is actually a pack of 1/4" Masonite leaves held together with two bolts. When the bolts are loosened, the leaves slide up and down independently, their bottom edges adjusting to match the shape of the form's waist. When the bolts are tightened, the leaves lock firmly into that shape. The center leaf is larger than the others. Its ends extend into the long vertical guide slots, keeping the shoe aligned squarely across the sides and centered on the waist.

Also riding in the vertical guide slots is a 1/4"x 1" aluminum bar, which supports the wood from beneath at the center of the waist where it might otherwise fracture as it is pressed down into the forms. The pressure (spring tension) of the bar pulling up against the bending wood increases as the wood is pressed down into the waist.

The wood to be bent is sandwiched between two pre-heated layers of thin (.020") stainless steel, which support the wood from beneath and pull it down tightly from above. The upper (outer) piece of steel has a wire bale pop-riveted to each end, and a leather cord tied to each bale. In use, the ends of the steel and wood sandwich are pulled down against the forms by drawing each of these cords tightly around a cleat.

The press is heated by three light bulbs installed in its base. Controlling the temperature is as easy as selecting the wattage of the bulbs - 150 and 200 watt bulbs seem to work well for all woods. There's no need to install a rheostat/light dimmer for any finer temperature control, but it is necessary always to run the unit through an electric shut-off timer. The timer precludes the possibility of this wooden machine overheating and catching on fire if it is left "on" and forgotten. This has never happened in my experience, but it certainly could happen without a timer set to turn the machine off after 10-15 minutes.

Across each bout is a hardwood hold-down block which is under enough spring tension to pull the wood & steel down against the forms. The block is pulled out from the waist to the far end of the bout, forcing the wood down against the form and holding it there firmly throughout the heating and cooling cycle. The leather cords take up whatever slack is left, closing any gaps between the wood and the forms. The machine is built of 5/8"-3/4" thin ply solid birch laminate - "Finnish" or "Russian" ply. The material is stable enough to last a lifetime. Its parts are joined with 1-1/4" x #10 wood screws. [Drywall screws]

MAKING A SET OF FORMS

1. PREPARE THE MATERIAL

Begin with two pieces of 1/2" plywood, each as long as the body of the guitar being built and as wide as half the guitar's greatest width - for instance 8"x 20" for most full sized steel string guitars. Tack them together temporarily, flush at their edges, with two 7/8" brads.

2. SHAPE THE FORMS

Trace the half pattern of the guitar body onto the set of boards, aligning the pattern's center-line with one long edge of the boards. Bandsaw inside the line and smooth the curves with a disc or belt sander; a sanding drum for the waist. This will produce a set of forms slightly smaller than the full half pattern, which is appropriate since the thickness of the sides falls inside the pattern's outline.

In the past I've modified the shape by tightening the curves at the ends to compensate for some "springback" of the bent sides. If the wood being bent is left to cool fully in the form though, this doesn't seem necessary. Even with a bit of springback the sides will easily conform to the shape of the mold or workboard during construction.

3. DETAIL THE FORMS

At the center of the waist, the deepest point, bandsaw a slot 1/4" wide x 1" deep, and square to the center-line. The aluminum cross bar that supports the wood and steel from beneath the shoe drops into this slot as the shoe presses the wood and steel down into the waist.

With a 1/2" boring bit, create ten holes around the curved edges of the form, tangent to the outline. Locate a hole at each end of the forms (A & J), a hole to either side of the waist slot (D & E), and a hole at the widest point of each bout (C & F). Space the remaining holes evenly around the edge (B, F, H, I). Use a narrow "finger fence" clamped to the table of a drill press to locate the forms such that the bit bores right on the edge. The holes will necessarily break out through the edges.

Once the holes are bored, separate the forms.

4. SURFACE THE FORM

With rubber cement or spray adhesive cover one side of each form with aluminum foil. This reflects the heat back into the machine and keeps the forms from becoming charred. Leave the most reflective surface of the aluminum foil exposed. After you've trimmed the outline with a knife, just punch through the holes with a finger.

An alternative to all this is to coat the plywood pieces with foil even before shaping the forms, tacking them together with the foil coated surfaces face to face inside the pack.

USING THE PRESS

1. PREPARE THE SIDES AND BINDINGS

True one edge of the sides, the edge that will be glued to the soundboard. With a few strips of masking tape, attach a pair of bindings to the trued edge of one side panel. If purflings are glued to the bindings, sandwich them between the two bindings. Taping the bindings tightly to the sides will allow the purflings to bend edgeways without distorting out of plane during the bending process.

