Drawing from late 80's confusion relating to Concentricity Call-out

[brandnew1]

Hi All,

Question relating to the concentricity call-out between the .3750 & .250 diameters. .3750 diameter called to be concentric to Datum D but when looking at the Datum D the datum is comprised of both diameters (.250/.3750?).

So i'm confused as to what .3750 needs to be concentric to given this call-out. Is this more of a call-out in reference to the .750 distance where Datum D comes off of and somehow .3750 needs to be concentric to this distance?

Can anyone shed some light as to what exactly is this callout for?

thank you

 

[KENAT]

In case it makes a difference, does the drawing claim what drawing standard it was made too?

Posting guidelines faq731-376

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

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

 

[brandnew1]

Looking into the file, it indicates Y14.5-1966

thanks

 

[Tenkan]

Datum D is shown from the C/L of the orthographic view derived from the right side view showing the Ø.250 hole. So the Ø.375 hole is called out to be concentric to the Ø.250 hole axis which is Datum D.

If the drawing was done in 1989 the standard might be ANSI Y14.5M 1982.... I think. Regardless the datum's are not called out correctly but it is extremely common to see them attached to a center-line. And such is the confusion as to which diameter that center-line belongs to...

lightweight, cheap, strong... pick 2

 

[KENAT]

Koda94 - per brandnew1's follow up post it is to the -96 version of the standard, not 82. I don't go back that far though so am not sure what acceptable drawing practice may have been.

Posting guidelines faq731-376

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

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

 

[3DDave]

Expose the print to a shredder. Claim you never got a copy. If someone manages to get another copy and asks again, feign sudden onset explosive diarrhea and hope they take it to someone else, preferably any machinist who thought he could guess what the mess meant. Eventually it should bubble back to an engineer who will, as I did, cringe at the ignorance** of the original engineer(s) and create changes to this turkey so that it is inspectable.

So what's wrong?

Of the four datum callouts, only [-C-] is apparently correct.

None of the others is unambiguous. Centerlines are not available for attaching datum feature symbols, for just this reason. I've just looked at a pre-1965 explanation by Lowell Foster and it clearly shows that either the datum callout is applied to the dimension, or to a specific surface; not to centerlines, so it's not as if it was OK in 1965. I don't have a copy of the 1966 version of Y14.5.

The book treats concentricity almost the same as position. While not a copy of the standard, it was a company reference for Honeywell.

Without the tolerance analysis to tell what was supposed to by controlled, this drawing is a lost cause.

**Ignorance in 1965 is hardly unexpected given the likely lack of decent explanation and examples. 40-50 years should have improved things.

Just a weak nit to pick, "comprise" is a synonym for "composed of" so "comprised of" is the same as "composed of of." In particular the past tense "comprised" is a synonym for "used to be composed of" as in "The Soviet Union comprised several republics." from

http://www.thefreedictionary.com/comprised.

I only know this from resolving the ambiguity in many sentences.

 

[Tenkan]

I get poorly done drawings from customers all the time. Datums to centerlines are common even today. Challenging the customers drafting ability isnt an option, making the part they want is an opportunity to make money. Call them up and ask them which hole Datum D is defined from and get the project sold. Once you have the PO ask the customer what standard they are using and let them teach themselves their drawing is wrong. Ask them if you can redraw their part current for a reasonable NRE cost so you can assure them they will get what they want and make even more money. Happy customer comes back again.

lightweight, cheap, strong... pick 2

 

[3DDave]

Accepting a drawing like this is accepting a blatantly defective contract.

 

[brandnew1]

3DDave: Thank you for the correction for my grammar, i did not know that but now i do and knowing is half the battle.

thank you everyone else for the input =)

 

[Tarator]

I'm not sure if it is because it is the 1966 standard or not... You can't put datum features on axis lines, center planes, etc... datum features belong to the part, they are not theoretical, they are actual features you can touch... so i don't understand where or what Datum A is?? is it an axis, a center plane?? how about B, is it the axis of the inner diameter or outer?? to me this drawing is a very bad practice...