DOF constrained by datum feature reference in FCF 2

[AM Engineer]

Hello all.

I have a question regarding degree of freedom constrained.

Does the datum feature referencing in Feature Control Frame constrains Degrees of Freedom of a part or tolerance zone with respect to datum reference frame?

Any help shall be highly appreciated.

Best Regards

Waqas

 

[chez311]

AM Engineer,

Thats sort of a weird question (or at least strangely worded - to me anyway). I'll try to answer it as best I can - the DRF constrains your tolerance zone, within which the features (surfaces, axes, centerplanes, etc...) must fall. When inspecting the part to establish a DRF (or really to locate/orient it relative to the features to be measured) however each datum feature on the part is mated to a corresponding datum feature simulator, and constrains the part as such.

Hopefully that makes sense?

 

[mkcski]

AM Engineer,

The complete answer is complicated and way too much for typing in a post. I suggest looking at figure 4-3 in 2009 as a start. The motion constrained is determined by the type of datum feature, the "perfect" datum and whether it is in the primary, secondary or tertiary position in the FCF. Fig 4-3 is primary only.

The tolerance zones are "located" from the DRF - typically by BASIC dimensions if the feature is not "sitting" on the datum.

Certified Sr. GD&T Professional

 

[AM Engineer]

mkcski

As an example we consider Fig 4-8 of ASME Y14.5-2009.

Lets first discuss this.

How many degrees of freedom of a part are locked with the given datum feature references?

My concept is since the datum feature B is modified at MMC so a fixed size MMB is established. If the actual produced part has a RAME less than that of MMB then there will be a mismatch between datum feature and its simulator. In that case, no degree of freedom is locked. Part can translate, rotate or do whatever motion is allowed within the gap between RAME and MMB.

If however, RAME= MMB then 5 degrees of freedom are locked. Rotation about z is unlocked.

Am i right in my concept?

Please respond.

 

[chez311]

AM Engineer,

I know you addressed your question to mkcski, however since its rather general and we both responded to your question I'm going to take a shot at answering as well.

The concept you are referring to is datum shift. This is introduced in para 4.11.9 in Y14.5-2009 and which allows the datum features on the part to shift in translation/rotation in relation to the perfect (or near perfect) datum feature simulators (and therefore the theoretical datums) proportionate to the amount which the datum feature deviates from the controlled boundary condition (MMB or LMB).

This is a separate discussion from DOF and constraint of a DRF, or at least slightly more nuanced. The datum features referenced still constrain the same DOF as if they were referenced RMB - as they would behave similarly when produced at the limits of size (MMC/MMB or LMC/LMB) however as I mentioned before they allow some shift as they depart from the controlled boundary condition.

 

[mkcski]

AM Engineer:

Chez311 cover it as I would have.

Certified Sr. GD&T Professional

 

[mkcski]

AM Engineer:

Be reminded that Y14.5 is for Product Definition. The symbols communicate part "function and fit-up" requirements as I call it. So you have to understand how your parts interface and then select symbols to communicate the intent. Trying to understand how to interpret and apply the symbols can be overwhelming if you do not have a good understanding of how the part "works". This is the foundation upon which datum selection is determined and defined and the first question I ask when critique GDT on a drawing.

Certified Sr. GD&T Professional

 

[AM Engineer]

Thanx Chez311 and mkcski.

Chez311

Sir you said in your second paragraph or this is what i undertsood from it is " Datum feature references still constrains the same degree of freedom as if refered at RMB".

But in Clause 4.3 of ASME 2009, it states that the relationship between the datum feature and its simulator constrains the DOF according to the material boundary condition referenced in FCF.

So my concept is it does make difference in DOF being locked if material boundary condition is changed.

Your take on this.

 

[3DDave]

ASME needs to create a picture book for this, maybe with cutouts to play with. Words just are too hard to interpret all the time. Then pictures showing a number that is bigger than one and less than another can be clearly constrained between those two values, because sometimes constraint doesn't mean locked in a vise and frozen against all motion. Simple really.

