Understanding Drawings 26

[AndrewMech]

Hey,
I have diificulty reading drawings, my understanding is improving the more I look at them but I was wondering if anyone has any methods or tips that helped you when you started?

 

[eadwine]

Ahh..., the mistake of not taking care of basics & fundamentals in the modern engineering curriclum. Whatever happened to Drafting and Engineering Graphics & Technical Communications? Oh yeah! The computer took over and now it's supposed to do everything. One problem, "garbage in equals garbage out".

Time too return to basics and the Professional Engineering Degree Program in the Universities instead of the simplified B.S. Programs.

 

[TheRocketScientist]

I had trouble interpreting a drawing when working at "Commercial Machinery Fabricators" corporation. I am a degreed engineer with a background as mechanic, machinist and machine builder as well... between jobs at the time, I was hired as a welder, then moved to the lathe when the lathe operator had a heart attack.

"Cut these shafts" said the stuporvisor [sic]... "here's the drawing."

3" dia. steel about 18" long, to be turned down on the end to.... get this...

2.500" +/- 0.000"

I kid you not.

I laughed and laughed. The stuporvisor tried to insist that the drawing was correct, and to follow it. I tried to explain that machining to a tolerance of "zero" is not even possible, but if one were lucky enough to get one part to measure exactly on spec within 0.0005" then after removing it from the machine and cooling it, the dimension would differ to outside the "spec".

Good grief.

Examining the pile of parts left by the usual operator led me to the conclusion that the actual dimension was 2.500 +0/0.020" or so. Basically, to fit into the wheel bearing w/o too much slop, er, "clearance."

 

[racookpe1978]

KNOW - and even more important !! - fully understand the the way we (engineering and draftsmen) define the section views, plan view, elevation and detail views.

EVERY one of those little arrows and "notes" and letters and numbers and abbreviations on the piece of paper you are looking at has a meaning. The "intelligence" of the whole comes from how you integrate those details together (looking from the left side towards the right, looking from "above down" in one view and then loooking from the "right side" of THAT section view to "see" what is in the tird view of the assembly.

Also: the missing dimension is often on the manufactoring or prefab drawing, but is not repeated on the final assembly drawing you are looking at. A forged piece may get many of its dimensions from the orignal "casting" drawing, then the forged piece gets machined (with many hundred other dimensions), then parts get added to the machined piece on a third or fourth "level" of the drawing. So the actual dimension for length overall (LOA) may be on any of several earlier drwaings.

Also - notice that there always seems to be another "also" ?? - there are very specific rules for naming and displaying and plotting these projection views: but those "specific" rules for drawings are exactly OPPOSITE each other in Europe and in the US. This is NOT the metric/ANSI/inches/gage/pipe/steelshape/threads/sheetmetal/wire/plate/rebar measurement problem, but that European-drafted plots will be present the "opposite" side of the machine than what you are trained in if you started in the US. Alternate: they see our drawings and will produce "mirrir-image" parts because they have been properly trained in the opposite methods.

 

[PeterCharles]

but that European-drafted plots will be present the "opposite" side of the machine than what you are trained in if you started in the US

Do you mean the PROJECTION according to which a drawing is produced?? Any competent organisation will ensure that their standard drawing border will include the projection in words, with the standard 'conical frustum' icon or both.

And it's a mistake to think all those on the East of the Atlantic draw in the same way. We standardised on third angle more than 50+ years ago because it made more sense (and maybe something to do with WW2). Of course the Germans all seem to OBEY the DIN standards

 

[cntw1953]

The best drawing should be easily understood by everyone, in any place, without the need of interpretation beyond basic knowledge on the object been presented. Too often, we are forced to guess a piece of work by its origin, trade standards, covnentions ... What a waste.

 

[PeterCharles]

2D engineering drawings have always needed conventions. Without them who would know what was the meaning of the different line styles!

 

[cntw1953]

Legend.

 

[KENAT]

Sorry cntw1953, I'll have to disagree slightly. At least in my little part of the mechanical world, to create a drawing that captured all the requirements, without relying on various drawing and dimensioning conventions set up in standards that the average joe on the street wouldn't understand would be almost impossible much of the time.

