Part 10: Deciphering technical drawings.

W

hich way is up? Is that the front or the back? Stupid drawings! If you’ve been bewildered by some of the drawings you got with your kit, you’re not alone. Some of the designers appear to have been intimidated by the more esoteric presentations and, for that matter, even made some very basic goofs. We won’t go into a full-blown explanation of the entire subject of drafting standards here—that requires several semesters of college-level courses and it is, even now, undergoing major changes on how dimensioning is done. What

In third angle projection you should imagine yourself standing inside not only the glass box, but inside the object as well. You could face the 12 o’clock position and you’d see the wall of the object. That view is the one projected onto the 12 o’clock wall and when that wall is laid out flat by hinging it up you would have the view you see.

Photos: Bob Fritz and Courtesy the Manufacturers

BY BOB FRITZ

we will do, however, is offer a couple of basics that should give you a shot at understanding even those drawings that bear an embarrassing resemblance to the runes carved on a Viking headstone. Isometric versus orthographic? You’ve heard the terms but probably can’t remember which is which. There’s an easy way: “Iso” and “graphic” translate to single view. And that, in turn, can be interpreted to mean one view shows almost everything. Having knocked off that definition, we’re left with orthographic. That breaks down to “ortho” and “graphic,” which translates to upright view. Simple, no?

In the upper left corner we see a single isometric representation; a single view shows all. The group of six views is all orthographic; each view is upright.

Well, no. There are types of each projection within those categories, so that’s a gross simplification. But for the moment we’ll leave it at that because these styles of ortho and isographic cover almost everything you’ll see.

The View The question then arises: If, in an orthographic drawing you can have several views, how do you know what their relationship is to one another? Are you looking at the right or left side of the thing? Can’t you just slap them anywhere on the paper? The answer is no. There are two principal methods for

This graphic is rotated 45° counter-clockwise so you can see the bowl and all the positions. That means that the 12 o’clock position is in the upper-left corner. To get a handle on the pictorials and positioning of a group of third projection orthographic views imagine the part sitting in the bottom of bowl. Sliding the part up the wall of the bowl lets us see the other sides. And if we slide it up the left side, we need to place that view to the left of the plan view. KITPLANES January 2008

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The Home Machinist, Part 10 continued

The lower view is the face of an RV-6 panel I designed a few years ago. The view just above it is the top view. Visualize the bowl, and you’ll see that this orientation is a third angle orthographic. But you’ll note that the altimeter, airspeed and artificial horizon hide everything on the switch panel below them.

In this sectional view we’ve sliced off the upper half of the panel and can now see the switchboard at the bottom of the panel. The slice-line is identified on the first view by a dashed line, two arrows and “D” at each end. That’s telling you three things: The horizontal line says where we cut the part; the arrows indicate the direction we’re looking; the “D” identifies this particular section. There could be several sections, so each is identified as “A” or “B” or whatever letter you like, and the section identifier is listed underneath.

placing the figures, and they result in confusingly similar but different layouts. They’re called first angle projections and third angle projections. The basic idea behind both is to float the object in a six-sided box, project the six faces of the object onto the walls of the box, and

then open the box to flatten it. The difference comes in the projection: First angle projection assumes that the side you see is projected onto the wall behind it. Third angle projection has a light bulb inside the object casting shadows of the object onto the wall nearest that side. Third angle projection is the standard for North America, so until the recent influx of kits from Europe, where first angle projection is the norm, things were not too confusing. The version used, first or third angle, is denoted by a symbol you’ll sometimes see on the drawing that probably left you wondering, “Huh?” Now before you go completely blurry over this, there’s another, and probably easier way to visualize the North American format, i.e., third angle projection. I call it the bowl method.

The Bowl Method Put the part in a bowl and look straight down at it. That’s the plan view. Now, keeping the part in contact with the bowl, slide it up the right side. That’s the right side view. Slide left, left view; slide to the upper side, top view. If the bowl were a clear glass sphere, you could keep

We can go further with this technology and quite easily get an enlargement of a specific area. That could result in too large a drawing, so we can select an area and generate a Detail View. In this case you see that the uppermost view, the sectional, has a dashed circle with an E. The degree of magnification is a simple push of a button, and now we can see the detail of the placement of the switches. Unlike the other view, section view placement is entirely arbitrary, though having it close to the source makes it easier to find.

coming around to the top of the sphere and you’d be looking at the bottom of the part. Much easier. The only wrinkle here is that if you’ve gone all the way over 180°, you should place that view on the paper just beyond the view it passed through. So let’s try that again. You want to see the bottom?

Sectional and/or detail cut-lines don’t have to be quite as tidy in their positioning as the instrument panel example; they can pass right through a “solid” part. This is handy to show a feature such as that curved interior that might otherwise never be suspected.

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On the left is a first angle projection; on the right is a third angle projection. Don’t try to understand the concepts by those names; there’s a logic to it, but it’s not important here. Just remember that Europeans use the bowl upside down and take it from there.

Slide to the right, get a right view and place it to the right of the first view. Now keep sliding in the same direction until it’s upside down. That’s the bottom view. You’ve been sliding to the right and putting the view to the right, so place this view even further to the right. What makes this last view a bit tricky is that for a part that is not symmetrical, you’ll get a different view if you go in a different direction. That’s why it’s important that you put the view in the correct position. Look at the drawing of the bowl and form a picture in

Viewed straight down that view of the part that’s centered on the bowls is identical. And when the part slides down the right side of the gray bowl (first angle) we see the figure 8. But when we slide the part up the inside of the blue bowl (third angle) we see the other side of the part, the side with the squared-off C. Take away both bowls and you have the same “front” view but a different “right” view.

