I have some very basic GD&T questions....working to ISO/BS8888

[ska7ch]

Hi guys,

I consider myself to have a fairly good handle on GD&T (except for these little queries) and have been trying to implement it over the past few months, however got a couple of niggling questions hopefully someone can answer.

1.) Say you have a rectangular block with a size dim across 2 of the flats of 10mm +/- 0.1mm. Is there/what is the indirect parallel tolerance of that part? I understand the ASME standard has rule #1 which I understand but I don't believe there is such a rule for ISO - Except for the "envelope" requirement which is a different kettle of fish that I'm still getting my head around using also.

2.) From everything I've read and been advised, anything that is a FOS can have a size/tol dimension....Now what do I do with size dimensions for things like a blind hole where the depth dimension isn't a FOS, how should I treat those particular "features" (bottom of the hole surface, whether they be crucial of non crucial) seems a bit overkill to add a basic dimension and use position or surface tolerance?

Hopefully the questions are fairly self explanatory, when I'm back in work tomorrow I may upload some diagrams to help illustrate my questions if required.

Kind Regards

 

[Belanger]

My take on Q #1 ... There is no indirect parallelism tolerance. If the envelope rule is not invoked, then the ISO requirement is only that each pair of opposing elements needs to meet the 10 ± 0.1 independently. In other words, that block could be bent into a horseshoe shape and its size requirements would still be OK as long as the cross-sectional thickness is between 9.9 and 10.1.

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

 

[ska7ch]

Thanks for the reply Belanger, as I suspected regarding the indirect control...say the opposing face was opposite a Datum would there be any implied parallelism then, if so what?

Have you any thoughts on my 2nd question?

Thanks

 

[Belanger]

Even if one face were labeled as a datum feature, it doesn't mean that the size tolerance controls parallelism. This is because a datum only has meaning for those callouts that specifically invoke it.

As for Q #2, there might be some legitimate disagreement about this, but I am in the camp that plus/minus tolerancing is really only for size dimensions (as well as chamfers and radii). But not for location. And as you say, the bottom of a blind hole is not a feature of size, so I would use profile of a surface (or position in BS/ISO), along with the basic dimension ("TED" in ISO). I don't think it's overkill; it's clearly stating what is needed, since a caliper-type size check can't apply there.

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

 

[CheckerHater]

J-P,
In your opinion, what is the correct way to specify depth of the hole, one to the left or one to the right?

 

[pmarc]

CH,
On the picture to the left the diameter of the hole can, but does not have to be basic.

 

[CheckerHater]

But... but then we will have Profile used with directly toleranced dimension!

 

[pmarc]

And?

I am sure we both can immediately find at least two figures in the standard where such a practice is shown.

 

[Belanger]

If the profile tolerance in the picture on the left was intended for all surfaces of the hole, that should have been indicated. But then I'd say it's overkill; the OP's question had nothing to do with the diameter of the hole because that's a FOS.
On the other hand, if the profile tolerance was indeed intended for only the pointed bottom of the hole, then the diameter wouldn't be basic.

That aside, I clearly said there can be legitimate disagreement about this. The depth symbol exists in the standard, so there's a reason for it. But I would ask how we measure the depth as expressed on the right picture -- is it take from a plane hanging over the top of the hole?

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

 

[CheckerHater]

I just want to draw your attention guys, that profile will do very bad job locating bottom of the hole unless diameter is made basic.