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Metrology

The GD&T Encoding Process—Final Steps

Smart GD&T workshop No. 8

Published: Wednesday, May 7, 2008 - 21:00

Encoding the mating flange:


In Workshop No. 7, we used Smart GD&T processes to encode the operation, assembly, and other functions of a flange. In this workshop, we take a deeper look at the mating aspect of the game by encoding the mating flange. The additional steps, 7 through 9, are listed in the expanded encoding process steps shown below.

Figure 1. Smart GD&T encoding process steps

1. Create a feature inventory for the selected part.

2. Analyze the function of the part.

3. Determine the function of each feature and create a feature hierarchy.

4. Encode the function of each feature of the part by determining what can go wrong, and then partially “stuffing” the necessary Feature Control Frames step by step, to begin to impose limits.

5. Finishing the feature control frame “stuffing” process, by selecting tolerance values and the proper Tolerance Zone Size and Mobility modifiers.

6. Decode the initial feature control frame set to determine if the functions they assign to each feature are the most effective for the overall function of the part. If not, modify the code to represent the better understood, newly assigned functions.

7. Repeat steps 1–6 for the mating part(s).

8. Determine the virtual condition of mating features.

9. Balance the Tolerance values to guarantee assemblability.

We start encoding the mating flange by reviewing its features, and discover some significant differences. As illustrated in figure 2, instead of through bores, flange 2 has threaded bores for the six bolts that will secure it to flange 1, and, in addition, an O-ring groove.

Figure 2. The mating flanges

Although we believe that flange 1 is fully and properly encoded, a bit of humility may be in order as we begin the encoding process for flange 2. Of course we’ll have to be well on our way with flange 2 before possible improvements in flange 1 code can be detected. Nevertheless, it’s wise to be looking for such opportunities. Placing encoded flange 1 next to still “naked” flange 2, as we have in the figure above, allows us to begin to see connections and so take advantage of all the learning with did with flange 1.

Getting started
We proceed as we did with flange 1.

Step 1. Create a Feature Inventory
Based on what we learned with flange 1, we can create a rather complete feature inventory for flange 2 very quickly. In fact, flange 2 consists of three planar surfaces, four cylindrical surfaces, six threaded bores, six blind hole bottoms, seven chamfers (six for the threaded bores, plus the symmetry-destroying chamfer on the outer periphery of the flange), and quite a collections of broken edges.

Step 2. Analyze the Part Function
The function of flange 2 is as follows: The planar surface containing the O-ring groove and the compressed O-ring itself will mate with the opposing planar surface of flange 1. Next, six bolts will be loosely inserted through the bores in flange 1 to be loosely screwed into the threaded bores in flange 2. Finally, an assembly tool will be used to coaxialize the outer perimeters of the two flanges, after which the bolts will be tightened and the job is done.

Step 3. Create a Feature Hierarchy
Paralleling our experience with flange 1, the planar surface containing the O-ring groove on flange 2 will constrain pitch and yaw and one degree of translational freedom, and must therefore be encoded as Datum Feature A. Furthermore, with the help of the bolts, the six threaded bores will preliminarily locate the two flanges relative to each other and also constrain mutual roll. In the end, however, it’s the outer periphery of flange 2 that, with the help of the coaxializing tool, will accomplish final constraint of the last two degrees of translational freedom, requiring it to be labeled as Datum Feature B. These facts, together with preliminary nominal dimensions are recorded in figure 3.

Figure 3. Nominally dimensioned flange 2 with Datum Features labeled.

Step 4. Preliminary Feature Control Frame Stuffing:
The most important planar surface: Datum Feature A in flange 2 must meet a similar but much less strict requirement than Datum Feature A in flange 1, because it isn’t responsible for creating a reliable seal. We therefore control it with the Flatness tool, but with a much larger, still to be determined tolerance, as shown in figure 4.

