Today I want to talk about a common definition for something we all talk about when the subject of GD&T comes up. I think it’s safe to say that the vast majority of people reading this series earn their livings on the floors of American factories. We design, machine, assemble, weld, measure, probe, form, punch, plate, paint and praise or curse the unimaginable number of “things” we all get our hands on during the day to day workings of our lives. We work in the various industries of our nation and world sometimes working to digest the big picture and sometimes focused on the minute details of the inner workings of things we can only see with the help of other machines.

What Constitutes a “Part” in GD&T

But before any concert of efforts can accomplish a Boeing 787 jumbo jet all our attentions must focus on a part. A part is the simplest form of usable geometry, upon which, we can focus. In this case the word usable can be argued with but for these purposes the term is apt. When I checked with Google on dictionary definitions for the word part, the only one of over 25 different definitions I found was for a component, that being something that can be attached or separated from a system. That’s pretty close to what we can use for a GD&T definition of a part. A part is an item, or two or more items, joined together, not normally subject to disassembly without the destruction or impairment of its designed use. Another wording for a part is the combination of two or more features that when separated defeat the purpose of its assembly. I like both. I have tried to think of a part that has only one “feature” or “item” but I have never come up with anything, which is not to say a one feature part does not exist, as I adhere to the saying that you cannot prove a negative. There are philosophers who are tricky at wording that say otherwise but I accept the fact that our universe is infinite and the term infinity points to a number large enough to contain any possibility. So I will accept that there may be things outside my imagination.

What Constitutes a “Feature” in GD&T

So, in GD&T, we have parts and we have features. Now we get to a level where much of our GD&T is practiced, on the feature level. A feature is a physical portion of a part. That includes holes, slots, bosses, sides, tops , bottoms, corners, chamfers and the list goes on. There are things that many of us might not recognize as features on our parts. The bottom of a hole can be a feature. The line where two planes of a part converge can be a feature; even if you don’t accept that a geometric line can be a feature, (although many people do,) because most modern drawings include statements about allowable inside and outside radii and chamfers. The corner of a part, where two planes converge, when it is radiused or chamfered, becomes a recognizable and definable feature. A datum, when applied to a part, becomes a datum feature, and with that in mind our GD&T system allows the definition of datums to include even a point of geometry. Now we have definitions that include and cover the lowest common denominators of our manufacturing world, parts and features.

With the previous topic, Rule #1, we discussed the need for a guiding principle to govern the need for geometry to follow rules that allow things to fit under all conditions, given that the proper engineering has been established for the condition being considered. That principal was stated as “size controls form.” In it the statement says that “at maximum material condition” the geometry of all features of size must have perfect form.    

What is Maximum Material Condition?

Maximum material condition, as it is stated or used in GD&T is a definition of limit not geometry. It is not the same thing as rule #1, it simply states the limit at which a feature of size achieves the MMC concept, which is, for any part that has in it or on it a feature of size, that feature of size is at maximum material condition when the part it is married to is at its heaviest, when it weighs the most. Put simply if a part has a hole in it that hole is a feature of size. The hole must have a tolerance associated to it by the fundamental rules of Y14.5. That part is heaviest; it weighs the most, when that hole is at the lowest possible end of its size tolerance. A part with a smaller hole in weighs more than the same part with a bigger hole in it. The hole is at MMC when it is at its smallest. The same principle is at work when a part has an external feature of size like a boss or protrusion. When the boss is at the largest end of its tolerance, it and the part it sticks out of are at their heaviest so external features of size are at MMC when they are at the highest end of their tolerance.

How Is MMC Utilized?

MMC is also used as a modifier in GD&T. Modifiers are a way engineers can add information to tolerance statements. When MMC is used as a modifier it adds a layer of complexity to the outcome of the manufactured part. The MMC modifier provides a choice for the maker to decide whether the values of some features, when chosen at a certain tolerance level (ultimately, tolerance choices, for all features and features of size in a given part, must be made,) can affect a value in the manufacturing cost of a part. These modifiers allow everyone in the manufacturing chain of events to make choices that lend the most quality, longevity, value and profit to the organizations that make those choices wisely.   

