Concept of Zero Line,Fundamental deviation & GD&T Notes 1

[xcllyf]

Hi guys,
I went through the concept of limits and fits. I was not able to understand two terms in the nomenclature which is "Zero line" and "Fundamental Deviation". What are these, after much probing i am still unable to get a fair idea of these. And in the deviation chart I saw that the "H" fit has 0 fundamental deviation, so what does this mean? If anyone is having a good explanatory notes of Fits and tolerances + GD&T basics please give me.

 

[pmarc]

Zero line is just a graphical presentation of common nominal size of both features being mated together.

As for fundamental deviation, per ISO 286-1:2010 it is:
Limit deviation that defines the placement of the tolerance interval in relation to the nominal size.
NOTE 1 The fundamental deviation is that limit deviation, which defines that limit of size which is the nearest to the nominal size.
NOTE 2 The fundamental deviation is identified by a letter (e.g. B, d).

This means that if you consider for example a hole with a size specified as [Ø]40G6:
- zero line is 40;
- hole's lower limit of size is [Ø]40.009;
- hole's upper limit of size is [Ø]40.025;
And since 40.009 is closer to 40 than 40.025, 0.009 is the fundamental deviation in this case.

If you take a hole specified as [Ø]40H6:
- zero line is 40;
- hole's lower limit of size is [Ø]40.000;
- hole's upper limit of size is [Ø]40.016;
Since lower limit of size is equal to the nominal size, the fundamental deviation is 0 (zero) in this case.

 

[xcllyf]

pmarc,
Hi got ur explanation. Making sense to me now. Just one more doubt - in all cases except "h" fit there will be fundamental deviation right?
By the way is there any notes or pdf you are having for covering this subject of fits, tolerances and GD&T?

 

[pmarc]

Just one more doubt - in all cases except "h" fit there will be fundamental deviation right?

Not exactly. See attached picture taken from ISO 286-1:2010.

http://files.engineering.com/getfil...9ea-4850-907a-a902d5926458&file=iso_286-1.png

As you may notice, there are some situations other than "h" or "H" where fundamental deviation equals 0.

By the way is there any notes or pdf you are having for covering this subject of fits, tolerances and GD&T?

I use officially released standards, but for sure you can find materials on the web.