The opposing face: As in the case of the opposing face in flange 1, we realize that only loose control of flatness, parallelism, and location relative to A are required, and decide to do so with the width tool, again leaving the tolerance value to be determined later. Furthermore, in spite of the fact that the O-ring groove breaks the symmetry of flange 2, we retain the big chamfer on the backside to further emphasize the fact, given its probable low manufacturing cost. Again, see figure 4.

The bottom of the O-ring groove: This is a new feature, whose form, orientation, and location must be controlled, but for which the size tool is inappropriate, because it faces in the same direction as Datum Feature A. Instead, we use the Dimension Origin tool, a largely overlooked, but very important and simple tool, which is perfectly suited to the job.

The outer periphery of the flange: Having decided to make this Datum Feature B, there are only three variables to control, namely size, form, and perpendicularity relative to A, and we again choose the typical tools, but leave the assignment of tolerances and modifiers for step 5. See figure 4.

The central bore: In this case, we must control the size and form, the orientation relative to A, and the location relative to B, for which we select the size and Position tools, and again leave all tolerance values to be determined later, as shown in figure 4.

Figure 4. Partially stuffed Feature Control Frames for flange 2

The edges of the O-ring groove: These serve merely to constrain the O-ring from collapsing or expanding under the influence of gas pressure, and, like the inner bore must also be constrained in size, form, orientation, and location. We could use the Diameter and Position tools, but why not try the “everything”—surface profile—tool for fun, the tool the machine shop and the quality department like so much? If we go this route, their two radii must be made basic. Again, see figure 4.

The threaded bore pattern: In this case, we must control the thread size and form, the mutual orientation and location of the six bores, their perpendicularity to Datum Feature A, and finally their location relative to Datum Feature B, as well as their depth. Their size and form are addressed using a thread standard. Next, their mutual orientation and location, as well as their orientation and location relative to Datum Features A and B, are controlled with the Position tool. Finally their depth is controlled with the Depth tool. Again, we leave the tolerance values, the Tolerance Zone Size (TZS) modifier and the Tolerance Zone Mobility (TZM) modifier, associated with Datum Feature B, to be determined later. Finally, because only Basic dimensions can orient and locate tolerance zones, we make the 80-mm radius Basic, and add an “EQUALLY SPACED” note to deal with the required Basic angles. Again, see figure 4 for details.

Step 5. Final Feature Control Frame Stuffing:
It’s time to finish the stuffing process by checking the validity of the preliminary code and filling in the missing tolerance values and modifiers.

The most important planar surface: Due to the fact that Datum Feature A in flange 2 is merely required to constrain pitch, yaw, and one degree of translational freedom, and no longer serves a sealing function, we select a much looser flatness tolerance of 0.1 mm. See figure 5.

The opposing face: As in the case of flange 1, thickness isn’t critical but parallelism is, this time not to ensure that bolt heads make full contact, but rather to ensure that a mating, welded pipe is adequately perpendicular to Datum Feature A. We do this mimicking the controls used for flange 1. See figure 5.

The bottom of the O-ring groove: This is the critical sealing surface. It needs to be reasonably tightly location-controlled to ensure proper compression of the O-ring, but much more tightly controlled in terms of parallelism to A and flatness to achieve a reliable seal. This requires beefing up the Dimension Origin tool with a much tighter Parallelism control. See our choices in figure 5.

The outer periphery of the flange: Because this is our coaxializing feature, its diameter must be very close to its counterpart in flange 1. Furthermore, although excellent perpendicularity relative to A is an essentially free gift of the manufacturing process, we set a reasonable limit just to complete our controls. Furthermore, we choose a regardless-of-feature-size TZS modifier for the perpendicularity, because a change in size brings no functional increase in our tolerance for poor orientation, and it’s foolish to entice the machine shop with an (M) if there’s no benefit. In this case our code is a mirror image of what we did for flange 1.

The central bore: Because the function is identical to that in flange 1, we use the same tolerances, including the cylindricity refinement justified in Workshop No. 7. All this is shown in figure 5.