Today our featured GD&T term is “feature control frames.” We at Metalcraft decided some time ago to put together a glossary of sorts. It will be alphabetized in proper fashion but as various terms are added they will not be added in A to Z fashion, but more in order of importance, or more importantly, how we feel that day.

Approaching GD&T As An Outsider

 As we’ve explained before learning GD&T is like learning a new language. First of all it is composed of an array of symbolic characters that make up a system of mechanical drawing engineering controls. The system serves our world to make the task of engineering definition more precise, more specific and as abbreviated as possible without suffering loss of detail.

I begin today’s term with this synopsis of our wider subject because feature control frames play a specific role in the administering of our symbolic controls, not the only role, but probably the most important one.  Take a moment to think about our world as the conglomeration of all the physical amenities and goodies we dote on and ride around in and pay a great deal of our attention and money to.  We have automobiles, airplanes, computers, cell phones, appliances, implements of social enforcement and the list goes on. These things are the focus of our human energy and they all require engineering. They require more than engineering but our focus, here and throughout our sphere of influence, is making these things come to fruition and at the very heart of this effort, we believe is the proper use of advancement of GD&T.  

What Are Feature Control Frames?

And so we are here to define the feature control frame. The feature control frame (FCF,) is what I like to call the engineer’s statement. It is a method that arranges a number of our GD&T symbols to say something. Actually a properly formatted feature control frame makes a demand; it states an instruction about a feature or feature’s relationship as a constituent of a part. Here is an example of a simple feature control frame.   The way we should read this symbology is like this:Or in words the engineer is stating to “ hold position within a cylinder shaped tolerance zone to .03 inches at maximum material condition relative to datum A.”

How Do Feature Control Frames Work?

This frame has three compartments. The first is the characteristic control compartment. It communicates the type of geometric control the engineer wants to apply to the feature or features. The second compartment is the tolerance compartment. Along with the value of the tolerance the engineer is applying to the control it can and often does have a number of symbols in it. The reason for that is because controls are often more complex than just stating that there is a tolerance to be held, like + or – some value. In GD&T tolerances are 3 dimensional. Some people may disagree with that because there are a number of characteristic tolerances that have both 2D and 3D versions of a control, like the profile tolerance. There are 2 versions of that characteristic tolerance; there’s profile of a line and profile of a surface. Profile of a line is considered a 2D control but it actually controls each line item of a subject surface, so the way I look at it, its effect on a surface is 3 dimensional. It affects control on the whole surface.

Tolerance and Feature Control Frames

So now back to the tolerance compartment of our feature control frame. The frame above first has a diameter symbol in it. That is known as the tolerance zone descriptor. In this case it states that the zone for the position control will be a cylinder. Next there is a value for the tolerance zone. In this case it’s .03, which means the cylinder, within which the axis of the feature being controlled will be contained, will be no more than .03 units of measure or in this case inches.

Condition Modifiers (Maximum Material Condition)

The last thing in the tolerance compartment may be (not all FCFs have one,) a condition modifier. In this case the feature condition modifier is maximum material condition (MMC.) This modifier adds a layer of complexity to our statement. The engineering behind this position control allows tolerance on both the controlled items in our FCF but also the items in the parts or assemblies that will become assembled to this item.

Flexibility Matters

Flexibility is a necessity in manufacturing so as complex assemblies make adjustments to suit the people, tools and conditions of their maker’s the suitable circumstances for how these parts and mechanisms fit together allow choices in the processes. If a part with three fastening bolts has to assemble with a bracket that has three corresponding holes for the bolts, the largest allowable holes in the bracket naturally allow a greater range of position control for the placement of the holes that will still allow the bolts to pass through them when the two parts get assembled. So when the bracket is manufactured with the 3 holes in it and those holes are chosen to be bigger than smaller, the position of their placement becomes easier and hence less expensive to make because it is true that tighter controls usually cost more money.  