Along that same edge, locate the point that will fall into the center of the waist, being sure to leave the sides and binding over long at both ends. Mark that point on the narrow edge of the bindings, boldly enough to be easily seen through the narrow vertical guide slots when the wood is inserted into the machine.

2. SET UP THE PRESS

Assemble the forms into a rigid structure by inserting the ends of the ten 6"x 1/2" steel rods into the holes around the forms' edges with the foil faced surfaces inwards. Set the forms into the press with their waist center slots aligned with the machine's vertical guide slots. Loosen the bolts that hold the shoe in shape and lower the shoe into the waist until the center (fixed) leaf is flush with the outline of the waist. Press the other leaves down into contact with the forms and tighten the bolts. Raise the shoe, Insert the steel pieces, lower the shoe tightly into the waist, and wrap the ends of the steel pieces down around the forms. Draw them tight with their cords.

3. PREHEAT THE STEEL

With the steel pieces drawn tightly around the forms turn the heat on (the shut-off timer is your switch) and leave it on for 10 minutes. Neither the hold-down blocks nor the aluminum cross bar are in place now. 4. MOISTEN THE WOOD

When the steel is heated, moisten the side and bindings with water from a spray bottle. The water will turn to steam and be driven deep into the wood when it hits the heated steel. Wet the surfaces completely.

5. RELEASE THE PRESS

Raise the shoe high enough that the side and bindings can be laid flat across the two bouts of the forms. Release the leather cords, allowing the steel to spring free.

6. INSERT THE WOOD

When the steel is quite hot, release the ends of the steel pieces, raise the shoe and slip the wood (its "outside" surface up) between the steel pieces, so that its trued edge is parallel to the steel pieces and its waist mark (Step 1) is visible through the vertical side slot. You might want to use gloves here while handling the hot steel.

7. BEND THE WOOD

Bring the shoe down into contact with the wood and steel. Insert the aluminum cross bar beneath the wood and steel and hook the spring bar ends to it. Press the shoe down firmly into the waist. Place one hold-down block across the upper bout near the waist, hook its springs in place and pull it firmly over the bout and down until it is at the end of the form. Draw that end's cord tightly around its cleat. Repeat for the lower bout.

8. HEAT AND COOL

The heat should be "on" through the process of inserting the wood. Set the timer for ten minutes and walk away. Let the wood and steel cool completely before removing the bent side and bindings.

Charles Fox is former director/owner Of the School of Guitar Research and Design Center of South Stratford, Vermont. He now resides in the Goldrush country of Northern California where he continues to teach.

SPBU (Luthiers' Mercantile Catalogue code) Universal wood bending machine, kit. This tool has probably generated more interest and enthusiasm than any tool we carry. We supply the parts that are difficult to obtain or make and we leave those that can be found at your local hardware store or Seven-Eleven for you to obtain. The kit includes two full-size drawings, a bill of materials, the necessary springs, the stainless steel sheets with bales riveted in place, spring tie-downs, cold-rolled round stock (cleaned, cut and chamfered), the press screw, and the aluminum waist support bar. The materials that remain for you to obtain include the plywood (Finnish birch is suggested), the shut-off timer, light bulbs and their receptacles, power cord, masonite, tie-down cleats, and leather laces. When assembled, you will have a universal wood bending machine, as described in Charles Fox's. InstructIons for use and assembly are included in the article.

The advantages of this machine over the conventional bending irons are many: Because the sides are supported on both the tension side and compression side, there is much less chance of fracturing. Sides that are off the quarter, or flat-sawn for that matter, can be bent and will conform to the proper shape. And, rippling at the waist, or bubbling in roey woods (mahogany) is all but a thing of the past.

We've sold these machines to many well known makers, as well as several guitar factories, Including C.F. Martin. Martin tells us that they use the machine exclusively for bending Brazilian rosewood sides. We've bent hundreds of sides on our machine without fracture, including some quilted mahogany sides that could not be touched with a conventional bending iron. This is not like a standard heated outside form, not like an Overholtzer mold with the heated waist shoe, not like Watlo heating blankets. The support at the tension side of the bend is what makes this gizmo the wonder machine that it is!

