what is the difference between continuous feature and pattern feature

[bxbzq]

Hi,
Use fig.2-8 as an example. If I specify 2X ø22.1, 22.2 to the shaft diameter, would it be different from ø22.1, 22.2 <CF>?
Thanks in advance.

 

[Belanger]

Yes, because without the CF, each diameter could be the correct size, yet they might not be in line with each other (coaxial).
One of those diameters could be up a little, and then after the groove the next one could be down a little. Using CF treats them as one feature, thus Rule #1 will keep them coaxial within the size limits.

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

 

[bxbzq]

OK Belanger, your explanation is about the size tolerance.
If I specify same positional tolerance to the two different size callouts, ie,
2X ø22.1, 22.2
|pos|ø0.5|A|B|C|
vs.
ø22.1, 22.2 <CF>
|pos|ø0.5|A|B|C|

Would the positional tolerance zones or inspection process in the two be different?

 

[Belanger]

Yes, those would still be different. This is because the CF modifier translates to Rule #1: Size dimensions also control form (to the same tolerance).
So with your position tolerance of 0.5, but no CF, one of the diameters could be 0.25 up and the other diameter could be 0.25 down (yet both axes could still be horizontal). In other words, this step would be allowed; it would not be seen as a form error violating Rule #1.

But the position tolerance of 0.5, and WITH the CF, they couldn't have that step of 0.5 -- they would be controlled as in-line with each other to the amount of the size tolerance.

Without using CF, we could take your suggestion and transform it into a composite feature control frame:
2X ø22.1, 22.2
|pos|ø0.5|A|B|C|
| |ø0.1|

I think this would be identical to having CF with the single position tolerance.

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

 

[CheckerHater]

What will happen if on fig. 2-8 we specify

2X DIA 22.2/22.1
POS|DIA.00(M)

?

 

[Belanger]

Then it becomes ... wait for it ... a continuous feature!

I mentioned the composite FCF only because I was trying to show how the position tol suggested by bxbzq wouldn't achieve the intended result.

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

 

[ModulusCT]

It does make a difference because the CF FoS is treated as ONE FEATURE, whereas the 2x diameters are treated as two separate features. At least, that's my understanding.

I'm not a vegetarian because I dislike meat... I'm a vegetarian because I HATE plants!!

 

[bxbzq]

OK, thinking it loud,
If the positional tolerance is applied on rfs basis, as long as the positional tolerance is smaller than the size tolerance, the two different callouts (2X vs. <CF>) would mean the same.
If the positional tolerance is applied on mmc basis, the positional tolerance has to be zero to make the two callouts mean the same.

 

[Belanger]

In looking back, I realize that my composite FCF suggestion wasn't totally correct (the numbers), but I think we all agree that the position tolerance of 0.5 wasn't getting the intended result anyway. Of course, CH's suggestion was more direct to what CF is doing.

As for the latest query --

If the positional tolerance is applied on rfs basis, as long as the positional tolerance is smaller than the size tolerance, the two different callouts (2X vs. <CF>) would mean the same.

No, because without the CF, each of the diameters (separated by the groove) could be within position tolerance and within their own size tolerance, but the virtual condition could be larger than the maximum size.

If the positional tolerance is applied on mmc basis, the positional tolerance has to be zero to make the two callouts mean the same.

Yes, I think this would be correct.

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

 

[ModulusCT]

Any mfring eng. would have plenty to say about the differences regarding setup and inspection of a CF feature vs a 2X feature like the one bxbzq speaks as well I think.

I'm not a vegetarian because I dislike meat... I'm a vegetarian because I HATE plants!!

 

[CheckerHater]

Modulus,

Could you please take a look at Fig. 2-8 ASME Y14.5-2009 (The one that actually started this conversation) and tell if it looks like 1 feature to you?

(Same applies to Fig. 2-9 and 2-10 as well)

The whole point of this thread is to discuss different ways to make 2 or more features behave like one. (Or at least that's how I see it)

 

[bxbzq]

No, because without the CF, each of the diameters (separated by the groove) could be within position tolerance and within their own size tolerance, but the virtual condition could be larger than the maximum size.

Did you mean outer boundary? If this is the case I think you are right.
Think it through, CF is a more restrict requirement. When applied at rfs, if the two diameters are produced at mmc, the CF callout would require the two diameters to be perfectly aligned to each other, while the 2X callout still allows one diameter offset 0.1 from the other one.

 

[Belanger]

Yes, that's the outer boundary. (VC is technically the term only if the outer boundary is a constant, such as with the MMC modifier).

And yes, CF is more restrictive, but that's what it's intended for. (It's pretty much Rule #1 extended to multiple features.)
We should only use CF when we want the alignment to operate within the size; if it's OK for the two diameters to stray apart beyond an MMC envelope, then position of some non-zero value should be used.

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

 

[bxbzq]

Outer boundary is not the correct terminology here either. I think UAME may be better one.

 

[ModulusCT]

No, I think Outer Boundary is right... An OB is the max size dim for an external feature + any geometric tolerance. The unrelated actual mating envelope of an external feature is simply the smallest envelope that can be contracted around a feature, at a given size.

I'm not a vegetarian because I dislike meat... I'm a vegetarian because I HATE plants!!