 

[AM Engineer]

Not getting you

 

[pmarc]

AM_Engineer,
What you refer to is called by some people "partial DOF constraint" (where there is a loose between actual datum feature and its datum feature simulator) as opposed to "full DOF constraint" (where such loose doesn't exist).

So from that point of view the better answer to your original question seems to be that datum features referenced in a feature control frame constrain tolerance zone(s) with respect to the datum reference frame. However, notice that while for a FCF like |POS|dia. 0.2|A|B|C| all translational and rotational DOFs of the tolerance zone(s) are constrained, this is not true for |ANG|dia. 0.2|A|B|C|, where only rotational DOFs are constrained.

Long story short, the answer to question is not that simple as the question itself.

 

[AM Engineer]

Thanx pmarc for your valuable response.

From now on, is it safe to remember the concept that we should apply the concept of locking DOF to tolerance zones, not parts?

 

[AM Engineer]

Pmarc

Also what does the first 3 lines of para 4.3 of ASME 2009?

Here what it writes

"The relationship between the primary datum feature and its datum feature simulator constrains the degrees of freedom according to the material boundary condition applied to the datum feature in the feature control frame."

 

[pmarc]

From now on, is it safe to remember the concept that we should apply the concept of locking DOF to tolerance zones, not parts?

No, I don't think I said that. The point of my comment was that the answer to your question was not that simple.

Also what does the first 3 lines of para 4.3 of ASME 2009?

The only problem I would see with the first sentence of para. 4.3 is that it seems to be of generic nature when it comes material boundary concept (it says "according to the material boundary condition applied"), while the paragraph is explicitly about degrees of freedom constrained by datum features regardless of material boundary.

 

[axym]

Waqas,

Going back to your original question, I would say that the datum feature references in the feature control frame constrain the degrees of freedom of the part with respect to the datum reference frame.

The degrees of freedom of the tolerance zone with respect to the datum reference frame are controlled by what type of tolerance zone it is:
-Position and profile tolerance zones are fully constrained with respect to the DRF
-Orientation tolerance zones (perpendicularity, parallelism, angularity) are oriented to the DRF, but can translate relative to it

There is much more to this, of course. As pmarc pointed out, different boundary conditions influence how "completely" the degrees of freedom of the part are constrained.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[3DDave]

ASME Committee members purposely make the language in the standard hard to understand and then cherry pick examples that only barely meet that language, leaving everyone else to ask questions that seem comparatively ignorant.

The phrase - a picture is worth a thousand words is dwarfed by putting functional examples into the hands of the learners.

 

[AM Engineer]

Thanx axym and pmarc. Much of the confusion is over but some is still there. I am still reviewing my GDT concepts as i was involved in this subject long ago. I am sorry for my foolish questions.

I shall ask one more.

Why we use the phrase " DOF locked wrt Datum reference frame"? Why not "wrt Datum Feature Simulators"?

 

[Sem_D220]

How about "degrees of freedom are locked by the interaction between the datum features and datum feature simulators wrt datum reference frame"?

 

[pmarc]

Datum reference frame is established from datum feature simulators, therefore, as long as we are not going into details about differences between theoretical and physical datum feature simulators, both statements are ok, I think.

 

[chez311]

AM Engineer,

Just to add just a little to what pmarc said without getting too far lost in the weeds - you can think of them as two sides of the same coin, they are intimately interrelated. Your DRF is a set of theoretical mutually perpendicular planes which is established by the physical physical datum feature simulators. So since your DRF coincides with and/or is fixed in relation to your datum feature simulators saying " DOF locked wrt Datum reference frame" or "wrt Datum Feature Simulators" is almost like saying the same thing aside from the understanding that a theoretical DRF obviously cannot contact/directly limit translation/rotation of a physical part.

For some visualizations of this interrelationship of DRF vs. datum feature simulators you can take a look at 4-9 and 4-26 in Y14.5-2009, the latter of which shows an MMB example.