3D Math Data, GD&T and FAI - My issue lies more with Profiles

[SArun Kumar]

Criteria for MMC

Dear All,

Could anyone suggest me when should use MMC and RFS.
I heard this depends of fixed or moving fasteners.
Could anyone explain this concept?

Thanks

Arun

 

[Stijloor]

Re: Criteria for MMC

Dear All,

Could anyone suggest me when should use MMC and RFS.
I heard this depends of fixed or moving fasteners.
Could anyone explain this concept?

Thanks

Arun

Do you mean (broken link removed) for engineering purposes (prescribe) or inspection (verify)? Can you clarify?

(broken link removed) is a concept used to calculate positional tolerances.

Stijloor.

 

[SArun Kumar]

Dear Stijloor,

Actually i want to some positions are given in MMC and some in RFS.
What is the design or fictional criteria for selecting MMC or RFS?

Thanks
Arun

 

[Crash Not]

Hi,

What is my issue is that if u go through to attachment file page 2 of 2 there are some Profile GD&T for form & trim edges 0.8 @ Datums A B C are shown in general notes. So it means two parallel profile surface with displacement of +/-0.4 from mean (3D model) is our tolerance zone. I got it checked in CMM with Point to Point comparison with 3D model.(after best fitting with 3D model) the observations of some of points are available with attachment file page 1 of 2. In these observations I have max displacement observation is - 0.6737. (Highlighted in attachment) means the actual out of tolerance is -0.6737-0.40 = - 0.2737.
But when I am going making report against this
(Profile 0.8 against Datums A /B /C)= 0.6737 it seems ok.
How could i make observations report correctly?

After reviewing the attachment, I would assume that the report was created using Virtual DMIS software. If that is correct, the report shows POINT profiles. The customer is has toleranced SURFACE profile on the attached print. I would go back to the CMM and scan or check the surface (AS A SURFACE) and report as "surface profile" and not "point profile". I would use the CAD model as nominal. Another option might be to use "profile of a line" in Virtual DMIS. I hope this is helpful.

 

[Paul F. Jackson]

SArun, You asked,
"What is the design or fictional criteria for selecting MMC or RFS?"


Tolerances specified RFS (regardless of feature size) provide a constant tolerance amount for the feature's form, orientation, or location... whereas MMC (maximum material condition) or LMC (least material condition) permit the upper specification limit of geometric tolerance to vary with respect to size.

The selection of these modifiers should be based on the function of the features and/or the liabilities to function when they deviate. Answering a simple question can lead you to the appropriate modifier selection.

"Does function worsten as the feature deviates from its ideal form, orientation, or location or will function be uncompromisd by increasing the allowable form, orientation, or location with respect to size?"

Typically fastener clearance holes would not suffer functional degradation as they are permitted additional location deviation due to their size so long as they are constrained within their size and virtual condition boundaries. However, if there are other functional considerations related to the fastener clearance hole... i.e. (a pattern of holes securing an oil pan along a narrow rail surface where maintaining a minimum gap between the hole and the edge to insure gasket sealing)... those liabilities should be examined in permitting a larger hole to deviate more for location and therfore compromising that functional gap requirement.

RFS is typically selected for features that provide alignment or maintain critical clearance or fits among features. Say for instance that there are a pair of dowels and dowel clearance holes for that locate and align that pan and mating rail. The fit "spread between the dowels and dowel holes" is functionally a MMC problem with a relatively miniscule amount of variable tolerance but the location of those dowels and holes on the pan and rail structures is functionally a RFS problem because as the dowels are permitted to deviate from their ideal location relative to the structure they compromise the mating gasket surface locations and alignments.

LMC is largely shunned by most because they cannot imagine its functional use and many claim that it cannot efficiently be gaged. That is unfortunate because it does match certain functional scenarios and gaging it is just as easy with CMM's and other measurement devices as it is with MMC or RFS. For attribute gaging "overlays" can be employed with LMC as easy as hard gaging is with MMC. An example of its application can be demonstrated with an engine "head gasket" (the gasket between the block and head). There are threaded holes in the block and fastener clearance holes in the head that mate together in a MMC relationship...dowels and dowel holes between them that have both a MMC relationship for spread and a RFS relationship for location... but also there are "oil and coolant" passage holes that functionally demonstrate a LMC mating relationship. If oil passage holes in both the block and head are cast or drilled at their minimum sizes they may functionally have more room to deviate from their basic location than ones at their maximum size without violating the gasket boundaries. An attribute gage for these LMC modified features would look like the gasket itself providing a virtual condition "overlay" boundary for the passage locations at the surface. Naturally location and orientation of the holes deep within the structure to predict "thin wall" conditions could be accomplished as desired with variables measurement devices (CMM's etc.).

The point is that function should be the primary consideration in selecting tolerance modifiers. There are many that advocate the use of MMC on most geometric tolerances to enable "functional gaging" but... unless the modifiers have been selected according to function the gages are just "hard attribute gages" and should not be labeled "functional".

Paul F. Jackson

I wrote a paper quite a few years ago that may be helpful in selecting the appropriate modifier.

