# engineering drawing question

• posted

Hi,

I got the following question on my first year engineering design course's midterm, and I was wondering if anyone agrees with the answer (I think the question are very ambiguous and really has no proper answer).

Which of the following types of drawings show all dimensions on the principal axes as true dimensions? a) Isometric projection b) Persepctive drawing c) Three view drawing (orthographic) d) None of the above do e) They all do

I think there are no correct answers (becasue both Iso and 3-view drawings have true dimensions on x,y,z axes). What does everyone else think?

Thank you

• posted

I think you're right.

Don Kansas City

• posted

• posted

But don't isometric view show true dimensions on the principal axes ? (any length parallel to x, y, z axis)?

• posted

Isometric drawings *do* show true sizes in all three dimensions . 3-D CAD renderings are the ones that can't be scaled on paper.

I, too, started on the board many years ago and have drawn thousands and thousands of isometric pictorials (I still favor them over 3-D). How could you draw them to scale if they weren't true in all three dimensions? Perhaps you are confusing them with "two-point perspective" or maybe "oblique" drawings (which, ironically, are also scalable in the three axes). A true isometric drawing is based on 30 deg. and vertical drawing angles and all dimensions in the three principal axes are true and scalable.

You might start some semantic argument that an isometric can be drawn based on other angles or variable scales, but you would be wrong. This is why they make templates of 35.26 deg. "isometric" ellipses: so that you can draw circles on an isometric drawing in all three axes (the circles are scalable in these axes as well; that's why their diameters are given on the template).

Extra Credit: can you show why the isometric elliptic angle is approx.

35.2644 degrees?

Don Kansas City

• posted

I read "ALL" dimensions and "axEs".

That means all "dimensions" shown on the x, y, and z (or the r, rho, and theta, etc.) axes must be true.

In an isometric projection (basically, a projection from a point), some geometric depictions ("dimensions") must be foreshortened.

If the question had said "all dimensions on the principal axIs", you would have a case.

Oddly enough, the way I first read it, I thought "What the hell kind of question is that? Some new kind of dimensioning where they change the dimension (value) because they draw it from a different perspective using a different method?"

In my 40 years-of-engineering-world (PE, in machine design, architectural, and structural; from design eng to VP engineering and CEO of a design-build and of a consulting firm) a "dimension" is the value describing a geometric depiction, not a geometric depiction being described by that value. In lay terms: you cannot get a dimension off a drawing by using a ruler - the dimension is the number inside the arrows that communicates size/value and location to the maker of the part.

To have it otherwise implies one can scale the drawing; scientists, engineers, designers, or shops do NOT scale drawings. Ever. No matter how true. Art majors, however, may scale.

Charitably speaking for your professor then and for your future awareness, one should be careful to remember that the term "dimension" may have different meanings in school than in the world, and needs to be taken as very context-sensitive term. Thus, help us out here and minimize one of the big retrainings we have to do with new engineers - by differentiating early between "dimension" and "geometric depiction". (For the pedantists, Webster's New World Dictionary can be read either way on this.)

Just for curiosity, how does your course book define "dimension" for purposes of its own discussion of the subject of drawings?

• posted

It says all dimensions on the *principal* axes. That's how an isometric generally works. The principal axes are the only axes where dimensions are true, but all dimensions along them are true and scalable. I would assume that the Cartesian x, y, z axes are what are implied, not cylindrical or spherical ones. In that case "d: none of the above" would apply.

Isometric pictorials are usually drawn with lengths along the three axonometric axes to scale, for convenience. I've never drawn them any other way. If it were simply an auxiliary view in orthographic projection, the dimensions would appear shorter because of their angle with the viewing plane. But that's not how isometric drawings are usually constructed. If they were, the draftsman would have to multiply all dimensions by the cosine of the angle with the viewing plane, or construct the view as an orthographic auxiliary from other views, which is not what he does. For this reason they appear somewhat larger than in reality, but are directly scalable.

Actually, all lines are NOT necessarily true in an orthographic projection, whether along the principal axes or not. What are you talking about? For a line to be true in an orthographic projection it must be parallel with the viewing plane. Unless you're talking about an object that is made up entirely of simple right angles, I don't see how "c" can possibly be the correct answer. If a line in an orthographic view is tilted away from the viewing plane, it will not be of true length. You have to construct an auxiliary view (or, often, compound auxiliary views) to show such a line at true length. This line would not be true in *any* of the principal views (top, front, side).

Dimensions are casually scaled all the time in engineering. That's why it's drawn "to scale". Otherwise you could just scribble a sketch and put dimensions on it.

Albeit, such scaled values should not be relied upon for manufacture or for critical calculations. But I and every engineer I've known for decades (except for you, apparently) does so regularly when discussing a design. And it's more reliable now with CAD drawings, as long as they are plotted to scale. With paper and pencil, it is usually easier to change the number in a dimension and add "NTS" without having to erase massive portions of the drawing and redraw them to make it truly to scale. But with CAD the dimensions are usually dependent on the lines (or vice versa in the case of 3-D modeling) and the drawing should usually be to scale, even after a revision, if done properly.

