GD&T Workshop: Decoding Feature Control Frames
Continuing with our analysis of the Greatest Design Tool ever, our objective in this article is to take a detailed look at the structure of Feature Control Frames. In particular we will analyze the content and functional objectives of each compartment in order to illustrate the impact of the various Geometry Control Tools and Modifiers on real part features. The objective in the end is to demonstrate the ability of Feature Control Frames to unambiguously specify permissible limits of imperfection, and to illustrate how, if they have been properly “encoded,” they can be uniquely “decoded” to completely eliminate the business of “interpretation.”
Because of the intricate complexities of imperfect geometry and the resulting, necessary complexity of GD&T, however, far too many CAD models are unfortunately “decorated,” rather than “encoded” with GD&T. Furthermore, like any work of fine art, these decorations must then be “interpreted,” but now at the peril of misleading the machine shop, and confusing the inspection department. In keeping with the opinion of many a metrologist, that machine parts are merely evidence of violence committed against innocent material, the GD&T found on machine part drawings is therefore all too often
evidence of an unparalleled condition of maximum confusion in design,
representative of a composite set of virtually immaterial conditions for manufacturing, and
nothing more than a Derived Median Line of BS (Bodacious Snafus) for inspection
Referring to the enhanced Feature Control Frame in Figure 2, the first compartment is dedicated to identifying the Geometry Control Tool, the second to defining the shape, size and any modifications of the tolerance zone, and the third is a set of instructions for establishing a Datum Reference Frame.
Compartment 1) The “Geometry Control Tool” determines the controlled component of the feature under consideration, namely in the case of the Position tool and a cylindrical tolerance zone, the bounded portion of the axis of a bore, and also the permissible constraints, namely orientation and location.
Compartment 2) The definition of the tolerance zone starts with a specification of its shape, continues with a definition of its size and ends with possible instructions for its modification. In the current example, the diameter symbol represents a “Tolerance Zone Shape” modifier which serves two purposes: a) it specifies a cylindrical tolerance zone and b) it also makes clear that the tolerance value is the diameter, not the radius of the zone. [Note: The ASME Y14.5 2009 Standard defines only two Tolerance Zone Shape modifiers, namely Ø and SØ, in spite of the obvious need for several more, for example, for “skin-like,” “ribbon-like” and “slab-like” zones.] The next entry specifies the size of the tolerance zone: the diameter, the width or the thickness. The next entry shown here is a material condition modifier, or, in more descriptive terms, a “Tolerance Zone Size” (TZS) modifier-in this case (M)-which allows the tolerance zone to expand as the Considered Feature departs from Maximum toward Least Material Condition, and which encodes the “clearance” function. The next entry is the Free State modifier (F) which requires that the indicated Position requirement be met in the so-called “Free State” of the part, in contrast to the remaining controls imposed on the part, which a drawing note requires be met in a “constrained” state.
Compartment 3): The last compartment consists of several sub-compartments containing a collection of Datum Feature Labels (not Datum labels-see Question 1) below) and their associated “material boundary” modifiers , or in more descriptive terms, “Tolerance Zone Mobility” (TZM) modifiers (S), (M) and (L), and the 2009 Standard defined “Degrees of Constraint” modifiers [u,v,w,x,y,z]. Together these make up a set of instructions for establishing a Datum Reference Frame relative to which the specified tolerance zone is to be oriented and located by Basic dimensions. In the illustrated case the “TZM” modifier (S) associated with Datum Feature B requires the Datum Reference Frame to be (S)table relative to B, whereas the modifier (M) associated with Datum Feature C permits taking advantage of potential Datum Reference Frame (M)obility relative to C. Furthermore, the Degrees of Constraint modifier [u,v,z] associated with Datum Feature A makes clear that A shall serve to constrain pitch, yaw and one degree of translational freedom, the modifier [x,y] associated with Datum Feature B makes clear that B shall constrain the two remaining degrees of translational freedom, and the modifier [w] associated with Datum Feature C makes clear that C shall serve to constrain roll.
