Thread: positional control and datum references GD&T

1. positional control and datum references GD&T

Referring to the picture, the part consists of a hole extruded through a rectangular plate and is located with basic dimensions, with respect to datum B and C (shown in the picture). Datum A, B, and C are primary, secondary, and tertiary datums respectfully.
My question is, if a positional control is applied to the circular feature of size, then why is datum A referenced in the FCF? To me, datum A is irrelevant for the reasons I mention in the following:

1. The focus is to control the position of the feature which is located with respect to datums B and C alone.
2. The positional tolerance zone resides within a region measured about true position relative to datums B and C.

What I’m trying to point out, in 1-2, is that the main features, true position & tolerance zone, that define the positional control have nothing to do with datum A.
The only reason I can think of for controlling the location relative to datum A is because datum B is typically oriented with respect to datum A, and C to A and B. Therefore datum A must be referenced because the orientation of datum B to A and C to B & A will affect the accuracy of locating the FOS to its intended true position. BUT if this is the case, then why do books on tolerance analysis not take this into account? My “reason” for why datum A is consider is no longer making sense, given the statement in the previous sentence.

Can someone help me make sense of all this?
Thank you.

2. Yes, your thinking about the datums' interrelationship to each other is the reason that A is important. Realize that while we say that datums A, B, and C are all mutually perpendicular, in reality the physical surfaces -- datum features A, B, and C -- might not be perfectly perpendicular.

So picture a part where all of the corners aren't quite 90º -- maybe 89.5º on some and 90.5º on others. If datum A were not referenced, then datum B would become the primary datum, and the fixturing of the part would be flush against datum B (needing a minimum of 3 points of contact). But with datum A invoked as the primary datum, the actual part will be most flush against A, and the actual part might then only touch 2 points on B.

Recall that the hole's position is to be measured from the theoretical datums (simulated by the fixture), not the physical surfaces. Thus, the different fixture scenarios that I describe will indeed result in different position results for the hole from B and C.

With all that in mind, I'm not sure what you mean by saying that tolerance analysis books leave out the orientation idea. Are you referring to tolerance stacks? If so, they don't leave that out, although it's probably assumed by the authors since those books focus on one-dimensional stacks. But please elaborate.

3. I almost get that you think Position tolerance only controls location or the distance that a tolerance zone is relative to the specified datums.

This assumption would not be correct as a position tolerance that includes a primary datum surface that is perpendicular to the feature would also control that geometric relationship - perpendicularity.

If the Datum A surface assembles to another part then a geometric relationship between the surfaces should be considered - maybe a bolt or another part must clear the hole and Datum A surface must mate with the surface or a gasket seal must be facilitated.. The DRF you specified primary Datum A, secondary Datum B and Tertiary datum C indicates that the tolerance zone is oriented perpendicular to the Datum A and then located to both Datum B and C.

When parts mate or assemble together all of those related surfaces should be considered in the tolerance specification.

Originally Posted by michael_bull
Referring to the picture, the part consists of a hole extruded through a rectangular plate and is located with basic dimensions, with respect to datum B and C (shown in the picture). Datum A, B, and C are primary, secondary, and tertiary datums respectfully.
My question is, if a positional control is applied to the circular feature of size, then why is datum A referenced in the FCF? To me, datum A is irrelevant for the reasons I mention in the following:

1. The focus is to control the position of the feature which is located with respect to datums B and C alone.
2. The positional tolerance zone resides within a region measured about true position relative to datums B and C.

What I’m trying to point out, in 1-2, is that the main features, true position & tolerance zone, that define the positional control have nothing to do with datum A.
The only reason I can think of for controlling the location relative to datum A is because datum B is typically oriented with respect to datum A, and C to A and B. Therefore datum A must be referenced because the orientation of datum B to A and C to B & A will affect the accuracy of locating the FOS to its intended true position. BUT if this is the case, then why do books on tolerance analysis not take this into account? My “reason” for why datum A is consider is no longer making sense, given the statement in the previous sentence.

Can someone help me make sense of all this?
Thank you.

4. Read what is in the attachment

5. First, let’s define the word “Byproduct”:

· an incidental or secondary product made in the manufacture or synthesis of something else. "zinc is a byproduct of the glazing process"
· a secondary result, unintended but inevitably produced in doing or producing something else. "he saw poverty as the byproduct of colonial prosperity"

By specifying datum A as the primary datum in your example is to specify intent. That intent is that a geometric and mathematical relationship between datum A and the hole feature must be considered in the fit, form and function within the target assembly or other application. Fit, form and function are achieved by proper tolerance requirement analysis, design and specification and then application of sufficient manufacturing capability on the engineering material.

Therefore, to say that “Perpendicularity is the byproduct of applying positional control” is not a correct statement as the perpendicularity control included in the position tolerance specified was facilitated with design intent - functional intent if you prefer.

Additional flaws in the statement include “Circular tolerance zone” wherein the specification in this discussion is a cylindrical tolerance zone.

Other flaws of understanding include “Perpendicularity is controlled only in the case an axis being controlled”. Dimensionally controlling an axis is an academic or as I like to say CMM interpretation of orientation or position where most correctly we are geometrically controlling the as-built surface of the FOS. We derive an axis by measuring selected surface elements of the as-built surface. Moreover one can the control the perpendicularity of a surface, complex FOS or even a complex surface profile.

There’s even more… but, let’s stop there.

Dimensioning and tolerancing per. ASME Y14.5-2009 or any other dimensioning and tolerancing standard (ISO 1101) is best understood in context and similarity to physics, geometry and mathematics than to philosophy.

I suggest that you drop the philosophical perspective or approach to dimensioning and tolerancing and assume a more scientific or design engineering view.

Originally Posted by michael_bull
Read what is in the attachment

6. read what's in attachment

7. Michael,

I've been certified as a Senior Level Geometric Dimensioning and Tolerancing Professional by the American Society of Mechanical Engineers since 1997.. My certification number is GDTP S-150. I believe Mr Belanger has been certified since 1997 as well.. I also have over 37 years as a design engineer of that 22 in the trenches designing hardware for aircraft, spacecraft and other applications.

See: List of ASME Certificate Holders

I've written and published six books on GD&T to the ASME Y14,5M-1994 standard, ASME Y14.5-2009 standard as well as the ISO 1101 - 2003 and 2009 standards. Additionally, I have consulted and trained thousands of professionals all over the world to the various GD&T standards.

You would benefit considerably by taking a course on ASME GD&T or ISO G&T with an ASME certified instructor. You should start with a fundamentals class and work your way toward more advanced classes. There are many public and onsite seminars available.

I wish you the best...

Originally Posted by michael_bull
read what's in attachment

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