This question is not at all trivial

[edwidder]

This question is not at all trivial, especially in the international environment. The two standards documents are ANSI Y14.36 and ISO 1302. In Y14.36, specification of a single value is implied as a maximum value. (NOT a target value!). However, in ISO 1302 (& ISO 4287), the simple specification applies the "16% Rule" as a default. Per this standard, if one wants the value to be an absolute maximum specification, then "MAX" must be added to the parameter designation ("Ra Max"). [Thankfully, Rmax is not recognized within ISO, but is within ASME B46.1.] This 16% Rule is a quirky blending of an acceptance criteria within the parameter definition document. There is an opinion that quality acceptance criteria are properly contained within the measurement plan for the part and not on the part drawing - or indirectly through the parameter definition. There is some uncertainty in surface roughness values - but there is uncertainty present in ALL measurement activities. Why single out surface roughness? One of the problems with the 16% Rule is that it would allow 16% of the measured values to be outside of the specified value, but doesn’t specify how far out any single measurement value can be. On some critical surfaces, outlier areas may be functionally inappropriate.

 

[MikeHalloran]

Well, okay, there is a question buried in there, but perhaps it should be asked of ISO, not us.



Mike Halloran
Pembroke Pines, FL, USA

 

[powerhound]

There is probably no real reason for the difference. I like the ANSI method better because the only deviation from the specified value goes in the direction of a better roughness value. I understand that sometimes you don't want the roughness to be too fine but there's a way of dealing with that too. Per ISO, you can wind up with a worse value than specified and that's something the designer has to take into consideration. I like working with absolutes (ASME/ANSI) rather than averages (ISO).

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[powerhound]

Okay, so I read and replied to this thread BEFORE I read the one KENAT posted. Now I see the context and can be a little more specific in my response.

I don't think the use of the symbol should make any difference here. You can state "ALL UNTOLERANCED DIMENSIONS ARE BASIC" on a print and you don't have to use the box around the dimension. The little checkmark means surface roughness so if you actually spell out "SUrface Roughness" I don't think it makes a bit of difference. That being said, I think a stated value, with or without a checkmark thingy, is a maximum value.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II