Tek-Tips is the largest IT community on the Internet today!

Members share and learn making Tek-Tips Forums the best source of peer-reviewed technical information on the Internet!

  • Congratulations cowski on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Y14.5-2009: Surf. Profile Control; combined with a position control

Ladico91

Mechanical
Joined
May 22, 2025
Messages
1
Hello,

Long-time lurker, first-time poster.

I have a couple of questions pertaining to paragraph 8.8 "Combined Controls" (pp 175-176) in the Tolerances of Profile section in the ASME Y14.5-2009 standard. It mostly has to do with the first example to illustrate the combination of a surface profile callout with a position callout "to control the boundary of a noncylindrical feature". Fig. 8-24 (p176) describes the situation and interpretation:

1747948478046.png

While I understand the calculations for the tolerance zone and the intent; I have still some doubts about the "validity" of such pair of controls.

1-First of all, why is the outer boundary of the internal irregular feature of size dubbed LMC; while its inner boundary is called MMB? I have always used MMB, LMB and RMB when talking about datums but I understand why that can be used on any feature.. Are LMC and MMC terms interchangeable with LMB and MMB? If that is the case, I do not understand what the distinction is in section 1.3 p2.
1747949671603.png

2- If the end goal of the positional control on the surface is to ensure no element of the surface feature lays within an inner boundary contour .6 +.25 = .85mm off the true profile towards the inside, when the cutout is at its MMC ("smallest"); cannot the same goal be attained by using a set of two separate segments FCFs: first with a surface profile callout of 1.2mm; and second with a surface profile control of .85 U .85 back to ABC?
Is this even "grammatically" correct from a GD&T POV; to have a first surface profile segment with no datum called out with a greater tolerance value than the second surface profile segment with datums called out?

3- Is the combination of the profile control and the position control just a subterfuge to achieve the same results as calling out the position of a regular feature of size at MMC, since calling out a surface profile at MMC (or LMC) is not legal? In this case, if the cutout was of a cylindrical shape for instance, that would directly translates to a positional control of .25mm with a (mating) diameter tolerance of +/- .6mm?

4- If 3- is true, then the illustration on the bottom RH corner of figure 8.24 should depict the virtual condition boundary of the irregular feature of size. What about the resultant condition boundary? Is it defined? Would it be correct to assume that no element of the surface feature shall lay outside the outer boundary contour .85mm off the true profile towards the outside? It does not seem to be covered anywhere in the 2009 revision.

5- Then, if 4- is true, wouldn't the same control be achieved by a pair of single segment FCFs (or composite, would not matter) with the upper segment being a surface profile control of 1.7mm back to ABC, and a lower segment of a surface profile w/o datum of 1.2mm? In my opinion, it would make interpreting this specific requirement quite easier.

6-In 2018, I am wondering if there is any way to leverage the dynamic profile modifier?

Thanks a lot!
 
Piece is unsolicited advice:
If you want answers on all of your questions, please break your thread in multiple discussions/ multiple threads.
I guess, I might be wrong however, nobody is going to answer all of your posted question .......unless broken down based on each subject.
 
Regarding question no. 1, I'd say MMB is a mistake and it should have been MMC. It is shown on the part of the figure that describes the profile control, which only limits the size and form of the opening. MMC is a size related term, while MMB takes into account also the location (and/or orientation). But that profile tolerance references no datums and doesn't control location (and/or orientation). The outline of the Positional Boundary on the other part of the figure could be described as the MMB or Virtual Condition of the opening (these two, MMB and VC are often essentially the same thing but the choice of term is context-dependent).
 
Regarding question no. 2 - no that would not attain the same goal. The second segment of profile within .85 U .85 would simply override the possible variation that the first segment would provide by the 1.2 mm wide tolerance zone. It would mean the total usable space for size, form, orientation and location variations are just the .85 zone provided by .85 U .85 relative to A,B,C. The current specification allows 1.2 mm for form and size, and it allows dislocation and disorientation as much as possible without violating the positional boundary that is set at true position and sized according to the MMC of the feature and the 0.25 mm offset from it all-around.
 
Question 3 - had the feature been cylindrical, then the most similar scheme would be size (diameter) limits within +/-1.2 (not +/-0.6) and position within 0.5 at MMC (and not 0.25). Think of two opposed faces of the cutout each allowed to dislocate 0.6 mm from true profile either in opposite directions or towards each other. That could enlarge or reduce their distance by +/-1.2 mm, not just by +/-0.6 mm. Also the amount of positional tolerance at MMC is always twice the radial gap between an MMC diameter at true position and the Virtual Condition boundary, which is very similar to what the figure shows.
 
