DML straightness material condition modifiers and relation to position/orientation at MMC 1

[Burunduk]

When the straightness tolerance at MMC is used in conjunction with an orientation or position tolerance at MMC, the specified straightness tolerance value shall not be greater than the specified orientation or position tolerance value and does not contribute to the IB or OB of the position or orientation tolerance. The collective effect of the MMC size and form tolerance produces a VC, OB, or IB resulting from the form tolerance but does not affect the IB or OB created by any orientation or position tolerances on the feature.

Why is the above rule of not specifying larger tolerance for straightness than for position/orientation applies only to DML straightness @MMC & Position/orientation @MMC, and does not include DML straightness @RFS & Position/orientation @MMC?

I think that if DML straightness @RFS & Position/orientation @MMC is applied the size of the unrelated AME may be greater than MMC size, however we wouldn't want it to become greater than the VC for Position/orientation @MMC, since we want the surface of the cylinder to be limited by the VC gage, without any unrealizable form allowance - same logic as for not wanting the VC for DML straightness @MMC to become larger than the VC for Position/orientation @MMC that will limit the surface anyway. Is this not the guiding logic?

 

[Belanger]

Would this earlier thread be of any help?...

[URL unfurl="true"]https://www.eng-tips.com/viewthread.cfm?qid=302375[/url]



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

 

[Burunduk]

John-Paul,
In the post you referenced, the reply by Evan and the correspondence that preceded it clarifies why when both @RFS, it makes no sense to require any dependency between DML straightness & position.
It is also clear why when both @MMC, Straightness should be within smaller value.
But when straightness is @RFS & position is @MMC, shouldn't the same principle apply?
If the RFS specified straightness tolerance value is greater than the MMC specified position for a pin for example, the OB for straightness + MMC size will exceed the VC calculated by MMC size + Position, which unlike the OB related to RFS specified position, doesn't get to grow to contain the straightness allowance. Then some of the straightness tolerance becomes unusable, which is what I think such rules are trying to prevent. This is why I don't get limiting the requirement only to when both @MMC.

 

[3DDave]

How does the feature function simultaneously at RFS and MMC?


The reason for this rule seems obvious - this rule, which can be derived from other rules that anyone performing an adequate tolerance analysis would already have observed, that allowing more straightness variation would not be a limiting factor - the only benefit is to be able to add this to training material tests given to those who parrot the contents without understanding them.

 

[Burunduk]

How does the feature function simultaneously at RFS and MMC?

How does having two different and unrelated worst-case boundaries make more sense functionally?

 

[3DDave]

You first.

 

[Burunduk]

VC for DML straightness @MMC is to provide additional tolerance as long as a boundary for unconstrained assembly fit is maintained. VC for position @MMC is to provide additional tolerance as long as constrained assembly fit is maintained. Most chances, if you need constrained assembly fit you don't care about unconstrained assembly fit and vise versa. When a component has a cylindrical feature intended for constrained assembly fit and limiting the form by rule#1 is from some reason impractical, yet the amount of deformation that may be caused by allowing size-dependent addition to form allowance is detrimental, limiting the DML straightness RFS may be preferable. Since the VC for constrained fit is based on the MMC size + position tolerance available @MMC, any straightness variation violating the boundary of perfect form @MMC will narrow the usable size range, but overall the scheme can work just fine.

 

[3DDave]

"if you need constrained assembly fit you don't care about unconstrained assembly fit"

is contradictory. Either the fit is constrained or it isn't.

"When a component has a cylindrical feature intended for constrained assembly fit and limiting the form by rule#1 is from [sic] some reason impractical"

And when would it be impractical?

Maybe you should make a drawing showing the sort of variation and how the mating part interacts with this feature to clarify.

 

[Burunduk]

is contradictory. Either the fit is constrained or it isn't.

Oh my. I don't think it really requires clarification but here goes: You generally need either a constrained fit or unconstrained fit. You care for the one you need and don't care about the one you don't.

And when would it be impractical?

It could be an integral pin with large height-to-diameter ratio, when rigidity is somewhat compensated and rule #1 is difficult.
Use your imagination.

 

[3DDave]

1) So, RFS is typical for constrained fit. MMC is for unconstrained fit. You want to explain how to use them simultaneously.

If you craft your responses better it will be more clear what you mean, hence the request for a drawing showing the exact case you started this thread about.

2) That isn't impractical is it? Sounds more like an argument for defining forces to constrain the part with a fixed diameter (MMC) boundary gauge.

 

[3DDave]

To better isolate your original problem statement, why does the standard "not include DML straightness @RFS & Position/orientation @MMC?"

 

[pmarc]

A food for thought (my apologies, it's not about the RFS case):

According to the rule from 8.4.1.3, when a non-zero DML straightness @MMC tolerance is applied, the feature is not allowed to receive an additional position or orientation tolerance of zero @MMC. But should this really be prohibited?

 

[3DDave]

Sort of an anti-refinement?

 

[pmarc]

Actually, sort of a question on whether the form and orientation/location should even be dependent in MMC/LMC-only cases.

 

[3DDave]

Same question as for B - diagram and practical use case?

 

[pmarc]

A secondary datum pin B of say dia. 10 +/-0.1 with a 0.1 @ MMC DML straightness addtionally applied (in order not to have to be unnecessarily overconstrained by Rule #1), nominally perpendicular to primary datum plane A controlled with a 0 @ MMC perpendicularity to A in order to assure assembly with a counterpart containing a hole (nominally perpendicular to the mating face) for which the VC related to the mating face is dia. 10.2.

I hope the description is clear enough without a diagram.

 

[Burunduk]

1) So, RFS is typical for constrained fit.

I didn't say it was typical. I suggested that RFS for straightness & MMC for position is possible.

The question was about a rule in the standard and in a general context.

Sounds more like an argument for defining forces to constrain the part with a fixed diameter (MMC) boundary gauge

Why do that? If it is a constrained fit at assembly and rule #1 is not intact, you don't need an MMC gage. You need a VC gage for position. You only need a VC gage for straightness if it's specified @MMC.

I don't see a reason to preclude straightness RFS in conjunction with position @MMC. It may not be the best design decision for every single application but there could be some circumstances that might support it. As I said you may want to allow violation of perfect form @MMC yet have the straightness limited within just this or that value regardless of local sizes. Never underestimate the amount of different applications and circumstances out there.

 

[3DDave]

A secondary datum pin B of say dia. 10 +/-0.1 with a 0.1 @ MMC DML straightness addtionally applied (in order not to have to be unnecessarily overconstrained by Rule #1), nominally perpendicular to primary datum plane A controlled with a 0 @ MMC perpendicularity to A in order to assure assembly with a counterpart containing a hole (nominally perpendicular to the mating face) for which the VC related to the mating face is dia. 10.2.

I hope the description is clear enough without a diagram

OK - that's an anti-refinement.

The position potential bonus tolerance is increased because of the increased form tolerance for the pin. As the pin is straighter and as the pin is reduced from MMC the "bonus" goes up. This is in contrast to the more typical case where the pin form tolerance is restricted by the position tolerance applied to it.

 

[3DDave]

From the lack of on-topic response Burunduk, my discussion with you on this topic is closed.

 

[Burunduk]

3DDave,
It's ironic because you haven't said anything on topic anyway and only tried to move the discussion away from the original question.

pmarc, in the example you brought, the VC for straightness is dia. 10.2 (due to 0.1 straightness @MMC & 10.1 MMC size) and the VC for position is dia. 10.1. It appears that some portion of the form allowance is restricted by position, and I think that this is the reason why the standard prevents this.