 

[SArun Kumar]

After reviewing the attachment, I would assume that the report was created using Virtual DMIS software. If that is correct, the report shows POINT profiles. The customer is has toleranced SURFACE profile on the attached print. I would go back to the CMM and scan or check the surface (AS A SURFACE) and report as "surface profile" and not "point profile". I would use the CAD model as nominal. Another option might be to use "profile of a line" in Virtual DMIS. I hope this is helpful.

Dear Crash,

What I am getting from this massage is that through surface scanning i can assure each element of surface and by comparing through points profiles I will skip some of points on the surface. This is draw back with this method. As we are doing CMM (Virtual DMIS software) inspections outsource. Scanning will cost us double of profile Point comparison.

We will try to use another option "profile of a line" in Virtual DMIS.

Arun

 

[SArun Kumar]

SArun, You asked,
"What is the design or fictional criteria for selecting MMC or RFS?"


Tolerances specified RFS (regardless of feature size) provide a constant tolerance amount for the feature's form, orientation, or location... whereas MMC (maximum material condition) or LMC (least material condition) permit the upper specification limit of geometric tolerance to vary with respect to size.

The selection of these modifiers should be based on the function of the features and/or the liabilities to function when they deviate. Answering a simple question can lead you to the appropriate modifier selection.

"Does function worsten as the feature deviates from its ideal form, orientation, or location or will function be uncompromisd by increasing the allowable form, orientation, or location with respect to size?"

Typically fastener clearance holes would not suffer functional degradation as they are permitted additional location deviation due to their size so long as they are constrained within their size and virtual condition boundaries. However, if there are other functional considerations related to the fastener clearance hole... i.e. (a pattern of holes securing an oil pan along a narrow rail surface where maintaining a minimum gap between the hole and the edge to insure gasket sealing)... those liabilities should be examined in permitting a larger hole to deviate more for location and therfore compromising that functional gap requirement.

RFS is typically selected for features that provide alignment or maintain critical clearance or fits among features. Say for instance that there are a pair of dowels and dowel clearance holes for that locate and align that pan and mating rail. The fit "spread between the dowels and dowel holes" is functionally a MMC problem with a relatively miniscule amount of variable tolerance but the location of those dowels and holes on the pan and rail structures is functionally a RFS problem because as the dowels are permitted to deviate from their ideal location relative to the structure they compromise the mating gasket surface locations and alignments.

LMC is largely shunned by most because they cannot imagine its functional use and many claim that it cannot efficiently be gaged. That is unfortunate because it does match certain functional scenarios and gaging it is just as easy with CMM's and other measurement devices as it is with MMC or RFS. For attribute gaging "overlays" can be employed with LMC as easy as hard gaging is with MMC. An example of its application can be demonstrated with an engine "head gasket" (the gasket between the block and head). There are threaded holes in the block and fastener clearance holes in the head that mate together in a MMC relationship...dowels and dowel holes between them that have both a MMC relationship for spread and a RFS relationship for location... but also there are "oil and coolant" passage holes that functionally demonstrate a LMC mating relationship. If oil passage holes in both the block and head are cast or drilled at their minimum sizes they may functionally have more room to deviate from their basic location than ones at their maximum size without violating the gasket boundaries. An attribute gage for these LMC modified features would look like the gasket itself providing a virtual condition "overlay" boundary for the passage locations at the surface. Naturally location and orientation of the holes deep within the structure to predict "thin wall" conditions could be accomplished as desired with variables measurement devices (CMM's etc.).

The point is that function should be the primary consideration in selecting tolerance modifiers. There are many that advocate the use of MMC on most geometric tolerances to enable "functional gaging" but... unless the modifiers have been selected according to function the gages are just "hard attribute gages" and should not be labeled "functional".

Paul F. Jackson

I wrote a paper quite a few years ago that may be helpful in selecting the appropriate modifier.

Dear Paul,

Thanks for defining the concept of selecting MMC, LMC, and RFS.

Is there any standard rule for selecting MMC, LMC, and RFS?


Regards
Arun

 

[Stijloor]

Dear Paul,

Thanks for defining the concept of selecting MMC, LMC, and RFS.

Is there any standard rule for selecting MMC, LMC, and RFS?


Regards
Arun

No, it's all driven by design intent.

Stijloor.

 

[David DeLong]

No, it's all driven by design intent.

Stijloor.

I agree with Stijloor on this one.

 

[QC Kid]

Other than specifiying the profile and tolerancing is always going to require extra steps.
One approach is ordinate dimensioning. Specific points of interest along the profile can be specified by coordinate values from some origin. These values can then be used in a more meaningful manner. The usefulness of the ordinate
data will be dependent upon your selection criteria of the points along the profile.

There is however another approach, one I have used, that may be of interest to you.

All profiles are the composites of individual geometrical features. Individual features such as a radius, line, tangent, angle, taper, etc...
You can treat the subcomponents of the composite profile individually.
ex:
|Profile| .010dia MMC|
.500 R Center(X= 0.0 , Y= 0.0), Size(.498)
1.00 R Center(X= 2.0 , Y= 3.0), Size(.997)

Each subcomponent will have characteristics that can be expressed more meaningfully.

Now we must only ask ourselves, "To what level of detail"...