Question 1) Why are the letters in the third compartment of the Feature Control Frame referred to as “Datum Features” instead of “Datums?” Answer: First of all, Datum Features are the physical surfaces of real parts, and Datums are imaginary reference points, lines and planes. Next, if the letter “B” represents a Datum, since it is referenced regardless of feature size, Datum B would have to have “size,” but mathematical reference points, lines and planes have no size. Similarly, if “C” represents a Datum, Datum C would have to have “material’ in order to be simulated at its Virtual Maximum Material Boundary, but mathematical points, lines and planes have no material. Only Datum Features have “size” and “material.”
Question 3) If the Tolerance Zone Mobility modifiers for Datum Features B and C are shown explicitly, why isn’t the modifier for A? Answer: Unfortunately the 2009 Standard hasn’t defined a modifier for specifying how a primary planar datum feature is to be simulated, even though it should (maybe someday). Instead the manner in which A is to be simulated (see the Feature Control Frame “decoding” verbiage below to find out how the word “simulated” is used), is defined by the Rule of Rocking & Rolling Datum Feature Simulation (ASME Y14.5 2009 §4.11.2 p.59).
Question 4) Why are the degrees of constraint modifiers [u,v,w,x,y,z] used, when in this case the degrees of freedom they specify are the very same which the Datum Features would naturally be required to constrain? Answer: They are shown mainly to indicate their availability as an important new tool in the Y14.5 2009 Standard, but also to aid in the “decoding” process addressed below.
To demonstrate how all these very precise requirements are actually communicated by the Feature Control Frame, let’s first read, and then decode it.
Reading: “Position within 0.15 millimeters at MMC in the Free State, relative to A, constraining u, v and z, relative to B RFS, constraining x and y, and relative to C at MMC, constraining w.”
Decoding: “With the part in the Free State, Position requires the bounded axis of the considered feature to lie within a cylindrical tolerance zone of diameter 0.15 millimeters at MMC, expanding by the absolute value of the difference between the unconstrained Actual Mating Size and the MMC size of the considered feature as it departs from MMC toward LMC, which is oriented and located by Basic dimensions relative to a Datum Reference Frame established using Datum Feature A simulated “rocking” to constrain pitch, yaw and one degree of translational freedom, Datum Feature B simulated “stably” regardless of its boundary to constrain two more degrees of translational freedom, and Datum Feature C simulated “mobly” at its Virtual MM boundary to constrain the last degree of translational freedom.”
In review, based on the presence of the Tolerance Zone Size modifier (M), the code tells us that the Design team understood the function of the considered bore to be “clearance” for a mating shaft. Furthermore, based on the “Rule of Rocking & Rolling Datum Feature Simulation”, they also understood that Datum Feature A might be slightly convex, and were willing to accept the limited instability it might induce. Next, based on the presence of the Tolerance Zone Mobility modifier (S) associated with Datum Feature B, the Design team made clear that even though residual play might exist between B and its mating Datum Feature, they did not wish to allow manufacturing to take advantage of it, since the function of B is to precisely locate the considered part relative to the mating part. Finally, use of the Tolerance Zone Mobility modifier (M) with Datum Feature C made clear that potential residual mobility between C and its mating Datum Feature did not negatively impact assembly or operation and was therefore a legitimate gift to the manufacturing and assembly teams. The code clearly informs the manufacturing team of these objectives, and encourages them to favor the Least Material Condition for the considered feature and of Datum Feature C, regardless of the well understood need to simulate them Regardless of their Material Boundaries (RMB) during manufacturing. Finally, the code specifies exactly how Datum Features A, B and C are to be simulated for functional gage design and/or for coordinate metrology, and also suggests that the assembly team not forget to take advantage of the play before locking things down.
This is code. There is no room for interpretation!
In June we’ll cast much more light on the confusing world of GD&T concepts with precise definitions of Datum Features, Datum Feature Simulators and Datums in preparation for a look at the most important rules we can extract from the Y14.5 Standard.