OBSERVATIONS

Most of the suggestions and detail photos below were provided by Paul Jacobson, a builder of excellent classical guitars, based on modifications he made to his machine. In our experience with the side bending machine and discussions we've had with several luthiers we've come up with several suggestions, too. We've pooled our observations as follows:

Paul uses solid linings in his instruments, which he said were always difficult and time consuming to hand bend, especially the thicker (5/32") back lining. The modifications he made to reinforce and generally upgrade the machine allow this troublesome process to be accomplished in about a fifth of the time required for freehand bending (he now bends 6-10 linings at a time). The improvements probably will make bending sides and other parts more efficient, too. Now that Paul has done the R & D for this upgrade, you can make these modifications easily on your machine.

The plans suggest screw eyes for attaching the springs. Screw eyes tend to list toward the spring, and eventually may pull out. We supply brass loops with the kit, which are secured by two screws, eliminating the problem. An alternative is to use 1/4" hex-head bolts secured with a nut and washer both inside and outside the body of the bender for each four positions where the springs for the hold-down blocks attach to the body (see detail photo).

The nut and washer on the inside of the body are recessed (using a Forstner or spade bit) to create clearance for the bending form, and the outside nut is screwed out to the end of the threaded portion of the bolt. By securing the bolt both inside and outside the machine, you create a near indestructible post for the end of the spring. With 3/4" to 1" of unthreaded bolt between the hex head and the outside nut, you can easily slide the ends of the springs on and off, which make it easier to load and unload a side. You also may want to make your machine wlth 3/4" plywood instead of 5/8", which will help firm up all the spring connections and strengthen the upper part of the machine (see discussion below on waist-bar springs).

We are sometimes asked why the slats we supply are not spring steel. Spring steel, particularly stainless, is both extremely difficuit to find and much more expensive - and unnecessary, if not downright counterproductive. The advantage of inserting the wetted side into "straight" (i.e., spring steel) slats before bending becomes a more-than- offsetting disadvantage when removing the side and slats from the form after bending. With the standard stainless slats we supply, when bending is completed and everything has cooled off, simply untie the laces and raise the shoe as high as it will go. Then you can ease the whole thing, side and slats, out of the bender quite handily while still retaining the bend.

Some customers have asked for slats wide enough to bend two sides at a time. Not a good idea! You must make sure that the bend is 90 degrees to the straight edge of the side. This is easily done by lining up the guitar side's true edge with edges of the slats, which in turn, are pushed against the inside of the form. This would be virtually impossible in attempting to bend two sides at a time. Anyway, it takes only 15 minutes to mark, wet and bend a side. The folks at C.F. Martin told us that they bend 3 sides every 45 minutes.)

The Universal Bender produces a bend by applying relatively low heat evenly over a large area, which is why it produces a smoother bend than freehand bending, with no scorching. Bending a side should, however, be done rather slowly in order to avoid fracturing the wood. But how slow is slow? Rule #1: The sharper the bend, the slower the bending. The waist needs more care than other bends, especially on classical guitars and other instruments with sharp bends. Lower the shoe until the slats make contact with the form, then crank it down more and more slowly as the bend takes shape. For example, when the bend is about a third completed you should be making a half-turn on the crank followed by a one- second delay; when the bend Is almost done, do a quarter4urn with a one-second delay. When applying a hold-down block (do the upper bout first), slide the block smartly away from the waist area, then slow down as you start into the bend, scooting the block along by moving it from side to side where the bend is sharpest, speeding up as you come out of the bend. You probably will find this step easier if you opt for the blocks with the built-in handles as shown in the photos.) Then tie the laces, pulling them firmly onto the cleat. When bending solid linings, you must do all this more slowly still, especially with linings thicker than 1/8".

To eliminate any possibility of rippling in the sides and to get the top slat to seat firmly against the wood on all parts of the bend, do the following: Lower the waist shoe all the way to make a complete waist bend, then raise the shoe so the bottom slat lifts about 1/8"-1/4" off the form; secure the hold- down blocks and laces; then lower the shoe back down all the way.

After bending rosewood or any other wood where the extractives are baked out onto the slats, clean the slats with Scotchbrite (not steel wool) and/or acetone. If you fail to do this and you bend a light colored wood you may cook a nasty stain into the side. Also, to avoid staining maple, use butcher paper between the slats and the wood, regardless of whether the slats are clean or not. (Extractives are baked out of maple and cooked back onto the surface of the side.)

If you build in the Spanish method, without molds, we suggest that you build your templates to compensate for spring back. It also helps in either method to "cook" the bent side more than once. A cycle or two of heating and complete cooling helps set the bend. One customer tells us that his sides have had minimal springback from arranging lights on the outside of the form. If you use a mold for the instrument assembly you do not have to worry about the minor springback.

