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SIM REQT with multiple composite tolerance FCFs in one view with multiple feature patterns 5

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Tunalover

Tunalover

Mechanical
Mar 28, 2002
1,179
Suppose we had a flat, rectangular plate of, say, aluminum. And say we had three datums A, B, and C as primary, secondary and tertiary datums assigned to the largest flat surface (A), the longest edge (B) and the shortest edge (C).

Lets call out a 12-member pattern of simple holes at diameter d1. Then lets call out, say, a 6-member pattern of simple holes at diameter d2. Now let's add a 8-member pattern of identical slots.

If each pattern possessed its own composite positional tolerance FCF with the same datums in the same order of precedence, will ALL of those features default to a single pattern even if they are geometrically different? Or will each pattern default to a SEP REQT? Or is it something else? I've always been confused on this.

ElectroMechanical Product Development
(Electronics Packaging)
UMD 1984
UCF 1993
 
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greenimi,

Here is how I understand it:

The 12 holes with diameter d1 have a 12X multiplier. The 12X multiplier "groups" the position zones for the 12 features into a "pattern" (see section 1.3.42 in Y14.5-2009). The property of grouping into a pattern is that the zones must be evaluated simultaneously (in the same candidate datum reference frame).

The INDIVIDUALLY annotation "ungroups" the pattern, so that the zones don't have to be evaluated simultaneously. There is no example of this in the position section, but there is one in the profile section (Fig. 8-23). The 2X multipler groups the profile tolerances for the two irregular cutouts, and then the INDIVIDUALLY annotation ungroups the 0.1 profile tolerance with no datum features.

Keep in mind that grouping, patterns, and simultaneous/separate requirements are subject to heavy use of "definition by example". Some of the above explanation represents my own interpretation, but I believe it is consistent with the text and figures in Y14.5-2009.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
 
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There is verbiage somewhere that says whenever features are controlled with FCFs containing the same datums, in the same order, and with the same material condition modifiers then all those features are understood to be simultaneous. That's how it was in Y14.5M-1994. Has Y14.5-2009 changed in that regard?

ElectroMechanical Product Development
(Electronics Packaging)
UMD 1984
UCF 1993
 
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Tunalover,

The verbiage that you mentioned is from the section on Simultaneous Requirements. I don't think that anything fundamentally changed in that regard from Y14.5M-1994 - the concept is still the same.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
 
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Thank you Evan for the detailed explanation about INDIVIDUALLY.
 
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Then by that verbiage all 26 features here are controlled simultaneously? Then what is the purpose of three separate FCFs if there are no separate requirements? Excuse me, but this post seems to show that this example, as close to real life as can be for sheet metal parts, is highly complicated. Or it shows that people just don't share the same interpretations. For many years I've been believing groups of patterns like this were functionally independent. That is, there is no mating part that needs to engage more than one of the three feature patterns at a time. Two of the GD&T trainers/experts I know (the guys who have to know the material in order to put food on the table) are too busy to look at it. I supposed I'll try Greg Hetland next (I've gone to Mark Foster and Norm Crawford).



ElectroMechanical Product Development
(Electronics Packaging)
UMD 1984
UCF 1993
 
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There is the chance that each group has it's own top level tolerance and each group has it's own size range for its features, so there is purpose to the separate FCFs for the separate groups/patterns.

The convention established by the standard that their nominal locations / true positions / true profiles are established in a single setup from the related datum feature is a choice the authors of the standard made to simplify dealing with that case. This is especially the case when different shapes need to be coordinated - such as a hole pattern needing to be coordinated with feature with an irregular profile. The only case I can imagine it makes a difference is if some feature or features would only be acceptable if the component was repositioned relative to it's location/orientation that otherwise accepted the features in other groups.

For example, if the mutual orientation of cross-drilled holes in a shaft didn't have to be controlled so the shaft could be re-oriented between checks.
 
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From a practical viewpoint, it is most likely that there are three different parts mating with the part in the OP. Let's say Part 1 has an array of 12 cylindrical rods (diameter sufficiently less than d1) that are intended to mate with that group of holes. Then there's a Part 2 with an array of six cylindrical rods (diameter sufficiently less than d2) that are intended to mate with that group of holes. Then there's a Part 3 with an eight-member array of oval protrusions (sufficiently smaller than the slots) intended to mate with eight-member pattern of slots.

Each hole pattern in OP part needs to be controlled separately so that Part 1, Part 2 and Part 3 can be functionally unrelated. Is that not the purpose of composite positional tolerancing? To describe functionally different hole patterns in a surface using the same DRF? I sure hope so because that's the way I've been seeing it (and the way it has always been described to me) since 1985!


ElectroMechanical Product Development
(Electronics Packaging)
UMD 1984
UCF 1993
 
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Well I contacted Greg Hetland and he wouldn't clear it up. Rather he said that my understanding was all wrong so I needed to take his course. Go figure!


