Nuts & Bolts

Craft & Technique

Tools, Terms & Techniques

sign, its handmade man at the old Harrah's parts serve as models Auto Collection back in for the full production 1972, I spent much run of parts made by time in the Boat and industrial forming Aircraft Shop learning methods. to make and repair An introduction to aircraft many different shapes Bucks, Mock-ups, metalworking traditions made from aluminum, Forms & stainless, and mild 3-D Patterns KENT WHITE steel. The pressbrake Making a sheetoperator there often told me, for metal part is similar to making he had worked on the brake for a gentleman's three piece suit. many years at Boeing, "This is Tailors use patterns and a dressaircraft work son, not that car maker's form, and aircraft stuff. Learn to do it right!" metalworkers use a three-diFussy and exacting didn't mensional (3-D) mock-up or a begin to describe the trainsolid form (Photo 2). They lay ing I got from the German out the patterns and fit them fellow at Interstate Aircraft, to the original "body" and either. But the marvelous then use the patterns to adjust thing was that after I got the the form, "buck", or (in aviarhythm of these careful protion terminology) "mock-up" cedures, my "car stuff" was to match the solid dimensions much higher quality, and These "mystery" pants have been refit to a very of that original. The "original" the tools and methods I used custom Stearman. can now leave to go about its were more accurate and a lot business while you cut and more fun to work with! Curform the material, assemble rently, even Formula One the pieces, fasten them toand AA Fuel dragster congether, with frequent re-fitting struction are on a par with to the mock-up. Occasional fittraditional aircraft procedures. ting back to the original part might be necessary, but the Aircraft procedures is what mock-up is your reference for we're interested in, and this most of the work. month we'll explore the termiKeeping this process in nology, tools, and techniques mind, let's make a wheel pant. used to make a "typical" part, such as the wheel pant in Dividing the Panel Photo 1. To some this will be Breaking down the original an introduction to aircraft 3-D mock-ups can be made of wood, fiberglass, both, metalworking, and to others it or anything. The black one is an Engineering/Manu- part into bite-sized pieces is a stumbling block to many may be a review, but the goal afacturing Template (EMT) for a Twin Commander. craftsmen. I look for pieces is to explore these tried-andthat are a handy size to work with true methods for making and When a manufacturer creates a new repairing aircraft sheet-metal parts. design, it builds a prototype to prove and I try to have breaks, joints, or It's important to remember that the the new craft. In most cases it's built seams on high crowns, if possible. skills discussed here are staples of avia- by hand, and its metal parts are cre- Because distortion caused by weldtion craftsmanship, including the ated using the methods we'll discuss. ing can be a problem, centering the creation of production airplanes. When the prototype proves the de- seam on the high crown limits the AS AN APPRENTICE METAL-

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distortion to the immediate area (Photo 3). Maybe it's Murphy's Law, but making a new part is always a learning experience because we always seem to find a better way on the second one!

Patterning

In school everyone should take Paper Dolls 101, especially if they want to build or restore an aircraft.

The skills learned are the basics for creating paper patterns that not only

dyes, remember that heavy scribing causes stress risers. It's quite possible that these will cause premature failure of the part when operating

stresses and vibration rise along that nick, gouge, or cut, happily growing it into a crack! For this reason, sharpen the scribe or dividers carefully and use only very

light pressure when scratching the

surface. I tend to use a scribed line

only when shearing or cutting and use the marking pens for hems, bends, and folds. Be sure to mark every detail possible—bend up, fold down, inside, right, and up. Such explicit notes

enable you to lay out the blanks accurately, but can also show the craftsman whether an area needs stretching or shrinking—and how might make you feel foolish about being so anal, but think how you'd feel much is necessary. Typical layout tools include a black if you can't remember exactly what marker, soft lead pencil, ballpoint you need to do and where—and you pen, and layout dye. If using fluids or end up making two lefts.

Blanking

After cutting out the pattern, lay it on the sheet and tape it down. Allow for about a half-inch of excess. Cut out the pieces with snips and shears (there are many varieties), but pay attention to keep the cutter fully engaged into the material, and to make one long continuous cut. If you don't, the cut becomes jagged with "Dutch daggers," the result of re-engaging the cutter in a series of new direc-

tions, segmenting the curves instead of flowing smoothly (Photo 4). If you want to save some blood

and have presentable hands at the

dinner table, wear some l i g h t weight Kevlar gloves when deburring the edges after cutting

out the pattern pieces.

