ISO Standards Not Recodnizing Rule #1 5

[SpaciouS]

As I understand it, the ISO 1101 Geometric Tolerancing standards do not recodnize Rule #1. How doe this approach work? I'ts hard for me understand since Rule #1 is such an important concept when using the ASME standard.

There is no such thing as perfect form at MMC?

All envelopes are to be looked at in terms of worst-case boundaries?

Thanks,
Sean

 

[DaSalo]

Sorry, misspoke in that last post. The reporting is min/max inscribed circle and in min inscribed cylinder. No max circumscribed on a hole. I also report form.

CATIA V5 R20
PC-DMIS 2011 MR1

 

[dingy2]

DaSalo:

To describe form size, let's take a 3.000 +/- .010 tube with and and ID. We have 3 features of size, OD, ID and length. You may take many readings on the OD and report that it is 2.996 - 3.002 but this is size.

Form size is difficult to check but imagine that the tube has a .013 bend in it and it has an average size of 2.999 (using the values that I have given). We are going to use tool room equipment here so we place the tube on the granite table, zero off the table with your indicator on a height gauge (which is the bottom of the tube) and rotate the tube. Place the indicator in the centre top of the tube and measure the distance from the bottom of the tube. As you rotate the tube, the average (or about there) + the bend to give us a form size of 3.012 (2.999 + .013). That is form size and it is out of specification since it is above the upper tolerance of 3.010.

Is this done on the shop floor on each feature of siz?. Not that I have seen yet Designers put a lot of faith in Rule #1 thinking everyone understands it. They don't.



Dave D.

http://www.qmsi.ca

 

[axym]

Hi All,

It sounds like what Dave means by "form size" is equivalent to what Y14.5 calls the "actual mating size". In the tube example, this would be the size of the minimum circumscribed cylinder. The surface plate and indicator setup would give a reasonable estimate of the actual mating size, as long as the magnitude of the bending variation was large compared to the circularity and local size variation.

I agree that the actual mating size is often left unchecked.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[dingy2]

Hi Evan:

Could you give me an example how you would check form size in my example noted. Use tool room equipment or a CMM with computer control. I would appreciate seeing the difference in our methods.
Thanks

Dave D.

http://www.qmsi.ca

 

[DaSalo]

Do we report this information for every FOS? No, absolutely not. Do we report it when the feature is critical or when the manufacturing method allows an elevated risk for this type of error? Yes.

CATIA V5 R20
PC-DMIS 2011 MR1

 

[powerhound]

Dave,

I'd like to interject here. I believe checking form per rule #1 should be a matter of GO/NO GO. I don't believe the amount of form error should be reported. In your scenario, it seems like a number of actual local size measurements should be made along the length of the tube to ensure they are between the MMC and LMC size, then--if the tube isn't very long--it should be able to pass through a hole 3.010" in diameter (gage quality) that is at least the length of the tube. Shouldn't that suffice if the tube is of reasonable length to justify it (say 6")? If rule #1 is to control the straightness then who cares how much the error really is? Now, if straightness is actually specified then the amount of error probably should be reported but personally I would still lean towards a GO/NO GO if it is specified at MMC.

I do have a question about your post: you mentioned an average size of the tube. Does this value matter? Is there ever a scenario where the functional dimension of any feature of size is its average dimension? I'm genuinely curious. I've worked with many inspectors who insist on reporting average sizes and I have never gotten an answer past "That's just what they want." as to why. An average hole size is still going to be larger than the largest pin that will actually go into it.

To answer your question from a few posts back. I have never seen that kind of reporting done.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[DaSalo]

"Could you give me an example how you would check form size in my example noted. Use tool room equipment or a CMM with computer control."

I realize you only directed this to Evan but I'm going to kick in my method here too:

I would scan the tube using a spiral type path with a pitch determined by the length of the tube. Then I would report the min and max values for the max circumscribed circular diameters, max circumscribed cylindrical diameter, and cylindrical form error. Max circumscribed cylindrical diameter would be you actual mating size and is what you are driving at here, I believe. This doesn't require any additional programming work all when done on a CMM. It does require additional machine time to get full surface coverage on the scan. That must be weighed against the function of the feature and the risk level of the manufacturing method to determine if it is justified.

CATIA V5 R20
PC-DMIS 2011 MR1

 

[dingy2]

Hi John (Powerhound)

You are correct that a Go/No-go gauge is better but could you imagine, we would need 3 gauges for a tube. Form derived from Rule #1 does not happen yet many Designers here believe so much in Rule #1 that when they really need it confirmed on a feature they feel Rule #1 would cover it. Placing a FCF on the tube OD with a 0 straightness tolerance at MMC forces one to confirm Rule #1. That, absolutely, will be confirmed.

Frankly, I think the ISO method is better. We do not understand Rule #1 on the shop floor in North America.

DaSalo:

It certainly appears that you have confirmed it but would you also perform this on the length of the tube since it is also a feature of size. The ID is really tough to perform using a CMM if there is any length (can't get in the centre) and one would most definitely have to use a Go gauge of its minimum size. Does it happen? No.

Dave D.

http://www.qmsi.ca