critical dimensions

[drayflatt]

Critical dimensions on drawings have become an issue at my company and my question is: Is there a standard symbol or method of indicating specific dimensions on a drawing are critical to the design form, fit or function of a part to the supplier as well as incomming inspection?

 

[arto]

How about a box or Flag with "CTQ" in it for "Critical to Quality" dimensions. "Must have" dimensions [i.e., it's scrap if it's not correct, don't even think of asking for a waiver ]
They usta do this on some GE drawings.

 

[drayflatt]

Thanks for your timely response. That is a possible solution but I was looking for something "bullet proof" that would have the same meaning to the mom and pop shop as well as the hi-tech CNC operations. I would have thought that the ASME would have adopted a standard to address this situation so as to not have company specific symbols that could be subject to misinterpretation. Once again thank you for taking the time to respond to the question.

 

[MadMango]

If you are familiar with GD&T, you can use Basic dims and feature control symbology to identify critical dims.

Also, some CAD packages have built-in options that are used during 2D drawing creation to identify dimensions that needs to be inspected to. I know SolidWorks makes the option available to create a oblong bubble around dimensions that are for inspection use only. you could use something like this in conjuction with a note on your drawings to identify critical dims. "The attempt and not the deed confounds us."

 

[abeltio]

arto's posting is particular to a company (GE) where there is a specific intra-company nomenclature.
Each company chooses it's own marking for critical dimensions.
E.g. a triangle, or a romboid with a C inside and a legend where its meaning is clearly stated: Dimensions with this marking are critical.

Also, indicating tolerances is pretty much indicative of criticality.
E.g. stating 2.000in +/- 0.050in
is totally different from
2.000 in +0.000in/-0.003in

This will be understood by any decent millwright.
HTH
Saludos.
a.

 

[swertel]

I'll reiterate abeltio's post.

Choose any marking you want to mark a critical dimension. Then, in the general notes, define that symbol. For example, we use a triangle with a solid square in the middle. In the general notes, we say "Where applied, dimensions marked symbol shall be validated for functional use."

 

[gjb1]

It sounds like either your manufacturers are making the goods incorrectly or your designs are adequate.

If a dimension is critical it should be toleranced as so. One such method of doing this above that of general tolerancing is geometric tolerancing.

Take a look through BS308 if you require further information.

What it sound like you are trying to do is create your own method of identifying important dimensions which manufacturers are failing to meet.

Attack the root cause of the problem - are your designs incorrect (if so correct them) or are your manufacturers failing to manufacture corretly (if so either find another manufacturer or get them to sort the problem out).

GB (UK)

 

[arto]

Ideally, making it to drawing would keep this from ever coming up. It's more of a method to let purchasing know which dimensional errors are "non-negotiable" & to keep the managerial heat off the engineers

 

[DangerMouse]

Due to customer demands and requiring to adhere to the latest practices.
We too have this system working in our factory. The way we indicate critical dimensions is to surround the dimension with a rectangle, but instead of a flat line at each end we have semicircles, sort of pill shaped. In the note section we indicate that all dim. marked with ???? is a critical dimension etc etc etc..
Hope this helps you.