ElectroMechanical Product Development
(Electronics Packaging)
UMD 1984
UCF 1993
 
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Tunalover,

When you're talking about "functionally unrelated" if those hole patterns have the same datum structure but do not need to be treated as a continuous pattern of 26 features (as a result of the default simultaneous requirements) and instead as separate patterns of 12,6, and 8 features then applying "SEP REQT" should do the trick. Note that as you probably well know simultaneous requirements of course only applies when all 3x patterns have EXACTLY the same DRF - both in precedence and material/boundary modifiers. This is just the default condition when conforming to Y14.5, and applies only to the PLTZF (upper FCF) on a composite tolerance.

Tunalover said:
Is that not the purpose of composite positional tolerancing? To describe functionally different hole patterns in a surface using the same DRF?
Click to expand...
I do not believe this is the purpose of composite tolerances. The purpose is to provide a method of refining a particular pattern's allowable positional tolerance by creating a tighter feature to feature relationship/tolerance (using the aptly named FRTZF or lower FCF) which can float within a looser tolerance zone (PLTZF or upper FCF). This could be used for a single pattern or 20 different patterns, and those 20 patterns could be "functionally different" - ie: 20x separate patterns with different DRF or the same DRF with SEP REQT or all 20x patterns could be held together in a single pattern with simultaneous requirements or anywhere inbetween, any of these conditions are possible with composite tolerance.

Hopefully that makes sense? I've tried to clarify it as much as possible but I realize I may have muddied the waters...
 
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Thanks chez311.

The most common situation I've seen is when we've had a single surface with any number of functional groups of mounting holes:
say one hole pattern for a terminal block, one hole pattern for a relay, and two different hole patterns for two dissimilar fuse blocks.

One DRF is used for all groups and all tolerances are called out at MMC. So do all here agree that if the top segment of three of the four FCFs had a SEP REQT note just to the right of it then that will make all four feature groups independent??

In this situation, the "pattern-locating" tolerances we can tolerate can be big (I've used Ø1.000 inch before) but the feature-relating tolerances are much smaller and calculated using either the fixed-fastener and/or floating fastener positional tolerance formulas.


ElectroMechanical Product Development
(Electronics Packaging)
UMD 1984
UCF 1993
 
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I am still unsure of the benefit to doing this. Unless the datum feature surfaces are really poor or the locations are right on the edge of acceptance it seems like a lot of work to re-establish the location of the part relative to the DRF for each group. There is nothing wrong with the theory of being able to do so, it just doesn't seem like there is a practical reason for it.
 
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3DDave,
The practical benefit is just this:

(1) Provides very loose tolerances for pattern-location. This is usually tolerable since we really don't care so much where a component is located on a surface just as long as it's roughly where we want it to be.
(2) Provides sufficient in-pattern (feature-relating) tolerances to insure each component fits. This tolerance is calculated using the fixed- and floating-fastener fit formulas.
(3) The in-pattern tolerances can be easily and quickly inspected with a Go/No-Go gauge (to check fit) and pin gauges (to inspect sizes).

Maybe I'm being a bit dense, but I can't see how this can't be beneficial.


ElectroMechanical Product Development
(Electronics Packaging)
UMD 1984
UCF 1993
 
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I was referring to using SEP REQT on the PLTZF to allow repeatedly establishing separate versions of the duplicated DRF.
 
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3DDave

OK, fair enough. I think was referring to that as well. In order to override the simultaneous condition that happens when the composite tolerance DRFs and material condition modifiers are the same, the SEP REQT note is required to the right of the top (PLTZF) segment of each composite tolerance FCF. For 'N' functionally independent feature groups on a part the SEP REQT note would be needed for 'N-1' FCFs to ensure none are tied together with simultaneous requirements.

ElectroMechanical Product Development
(Electronics Packaging)
UMD 1984
UCF 1993
 
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What is the advantage to restarting the inspection process for each group to verify the PLTZF requirement? Is there some process having trouble hitting a one inch diameter target where multiple DRF setups would accept them? That seems like a lot of datum rocking going on.
 
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3DDave,
When I design I don't obsess over inspect-ability. I focus on function. Besides, why would you have to restart the inspection process for each feature group? A CMM can probe one hole in each group in one operation since they're all tied to the same coordinate system.

ElectroMechanical Product Development
(Electronics Packaging)
UMD 1984
UCF 1993
 
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If you are going to inspect that way then why do you want to use SEP REQT? You still have to inspect all the holes in each pattern to ensure they meet the upper constraint, not just one.
 
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It is understood that the pattern-locating dimensions of all groups need inspection with a CMM. But the tolerances are usually so big that the inspector can do that very quickly. Now for each pattern the GO/NO-GO gauge and pin gauges make for quick work inspecting to the feature-relating positional tolerance and size tolerance.

ElectroMechanical Product Development
(Electronics Packaging)
UMD 1984
UCF 1993
 
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