Noobe Question on Design Intent 6

[erk1313]

This is a question about design intent vs. matching the steps a shop would use to create the part.

The example is a pin used to support a "table". The pin has a shoulder that is cut short to prevent it from showing through the top of the "table".

As the attached drawing shows, i've dimensioned the part from the edge of the shoulder (since that will be the critical reference point in determining the height of the table off the ground).

However, a colleague suggested I redo the drawing to match how the shop would make the part on the lathe: 1) the entire length of the shaft is measured and cut; 2)then the shoulder is measured and turned down. This would suggest an overall height dimension and a shoulder height dimension.

Which one is preferred?

Thank you for the help, and these great forums!

 

[KENAT]

Drawoh, I don't see the reference dimension as much of an issue. Just pointing out that they can have a down side. Of course once you start second guessing the people making the part etc. where do you stop, which is why I qualified my post.

In my OP I made it clear only the OP fully knows the function and that my comments related to it were just guesses.

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[erk1313]

Thank you for all the thoughtful responses. The consensus IMO: if the tolerance of a particular dimension is critical to function or manufacturing, then it deserves priority; otherwise, consider dimensioning that would help purchasing/manufacturing/or an overall understanding of the part.

For example, the length of the bottom shoulder determines the table height, and is critical. However, the smaller pin height just needs to be (.020) shy of the table surface, to keep it from poking through. This is easily achieved and could be left off so that the overall dimension could be listed instead. Sound good?

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Besides Machinery's Handbook, is there a noobe "guide" to AMSE practices that you can recommend? I apologize if this deserves a separate post... I was trained in Biomedical Engineering, but am now pursuing ME. I never realized ME's received so much training in standards. I really thought we were taking pretty much the same courses. Of course, this was back in 1995-9.

 

[MechEng2005]

I didn't receive training in standards as part of my education. I suspect most posting here did not learn the standards from their BSME courses.

 

[KENAT]

Your idea sounds reasonable given typical tolerances. As you're still learning may be worth a conversation with the machine shop if possible just to get an idea on their specific capabilities, might learn a bit about what process they plan to use etc.

As to the other issue a separate post might be better but, I learned most of mine from having my drawings bled over, reviewing course notes from some ASME training (held just before I joined the company) and reading the actual ASME Y14.5M-1994 standard. I've seen some weird and wonderful ideas & questions posted on here based on secondary sources so while they can be invaluable, I'd always use the standard as my primary source.

I don't think mose ME's spend that much time on drafting standards as part of their degree, there may be exceptions though. I actually studied Aerospace Systems in the UK and took about half a dozen labs in drafting which and the only standard references was the BS version of standard shaft/hole combination's.

Much of the drafting stuff is more likely to be taught to people taking a drafting course, maybe an associates or something, even then I couldn't say how closely some would follow the industry drafting standards rather than just learning how to use various CAD packages, at least these days.

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[drawoh]

I didn't receive training in standards as part of my education. I suspect most posting here did not learn the standards from their BSME courses.

My "Engineering Graphics" training in college was very basic. I learned on the job, and I talked to machine shops. I have learned a lot in this forum.

JHG

 

[erk1313]

Thanks for the encouragement. I'll do a bit of research on ASME guides and see what I can find. I'm surprised this practical stuff isn't covered more in a $100,000 education!

 

[fcsuper]

I got general exposure to ANSI Y14.5-1982 in my trade school classes. It wasn't enough to make me an expert or anything. Actually, it was just enough to scare me away from using GD&T for a few years.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[KENAT]

erk1313, at risk of going off topic. Drafting isn't considered a 'hard core' engineering skill by many in the engineering field. It's seen as something to be delegated or picked up as needed. As such, it doesn't form a large part of most bachelors of engineering curriculum.

Most bachelors Engineering degrees try to give a broad foundation heavier in math and theory than in practical application. It’s part of the breadth V depth argument.

You can get associates degrees or diplomas etc in drafting at some trade schools and community colleges though at least some of these seem to concentrate more on teaching various CAD programs than the fundamentals of drafting & design.

Not saying it’s right, but it seems to be how it is.

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[fcsuper]

KENAT is right. The problem with the modern approach is that if particular types of engineers (usually the know-it-all) learns something wrong earlier on (from someone else that learned it wrong), it sticks with them their whole career, and its a pain to try to correct their errors because they really take it personally. It's the strangest thing. I've seen this repeatedly. These same people will take engineering advice from other engineers, but the lowly engineering services person (who was training specifically in the field) tries to correct them on detailing methodology or drawing technique, well, they must be put in their place.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group