Calculating virtual condition??

[Babu96]

Little bit confused with calculating the virtual boundary when the feature having datum reference at MMC. I know datums at MMC will result in datum shift but it really changes the boundary of the features or not???
I saw some calculation in some pages where they haven't considered the datum reference at MMC while calculating virtual condition.

 

[Burunduk]

If the drawing standard is Y14.5-2009 (or 2018?) a more suitable terminology for datum references is MMB, this may prevent confusion between maximum material condition of the considered feature (included in the calculation of Virtual Condition) and the Maximum Material Boundary of the referenced datum feature - deciding the size of the fixture. Datum shift does not enlarge the tolerances applied on the considered feature, therefore, it shouldn't affect the size of the VC boundary. Some describe datum shift as additional mobility of the same VC boundary relative to the datum reference frame. In practice, it allows looseness in fixturing that allows usable parts to pass.

 

[chez311]

Babu96,

You have posted 3 different questions in the past week on intimately related topics regarding MMC/MMB and Virtual Condition (VC). Perhaps if you followed up and engaged in conversation and asked follow up questions in the threads you started some of your questions would be answered and your understanding would be broadened.

First off, its been asked several times but what standard are you using? If its Y14.5-2009 or later the proper term for datum features referenced with the (M) modifier in the FCF is MMB or Maximum Material BOUNDARY, a small but important distinction.

Second, if you are using the Y14.5 standard - do you actually have access to it? It seems by your questions that you do not - if not I would highly recommend it. Secondhand sources help in initially learning some of the concepts as it can be daunting to digest a 200+ page standard however it only gets you so far. Reference to the primary document is a must when one strives to gain a deeper understanding of the topics at hand.

Finally to your question:

I know datums at MMC will result in datum shift but it really changes the boundary of the features or not???

Your question is somewhat nebulous and mixes some concepts (see above) - its not clear whether you are talking about the datum feature itself or the feature(s) which reference said datum feature in an FCF. Some specifics might help, or an example.

In the thread where you asked an uncannily similar question (

https://www.eng-tips.com/viewthread.cfm?qid=461299)

I tried to point you in the direction of a thread which discussed a similar topic and provided some examples of the effect of datum feature shift. Did you review that or any of the other responses?

 

[Babu96]

Chez311
I'm using Y14.5 2009 standard and sometimes i am getting confused with those explanation in the standards.It doesn't covers any explanation for profile callout with datum references at MMC.

 

[chez311]

Babu96,

Again, its MMB when discussing boundary conditions of a datum feature.

Sure it does - Y14.5-2009 fig 4-39 has a profile tolerance to |A|B(M)|. What exactly are you confused about in reference to MMB with profile?

Once again, an example of a drawing or textbook example which you are confused about would help.

 

[Babu96]

Chez311
Thank you so much for your response.my confusion is about what is the difference in the effect of using datum refernce at MMB and datum reference at RFS for a profile callout.
Why we go for profile datum reference at MMB.

 

[3DDave]

Typically, if the aligning feature on the mating part is a force fit or an interference fit then the datum feature that will mate with it is allowed to be RFS.

If the aligning feature on the mating part has clearance and allows the aligning feature to move around then the datum feature that will mate with it is allowed to be MMB.

 

[chez311]

Babu96,

The datum feature references in the FCF of any control describes the relationship between the datum features and their respective simulators. This is not dependent on the type of control (position, orientation, profile, etc..). The differences in this behavior for RMB vs MMB is generally as 3DDave has described.