Profile of Surface vs Profile of Line

[sqrt-grant]

I have a surface profile tolerance with 3 datums, controlling all 6 degrees of freedom. If we replace the surface profile characteristic directly with profile of a line, and leave everything else as-is, is the tolerance zone essentially just the same? If not what's the difference and if yes is it "illegal" as such or just lacking in common sense!

[Belanger]

No, they are not the same. If using profile of a line, there are many (infinite, really) tolerance zones which each follow the perfect profile in the view shown -- in other words, a series of two-dimensional slices only.

Each measurement of profile of a line must stay within the tolerance zone imposed at that slice, but there is no requirement that each of the slices going into the part will conform to the correct shape, depthwise.

The bottom line is that profile of a line is 2-D, and profile of a surface is 3-D.

[Kelly_Bramble]

John, the questions seems to be that they want to change the geometric tolerance from Profile of Surface to Profile of Line. Usually, we see Profile of Line utilized to control the variability of a surface section at a specific surface location. We normally specify a Profile of Surface tolerance when one wants to control the variability of an entire 3D profile surface.

If as they suggest that:

replace the surface profile characteristic directly with profile of a line, and leave everything else as-is

I believe the resulting tolerance boundaries (upper and lower) will ultimately control the total variability of the 3D surface identical.

I do agree the measurements are conducted differently however the as-built 3D surface will not be built larger or smaller.

I did at one point think that Profile of Line was technically not required in that one could simply write a note, specify Profile of Surface, indicate a surface section location where the tolerance was to be applied and the section as-built would have the same geometric tolerance control. However, it is possible to have applications where one may need to specify a geometric Profile tolerance that is different depending on the direct of measurement. The best similar example I can think of is illustrated with the Straightness example given in 5.4.1.4 of the ASME Y14.5-2009 standard.

[Belanger]

Kelly, I still say there could be a difference

I agree that profile of a line is useful when we just want to do the check at a specific station, but the OP seemed to be talking about merely changing the symbol with no other stipulations.
If they simply change the symbol, then the results might be different. See Fig. 8-27 in the standard: if profile of a surface is used then there would be a difference! Right now the depth aspect is not controlled by the geo tol, but by the height tolerance. That is like the straightness example you mentioned, but it's why I hesitate to say that the two profile symbols are identical even if no gaging location is given.

So I guess it boils down to whether the relationship to the datums is basic or toleranced in the OP's situation. Then we could give a definite answer.

[Kelly_Bramble]

In figure 8-27, if one replaces the Profile of Line specification with Profile of Surface - how would the concerned total surface variability be any different?

[Belanger]

It would be different in that the first slice of the part I measure (the front lip) might have the profile tolerance zone on the high side: 80.2 mm from the floor could be the top of the zone. But at another slice of the part (maybe halfway back, into the depth) the profile tolerance zone could be at the low side of the height: 79.8 being the very bottom of the zone.

So the variation of that top surface in the "z" direction could be bumpy to the tune of 0.4 mm, while in the "x" direction no bumps could exceed 0.07. However, if we change the symbol to profile of a surface, there can be no bumps anywhere that exceed 0.07.

[Kelly_Bramble]

It would be different in that the first slice of the part I measure (the front lip) might have the profile tolerance zone on the high side: 80.2 mm from the floor could be the top of the zone. But at another slice of the part (maybe halfway back, into the depth) the profile tolerance zone could be at the low side of the height: 79.8 being the very bottom of the zone.

So the variation of that top surface in the "z" direction could be bumpy to the tune of 0.4 mm, while in the "x" direction no bumps could exceed 0.07. However, if we change the symbol to profile of a surface, there can be no bumps anywhere that exceed 0.07.

OK, so I agree with your interpretation in this scenario where there are NOT basic dimensions constraining the feature to the Datum's. As you pointed out, "I still say there could be a difference" holds correct.

Clarification from the op would be required to sort out the most correct answer.

I would like to point out that I'm not sure I have seen a real-world application of the concept illustrated in Figure 8-27. Can you think of one?

[Belanger]

The only ones I can think of involved a two-segment callout where profile of a line was a refinement of profile of a surface. Not exactly like Fig. 8-27, but still a situation where each line profile's check could dance up and down within the larger surface profile.

Side note: If the OP's geometry is complex then there might not be any dimensions given on the print. In that case the relationship to the datums should resort to CAD, which would be basic anyhow.