The edges of the O-ring groove: Our main operational concern is to constrain the O-ring from collapsing under the influence of gas or liquid pressure. Our assembly concern is to ensure room for the O-ring to fit into the groove, and our manufacturing concern is to keep our machinists smiling. All this is easy with a surface profile tolerance of 0.2, which guarantees a minimum width of 3.6 and a maximum width of 4.4 mm. We need “break edges” notes for the transition of the walls of the groove to Datum Feature A and in several other cases as well, and therefore add them now.

The threaded bore pattern: As shown in figure 4, our current code requires the bounded axes of our threaded bores to lie within a set of six, yet to be prescribed cylindrical tolerance zones. However, because it will be the mating bolts rather than the threaded bores themselves that will interact with the mating part, we ought to focus on their projected, not their bounded axes. We do this by inserting a Projection modifier (P) into the Position Feature Control Frame, and adding a distance of 41 mm to accommodate the thickest possible mating flange. Next, we must select a TZS modifier for the position tolerance zones. Because our focus is on the axes of the pitch diameters of the bores, and the axes of the mating bolts will tend to line up with them under stress, there’s no forecastable position bonus, making a regardless-of-feature-size TZS modifier (S) the proper function encoding choice. Similarly, there’s no pattern offset benefit to be gained from a shrinking Datum Feature B, so that a regardless-of-feature-size TZM modifier (S) is also the proper choice here. When it comes to controlling bore depth, we’re only concerned that they’re deep enough to allow the bolts to tighten against the mating flange, and not so deep as to possibly poke through to the other side. A tolerance of ±2 mm ought to make everyone happy.

Figure 5. Feature control frame stuffing refinements

Manufacturing note: Even though our decision to use the Projection modifier (P) makes all the sense in the world for the assembly function, it’s potentially upsetting for the machine shop, because they must impose a much tighter tolerance on the bounded axes of the threaded bores to guarantee acceptance. Hopefully, they will see the benefit in the long run, because assembly and operation are by far the most important functions in the end.

Inspection note: The coordinate metrologists might also complain about the use of the Projection Modifier, but we must hope that their software is capable of dealing with it.

Final steps for encoding flange 2

Step 6. Feature Control Frame Decoding and Reassessment of Feature Functions
It’s time to assess the code we’ve generated. The decoding process is important, because it forces us to check for syntax errors and to reassess feature functions, and possibly change the code to better represent them. Because most of the functions of the features of flange 2 are identical to those of flange 1 and have already been reviewed, we will deal only with the threaded bores and the O-ring groove.

The groove: After a good review, we feel all is well here. Of course we could have used the Diameter and Position tools to control the two cylindrical surfaces, but the Surface Profile tool works beautifully, so we leave it in place.

However, do you detect a problem created by the groove, which we failed to notice at the outset? In fact, the groove turns what used to be one planar surface on flange 1 into two on flange 2. The Flatness tool no longer cuts the mustard, because we need the two surfaces to be flat and coplanar. This requires adding a “2X” to reference the two surfaces, and replacing the Flatness tool with the Surface Profile tool, because only the Surface Profile tool can define two slab-like tolerance zones, which are locked together as a simultaneous set. The necessary changes to affect this are shown in figure 6. These include moving Datum Feature label A from the surface of A to the controlling Feature Control Frame, to make it clear that A is in fact both surfaces.

