Common Zone modifier

21 / 03 / 2011

First post since November! It has been a busy few months, but I must try to do better than that. I’ll put up some more posts about what has been going on shortly.

Mas’ood occasionally sends me interesting (or awkward, depending on your point of view) questions about geometrical tolerancing.

I am going to put some of his questions here – feel free to comment if you have a view.

First question is about the use of the ‘common zone’ modifier, CZ.

This is placed in a geometrical tolerance frame to indicate that two or more features have to satisfy the requirements of a single tolerance zone.

fig428

Mas’ood’s question was whether the CZ modifier can be used with the envelope requirement.

Personally, I don’t see why not.

ISO devised the CZ modifier with a view to it being used in the tolerance frame only, but as long as the meaning is clear, I see no problem with applying it to the envelope requirement as well. It would be a way of indicating that two features-of-size, of the same size and in alignment with each other, should be treated as a single feature.

fig427

I would feel less comfortable with this approach if it was being applied to two features which were produced with separate machining or manufacturing operations, but we are well into the realm of ‘designer’s descretion’, as the standards provide no guidance in this area.

fig426

ISO have developed several different symbols in recent years, such as Common Zone (CZ), Separate Zone (SZ), United Feature (UF), Contacting Feature (CF), etc, and are currently reviewing these to see if they can minimise this ‘symbol inflation’. The ‘UF’ symbol was to be used to indicate that several features were to be treated as if they were a single feature, but ISO TC213 will be reviewing whether the CZ symbol could be used as an alternative, or even remove the requirement for the UF symbol altogether.

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