Tuesday, November 28, 2006

AV - The Wandering Prick Mark

The typical aviation apprentice, military or civilian, is a teenager fresh from high school. Homebuilders are rarely that young but unless their background has been in aviation, they too are an apprentice of sorts, at least with having to undergo the same Rites of Passage of an apprentice aviation machinist or metal smith. One of those Rites is layout work, long the bane of every aviation apprentice. Mature in years, albeit not in aviation, this particular Rite can prove especially trying for the homebuilder because any emphasis put upon layout work often appears to be a waste of time. Every adult is familiar with rulers and pencils; with the measuring and marking of things. Why should doing so for airplanes be any different?

In a purely engineering sense, airplanes are not different from other automotive machines whose design is optimized to yield the highest strength for the least weight, even though that achievement has spawned a body of procedures, techniques and specifications unique to aviation. I’ll address a couple of those aviation-unique things in a moment but in a philosophical sense airplanes will always be different because man can not fly. If a boat or car should fail us, we can swim or walk. But airplanes embody a form of implied trust not found in any other auto motive device, in that the mere use of the thing, properly and correctly, will not kill us.

Each new physician is required to swear that at the very least, he will do no greater harm. There is no Hippocratic Oath for aviation but if there were it would probably be: Let’s try not to kill anybody today. And that’s why airplanes are different.

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Note: Automotive means a machine capable of moving under its own power. The Society of Automotive Engineers encompasses everything from paddle-wheel steamers to the Lunar Lander.

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I hope this doesn’t come as a surprise but when using a machinist’s combination square, steel ruler, tape measure and so forth, your best efforts at laying-out will always be off, plus or minus, by some amount. This is normal. The error reflects the precision of your tools and your experience using them. For example, the width of the markings on your tools introduces some degree of imprecision, as does the way you look at the markings, as well as the manner in which you make marks upon the workpiece. So long as work must be laid out by hand there will always be some degree of error. For the sake of safety, that error is always taken into account in the design of airplanes and the layout of their parts.

(Personal Note: Always buy the best tools you can afford. A quality tool will last your lifetime. Or more. I’m in my sixties. Some of my tools belonged to my grandfather, others to my dad. A quality tool is a practical legacy and daily memorial.)

One of the realities of aviation is that airplanes are still built by people rather than robots and human abilities as tool users varies from one individual to the next. Much of the basic training for aviation sheet metal workers and machinists is devoted to teaching standardized methods that reduce human error to an acceptable norm. Scribing a line or pricking an intersection offers a good example of the subtle differences in our ability to use common tools. In a class of about twenty-five, when using a combination square, pre-set to a given dimension by the instructor (but each student using their own scriber), it is rare for any two lines scribed by the students to fall upon the same point.

A similar variation is seen in the student’s efforts to place a prick mark at the intersection of two lines. Magnified and displayed on a video screen, the crater of the prick mark will be canted to the left or right, depending on the handedness of the student, and however canted, seldom falls exactly upon the intersection. This is not a graded exercise but a demonstration, without which the tasks to follow might seem a waste of time. Everyone knows how to scribe a line and use a prick punch. Or think they do. The demonstration makes it painfully clear that all lines are not created equal :-)

As with any of the manual arts, practice makes master of the man. An experienced machinist, working with his own tools, will usually have a scribing error between one and three thou. For a student, the error is typically between eight and fifteen thou. Learning how to hold and use the tools will reduce the error. A useful exercise is to lay out a grid upon a coupon of aluminum or steel and to prick punch the intersections, an obvious waste of their time... without the prior demonstration.

Keeping a blade or straight-edge flat to the work while holding the scriber at the proper angle does not come naturally to all. Yet these seemingly insignificant details have a profound effect on the magnitude of your scribing error, as does the sharpness of the scriber’s tip and its shape. All scribers have a slightly different shape to their tip. Viewed with 3x glass, most define a ogee curve (ie, ogive, etc., similar to the nose of a Spitzer type bullet). That means the tip of the scriber will always fall some distance away from the blade against which it is pressed. That distance will vary according to the thickness of the blade the angle at which the scriber is held. Learning to polish a symmetrical needle point onto their scriber results in an immediate narrowing of their scribing error. (Some machinists stone a small flat on the side of the scriber’s tip, allowing the tip to fall closer to the blade.)

The fact scribing errors exist isn’t the object of the exercise - the errors are painfully evident when the workpiece is projected on the screen (or when viewed using a low power binocular microscope). The object of the lesson is for each person to understanding the factors that cause such errors. Once the problem is understood, we can move on to weightier subjects, leaving the student to reduce their particular error to an acceptable level through practice and self discipline.

Some error will always remain and while a small error is generally considered better than a large one, the real goal is consistency. Once their error becomes consistent we simply calibrate the student :-)

This calibration is nothing more than teaching the student to recognize their normal working tolerance - how large their particular scribing error happens to be. Once the error factor is known it can be dealt with by adding or subtracting that amount to your tool's settings when laying out a line. One way of doing this is through the use of shims, selected to match your rate of error. I'll have more to say about this in a minute. Right now, I want to address the Wandering Prick Mark.

