Do you use B4.2 a lot?

[bxbzq]

Hi guys,

If you work to ASME, for limits and fits applications, do you use B4.2 a lot?

The automotive company I used to work never uses B4.2. I even did not know the ANSI B4.2, not to mention ISO 268, until I moved to this company. Now our drawings are full of H6, f9, type of callouts whenever there is hole and shaft, or groove and tab fit. Our drawings reference ISO 286.
One thing I don't like with this type of tolerancing method is in the models, there is clearance in the mating pairs. All are perfectly fit.

 

[CheckerHater]

One thing I don't like with this type of tolerancing method is in the models, there is clearance in the mating pairs. All are perfectly fit.

B4.2 is exactly the reason. If you decide to change your fits, say, from H6 to G6 you don't have to re-model every hole in your model.
But you should only see things like that with tight fit situations though, like dovels.
Are you saying that your company is placing 10 mm bolts into 10 mm holes?

 

[Belanger]

For companies based in North America, it seems that the limits and fits system is almost never used. That's not to say I like or dislike the B4.2 method; I'm just answering the question


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

http://www.gdtseminars.com

 

[bxbzq]

oops, I meant "...there is NO clearance in the mating pairs."
Also, a lot non-symmetric tolerances in ISO drawings.

To CheckerHater, for bolt applications, we don't tolerance this way.

 

[KENAT]

We don't really use the ISO or B4.2 fits here. Occasionally but not systematically. Then we don't do that many 'fits' like that as such.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[pmarc]

One thing I don't like with this type of tolerancing method is in the models, there is NO clearance in the mating pairs. All are perfectly fit.

It is not ISO 286's or ASME B4.2's fault that 3D models in your company are prepared with no clearance. At least nowhere in the standards it is said that they shall be done in the nominals. This is probably the company's decision. I know companies that use limits and fits system and model their parts always in the middle of tolerance zones. Probably there are firms that follow other fancy rules - at least I wouldn't be surprised about that.

 

[CheckerHater]

At least nowhere in the standards it is said that they shall be done in the nominals.

In "standards" it says better then that.
Actually it is one of the areas where ISO and ASME agree.

 

[pmarc]

But the cited paragraphs don't say that the models should always represent nominals.
So if I, in my own company, decided to model all features always at their MMCs, I do not think it would be in conflict with what you attached.

Am I missing something?

 

[CheckerHater]

Absolutely not.

You just have to document it somewhere that your model is at MMC.

Giving that standards demand from model “ideal geometric form” as well as “perfect dimensionality and shape”, in MY own company I would model everything to BASIC, just to be safe.

That also would be more in-line with the way reasonably priced CAD software works. Say, in CATIA you can plug tolerance into your sketch, and then push model to MMC, LMC, or mean, but CATIA seat will cost you 10 times more than SolidWorks. In SW you model to nominal and then produce tolerances during dimensioning.

I was just trying to make point, that modeling to nominal is nothing to look down at.
Also, as I already said, it is easier to change fit in the dimension, than to re-model every hole.