Step 7. Repeat Steps 1–6

Steps 8 and 9. Virtual Condition Assessment and Tolerance Value Balancing
The threaded bolt-hole pattern: Upon careful consideration, we realize that the bolts, once threaded into their holes, must of course clear the mating bores in flange 1. Let’s see if they will. Based on the controls imposed on the through bores in flange 1 (see figure 2), we can calculate the diameter of the region inside them, which can be guaranteed to remain material free. It’s the minimum diameter of the bores, 15.5 mm, minus the diameter of the Position tolerance zone of 0.5 mm, for a total of Ø15. To guarantee assembly, no portion of the surface of a bolt may fall inside that zone. As a result, the diameter of the material-free zone outside the bolt must be smaller than 15 mm. Looking at the controls on the threaded bores, we see we have a problem, because the MMC diameter of our bolts is 15 mm and the projected position tolerance zone diameter is 0.5 mm. This results in a material-free boundary outside the bolt shank of diameter of 15.5 mm, which is 0.5 mm larger than the material-free boundary inside the mating bolt holes. To resolve this problem, we could either select Ø14 bolts, or increase the nominal size of the bolt holes to 17. In the latter case, the material-free boundary inside the bores will be 16, leaving a comfortable 0.5 mm of breathing space for the worst condition bolts. Furthermore, Ø17 bores are easier to drill than procuring Ø14 mm bolts. Whew! Lucky we did this little virtual-condition test before we placed our order for the flanges. For all the finally revised, flange coding details, see figure 6 below.

Figure 6. Both encoded flanges side-by-side

Standing back for a final look
Reviewing what we have done here could bring many reactions. Two at least are worth thinking about:

  1. “Ya gotta be kidding! All that work for two little flanges that my machine shop would make based on back-of-the-napkin sketches?”

  2. “As a pay-off for all the time, I know I’m gonna get exactly what I want, the very first time around.

Let’s remember that GD&T is a risk-management tool. If there’s a risk that the designer has failed to understand what he or she is asking for, that could get expensive. If there’s a risk that we might not like what we get back from the machine shop around the corner, even though it meets our stated requirements, that could get expensive too. Of course taking all the time to decorate CAD models with GD&T is also expensive, unless you’re good at it, in which case can you afford not to?

Future articles
In our next Smart GD&T Technology workshops we plan to investigate the impact of GD&T on manufacturing, and coordinate metrology, and hope you’re still enjoying the ride!

Feedback on topics of interest
We were happy to have feedback from Gabriel Robb requesting further clarification of the explicit use of the RFS modifier (S). As he points out, Rule No. 2a, found in §2.8b) on p. 28 in the Y14.5M 1994 standard, permits explicit use of TZS and TZM modifiers (S) in conjunction with the Position tool only, in spite of the fact that it is no longer required and in spite of the fact that it no longer appears in the tools table in Appendix C p. 212. Oh, well! A slight inconsistency, yes, but one we actually like very much, because we feel that explicit use of the RFS modifier speeds the decoding of GD&T and heightens our confidence that the choice of RFS was intentional rather than accidental. Gabriel also points out that earlier Workshop articles contain drawings in which an explicit RFS modifier has appeared in conjunction with tools other than Position. Yes, we admit to committing such sins, and apologize, but often do so (ouch!) to emphasize our strong desire to see the standard changed to permit the use of explicit RFS modifiers wherever TZS and TZM modifiers may lurk, thus with Perpendicularity, etc., as well. While we are on the topic, it might be interesting to state the rule (not explicit in the Y14.5M 1994 standard) specifying where TZS (not TZM) modifiers are applicable, namely in conjunction with all geometry control tools with address feature symmetry components, such as an axis, a median line, a mid-plane, a median plane, or a mid-point, and nowhere else.

We also had feedback from Robert Burroughs requesting more insights into criteria for selecting datum features. In response, we hope Workshops 7 and 8 have provided some of this and will give thought to a more thorough discussion in the future. In case anyone is interested, this subject is treated in depth in Multi Metrics’ web-based system, e-GAD (electronic GD&T aided design).

Discuss

About The Author

William Tandler’s picture

William Tandler

William Tandler’s professional experience includes three years at the Hewlett Packard R&D laboratory in Palo Alto, CA, where he helped develop a quadruple mass spectrometer; five years with the laser manufacturer Coherent Inc. as international marketing manager; followed in 1975 by the founding of Multi Metrics Inc.

Tandler is currently a member of the ASME Y14.5.1 Mathematization Committee; a subject matter expert for Working Group 4 of the ASME Y14.5 standard focused on datums; and has been a contributor to ISO TC213 in the realm of datum reference frame construction.