Don Kansas City

• posted

Actually, after re-reading my post I realized that even scaled isometric drawings may be suspect.

It all depends on how you interpret the semantics of the question. A "front" view in orthographic projection could contain vertical or horizontal lines that are not necessarily true. There is no way to tell whether they are or not unless you consult other views. A compound angle may not be true in any of the three views, but without an auxiliarly, there may be no way to tell.

Likewise, in an isometric, a line could theoretically appear to parallel one of the axonometric axes, but still not be at the standard angle from the viewing plane, and thus not be scalable.

I think it is a poorly-worded question.

Don Kansas City

• posted

I worked with a checker once who took "DO NOT SCALE DRAWING" to mean nothing could be drawn to anything other than 1X scale. He sat there with a scale, scaling the drawing, to make sure it wasn't scaled!

It was a very young company (the company itself, not the employees; the checker was ready to retire (or be retired)), and they had a lot of like issues to work out. My stint there was thankfully brief.

Rob Campbell

• posted

On further reflection and checking sources a bit farther and deeper, I think I agree with the student - and I think there is a bit of reference shifting as to "dimension" in the question.

When the question introduced dimension and an axis (which is a line), it limited any geometric dimension which could possibly lie on an axis to the line. Anything other than a parallel line to the axis can only intersect the axis at a point. That is, the dimension must be coincident, parallel, and occupy the axis to be "on the axis" - and the dimensions of a line lying totally on the axis of an orthogonal view is true.

1) isometric has true lines (dimensions) on the axes by definition (see Webster), and 2) any line (dimension) ON an axis of an 3- view orthogonal view lies only on an axis line and not in the plane made by two adjoining axis, so it must be true by the definition. (Note that any dimension crossing the axis is not on the axis, only a point on that axis)

The narrowing to "on the axes" made the orthogonal valid.

imho

dimensioning

• posted

I've seen that marking on drawings before, but it always appeared to me like a cover-your-ass disclaimer that isn't very practical in the real world. If a drawing isn't to scale, why in hell AIN'T it to scale?

If I'm looking at a drawing of a jig plate with 300+ holes of all different sizes in it and I want to know the approximate distance from one particular hole to some other cavity, I'll be goddamned if I'm going to waste time adding and subtracting a column of 35 figures when I can just slap a scale onto the drawing and measure it directly in two seconds. I would think that there is much more possibility for error in a complex calculation than there would be in reading a ruler. That's why I, and most every engineer I know, keep a triangular scale in the top desk drawer. That's what it's for. I also keep a second, architectural scale for scaling plant layouts.

Don Kansas City

• posted

Thank you very much everyone. The question baffled me because there were so many ways to look at it. I put "all of the above" because it was the best possible answer (even though there were no right answers). My argument is that if there was a dimension (arrows with value.. which is what we call dimension in this course) then all must be true. All dimension on any axes on any drawing of any kind must be true. I think they meant "geometric depiction," but even then there would be no right answer.

Thank you very much.

• posted

To recap the question: Which of the following types of drawings show all dimensions on the principal axes as true dimensions? a) Isometric projection b) Persepctive drawing c) Three view drawing (orthographic) d) None of the above do e) They all do The answer is probably in your notes or your codex. What the professor is asking is an answer that he/she talked about in your lectures. In this neck of the wood we call this Academic gymnastic. You have to provide the answer that the professor likes to see. In this case the answer could well be: c) Three view drawing (orthographic). However, the type and class of drawings are not mentioned. The intent of a drawing is to give all the information to accomplish the desire objective without having to do any calculations or guessing. As we all know there are many types of drawings. Layout, General arrangement, General Assembly, Details, Weldment, Machining, Fabrication, Casting, Tooling, Numeral Machining, Structural, Architectural, Civil, Piping, Electrical etc etc. Year ago, it was common to have the graduated engineer work as a draftsperson for the first 6 to 12 months. After that he/she was assigned project under the mentorship of a senior engineer.

• posted

Tolerances?

If you have to do that, the drawing is incorrectly drawn, period. AND it is not communicating what needs to be communicated and HOW it is to be done. AND both the mathematical and the measurement tolerances must be hell for a drawing needing 35 calculations in order to locate two points. E.g., proper format is to draw a bolt pattern referencing all holes in a pattern to one reference hole, and referencing that reference hole to the reference edge. In other words, any part has three reference edges. A group of discontinuities having a specific purpose (e.g., a hole pattern) is referenced in whole to an edge using one and only one of its discontinuities - not have each one in the group referenced to the reference edges. And that is EXACTLY how I want it machined. If I have to go out on the shop floor to show them how to do it per drawing, I will (and have)

Why? When I describe and approve a part, I approved the pattern shown to be within the dimensions and tolerances _I_ listed for the purposes _I_ deem appropriate, to be measured in the manner shown on the approved drawing, with tolerances referenced within the group as shown - not have someone down the line decide it is easier to make all measurements off an edge, or scale a drawing because "it's easier". They get paid to do it per the drawing which I decided was the easiest way for all concerned, not do it "their private easy way". ( It goes to large production statistical fits as well as to one-time tolerancing.)

