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How to read engineering drawings

salty

Well Known Member
When reading the left side of figures 3 & 4, is there an assumption that the part is sitting flat with the angled portion of the part extending toward you? I can't see any cues on the drawing that makes that certain, but I'm no engineer, I don't know if this is always an assumption or something.

You could also be looking at the part from the flat side with the angle extending away from you, which in this case, would matter since the part isn't symmetrical.

I'm just curious if I'm missing something, whether it is on the drawing, or a convention I'm not aware of.
 

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These drawings are usually based upon the X, Y, Z coordinate system. Different views are usually based upon rotating the part 90 degrees. Normally the view is perpendicular to the part in the basic views. This is a very standard way of showing different aspects of a part.

Think of having the part in your hand and rotating 90 degrees then that is what you would see. If it is a horizontal rotation normally the view is drawn to the side of the part. If it is a vertical rotation the view is normally drawn above or below the part.

I am an engineer and I have often wondered about the difficulty of someone who doesn't have any background at looking at mechanical drawings with learning this skill.
 
For 3 view drawings like this, you can interpret them as if the part was sitting in a bowl (or half sphere). Take the lower left drawing and assume it's on the bottom of the bowl. Slide it up the bowl away from your , look down from the top, and you get the top left projection (90 deg rotation on one axis); slide it from the bottom to the right rim and you get the bottom right projection (90 degree rotation on the other axis).

You can do this experiment with a "real" part to get the feel for how the three projections are done.
 
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My instructor in High School Drafting class had us visualize rinsing a dish under a water facet, "all drawings you make in this class will be laid out this way.”

We all could relate to this method as we grew up washing dishes after the dinner meal.:eek:
 
The easy way is if the flange was facing away from you, it would be hidden from view by the back of the part. As a hidden line it would be dashed. if the flange was coming out toward you it would be a visible line, and therefore would be solid. In this case it is a solid line, so it is coming out at you.

Tim
 
My high school instructor had a very intuitive teaching aid. It was a three panel frame with screen within the frame. Each panel was square. Flattened out, it formed an 'L' shape, with a hinge where each panel met the adjacent one. With the frames folded, it took on a cube shape, of course with the bottom, left side and back missing. An object was placed in this partial cube, and looking at the object through each screen, blackboard chalk was used to draw the shape of the object onto the screen. When the panels were unfolded, the object was depicted as it would be in the drawing we would make of the object.

This is called third angle projection and is the convention in North America, and is the projection shown in the OP's picture of the Vans bracket.

A further explanation of the orthographic and isometric views is shown in the Construction Manual page 03-06
 
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It isometric view is a bit confusing since it doesn't show the short flange with a tapered face. It shows a part that's symmetric, and the other views don't.

Dave
 
The easy way is if the flange was facing away from you, it would be hidden from view by the back of the part. As a hidden line it would be dashed. if the flange was coming out toward you it would be a visible line, and therefore would be solid. In this case it is a solid line, so it is coming out at you.

Tim

This is a good tip. I felt that might work, but I wasn't certain.

Now, a followup question.

What's the purpose of the dashed line running parallel, but very close to the solid line on the right side of the bottom left drawing in figure 3? I get the purpose of the dashes on the flange at the bottom, as they indicate the little kickout, but why do they go all the way up the part?

My guess is just to provide context. It makes it more obvious than if the dashed line where only on the flange, but does it mean something more?
 
And another, slightly different example of my confusion. In this particular case it doesn't really matter because the cuts are symmetrical.

However, it's still interesting to me because my wife held the part up one way and I held it up the other and we both said it matched, and after I looked at it closer, I realized we were both right. Nothing on the diagram indicates if the bend is towards or away from you.
 

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This is a good tip. I felt that might work, but I wasn't certain.

Now, a followup question.

What's the purpose of the dashed line running parallel, but very close to the solid line on the right side of the bottom left drawing in figure 3? I get the purpose of the dashes on the flange at the bottom, as they indicate the little kickout, but why do they go all the way up the part?

My guess is just to provide context. It makes it more obvious than if the dashed line where only on the flange, but does it mean something more?

That right end is cut at an angle (beveled) and you're looking at the front view. This front view shows you a solid line being the front edge and the dotted line as being the rear edge of the angle cut.
 
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That right end is cut at angle (beveled) and looking your looking at the front view. You see a solid line being the front edge and the dotted line as being the rear edge of the angle cut.

Holy cow, you're right. I didn't catch that before. It looked to me like it was squared off at the edge of the flange, but now I see the angle goes all the way back. Thanks! Now it makes perfect sense.
 
The irony is I was jumping through crazy hoops to fabricate it without the angle. It'll be a whole lot easier to finish it now. LOL

Great help!
 