To make forming and welding seams easier, divide the part by its contours. I divided this Cessna 195 wheelpant replica into five sections.

For accurate bumping, the contour of the mallets should match that of the part.

Using paper patterns based on the original part, cut out the blanks you'll form and weld together.

Making the form block for this Stearman duct required straightening an original first. Sport Aviation

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Craft & Technique

From the bottom to top: bench-top mechanical shrinker marks, benchtop using intermediary, hand shrinking (mallet and stake), air hammer, and electric hammer.

Tight fits give you accurate scribe lines.

Forming

Exactly which piece should you form first? I always do the easy ones first because I never know if I'll quit or get fired. Seriously, some parts may have a master piece, the one piece that must precede the others in order that proper contours be maintained more easily throughout the forming, assembly, and fitting processes. Once you've decided what your part's master piece is, shaping the metal into its proper contours is the next step in the process.

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Trim excess evenly. No daggers!

Stretching increases the surface area by thinning the metal and shrinking decreases the surface area by thickening it. Having the ability to "flow before fracture" enables the metal to simultaneously decrease in surface area and increase in thickness, despite our mental mulishness to the contrary. Working the sheet with a mallet into a leather bag filled with sand or shot is called "bouging" (pronounced "booszhing") in old traditions and "bumping" today. Both are words for stretching, and using it to form a part requires frequent fitting, which is called "working in space" or "working in air" (Photo 5). Hammering the metal over a stake with a steel body hammer not only stretches the metal— it also leaves hammer marks, something you may or may not be too concerned with. Hammering metal into a mold is another way of shaping. If you use a mold made of wood, plastic, fiber, or aluminum, the process is called "blocking" and the mold or form is called a "form block." Metal craftsmen use special form blocks to shrink ("shrink blocks") or to flange ("flange blocks"), but the process is still called "blocking" (Photo 6). (If the mold is concrete you can call it whatever you want.) When you drive metal down into the form block by hammering a secondary wood or steel striking tool shaped like a blunt chisel, it is called a "driver" or a "cocking tool" because it tilts or cocks the metal over the edge of the block. Shrinking the metal calls for either a mallet or special hammer, or various machines (Photo 7). It takes more skill to shrink metal than to stretch it, so many folks tend to rely heavily on the machines. Watch out! Some shrinking machines work harden the metal quickly. Second, and very important, is that some of them create terrible stress risers that you must sand or file out on both sides, which seriously thins the metal. If you're working on a part for a car, this isn't a problem, but aircraft can't "pull over" when a failure resulting from a stress riser occurs. The solution to this problem is simple: Boeing and many others use an intermediary that absorbs the jaw teeth while gripping the aluminum. The forming operation in total goes on in a straightforward process of roughing in the shape and then smoothing

Cardiovascular it out and checking the fit.

Fitting

During the forming process you need to fit the piece to the buck repeatedly to ensure it has the correct shape. Only when all the pieces fit nicely do you trim the master piece to size with nice even edges. Then you introduce the secondary pieces and scribe them off of the master (Photo 8). Occasionally, you may need to trim a piece several times before it drops right into place

Joining

After tacking the pieces together, it's a good idea to check and adjust the fit again because little slippages can accumulate into a large discrepancy. After you've tacked the assembled part every 1.5 inches or so and made sure the fit is good, weld the part completely. Usually, it doesn't matter from which end you start as long as you start at one end and weld to the other. I prefer gas to TIG because TIG weld-beads are thick and heavy and the penetration is only mediocre

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Thepressbrake operator there often told me, for he had worked on the brake for many years at Boeing, "This is aircraft work son, not that car stuff. Learn to do it right!" (Photo 9). Ultimately, using witness marks (little scribe or pencil registry marks, stepping from the panel over onto its best fitting location) and careful workmanship keeps the pieces properly aligned. When you achieve this then you can tack them together in preparation for welding (Photos 10, 11, 12, next page).

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