So that the slats are not tied down to the cleats askew, offset the cleats so that the leather lacing is pulled directly down, following the center line of the slats. If the lacing is pulled around the cleat at an angle this angle tends to get transferred to the slats and therefore to the sides.

DO NOT TRY TO STRAIGHTEN THE STEEL SLATS ONCE YOU HAVE USED THEM. You will only succeed in putting permanent creases in them, especially in the crucial waist area, and by that make them unusable. If you are making instruments with very different kinds of bends, say classicals and dreadnaughts, you may need two or more sets of slats.

Here is a nifty tip for aligning the slats quickly and accurately in the machine before bending, so you can more easily focus on getting the side lined up correctly. First mark the top surface of both slats for the exact location of the center leaf of the shoe by scuffing the steel with emory cloth and drawing a permanent line with a laundry pen. After pre-bending the slats in a dry run, tape them together wlth the laundry-penned marks aligned precisely with each other and drill two 3/32" holes through both slats spaced 5-1/4" apart and equidistant from the center of the slats. Then install two alignment pins made from 52 gauge wire nails in the waist-support bar, spaced to fit into the holes you drilled in the slats. (See photo detaIl. Drill holes in the top of the bar with a slightly smaller 1/16" bit, then tap the nails in with a tack hammer; trim/file to 1/2" long.)

You will also need to drill two relief holes at appropriate positions in the center leaf of the waist shoe. When bending a side, the laundry-penned mark on the inside slat comes in handy for lining up the waist-center line of the side precisely with the center leaf of the shoe using just ordinary shop light. (In our shop, we had always found a flashlight indispensable for this task.)

DO NOT DRILL THE ALIGNMENT HOLES IN THE SLATS BEFORE PRE-BENDING THEM. You will surely create a permanent crease in the slats where the holes are.

Another of Paul's tips is to use heavier and longer springs for tension on the waist bar and to attach them with an eye bolt to the top of the machine with two sets of nuts and washers, as with the bolts used for the hold-down-block springs. (See detail photo, below; you won't need to recess the eye bolts inside, however, since they clear the shoe when it is raised to its highest position). For sides and bandings, he uses Century C-329 springs to help assure a very smooth and fracture-free bend at the waist. For bending the stiffer linings, he uses very- high-tension C-333 springs to avoid waist fractures. The higher tension produced by either of these springs requires that the top block that supports the crank be beefed up substantially so the block will not pull out. In place of the four #10 screws called for in the plans, Paul used four 1/4" lag bolts 3" long. (On our machine, we solved this problem by bracing the block with two pieces of rosewood roughly 5/16"x 2-1/2"x 7-1/2" that span the two sides of the machine neck. It may not be a bad idea to use both!)

Paul tells us that bending solid linings with the Universal Bender presents some unique but very solvable problems. For the soundboard lining, he uses a 1/8"x 5/8" mahogany strip to which he doubles, in a separate gluing operation, a 1/8"x 3/8" strip for a total lining thickness of 1/4". For back linings, he uses a single strip 5/32" thick, and these are the ones that have always led to teeth-gnashing and garment-rending (what a difference in bending travail that extra 1/32" makes!) The Universal Bender makes it all... well, the closest thing to a breeze, but you must have a boiling trough. Paul boils the soundboard linings and doubles 1/2 hour before bending; the back linings get a full hour of boiling. Before putting them in the boiling trough, he stitches them together loosely at the ends with thin cotton string (see photo detail. Don't use synthetic string; it'll melt!). This keeps them together during boiling and while inserting them between the slats.

His bender is also equipped with a thirty minute timer so he can roast his lining for up to 25 minutes to dry them out thoroughly and assure a true bend. If you use a thirty minute timer, you probably should take extra pains to foil-over all areas exposed to the heat lights, especially because you will definitely need to use 200W bulbs for bending linings.

The slot in the body for the shoe center leaf should definitely be routed, not cut with a jigsaw or reciprocating saw. To do this accurately, rout the slot before cutting out the sides of the bender body, using a makeshift fence and a 1/4" router bit. You will have to make several passes. Keeping the same fence setup, use a 3/8" bit to enlarge the slot area where the center leaf is inserted.

Here are some suggestions on how to make the shoe. The inner leaves can be made with several materials (masonite, Finnish or baltic birch plywood are excellent). The center shoe, however, should be made wlth 1/4" Finnish or Baltic plywood, since it has to absorb quite a bit of stress especially if higher tension waist-bar springs are used. Don't use maple or other solid hardwood; it may warp and prevent the leaf from sliding freely in the slot. Check the plywood for thickness when you buy it; it will probably be thinner than the slot, but if it's much thinner there may be too much play. The outer leaves should be made from hard maple. You will need to tighten the bolt (which should be 3-1/2"...not 3" as in the plans) pretty hard to keep the shoe shape, and softer materials won't stand up to this.

