This cover panel, right, for a 1942 Interstate, was copied directly from an original, left, using a form 'block, top, and a little perspiration.

EVERY CRAFISPERSON APPROACHES

Form Blocks Parti

a task with certain anticipation, acquired skills, and personal insight. After assessing the situation, he or she works up a build plan. Because many choices always offer themselves, sometimes it is only a matter of barometric pressure, in combination with it being a Tuesday evening rather than a Saturday morning, that yields a chosen angle of attack. Parts you can't weld, like wing ribs, are certain to be made using a f o r m i n g method such as form blocks (unless production costs dictate press forming dies). Ideal for the small shop, craftspeople have used form blocks since the 1920s and made them from hard wood, aluminum, epoxy, Fiberglas, plastic, or a combination of any such materials. Detroit car guys are now u s i n g the term " h a m m e r forming" for the same process, but I t h i n k their term needs a little more pizzazz.

Giving metal an unweldable shape KENT WHITE Tools & Materials The tools necessary for the parts I'm making here are common in the small shop: a saber saw with blades and speeds for cutting steel and wood, an oxy-acetylene torch for h e a t i n g and w e l d i n g , f o u r clamps, a hammer, and a few "cocking" tools, such as a driver and a blunt chisel. The needed materials also are common: plastic body filler, paste wax, some 12-inch lengths of light steel angle and strap, some plywood, and the aluminum sheet for the parts to be made.

The Process For these parts that are a little

bit detailed, I'll mold a receiver into which I'll pound the shape. This is analogous to half of a press f o r m i n g operation, where only the female die is made and the male die is s i m p l y a few shaped striking tools you use to whack and drive the metal into the receiver in a carefully organized fashion. Rather than applying 20,000 pounds of force to the male pressing die only once to form the part, we are applying 10 pounds of force 100 times (or so) to "chase" the metal part gradually into shape. From a variety of molding materials available I chose Bondo or polyester plastic body filler, but the stuff must be supported in a die box made of welded steel to prevent shattering of the plastic. I use a small steel welding table for tasks like heating and bending metal, but you don't need to scurry out for the parts to make one of these if you don't have one. Just clamp one end of the steel angle to

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Curving the steel angle for the die box involves heating and bending to stretch the edge.

your wooden bench with two clamps (so it won't slip under bending loads). Leave the free end in space to be heated and then bent by pushing hard on the end of it. I marked off the curve of the part's major line and used it as a rough reference while bending the 1-by-l-inch 1/8-inch thick steel angle. Heating the steel to a cherry red color is the region of plastic heat, which allows the metal to stretch evenly and easily. Marking again for length, I cut the curved angle and laid it up to the pre-sawn angle for the opposite side, clamped it down, and tack welded. The flat strips are laid up on the angle frame as backing and then tacked as well, using the original part as a guide. When all pieces are in place you have an open-ended box that is ready for welding, though only enough welds for m e d i u m strength and for keeping most of the plastic filler from gooshing out under molding pressure. After welding I remove the loose scale with a wire brush and wax the o r i g i n a l part I wish to copy. Mixing the plastic filler to a 98

SEPTEMBER 2001

The steel die box, right, is made of four pieces welded together, along with two clamping tabs, and matches the original, left, fairly closely.

The waxed original is smooshed into the die box cavity and then clamped until the filler hardens, making the receiver an exact copy of the original.

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Craft & Technique

Rather than applying 20,000 pounds of force to the male pressing

die only once to form the part, we are applying A sized aluminum blank is first annealed and then clamped tight, using the wood as an upper form to hold the definition of the curve.

smooth consistency is a job for a triangle whipper whirled by a drill motor. You don't need air bubbles mixed into the mold, but we do need a very smooth batch of filler. Glop the filler into the die box and quickly smoosh the original down into the mess. I grab the

preset clamps and cinch 'em tight, making sure to keep the alignment true. After the f i l l e r h a r d e n s , I remove the original part and flip the receiver over to knock off the stalactites and goobers hanging out the back, where my skip welds

10 pounds of force 100 times (or so) to "chase" the metal part gradually

into shape.

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left a few gaps. Cleaning up the surface of the plastic is the work of a minute, and then the fresh a l u m i n u m panel is sized, annealed, and clamped in place for the fun part. D i f f e r e n t alloys work d i f f e r ently in this situation, with 5052 requiring many annealings and 1100 requiring none. Because I used 3003, I annealed it once in the beginning and yet had a part plenty hard and stiff enough for service at the end. The clamping tabs are a big help when the receiver begins to walk away in the middle of h a m m e r i n g out the part, so remember to weld them on before adding the plastic. Using a "cocking tool" or "driver" is very helpful and is an old tradition. One spelling has it as a "caulking" tool, but we are not replacing windows. Webster gives the satisfactory definition of cocking as, "to turn up or tilt." So, we turn up the edge by driving the metal down into the receiver with our "cocking" tool. I use a variety of hammers to work the tools and have a fine collection of 8-, 12-, and 16-ounce ball-peen hammers for this purpose. I do prefer a nice 2, 3, or 4x pneumatic hammer with the air carefully set for rapid work, however, and teach many satisfying techniques for this in workshops. This particular method is called "flow forming" and dates back to the 1930s aircraft environment. When you f i r s t see the metal move really fast with this technique, you know exactly why it is called flow forming. You must proceed with caution, as any heavy hit can tear the metal, especially down in the corners where 1 use a blunt chisel for the details. After working all of the metal down into the receiver, I go over it carefully to make a s m o o t h i n g or f i n i s h pass. Any sanding at this point endangers the thin metal, so I rely on planishing for a cleanup. Sport Aviation

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Craft & Technique The schnooble I've chosen to drive the metal Into the receiver is plastic over steel.

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