Tolerance and Feature Control Frames Continued

Take a deep breath as we head back to the feature control frame and our tolerance compartment. It has a zone descriptor, a tolerance value and a feature condition modifier. The modifier area of the compartment can have more complex condition modifiers with modifier zone values, modifier zone placement symbols and values and more. But that is for another time. If there is no material condition modifier the rule is that RFS is the default condition. Regardless of feature size means that the stated tolerance is to be held no matter what the controlled features measure.

The Datum Compartment and Feature Control Frames

The last compartment in our FCF is the datum compartment. Sometimes there is no datum compartment and sometimes there are a number of them. Theoretically there can be any number, depending on what the engineer needs. I’ve seen parts with dozens of datums so it depends on the circumstances. Another thing that might be found in the datum compartments is a condition modifier for a datum, also known as datum mobility. It shows up with the datum symbol as m, which in this case is not a maximum material condition modifier but a maximum material boundary modifier. When a symbol like this accompanies a datum that datum must be a feature of size and when used to advantage this modifier allows “datum shift” when calculating all the factors that make up the position mathematics. The datum feature of size is treated much like a feature at MMC; as it departs from MMB the actual datum feature can be arranged or shifted within the bounds of its size to see if a feature position can squeak into tolerance when this is used. In our GD&T language of symbols feature control frames are the engineer’s statement of tolerance.

Rule #1. In the world of engineering drawing there is an infinite amount of complexity to be detailed. All over the world there are parts and mechanisms that are dreamed up every day and for many of them there is a need for the assurance that a particular design will be flexible enough to work in either a specific duty as an assembled part or as a part that can be used over a wide category of common applications.

Commonality is Key

Whether a designed part is born in Mexico City or Vancouver Canada for it to perform its functions successfully it must have commonality with its intended uses or it will not be functional.

This means that a 14 mm bolt made in France must fit and assemble with a 14 mm nut made in Brazil. This task seems not too difficult but for these things to function, with all the possibilities of our physical world,  there needs to be an overarching concept that guarantees that all the possibilities can and will sing from the same hymnal.  

Size Controls Form

That concept is Rule #1. In its simplest iteration it means that “size controls form.” If we dig into what this means we see that no matter what geometry we might be analyzing there is an approach that says when something moves from one point in its tolerance to another, it must follow the concept that demands it to be more and more perfect in its form as it becomes closer to its maximum material state. That simply means for a part to change from one level of tolerance to another, in one direction it gets lighter and in another it gets heavier. Maximum material condition is when a part is at its heaviest and at that point Rule #1 states that it must have perfect form. Size controls form.

Perfect Form – Rule #1 in Action

To illustrate the value of this we can look at a simple metal shaft. It is designed to be 1.0” in diameter and it is 5” long. Those qualities, its diameter and length are given tolerance or we couldn’t make it. We couldn’t make it because we can’t make perfect things. Now back to the shaft. If it is made at .990” diameter and roughly straight then even when it is made in Barcelona it will fit in a 1.005” hole in a cast housing made in Atlanta. But!!! If it is 1.000” diameter and it is .005” NOT straight, not in perfect form, then it won’t go in that 1.005” hole. It will hang up because it does not follow the rule.  Whether a geometry feature is internal or external, rule #1 will affect the possible assembly criteria of a design in the same way, anywhere. A design at its heaviest must have perfect form. Rule #1 assures form, fit and function.    

Actual Mating Envelope – The Basics

Actual mating envelope is a term that refers to a machined / manufactured part’s feature of size “as is” condition. To define what that means specifically, we need a specific individual part to talk about. The reason for that is when you have a part, or bunch of parts, that are manufactured to be alike, the fact is that for any given feature of size, part number one of any number of parts won’t likely be exactly like any other.