Here are some tips for making the forms easily and accurately. Backs and fronts of all forms should be shaped and drilled for the steel bars together. Tack the rough blanks together with 3-4 dabs of contact cement. Cut them out with a band saw and smooth with a table sander (a drum sander on a drill press works best for the waist). Then drill the holes using a dowel finger, as Charles Fox suggests, for greatest accuracy. Use a Vermont American Forstner-COPY bit for a snug fit to the steel bars. Do not use a genuine Forstner or spade bit; they drill a slightly oversize hole, and your bars won't stay put. You won't be able to avoid all chipping when drilling so close to the edge on plywood, but you will be able to remove most chipped areas when you trim the open ends of the holes at the edge of the forms and the remaining chips won't hurt a thing except maybe your luthiery pride. When all this is done, simply pry the form components apart and continue the alignment work.

You may want to upgrade the accuracy of your bending form(s), especially if you will be using solid linings. Rather than adjust the shoe on the bending form itself, a separate pair of forms for this step compensates more accurately for the thickness of the part(s) to be bent plus the slats. All you need for adjusting the shoe is the waist section, but you should leave the adjuster forms long enough to extend out past the ends of the bender body so you can make easy-to-see alignment marks there. To make the adjuster forms for the sides and bandings, trace with your template and band-saw outside the line to compensate for the outside steel slat. To make the side/banding bending forms, draw a line 1/10" inside the template line and cut on the line; this will compensate for a side 2.0 to 2.2mm thick plus the thickness of the inside slat. The principles for making shoe-adjuster and bending forms for solid linings are the same: For a 1/8" lining (you can use the same forms for a 5/32" back lining with adequate accuracy) the adjuster form line should be 1/16" inside the template line, the bending form line should be 5mm inside the template line, both cuts on the line; for a 1/8" soundboard doubler the adjuster form line should be 3/16" inside the template line, the bending form line should be 11/32" inside the template line, both cuts on the line.

When using separate shoe-adjuster and bending forms, it is important for accurate and smooth waist bends that you align all forms on the bender to the same reference and keep the forms firmly anchored to the bender while adjusting the shoe or doing the bending. Your basic reference should be a line drawn on your template at the waist center and perpendicular to the upper-bout/lower-bout center line. This line should then be transferred with the greatest accuracy you can muster to the outside surfaces of all form components after tacking the blank pairs together and before cutting them out. This reference line will enable you to later align all form components individually to the center leaf slots on the machine. After you have a form component properly lined up to the machine, mark the component at both ends of the machine for easier alignment later. Then drill a hole between and above the hold-down-block spring fasteners, drilling through the body and the form (see detail photo).

After removing the form component, install a T-nut on the inside of the form. This lets you secure the form to the bender, using a bolt or an eye bolt with a nut (see photo detail). You can later apply your foil reflector right over the T-nut to keep it from getting too hot. After you have aligned and drilled all your form components, enlarge the hole in the body to provide some slack for adjustment when installing forms in the machine. There is some labor in all this setup work, but once you've gone through the process you can remove and install all kinds of forms and adjust the shoe with the greatest of ease and accuracy.

Here's Mark Campellone's addition to the machine. Several people have asked about bending cutaways, and we've heard several ideas on the subject. Mark is the first we've spoken to who has been doing it with success. As Mark says the cutaway attachment "works well, aside from the usual, inevitable snags that arise when you try to do with two hands something that's more easily accomplished with three or four."

The drawing pretty well explains the cutaway assembly. The shoe for the cutaway press caul is made from solid wood, that is, not adjustable as is the waist shoe. It is made to match the profile of the cutaway. To maintain symmetry and simplicity, Mark has made the form with an attachment which enables him to bend non-cutaways or cutaways with the same form.

You need a separate set of steel slats for the cutaways - they become too distorted to bend non-cutaways after using in the cutaway jig.

For the swivel, two (2)1/4" [5/16" might be better] bolts fit through the bottom of the 1/4" vertical slot and into either threaded "T" nuts or recessed nuts which are set in the template.

Text From The Luthiers' Mercantile Catalog - 1993

Variation On The Theme:
A Wood Bending Form Used By K. Matsushita