Visual acuity in humans varies over a wide range and declines with age. Some can see divisions as fine as one hundred twenty eight to the inch at arms length with perfect clarity while others have trouble with sixty-fourths held close up. Even when a fine line is visible, variations in hand-eye coordination result in errors when transferring that line to the workpiece, pricking an intersection or setting a tool. When asked to set the blade of their combination square for a projection of one inch, it's rare for even one of the class to hit it dead on. (And if she does, you simply reset her square and ask her to do it again.) An experienced machinist can usually come within plus or minus 0.003" of a mark with a reasonable rate of repeatability but it's a more difficult thing to do than most realize.

So don't do it. Not with your naked eye. Unless you're an experienced machinist.

At the very least, a magnifying glass should be used when picking up your points. There are inexpensive optical devices that allow you to prick the intersection of two lines with repeatable accuracy of about +/- 0.003", which is very good for even an experienced machinist. (Look under Optical Center Punch in the catalog of your favorite supplier [ie, Travers, MSC, Enco, etc.] You'll also find plans for do-it-yourself versions on machining-related newsgroups.)

In a similar vein, you would not use the naked eye to set the extension of the blade of a combination square unless the allowed tolerance was on the order +/- 0.015" (ie, about a sixty-fourth). Instead, you would use a known standard, such as a stack of Jo blocks on a surface plate and set the blade according to that. The resulting setting will usually be within a couple of thou (usually + zero, minus something). Which should be more than enough. If greater precision is required, you wouldn’t be using a combination square. (What would you use? A template or drill jig, created using something other than hand tools.)

But the odds are you won't have a surface plate or set of Jo blocks in your kit. If you're the typical homebuilder, what you'll have is a collection of tool bits of various sizes, measured and marked so their dimensions are known. And your scribing shim is liable to be a piece of cigaret paper (!).

A cigaret paper is about one thousandth of an inch in thickness - thinner than a human hair. The thickness varies from batch to batch and brand to brand. (Buy yourself a packet of Zig-Zag or Bugler and measure them. You'll see a similar packet in the tool box of most machinists.) Dry paper doesn’t make a very good shim. (Paper tends to compress.) But when paper is treated or filled it serves quite well, as shown by its use for gaskets. On the job, the handiest filler is to simply soak the stuff with kerosene or light machine oil. The dimension of oiled paper is more than stable enough to be used as a shim for casual layout work, setting the height of a sharpened tool bit and so forth. (Indeed, paper shims were the standard method of adjusting cutter depth in rifling machines for more than a hundred years.)

To subtract your scribing error to the setting of the blade, put the shim under the blade of the square where it contacts whatever you’re using for a surface plate. To add your scribing error to the measurement, as when scribing off the frame of the square, simply add the shim to the stack.

In addition to tool bits, which usually range between plus zero and minus two or three and vary for each face (ie, they aren't especially precise), another handy source of inexpensive gauge blocks is precision ground tool steel or tooling plate, which is often within .0005" across one dimension (ie, either thickness or width - more precise than you’ll need to build an airplane).

Unless you’re using real Jo blocks on a certified surface plate, the accuracy of your stack-up gauge will wander around a bit. Its saving grace is that it?s quick to set up, inexpensive, highly portable and more accurate than your eye. If greater accuracy is needed you may set the blade using a beam-type caliper or depth mike but for this type of layout, that degree of precision is rarely needed.

Which begs the question: How good is good enough?

All dimensions have a tolerance related to them. This is an inescapable reality of machine work. (Or life itself, when you think about it. Nothing is perfect.) A dimension and its tolerance is inherently linked; you can’t have one without the other; they are a paired set. And since the two can not exist apart, when tolerance is not stated, it is implied.

Until the creation of the International Organization for Standardization (ISO) in the late 1940's, the minimum accepted tolerances for working layouts for airplanes (as opposed to patterns, fixtures or jigs) were plus or minus 1/64th for fractional dimensions, +/- 0.015" for decimal dimensions and plus or minus one-half of one degree for angles. Manufacturers often had their own minimums but trade schools generally used the tolerances above, as you’ll see by examining any of the manuals from that era.

ISO changed all that. When the United States went metric in the mid-1970's we did away fractional dimensions, which today are rarer than fur on a turtle except in the homebuilt community, reflecting the tools and non-aviation background of the typical homebuilder. You run into fractional dimensions occasionally when doing repair work on pre-ISO airframes but most American aircraft manufacturers had already gone to decimal dimensions by the time ISO arrived.

Today’s homebuilders manage to escape most lay-out chores, thanks to simple CAD programs such as DeltaCAD, a simple 2D replacement for the traditional T-square, triangle and engineer’s scaled ruler. Now we need only print the lay-out full-scale, glue it to the part with a spritz of spray glue and use an optical center-punch to pick up our intersections with an accuracy equal to that of a skilled tool & die maker.


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