Metric title block tolerances 1

[tdculbert]

Our company has historically dimensioned exclusively in english; our standard title blocks contain the typical Tolerances unless otherwise stated:
.XX+-.01
.XXX+-.005
.XXXX+-.0005

Lately we've had to produce more metric drawings or metric variants of existing drawings. Without using dual dimensioning, this raises questions regarding the title block standard tolerances. If you were to do a direct conversion, your title block would contain fairly silly tolerances:
.XX+-0.25
.XXX+-0.127
.XXXX+-0.0127

In addition, in metric tolerancing trailing zeros are omitted. Thus, 0.240mm is written 0.24mm and then the decimal-place-specific standard dimensioning rubric becomes irrelevant.

What's the proper way to specify title block tolerances in metric? A single symmetric tolerance with every different tolerance specifically called out?

 

[KENAT]

Um, am I missing something. Tolerances surely give the dimensional pass fail critera? If the dimension is within tolerance it passes. If out side it fails.

Tolerances shouuld come from the part requirements which the drawing defines...

Maybe this is an ISO/ASME issue but it seems pretty dumb to me that as a general rule a part can be made not to drawing but that that isn't reason for rejection.

In practice I'm sure many of us have been in situations where parts have come in out of tol and due to cost/timescale we've had to determine if they can be used anyway or come up with some scheme to rescue them but that should be very much the exception. That wording in the standard makes it sound like the norm.

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

 

[JMarv]

I think you're misinterpreting that last paragraph and/or taking it out of context. Pass/fail criteria may be the most important thing for a drawing, but isn't fit/function the most important thing on the finished product? I think that part of 2768 is geared toward mass production. As a tier-2 automotive parts supplier, there is a BIG difference between receiving a Corrective Action Request and receiving a Defective Material Notice (8D rejection). If a product is found to be out of spec, but the fit/function is not impaired, a CAR is issued. The manufacturing process gets fixed, it's documented for the customer, and the issue is normaly closed. When the fit/funtion is impaired, then a rejection is issued. This may involve sending someone to the customer to sort tens of thousands of pieces (expensive!), paying the customer for each hour their assembly process was shut down due to the parts being quarantined (more expensive!), and paying the auto manufacture for each hour their assembly plant was shut down (don't even ask how expensive!). Not to mention all the months worth of paperwork to follow up afterward. Having ISO 2768 invoked keeps "reject happy" purchasing agents from making their customers jump through all these hoops when it's unnecessary.

In my situation, I'm refering metal stampings that are mass produced at 30-40 pcs per minute. In the industry that you work in, things may be viewed differently.

Joe
SW Office 2006 SP5.1
P4 3.0Ghz 1GB
ATI FireGL X1

 

[KENAT]

Jmarv, while there are some exceptions, I'd like to think most of the time that if the drawing is done properly (particulary tolerancing) it pretty well reflects the fit/function of the finished product. In fact that's kind of the point isn't it, to give a definitive requirement for the part. If it meets the requirement it's good if it doesn't then it's bad. My boss frequently talks about the drawing being a legal document defining what you'll accept.

If parts are coming in that don't meet the drawing requirements but are fit/function OK shouldn't the drawing perhaps be changed to better reflect the real requirement?

Like I said I know there are situations where out of spec parts are accepted or some how salvaged. In aerospace I got involved in this, we made some parts that were fairly high production and was once involved in scrapping several days worth of production which as you say is painful.

I hope you're right and I'm misinterpreting it, perhaps it is just meant to allow dispositioning of defective parts but I still don't like it.

Anyway this has got way off topic and I'm starting to rant, sorry.

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

 

[fcsuper]

Yes, drawings should detail form/fit/fuction accurately. If a looser tol is ok, then it should be used in the first place, or when discovered; just as it would be made tighter if the converse is discovered. That line doesn't make much sense, and I question why it is even in the standard at all.

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

 

[mwbaker]

Instead of being bound by the entire drafting standard, at my company we call out an exception in our notes as shown below:

1. APPLICABLE STANDARDS/SPECIFICATIONS:
ASME/ANSI Y14.5M-1994, DIMENSIONS AND TOLERANCES.
ASME/ANSI Y14.38-1999, ABBREVIATIONS.
EXCEPTION: TRAILING ZEROS DENOTE TOLERANCE.

I'm sure that everyone has written a requirement based on a specification and up front called out the exception to the standard. It should be OK to note your exception to the standard since you are declaring it up front.

 

[KENAT]

I don't see a problem with that in principle.

In fact we do something similar with metric threads, as well as calling up B1.13M we say that threads without pitch specifed are coarse pitch. This then brings our drawings in line with the rest of the world;-).

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

 

[CheckerRon]

mwbaker: Since this thread is addresssing metric tolerances, I hasten to add that in metric dimensioning, trailing zeros are dropped and do not define the tolerance. The last significant digit defines the tolerance.

 

[KENAT]

Ron, I think that's his point. By adding his note he's adjusting it so the same convention on number of decimal places invoking block tolerances can be used with metric & inch. I'd probably clarify with something like "EXCEPTION: TRAILING ZEROS INVOKE BLOCK TOLERANCE ON METRIC DIMENSIONS UNLESS OTHERWISE STATED." but i don't see the problem in principle, it's a bit like what we did with metric threads as I said.


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

 

[CheckerRon]

OK KENAT, but seems like confusion for convenience, and not something I would want to do. Here we have a separate metric format and an english(not really),imperial(not either), American (yeh!)format, as do you. After all it is not EMSE, it's ASME.