Symbols used in Harold Hall’s component details

[FatherWes]

At the risk of announcing my ignorance but in pursuit of reducing it:

What do the symbols (two concentric circles and the trapezoid) mean? They appear in most of the component details in Harold Hall’s books but I have not found any reference as to what they signify…
Thanks in advance!

 

[ChazzC]

That is the drawing projection legend. In this case, it indicates that if you look at the left end of the truncated cone (which looks like a trapezoid in 2D) you will see the small and round end. If you look at the right end of the cone you just see the large end, with the small end being dashed (hidden).

Not a bad question at all: many drawings do not show this and assume that you know enough about what it depicted you will know how the drawing unfolds from 3 dimensions into 2.

 

[whydontu]

As @ChazzC says, those two symbols define the type of projection. Not very common now.

http://mem50212.com/MDME/MEMmods/MEM30003A/7786AN_Eng_Drg_Interpretation1_10_to_11.pdf page 2

 

[Susquatch]

At the risk of announcing my ignorance but in pursuit of reducing it:

"The pursuit of reducing our ignorance." What a great way to ask the question. A very worthy goal for all of us.

I recall some advice from my early career which I have never forgotten. "Never overestimate how much the chief engineer knows. Never underestimate how smart he is."

 

[FatherWes]

Thank you @ChazzC and @whydontu - seems obvious now, but I was wracking my brain trying to figure it out… does that mean I’ll have to tap a hole and do some milling or reaming?? I should have clued in because I did see that the concentric circles could be a plan view of the conic section.

Susquatch - I was an instructor for a few years. “The only stupid question is the one not asked.” But ego can still get in the way.

 

[whydontu]

A concept that took me a long time to accept, and I still get caught.

You don't know what you don't know.

 

[Susquatch]

You don't know what you don't know.

My favorite version of that is: "The more you know, the more you know you don't know!"

And, I've reached that magic point in my life where I have learned so much that I have finally come to realize that I don't know squat!

 

[ChazzC]

Thank you @ChazzC and @whydontu - seems obvious now, but I was wracking my brain trying to figure it out… does that mean I’ll have to tap a hole and do some milling or reaming??

We would need to see the full drawing to answer your question about milling, tapping & reaming.

I should have clued in because I did see that the concentric circles could be a plan view of the conic section.

The symbol is just for reference, it is not part of whatever is the item(s) depicted in the drawing.

 

[PeterT]

Even by knowing that the drawing is first or third angle projection, you might still have to ponder what the physical part looks like depending on what views were displayed & how complex the part is. Sometimes you can have parts with features that have coincidental lines corresponding to features. Technically correct in their respective views, but still not exactly intuitive. I have seen many drawings like that which is just plain frustrating, especially if they skipped other views that would further define the feature. The objective should always be clarity, not an image IQ test. Anyway, careful when you get drawings of parts from Europe or Asia because they use different projection standards to N-Am. Its not better or worse, its just different. That's why CAD software typically watermarks the standard being used on drawing itself, so the designer cant mess it up by omission. The part itself doesn't change inside the CAD model, but the 'hardcopy' can. Projection is as important as stating the scale & dimensions.

Angle of Projections : Engineering Drawing

Angle of Projections : Engineering Drawing: We all know that engineering drawing is language of engineers. So every engineer must be able to read drawing ond decode the information provided at drawing. This exercise enables you to identify the projection system and you will never forget.

www.instructables.com