There should be little or no guessing in drawing interpretation, if you don't know, ask. To assume ...

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[cntw1953]

Don't meant to argue, but how do you like to see a civil engineering plan without N/S arrow?

Prints made in 19xx
Company out of business
Author unknown

Popular Convention:
A - North to the left
B - North heads up
C - North follow flow direction

Sure it finally gets figured out, just a little time and some frustration.

Just a few days ago I realized I have interpreted the stock up and down in a completely reversed manner by looking the color of the number indicated in China website:

Red - DOWN (in US), UP (in China)
Green - UP (in US), DOWN (in China)

Glad I didn't throw any money in the trap.

Best practice - provide some consideration to readers...
Maybe I am asking too much, but that's the way I do.

 

[flash3780]

Just remember, it's called detailing for a reason. If any piece of geometry is not explicitly defined on the drawing (or in a specification referenced by the drawing), then the drawing is wrong.

Luckily, there are a lot of great tools to help an engineer or designer communicate exactly how parts should look and fit together available in ASME Y-14.5M-1994 (newer standards are available, but 1994 is quite widely referenced). It's 250 pages long and explains how to use Geometric Dimensioning and Tolerancing quite well.

Nonetheless, it may be a bit difficult to pick up off of the bat. If you'll be dealing with drawings extensively, you may consider taking an ASME class. ASME offers some good online classes that will get you up to speed on working with drawings. Good luck!

 

[cmcbain]

For those of you who commented on Engineering Schools lack of education with regards to reading and generating drawings, here's a little food for thought.

The modern Bachelor of Science engineering program is literally stretched to the limit. Most schools have reached the required credits limit allowed for a bachelors degree (I think it's 128 credits) and are now having to make very difficult decisions about what to cut back, or cut altogether. There is simply too much information to teach a normal high school graduate in 4 years, especially now that humanities (core class) requirements are increasing.

The decision to cut some of the drawing generation and interpretation classes is a pain, but at least it is something that can be picked up quickly with a little encouragement from an angry machinist!

 

[KENAT]

cnw, aren't you arguing against yourself? Surely if the drawing had followed some convention, and what convention was clearly stated, it would have been easier?

"but at least it is something that can be picked up quickly with a little encouragement from an angry machinist"

Sadly not, or the drawings round here wouldn't have been so bad for so long.

Even in the UK where they don't have the same humanities requirement at least on my degree very little time was spend on design communication or whatever you want to call it.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[mfgenggear]

In the USA there needs to be more apprenticeship programs in the engineering, & metal fabrication fields. An Engineer graduates from college/Trade now put him or her on an apprenticeship for 4 years.
During which the Engineer, Machinist or Metal Fricator, is making a living.

Other countries have excelant such programs.

 

[ewh]

I think this discussion is turning into one about apples and oranges. It seems that cntw1953 is approaching the subject from a civil viewpoint, while many of the others are approaching it from a machinery viewpoint. On a machinery drawing, legends are seldom used and industry standards are usually specified. Projection method is also usually indicated, which should avoid problems like that mentioned by racookpe1978. While necessary dimensions may inadvertently be left off, other "end" dimensions are left off because they are not required to fabricate the part being detailed. It is poor practice to include dimensions that are the result of previous processes, and if they are included they should be reference only. A good drawing is a concise drawing.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[cntw1953]

ewh:

"...and industry standards are usually specified. Projection method is also usually indicated, which should avoid problems like that mentioned by racookpe1978."

True, the lack of above in many otherwise excellent drawings is disappointing, it causes confusion and frustration. (A personal experience after few years dealing with mech drawings from different vendors/regions)

KENAT:

Sorry I didn't make it clear. I meant when you have to "guess" from those non-consistent country/regional/company conventions, it frustrates.

The other example has occurred a few times somewhere in the forum - one mistakenly interpretated the thickness left out by the other was in "mm" because "m" & "kg" were repeatly mentioned in the post. Wrong! It turned out to be "cm", a regional practice/standard of the poster.

I believe most of us here wouldn't make the same mistake. But "never"? Especially when we accross disciplines around the globe.