The bolt pattern of the attached part does not care about the holes lining up from an edge - it needs the holes it uses to line up with each other. The location of the attached part on the part does not care if the bolt goles line up - just if the pattern's reference hole is in the right place.

If the drawing you describe is properly drawn, the accurate distances from pattern to pattern is quickly found by looking at two dimensions and one subtraction for each axis. If you want to find the distance between two non-reference holes, chances are you are moving the intent of the design itself off-track.

An architectural drawing that dimensions the column lines to each other, one that dimensions the overall size and lists column lines as typical, and one that dimensions an overall size with all but one line dimensioned all have different meanings. Yes, I have scaled them for purposes of my personal design approach; I check their accuracy of scale first, I use them in lieu of sketches, but I NEVER seal or approve a drawing using scaled information from a drawing.

I would hold the opposite, so perhaps it depends on the person's eyesight and number comfort. I do believe a benefit of discouraging scaling is that your error in calculation is your error alone, and that your doing the calculations also crosschecks the other persons' inputs to the drawing/design for possible errors not yours - before steel is cut or concrete is poured. But an error in scaling moves forward any one of several person's errors, and has no crosscheck. The other lemmings follow the first lemming.

Yes, scale your own sketches when designing. But don't scale drawings made by other people. Use the dimensions put on the drawing.

That's why they are there.

nuff.... I

• posted

Buddy you've been in management too long. Or maybe work for some anal-retentive Nazi =FCber-company. No drawing can anticipate any two points someone might want to know the distance between. The drawing would look crazy and unreadable. All my engineers have scales. It would be a pain in the neck without them. Like Don said "that's what their for". It's just a convenience when we are discussing a design, or troubleshooting in the factory. Nobody is machining parts using a scale.

-- Sid

• posted

anal-retentive Nazi über-company.

Or maybe I have had some real experience with "scalers " - the experience of more than once shipping field service men and several thousand bucks worth of parts halfway around the world and overtime remakes, because the former chief thought scaling it was close enough and almost took down the company. Ten thousand dollar mistakes (which come right off the bottom line) put in just-in-time response equipment because some engineer, designer, draftsman, and machinist all "scaled it"-- that is not well received by the customers or the investors. And I got more than a little tired of having the shop super complain about drawings unable to be fabricated unless he went to get dimensions from lonely power-tripping clueless engineers, and tired of meeting time wasted because some engineer's sketch/private-scale-PT-reading is, oops, a little off.

Sorry, but think about that - and then agree that that is about as stupid a statement as you have ever heard. 1) Every drawing with its points dimensioned (as I had indicated before) does specifically and accurately locate each point on that drawing, And it locates BY DIMENSION any point's location relative to any other point, either directly or indirectly, not to mention providing an accuracy within the accumulated tolerances. Thus if you are getting drawings where you can't determine one point relative to another by using the dimensions, such that you must scale it, you either need to send your draftspersons/cad system to school so they put dimensions on the drawings, or at least teach them where the "auto-dimension" command is and let the software do it, or you need take a course in reading dimensions on drawings. And once your engineers add a new point, do they make a new drawing with dimensions to that new point, or do you let them write "scale this" since they will anyway?

2) What do you scaling people do when you get a break line on a part? Do you use 16 foot long scaled drawings for piping connections? Kind of hard to scale those 0.5" holes, isn't it? 3) And scaling with a SCALE? Ever hear of CAD? If you want to find a dimension and you are too lazy to calculate, why are you using paper and wood? Don't you just point and click onscreen and ask it for the distance? You can even use the screen on the cad-cam machine in the shop. We had that one-command ability over 20 years ago. 4) Anyway, if a drawing is communicating to others what the designer/engineer wants, I have a hard time understanding why anyone wants to go off on their own with a wooden scale on dired pulp, other than to prove a point about drawing accurately. 5) BTW, just what exactly is it you measure "to scale" with your scale instead of your using the dimensions? The location of the main engine thrust pin? The hole for the blood pump? The heart valve diaphragm mounting sleeve? The column locations for the 70-story tower?

Equally as foolish a statement. How can it possibly be unreadable if each group is dimensioned and the group location is dimensioned? I seriously doubt that your company leaves off dimensions because they clutter the drawing.

The engineers or the scales? (Sorry, had to :-) ) (alternative: insert before the previous sentence "Sorry to hear that. My engineers have tans")

I think I said that about dimensions.

But they are discussing a PROBLEM with a design created by scaling, using a scale...... ?

Seem kind of odd.

No sarcasm was intended in my post, so if it sounded like it, cut me some slack because it is Friday and I have government reports due by Monday.

take care

-- Sid

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