CT 2

[Andera]

I am interested to find out if there's a commonality between CT in ISO and CF in ASME 2009/2018.
Moreover what would be the differences?
Reading ISO, looks like CT cannot be used for features of non-size, but still not clear to me if that's a rule or just some unintended consequences.

 

[Belanger]

I don't have the pertinent ISO standard in front of me, but take a look at the attached table, which is from the appendix of ASME Y14.5-2018. Apparently they don't see ISO has having an equivalent to CF.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[pmarc]

CT modifier is defined in ISO 14405-1:2016.

It is intended for features of size only - the full name of it is "common toleranced feature of size". If the intent is to treat multiple non-features of size, e.g. multiple coplanar surfaces, as a single feature, UF (united feature) is a better choice, in my opinion.

As for differences between CF and CT, ISO requires use of 'nx' prefix when CT is used, whereas Y14.5 don't use 'nX' in conjuction with CF.

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In my opinion, tables C-6 and C-7 in the appendix of Y14.5-2018, that J-P referred to, should be read with some caution for at least two reasons:

1. They show which of the symbols adopted by Y14.5 are contained in ISO GPS standards, and for some people this may create an impression that the two dimensioning and tolerancing systems are pretty similar, which is very untrue. If the table logic was reversed to show which symbols adopted/developed by ISO GPS are contained in Y14.5, it would become apparent right away.

2. They simply contain mistakes. Lack of ISO equivalent to CF is one example. The other is use of incorrect symbol in the ISO column for dynamic profile tolerance modifier. Instead of the triangle, it should show 'OZ' (offset tolerance zone).

 

[Andera]

Pmarc,

Do we need GN or GX to make CT and CF equivalent or simply
nx size+tolerance CT = size+tolerance CF ?

I know datumless pos0@MMC = CF, but how GN/GX will play or not a role in their equivalency I am not really sure. If I don't put them (GN/GX) will inadvertently change the meaning of the callout?

On the same token/differencs, I think CF could and should be used for features of non-size as the ASME does not have its UF equivalent symbol? Is it true?

Thanks
Andras

 

[pmarc]

Andras,

nx size+tolerance CT is not equal to size+tolerance CF (assuming Rule #1 has not been overriden). To make them equal, (E) modifier needs to be added to the ISO specification.

Alternatively, instead of (E), a combination of GN and LP (for external FOS) or GX and LP (for internal FOS) can be specifed.