The easy way is if the flange was facing away from you, it would be hidden from view by the back of the part. As a hidden line it would be dashed. if the flange was coming out toward you it would be a visible line, and therefore would be solid. In this case it is a solid line, so it is coming out at you.

Tim

Correct. I learned this the hard way. When I built my -8 I had never seen a blueprint before. The first piece I made from supplied stock was the bracket that holds the throttle, mixture, prop cables near the console on the left side of the fuselage. I was so proud. It was beautiful and looked exactly like the part in the drawing except that it didn't fit. That's when the lightbulb went off in my head -- oh, maybe that's what that dashed line means. The flange goes on the other side. If it's a dashed line, it means that part of the piece is on the other side from what you're viewing.

Homebuilding is educational after all.

Chris
 
As others have said, Vans uses the US drawing convention called 3rd angle projection.
It is not used worldwide. It is common to see 1st angle projection on European documents.
They read 100% mirror image from the above discussion. Watch out for this.

Rant mode on...

A proper title block will tell you in text and with a small model of a truncated cone (easy to visualize view directionality) what projection type is being used.

I blame the degradation of 2D documentation on two things. One, folks dont have time. Just get er done. I am guilty of this. Plus there have never been standards at my current employer.

I think more insidious is the unintended consequences of the advent of 3D modelling. The draftsman thinks he is done once the 3D model is complete because he can throw it over the wall to somebody else for manufacture. All the data is there if you want to dig for it. The automated systems to generate 2D drawings work fine, but they only pay attention to the part itself. The extra stuff that used to be very useful standards are a fading art.

The drawing in question from the OP is from a 3D model. The isometrics are true and correct. You can see the off angle flange if you carefully compare the left and right iso views.

Rant off.
 
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This is a good tip. I felt that might work, but I wasn't certain.

Now, a followup question.

What's the purpose of the dashed line running parallel, but very close to the solid line on the right side of the bottom left drawing in figure 3? I get the purpose of the dashes on the flange at the bottom, as they indicate the little kickout, but why do they go all the way up the part?

My guess is just to provide context. It makes it more obvious than if the dashed line where only on the flange, but does it mean something more?

the right edge of the part is tapered. Therefore the front edge is visible, but the back edge is hidden and slightly narrower than the front, so it is a dashed (hidden) line going all the way top to bottom.

Tim
 
And another, slightly different example of my confusion. In this particular case it doesn't really matter because the cuts are symmetrical.

However, it's still interesting to me because my wife held the part up one way and I held it up the other and we both said it matched, and after I looked at it closer, I realized we were both right. Nothing on the diagram indicates if the bend is towards or away from you.

I had the same issue with the rudder stiffeners. I was certain I had labeled them right but when I went to assemble them it was clear I had it backwards, luckily it doesn't matter for these parts because no matter how you see the drawing you'll end up with one of each length!
 
The views are Plan, which is from above looking down,
Front elevation, which is looking horizontally at the broad face, and Side elevation, also horizontally, but at 90 deg to the front elevation. The isometric view is just to make it clear
 
When reading the left side of figures 3 & 4, is there an assumption that the part is sitting flat with the angled portion of the part extending toward you? I can't see any cues on the drawing that makes that certain, but I'm no engineer, I don't know if this is always an assumption or something.

You could also be looking at the part from the flat side with the angle extending away from you, which in this case, would matter since the part isn't symmetrical.

I'm just curious if I'm missing something, whether it is on the drawing, or a convention I'm not aware of.

Looking at the drawing in the upper left, I believe the second horizontal line down would be dashed if the long arm of the angle was pointed away from you.
 
My experience

My day job (big jets) heavy structural overhaul has had me deep into drawings for the last 40 plus years, so much, I did several years in the engineering group. Typically, the production drawings are only for the L/H side, and standard notes the say "R/H opposite". It takes a while to learn working that way, because unless the R/H side has major unsymmetrical difference, there is no drawing for it. VANS drawings are really pretty darn good to work with, even the Jurassic's like the -4 (what I built) are superior to some experimentals I have seen. many production aircraft drawings don't even have dimensions anymore, and the drawing is superimposed over 10" grid blocks which have to be used as a measuring point for actual size..PITA for reproducing parts from them. In production, there is a CAD program that goes to automation. Fortunately, VANs puts all the dimensions in for us !I always encourage builders to look at every drawing of next higher assembly before starting to cut a piece, as to be sure they fully understand hidden or invisible elements of their orientation.
 
Fortunately, VANs puts all the dimensions in for us !

Maybe on the 3-4-6, but certainly not on the 7. More like illustrations for assembly. Usable only for required work, but not complete for my 45 yrs of looking at engineering drawings with datums, tolerance, radii . . etc.

It is not exactly how to read the drawing but understanding how that drawing was constructed (by hand of course), then one can not forget it.
 
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