 The outcome of the qualifier “won’t likely” will depend on the severity of how the word exact is applied to discriminate between the items of the sample. I believe these words are true but we can get caught up in semantics so let’s take a close look.

Unique Features and Mating Envelopes

 In GD&T we accept the premise that nothing can be made perfectly. It’s the reason we accept no condition as viable that doesn’t have an associated tolerance as an engineering definition. When we make parts, when we make a feature of size to a specification, each part in whatever the line of parts happens to be is made uniquely. Each pc of material has its unique properties. Each machine cycle, handling operation, plating application and even down to the time of day has its own unique effect on what we do.  These unique influences cause parts to have unique difference and this is where the actual mating envelope of a feature of size becomes important. When an engineer specifies a geometric tolerance to be held, such as the position of a hole, they are specifying a requirement that needs to conform to a complicated set of circumstances.  In this scenario we are imagining a hole, toleranced with position, to other features. The hole, when machined to its proper size has a unique shape, at a unique size, so in order to establish where it (the hole) is, we need to have a method to identify where its axis is or in the case of a feature at MMC, where its surface is.  That’s where we assess what the feature’s actual mating envelope is. It’s a method used to determine what the unique shape of a feature is that relates to the axis or surface of the feature.   

Determining the Mating Envelope of Unique Features

 The description common in the industry instructs the course of action to be “for an external feature a cylinder, larger than the feature, is placed around that feature and contracted until it stops on the external high points of the feature. It is assumed that the actual local size of the feature has already been assessed to meet the feature of size (FOS,) criteria. When the contracted cylinder stops on the high points of the feature it is now considered to be its actual mating envelope (ALS,) and the axis of that ALS is then considered to be the axis of the feature of size for determining its adherence to the tolerance requirement. In the case of a FOS at MMC or LMC, where the surface interpretation is used to assess the tolerance, the AME itself can be used to determine the conformance.  For internal features, like simple holes, the same process is used only in reverse. A cylinder is expanded on the ID of the feature and when it meets the high points of the internal surface it is said to be that FOS AME. Using the axis or surface of the AME is chosen like the above scenario.

Visualizing The Mating Envelope

 Another way to look at an actual mating envelope is a way I teach it in the college classroom and the corporate classroom. I explain that a person uses a “magic balloon.” My magic balloon is a virtual cylinder that can be expanded or contracted at will like using the dynamic zoom tool on a computer screen. You can mouse to the balloon, click and scroll up or down and the cylinder grows or shrinks in response to the mouse. You can place the magic balloon inside a hole or outside a pin and magically expand or contract it until it hits the coinciding high points on the surface of the FOS and when that happens the balloon becomes the actual mating envelope of the feature.    Now there is the complication of the phenomenon with the condition of the AME being either related or unrelated. This merely means that any feature has an actual mating envelope but whether the feature is a machine dowel pin lying on a table or that same pin is pressed in place in a part designed to mount a precision circuit board in place in the chassis of a fighter jet defines two different values.  The pin lying on a table can have a magic balloon contracted around its OD and the mating envelope is simply the AME of the pin by itself. However, when the pin is being assessed as a component of the fighter jet chassis its AME is not just meant to replicate the pin; it’s meant to assess the pin in its place in the chassis assembly.  Let’s remember that nothing we make in our shops is perfect so when we consider the pin hole position machined in place in the chassis it is going to be some dimension different from the basic dimensions that locate it and at the same time it will be less than perfectly perpendicular to the primary datum it is assigned to. Since the hole is not perpendicular, the magic balloon expanded into its ID, (which will be the resultant AME of the pin,) needs to be corrected for.  The relevance of the hole is assessed for its position in three dimensions and the related actual mating envelope of the feature is what needs to conform to the tolerance.