True position tolerance to a face 1

[Picklefactory]

Hello folks
Hope I'm OK posting same question in multiple forums, I'm new to this one and was recommended to post here rather than my original choice.
OK, my question-
We have shiny new designer in the company, who is producing drawings with faces toleranced by true position back to other datums/planes. I'm perfectly familiar with true position of holes, but I've never come across this style of tolerancing a face. I don't understand how a face can be toleranced in this way, am I thick, or is my new designer nuts?
I hope that makes sense.
Here is a sketch (If I've managed to load it correctly) where there are 2 faces toleranced as true position. One is the end face of a valve feature, where there is a threaded cap assembled up to the face, and the other is a milled face on the right hand side to accommodate the two ports drilled through. Both are toleranced back to a fictional plane. Does that make sense in this context?
Cheers

 

[dingy2]

Positional tolerances are only on features of size such as holes and pins and NOT surfaces. This drawing in not drawn according to ASME Y14.5M-94 and is wrong.

The Designer should use the geometrical symbol profile of a surface on faces rather than positional.

Good luck!

Dave D.

http://www.qmsi.ca

 

[drstrole]

The position tolerances on the surfaces are incorrect. A profile tolerance needs to be used to locate those surfaces.

David Strole
Engineering Systems Administrator
GDTP S-0132

 

[Picklefactory]

Thanks guys, that's what I thought, but I'm just a humble machine jockey, I just like to get my facts straight before arguing with the grads.
Appreciate the advice.

 

[ringman]

I belieive you might have other problems too. I don't believe the Std provides for datum identifiers to be attached to CL.

 

[vc66]

I could be wrong, but I don't believe it's acceptable to use a positional tolerance on any feature that cannot be captured at least 180° around, i.e. half circle or slot.

V

 

[dingy2]

Ringman:

You are correct here. Datums are developed through features that create the C/L rather than the C/L itself. Apparently, the ISO standard allows a centre line to become a datum though.

Dave D.

http://www.qmsi.ca

 

[weavedreamer]

ISO 1101-1983(E) uses position in that fashion.
Is the document stated to be ASME Y14.5M-1994 complient?

 

[Picklefactory]

"Is the document stated to be ASME Y14.5M-1994 complient?"

If you mean the drawing I'm working to, then no. We design/develop and manufacture our own products complete, so this is a drawing from my design friend 20yds further down the office. I will educate him on his drawing style, but I just wanted some expert opinion on this specific issue, as it was a new one to my dodgy eyes. As you've already noticed on that small section I showed, there are many other issues too. The fact that he has 14 datums specified on this product, being one of them. Features being toleranced back to 3 or sometimes 4 other features, needlessly, being another. Nightmare for the poor guy who has to try and create CMM prog to measure it. (We are still in the dark ages there, no model import function for us, all manual CMM programming).
I think you've all answered my question admirably, thank you again for your time and experience.
Cheers

 

[ewh]

If the drawing isn't required to be Y14.5 compliant, and lacking any company standard addressing this issue, then I think you just out of luck. If there is no standard, how can you accuse him of not following it?
I completely agree with the others though that it is not drawn per Y14.5.

Believe it if you need it or leave it if you dare. - [small]Robert Hunter[/small]

 

[Picklefactory]

"ISO 1101-1983(E) uses position in that fashion.
Is the document stated to be ASME Y14.5M-1994 complient?"

Errr, maybe it might have helped if I'd thought to mention (DOH!), but I'm in the UK, so we will be working to BS or ISO. I gather that makes a significant difference.
Oops!

 

[KENAT]

thread286-218414: was the original post.

Picklefactory - yep that makes a difference in some details. I used to work in the UK but didn't use GD&T as much & certainly didn't understand it as well as I do know (which aint all that!).

Do you formally work to BS8888, i.e. is it listed in your design room manual or referenced on the drawing or equivalent?