A simple way of determining if it’s a third or first angle projection is to again use the bowl; if the bowl doesn’t work, then it’s probably first angle or mis-drawn.

Mis-drawn. I can only imagine your screams of confusion. But, and this is most likely, especially if the drawing came from Europe, it’s a first angle projection. How can you tell? Easy. Let’s try the third angle bowl one more time. In a third angle projection the bowl was sitting on the counter in its ordinary open-side-up fashion, and the part was resting nicely down in the bottom of the bowl. Look straight down on it; that’s the front view, and we drew it on paper. Then we slid the part up the right side of the bowl and that gave us the right view, which we drew to the right of the front view. Simple. To get a first angle projection we start by turning the bowl upside down, and the part just fell on the floor. Now place the part on the bowl in the same orientation as before. You look straight down at the part, you get the same picture as

The symbols to tell you which of the drawing conventions is used.

your mind of that representation at 12 o’clock continuing on around to the top of the bowl; the thin section is in the 6 o’clock position. Now take the view shown at the 3 o’clock position and slide it up to the top of the bowl; the thin section now points back at 12 o’clock, just the opposite of the first attempt. Go try it to see the effect. Get a salad bowl and a small asymmetric component and slide it up the wall; you’ll see how the views naturally fall into place and how it’s important to place that top-of-the-bowl view in the correct position.

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The Home Machinist, Part 10 continued before and you draw it on the paper. But now when you slide the part to the right, it goes down the outside of the bowl and shows you a different side of the part. The same practices apply for placing each of the views on the paper, and you’ll get that same reversal of some elements when tracking the final view from two directions 90° apart.

The Reasoning Behind It These conventions have been defined for only a century or so. Prior to that it was a free-for-all, or something that required a very good artist to draw accurately. In the past few years, though, engineers have been able to draw the object on a computer and then literally turn the item around as if it were a real object. That led to the possibility of being able to position several of the parts into a realistic-looking assembly. The advantage of this is the ability to check clear-

An iso/ortho drawing is useful for giving the viewer a fast idea of what we’re talking about and the details of size. But note that the “bowl” analogy applies to the orthographic drawings, making this a third angle projection.

A top-level CAD program will allow you to create the individual parts, assemble them and then, as in this drawing, even show it with a perspective view. Note that it’s smaller at the higher, right end, giving the impression of depth even though it is in actuality just pixels on a flat screen.

ances accurately and, consequently, increase the density of the part placement without worrying about needing a fudge factor. An even more immediate benefit is that libraries of everything from bolts to bearings are available with CAD programs, and the manufacturers are supplying CAD drawings of their products in an effort to get them easily specified into the next assembly. This means the designer can insert one ¼-20 bolt or a commercially available hinge, and then 56

KITPLANES January 2008

Creating all the parts, even the switches and the diodes on that green circuit board, allows the designer to feel confident that it really is a what-you-see-iswhat-you-get design.

use it throughout the drawing with the assurance that there is room for a wrench and the hinge really works. Because the parts can be moved, rather than just popped into position, the bolt can be extracted to check for clearance. You might remember pre-CAD automobiles that required the removal of a fender to get one of the spark plugs out. While there is no guarantee of that not happening now, the potential is enormously reduced. One of the most useful features is the ability to slice open the assembly and see how several parts interface. Back in the bad old days of 2H pencils and vellum this used to be an expensive and timewww.kitplanes.com

THE WORLD’S BEST TUBE BEADER! consuming procedure of hand drawing. With CAD it’s a matter of pushing a few buttons and a couple of mouse clicks. By the way, CAD is an acronym for computer-assisted drafting; ACAD and AutoCAD are CAD products made by one of the industry leaders. Now that the iso/ortho drawings have given us the big picture, we’ll use another type of drawing to get some details. It’s the sectional view, and it’s a favorite of designers. It has two uses: You can, in essence, get around the parts that are blocking your view, or you can slice right through a component to see a cross section.

Drawing the Line Finally, let’s look at the issue of line styles. If every line were the same we’d have a very confusing situation, so there’s a universal consensus on at least a few of the basic ones. They are: • Solid lines indicate a visible feature. • Dashed lines indicate a hidden feature. • Dash-dot is used for the centerlines of holes or other features. After that it goes into line weight, meaning line width, and a plethora of cross-hatchings to indicate everything from aluminum to zinc. Fortunately, that esoterica is not prevalent outside the confines of mega-tech companies like Boeing or Bell Helicopter. As was noted at the outset, mastery of this is not intuitive. It is a specialty so much so that when I was a design engineer, we would whip out a component and then send it to a group of specialists to clean up our digital gibberish and make it conform to the company standards. But the one rule upon which I’ve always relied is that clarity is more important than any rule. If showing it in a non-standard way is the only way to get the idea across, do it. Just be sure you’ve exhausted the standard ways first.

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Editor’s note: If you have specific questions for author Bob Fritz, or if you have certain projects you’ d like us to cover, email us at [email protected] with “Home Machinist” in the subject line.  KITPLANES January 2008

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