Question 4 - yes that position boundary in the right hand corner is the Virtual Condition boundary. The Resultant Condition envelope has the dimensions of the LMC of the cutout enlarged by 1.2 (profile tol. as a "bonus") +0.5 (stated position tol.) = 1.7 mm. For example, the vertical dimension of the RC envelope is 37+1.2+1.7 = 39.9. The horizontal dimension is 30+1.2+1.7 = 32.9.

Here is a correction:
Let's analyze the width of the cutout, and its Resultant Condition. The nominal width is 30 mm, the MMC is 30 - 1.2 = 28.8 and the LMC is 30 + 1.2 = 31.2. The RC width is the LMC width + maximum allowable dislocation at LMC, resulting from the specified position tolerance (0.5) and the difference between LMC and MMC. So, the RC of the width = 31.2 + 0.5 + (31.2 - 28.8) = 31.2 + 0.5 + 2.4 = 34.1.

If you want it visualized for understanding, see the below sketch where:

- A is the width of the VC boundary, basically located and oriented in the A,B,C DRF, and could be implemented by a physical gage.

- B is the width of an LMC cutout, shown dislocated as much as possible to the right.

- C is the resulting maximum gap from the VC boundary to the right side of the cutout.

- C' is the the maximum gap from the VC to the left side of the cutout, applicable if the cutout was dislocated all the way to the left as possible.

The total width of the Resultant condition is A + B + C + C' = 34.1

1000023094.jpg

It goes without saying that the same analysis, with different dimension values, applies to the vertical dimension of the cutout (just substitute 30 by 35).
 
Last edited:
I am not sure which of the questions will be addressed by my response below.

Essentially, the technique shown in fig. 8-24 is similar to controlling position at MMC of a regular feature of size, such as cylinder, sphere, pair of parallel planes, controlled with a directly toleranced dimension for size. (One exception being that profile tolerance requires perfect form of the feature when at MMC and LMC, but the directly toleranced dimension requires perfect form at MMC only).

Typically, when people attempt to translate the set of requirements in fig. 8-24 into two surface profile callouts, the end results is as follows:
|PROF|1.2|
|PROF|2.9(U)0.85|A|B|C|

There is, however, one aspect that is usually unnoticed when performing this translation. The larger of the two profiles in the two-profiles solution requires the considered feature to fit inside two boundaries - LMB and MMB - whereas in the original scheme, the position at MMC callout requires the considered feature not to violate the MMB (VC) boundary only. The other boundary (LMB) is merely resultant (i.e., need not to be verified), and its size is not a result of simple RC calculation that would be performed if the considered feature had a regular shape.

Why is that? It's because the largest feature (within the 1.2 profile tolerance) may still rotate around the MMB/VC boundary without violating it, and for that condition the extreme points of the feature may fall beyond the LMB boundary calculated from the simple formula: LMB = LMC + 0.5 (pos. tol.) + 2.4 (total size tol.).
 
So where 0.85 tolerance zone is coming from?
Got to be a mathematical calculations, right? However, I am not able to figure it out.....
Maybe from 1.2/2 + 0.5/2 = 0.6+0.25 = 0.85?
So if the datumless profile is translated into size±1.2 then the VC is: size -1.2 - 0.5, right?
Then why we have too halve it?
Or we have to halve it only when we talk about the radii basic dimensions?
 
So where 0.85 tolerance zone is coming from?
Got to be a mathematical calculations, right? However, I am not able to figure it out.....
Maybe from 1.2/2 + 0.5/2 = 0.6+0.25 = 0.85?
So if the datumless profile is translated into size±1.2 then the VC is: size -1.2 - 0.5, right?
Then why we have too halve it?
Or we have to halve it only when we talk about the radii basic dimensions?
Because the 0.85 value in the profile callout is an offset on one side of the feature only.

Had it been changed to 1.7, the MMB size would be equal to (basic size - 2*1.7).

If you substitute, for example, 30 for the "basic size" in this formula, you will get 30-2*1.7=26.6, whereas if you substitute 30 for the "size" in your commonly known formula for VC, you will get 30-1.2-0.5=28.3. Assuming you accept the fact that the VC formula is correct, then, to get the same results, the left side of the MMB formula must be changed to 30-2*0.85.
 

Part and Inventory Search

Sponsor

Back
Top