I know it's a bit daunting with the 4-5 3" ring binders or whatever it is now but take a look if you have a copy. (For those unfamiliar with BS drawing specs, 8888 is basically a compendium of the relevant BS, BS EN & BS EN ISO specs).

I believe at least some of the drawing practices on that sketch also contravene BS/ISO standards.

If he's a grad from a UK university it's unlikely he had any significant drawing training - heck I'm surprised he know's what GD&T is, that wasn't covered on my short course at uni. As a grad I got taken in hand by some of the more senior Design Engineers (most of whome didn't have degrees) and had the pleasure of having my drawings picked apart as part of the check process. It may be your time to do this, unless management don't care/wont back you etc & it causes issues for you.

KENAT, probably the least qualified checker you'll ever meet...

 

[Picklefactory]

Thanks Kenat
No, management is not a problem, it's usually just an ego thing. The guy in question is nice kid, loads of brains and potential, just (As you surmised) not had much 2D drawing experience or direct involvement with manufacturing. I don't envisage any issues in knocking him into shape (The old "I'll make it son, then you draw it" policy), I just wanted to see if I was mental in my opinion that those type of features would be better toleranced another way. I've struggled greatly with his drawing, as have my boss, 2 other engineers working on the project, the inspection guy and another experienced application engineer who is quoting a turnkey package on the production set up. We'll review the drawing at the end of the prototype run we're making now. I just needed what you have all given me, agreement that this is a far from ideal method to use for those particular features. I think that pretty well wraps this one up.
Thanks again for your time and trouble, much appreciated.
John

 

[KENAT]

Having your ego crushed in your first job out of Uni is an important milestone;-).

KENAT, probably the least qualified checker you'll ever meet...

 

[powerhound]

ISO allows this and is interpreted in the same way as ASME uses Profile of a surface when used in this fashion. The rule of thumb with ASME is when locating surfaces, use profile, when locating FOS, use position. ISO has no such rule. This could be viable if he has invoked an ISO standard to interpret by, otherwise he's just wrong.

Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[MechNorth]

It's a bit of a dead horse now, but ... again ... if the drawing doesn't state compliance to any defined standard, it's a crap shoot.
ISO has a couple gaps that can cause some legal issues ... i.e. what feature of size is being used to generate the datum axis? Something to watch out for.

I am curious, though, why there are so many datums being indicated, and why are there different datum reference frames for coaxial features. Presumably z,y,x are your casting datums, and then a set of 3 machined datums based on those, but I see four machined(?) datums (S,T,U,Q)? Then there's a 10mm hole positioned wrt T/Z/Y while its counterbore is positioned to T/Q/Z, and there isn't any size or position tolerance directly on the 7mm hole coaxial with both of the preceding features. Generally, one set of machined datums generating a single datum reference frame, properly referenced back to the cast datums, will ensure an appropriate relationship between all machined features. Introduction of additional datum reference frames most often just adds cost and complexity where they are not intended or needed.

I know that wasn't your original question or concern, but when you flog a dead horse, might as well make it a good, thorough whipping.

Good luck with Junior!


Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca

 

[ewh]

Welcome back, Jim!



Believe it if you need it or leave it if you dare. - [small]Robert Hunter[/small]

 

[MechNorth]

Tks EWH. I love the emoticon!

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca

 

[Picklefactory]

Thanks for all that info (And I also applaud the emoticon, classic).
Yeah, I have a whole heap of issues with this drawing (The one I questioned was one I didn't fully understand), they go slightly below the heap of issues I have with the design in general............. it doesn't work :-|
The amount of datums (14 in total, yes .... 14) is ridiculous. I'm just waiting for Junior to get over the review regarding total lack of functionality, that we held this morning, before we go and review the 2D manufacturing drawing. Starting to feel rather sorry for him, he has an awful lot of work to do........very quickly.

 

[MechNorth]

OK, non-functional trumps bad drafting. A full day of shredding Junior's ego ... you're going to need a pint or maybe a shot tonight! Maybe one for Junior as well; it's easier to reinflate an ego with a couple brews.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca