Today’s manufacturing world is full of challenges and opportunities. Our shop technicians change hats hour by hour to download programs and accomplish their setups. Among other tasks, they pull material, load tools, and edit and rewrite code. They measure, adjust, and change offsets. They fill out their in-process inspection data and submit the first pieces, and that’s all before lunch.

On top of the many things of which our technicians must remain knowledgeable is the practice and translation of geometric dimensioning and tolerancing (GD&T). GD&T is a vernacular all of its own. It’s used to specify and control the definition of hardware geometry coming from the world of ideas and concepts to bring those projects into the real world of shops with chips and scraps of metal on the floors and hot summer days and changing work shifts.

Learning GD&T is like Learning a New Language

GD&T is not something a person’s intuition guides them through. It’s a set of rules and procedures that demands you to do it right or don’t do it at all. 

A person cannot learn simply how to read where a dimension begins and ends. GD&T is a complicated system of geometric symbology, a system that can describe, to any degree, the limits of form, fit, and function of literally an infinite set of physical circumstances. To that effect, once learned, GD&T becomes invaluable.

GD&T Requires Ongoing Education

We, as a company, try hard to “know what we don’t know,” but Metalcraft Industries exists in a world where GD&T often calls the shots. GD&T requires us to reassess drawing interpretations, before, during, and after parts are made on the shop floors.

Our shop personnel, engineers, and quality-control personnel participate in a company-wide training program to learn and practice the skills needed to read and interpret these controls and then how to apply it to their shop-floor practices.

Changes to GD&T Create Challenges

One of our biggest challenges is sorting out GD&T problems that make their way onto our estimating desks and sometimes onto our machines. We find that a good portion of the drawings that come through our door have GD&T issues and sometimes outright mistakes on them. 

Engineers that use GD&T in their designs and shops that make the hardware all must adjust as it changes, so we can all profit by a closer study of its application. 

Partner with Metalcraft

We’ve equipped our facility with the most diverse selection of machines in Colorado. Our engineer-minded machinists stay up to date with the changing industry, from GD&T to software, and beyond. Contact us for a quote, today.

This month’s installment of our definition of terms will be about characteristic symbols. These symbols are what most people in the manufacturing world recognize as being what GD&T is all about and, from one perspective that’s true.

Those Weird Hieroglyphic-Looking Things

When the average worker in our manufacturing world first sees an example of GD&T, it is likely they find it on a drawing. It’s much like the other drawings they’ve learned to interpret, only this one has some weird hieroglyphic-looking things mixed in with the normal lines, arrows, and numbers that they are used to. Upon seeing the hieroglyphics, they might look to their left and right to check if anyone is watching them, then nonchalantly mosey over to their computer to start Googling.

Target Thing = True Position

They might be thinking to themselves: “What the heck is this target-looking thing?” After some scrambling, they will find the “target thing” means “true position.” And so it begins. Our average worker who has entered the world of characteristic symbols now knows the term, true position.

Quality, Quantity, Variety, and Specification

Geometric dimensioning and tolerancing is a system designed and utilized to control, manipulate and enhance geometry. Words like “cylindricity,” “angularity” and “flatness” describe geometric qualities and those characteristic symbols represent a set of values or qualities that cover the extent to which our manufacturing and design world needs to give these things quality, quantity, variety, and specification.

There are 14 characteristic symbols in both our American Y14.5 system and the ISO system used prevalently in Europe and other countries. However, there are still some minor differences in how they are defined and utilized. The definitions in this blog are Y14.5 specific.

14 Symbols in 5 Categories

If we look at the graphic below, we can see that the 14 symbols each have a characteristic definition and they are categorized into five different types. These characteristic definitions and types are arranged so that the features on manufactured parts can be controlled for their shape, angular orientation, location, and dynamic relationships to other features and parts.

Just to keep our terms in perspective, let’s remember from previous posts that a “part” is a collection of features that cannot be disassembled without destroying the purpose or function of its original construction and a “feature” is any physical portion of a part. Features can be, among other things, edges, faces, holes, bosses, slots, cones, and spheres.

A Symbolic Engineering Language

Now, let’s revisit the definition of “GD&T.” GD&T is a symbolic engineering language (as in a system of communication used by a community of people) for defining geometry and communicating tolerance. Symbols are marks or characters used to represent objects functions or processes. In other words, symbols are a way to synopsize a volume of information. Symbols use a small amount of visual data to convey a package of meaning. They are the proverbial pictures worth a thousand words.

So our characteristic symbols give us an approach to the nuts and bolts of geometry. If something needs to be flat, there is a symbol for that. Its definition is specific for that characteristic, both in the way it applies the definition of perfect geometric form to real-world features and where it applies them. It also conveys in what proportions it applies tolerance to every conceivable possibility we engineers can imagine.

 

Introduction to Characteristic Symbols  

So, we have these 14 symbols to learn about and, as a part of this introduction to the characteristic symbols, I will start with one of them and try to establish a format that we can follow as we go through them. As I said before, each symbol has a definition of what it controls and each symbol will also fall into one of five categories.

Flatness

The first symbol we’ll study will be flatness. Flatness is a form control. Form controls are about shape and uniformity. Shape is shape. We relate to shape in the things we see. Balls, tree trunks, hot dogs and parking lots show us shape and we relate to shape when we learn to differentiate between things.

Uniformity

Uniformity is about consistency and conformity. It gives us more information to distinguish physical quality. How round is the ball? How flat is the parking lot? So our first characteristic deals with form and we’ll learn how to define how it’s used through how uniform any example of its use is required to be.

Planarity

Flatness is also planarity. Planarity, in geometry, is any surface in space of which two points are chosen (any two) and when these points are connected by a line, the line will be entirely in the plane surface.

Flatness, in engineering terms, is any surface (of points) in 3D (X, Y and Z) space that exists entirely in two of those dimensions, such as X and Y, but not in the third (Z).

Flatness Tolerance

The flatness tolerance can be applied to any surface by attaching the tolerance to the extents of it and applying a tolerance or definition of its allowable imperfection. That imperfection will be stated as a value of distance or space lying between two parallel, theoretically perfect, planes disposed about the plane in question (see the next graphic taken from the ASME Y14.5-2009 standard.)

Our example shows a perfect part designed to be constructed as in the upper frame. Then, it shows what it can look like with allowable imperfections in the second frame.

Our flatness tolerance is a form tolerance, one of four characteristic symbols that belong to the form tolerances. Since the shape of something can be defined as a standalone value form tolerances do not require datums in their feature control frames.

The flatness tolerance may be applied to either features or features of size. When it’s applied to a surface the evaluation is made of the space between the two parallel planes as shown earlier.

Regardless of Feature Size

When it’s applied to a feature of size, it may be applied at RFS (regardless of feature size) in which case the tolerance zone is two parallel planes disposed about a derived median plane assembled from the midpoints of the parts actual local size geometry and the outer boundary must be equal to or less than the part’s high size tolerance plus the flatness tolerance. (See graphic below from the ASME Y14.5-2009 standard.)

Maximum Material Condition

Flatness can also be applied to a feature of size at MMC (maximum material condition) in which case the tolerance is assessed using the surface condition or virtual condition of the effects of the tolerance. The method uses a fixed gauge which allows for the combined value of the flatness tolerance added to the size of the feature at MMC. (See graphic below from the ASME Y14.5-2009 standard.)

In future posts, we will examine the remaining form tolerances and move on to the orientation, location, profile, and runout tolerances.

This month our definition of terms will look at regular and irregular features of size. Looking back we can see that a previous installment of our “definition of terms” looked at “features and features of size” which discussed features of size as they relate to parts and our larger scheme of geometric tolerancing.

This discussion will delve a little deeper into these features of size to differentiate between how these critters can be classified and used. There is a fairly wide variety of opinion about this in the GD&T user’s universe so I will try to enumerate some of the translations and practices we all use. First of all we should start with our ASME Y14.5’s definitions. A regular feature of size is said to be “one cylindrical or spherical surface (or circular element,) or two opposed parallel elements (or parallel surfaces,) associated with a directly toleranced dimension.” Now we need to consider another term that affects the results of our discussion and that is “size.” Both the terms regular feature of size and irregular features of size actually depend on how the word “size” is defined. This is fairly straightforward.

The vast majority of users accept that this identifies internal and external cylinders, planar thicknesses, and certain forms that have opposed parallel elements or surfaces, which all have a directly associated size dimension. Now before we go any further with this let’s look at our standard’s definition of an “irregular feature of size,” which is defined as two types: Type (a) “a directly toleranced feature or collection of features that contain or may be contained by an actual mating envelope that is a sphere, cylinder or pair of parallel planes.” Type (b) “a directly toleranced feature or collection of features that may contain or be contained in an actual mating envelope other than a sphere, cylinder or pair of parallel planes.” As we said earlier this is also fairly straightforward. By the words in this definition irregular features of size include those that are not spheres, cylinders or parallel planes, that are either internal or external features that can be contained by an actual mating envelope.

By now many people get rightfully weary of all the words, and I care about that but can do little to simplify the discussion because I study this stuff a lot and what is and is not acceptable hinges on these definitions. And the truth is many people agree that the Y14.5 standard has gaps in how things are defined and can be translated. I suppose if everything in the standard was clear and everyone agreed on its contents they wouldn’t need new updates. So here we might stop discussing words for a bit and look at some pictures that are worth a thousand words. The following graphics show examples of what most people accept to be regular and irregular features of size.

Regular features of size 

 

Irregular features of size, type A

Irregular FOS Type B

Now let’s go back to our Y14.5 definitions: #1 is a direct quote from the standard under Regular feature of size, whereas #2 is a direct quote, again from the Y14.5 standard for irregular features of size.

1. “one cylindrical or spherical surface (or circular element,) or two opposed parallel elements (or parallel surfaces,) associated with a directly toleranced dimension.”
2. “a directly toleranced feature or collection of features that contain or may be contained by an actual mating envelope that is a sphere, cylinder or pair of parallel planes.”

Looking at them this close together allows us to dissect their differences as concisely as possible to determine what the differences are between both number one’s and number two’s meanings. “Apparently there is only one difference, which is for regular features of size only one allowable directly toleranced cylindrical or spherical surface or element or two opposed parallel surfaces or elements. And for irregular features of size there can be one or a collection of directly toleranced cylindrical, spherical or parallel features of size.” Breaking this down even further we can say that there can only be one regular feature of size that fits the definition but there can be one or a collection of irregular features of size that conform to the definition. So going one step further is it true that regular and irregular features of size can be the same if there is only one of them but their definition separates when irregular feature of size become a collection? This seems wrong or at least ambiguous. Ambiguous meaning unclear or inexact because a choice between alternatives has not been made. If that’s true then let’s look at some examples of what people in our field call irregular features of size. Alex Krulikowski, in his Fundamentals of GD&T, says that an obround, such as the one below, fits the definition of an irregular feature of size. The two linear sides of this shape can be contained in a pair of parallel planes but that ignores the two half cylinders on either end of the feature. Is this OK? This is where I start to get off the bus. The definition in the standard says “may be contained by an actual mating envelope.” Well let’s look at the graphic on the previous page that shows an obround slot inside a circle. Isn’t that the same thing as above where this irregular feature of size, which is a collection of features, no it’s not a collection of holes or squares, it’s 2 radii or half cylinders and 2 opposing elements or planes, is meant to be a viable feature of size. We can contain it by contracting a circle around it  but the parallel elements or planes of its long sides are really not relative to the mating envelope, except that they are inside it.

How about this collection of features?

Or this one?

They can both be contained by the same mating envelope as the obround we discussed earlier. So where does that leave us? Back to the words of the definitions in the standard. Irregular type A can be contained by a mating envelope that is a sphere, cylinder or pair of parallel planes and B can be contained by a mating envelope other than a sphere, cylinder or pair of parallel planes. So what is an “other?” And if we think of a way to properly define “other” how can we agree on how it should fit the feature? Below we see two pentagons. If they are proper type B irregular features of size, they must be contained by a mating envelope that is other than a sphere, cylinder or pair of parallel planes. The concave example, if it can be, can be contained by a really weird shape of envelope. The convex example can be contained by an ellipse maybe? The first is not very conventional while the second is more conventional. Is this what the standards committee intended? I’ve spent more than a few hours Googling and looking for astute opinions on this subject because I teach GD&T and my students usually like static answers to the subjects we study. By static I mean ones that don’t wiggle around. So one thing I read about the subject of irregular features of size simply said Type A features are of a sort that can easily be constructed and then be contained in “envelopes that are spheres, cyl…yadda-yadda.” Things like triangles and symmetrical pentagons. While “type B irregular features of size” are everything else. I want to argue with this, but based on the definitions I can’t. The phrase “contained in other than a sphere, cylinder or pair of parallel planes” is succinct. If we accept that the word other rules the alternatives then other means every mating envelope that’s not a sphere, cylinder or pair of parallel planes, which includes all possible envelopes. So have we nailed it down? I can easily say No! There are going to be people who say that by the way they read the standard that some part of this is way off base. I’m hoping someone does because I don’t get a lot of response to the words I write for this blog and even words of disagreement will be welcome. The fact is that I think the wording in the standard leaves room for “wiggling.”

I’ll close with a comment I read last week made by an ASME “Senior Certified” member of our GD&T community of users. He said that he passed the Senior level exam on the first try, with only one mistake. Words with which he meant to impress, and I was.  He was recommending that prospective aspirants to the Senior Certification process study the ASME Standard not the vast array of engineers and specialists that give their opinions of what the standard says.  I can’t argue with that other than to say I consider myself to be well educated. I write a lot. I teach 6 college subjects at a specialized community college manufacturing program. I’ve written 3 novels. I work a full time job as a manufacturing engineer and mechanical parts and assemblies estimator. I know this sounds like a buildup to “you should listen to me because.” But it’s not. I have and use my well worn and dogeared copies of the 1982, 1994 and 2009 Y14.5 standards and one thing is abundantly clear to me. I have to read nearly every paragraph in them at least three or four times and often over a period of days to confidently digest what they say. I have to pour over diagrams and examples and I often copy a paragraph on page 34 and tape it beside one on page 284 so I can compare what they’re saying.  And following that I spend time wondering if I really understand what I’ve translated. So I’ll close with a thank you sent out to all the brave members of the standards committee and all the people out there that work hard to promote the adoption of this system we all use. These thoughts are my interpretation of the subjects discussed and I’ll bet I end up with more than a few new perspectives on them in the future.     

In our last blog post, we looked at the six common tolerance zones that appear in today’s engineering drawings. While these six make up the broad majority of tolerance zones in geometric dimensioning and tolerancing (GD&T), you may encounter another zone in a given tolerance scheme—the spherical tolerance zone.

The American Society of Mechanical Engineers (ASME) Y 14.5-2009 standard, paragraph 7.4.6, outlines the use of location tolerancing with spherical diameters stating that a spherical diameter symbol precedes both the feature and the positional tolerance value (shown in fig. 7-35 on page 127 of the standard.)

A spherical diameter feature’s center is identified with a point, which engineers can use as a datum or simply the location of a part feature. The illustration below depicts a 2” diameter spherical feature called out as a spherical diameter with a positional tolerance imposed to datums D and P. 

Source: University of Florida

The example below shows a part with a spherical feature with its location controlled by the feature control frame to a .2 mm spherical diameter tolerance zone at least material condition.

 

Source: University of Florida

Additionally, this example shows the feature itself identified in what may be a basic dimension box and the dimension locating the spherical radius of the feature is shown as 40mm with an applied distance tolerance, which may interfere with the geometric position frame. 

The Y 14.5 standard also explains that a sphere’s location can be controlled with other than a spherical diameter zone if the spherical feature needs to be located in a bidirectional zone (shown in Fig. 7-28 on page 124 of the standard.)   

You can use the same formula for regular feature locations as you can to solve positions involving spheres, holes, and bosses, only using 3 deviations to sum the squares instead of 2, such as in this formula:  2* √(A2+B2+C2). 

According to David Slopsema, head of Dimensional Consulting,

“You just need to find the true distance between the actual position of the feature and the true position.  That distance must be smaller than the radius of the spherical tolerance zone.”

Source: David Slopsema

You may notice the formula above is missing the 2X multiplier, but Slopsema’s comment about the radius of the spherical tolerance zone explains its interpretation to be half of the diameter zone.

Slopsema also wrote Dimensional Consulting’s Free Online GD&T textbook. This entertaining is very easy to understand and we recommend it to anyone looking to learn more about GD&T. 

GD&T is a convention that gives the engineering world a tool that enables designed components to have both an unlimited variety of both geometric definition and tolerance. Consequently GD&T is about many things but it is ultimately about the disposition and appraisal of tolerance zones.

This month our definition of terms will be about tolerance zones: what types there are, how they’re arranged and the different ways engineers give them value.

We can get started with the zone called “2 parallel planes.” I’m beginning with this because it’s a very basic zone and is used as the default for many tolerances.

We can see a graphic depiction of it here.

2 parallel planes describe the volume of space used to control things like the orientation of planar surfaces (angularity and perpendicularity etc.) It can also be used to control form, as in flatness or symmetry, as in the location of a planar feature like a slot or a rectangular boss. The runout tolerance uses parallel planes to control dynamic feature tolerance situations, which manifest as the wobble of planar surfaces perpendicular to axes of rotation.

There is also a version of 2 parallel planes used for complex features like the shape of an airplane wing or a complex cam surface as shown below.

The 2009 Y14.5 standard expanded the allowable arrangement of this tolerance zone to include values disposed on part profiles either on the inside or outside or any combination of both, as shown below.

This version of the 2 parallel planes tolerance zone (used in the surface profile tolerance) is considered to be the most powerful of all the characteristic controls in that it can be used to simultaneously control size, form, orientation and location in one control.

The 2009 standard also gives engineers the ability to change the values of the zone in non-uniform ways to create an almost infinite variety of shapes and conditions.

Now, we’ll look at our next zone which is a cylinder. A cylinder is also a common tolerance zone in GD&T. The volume inside a cylinder is used to define the allowable deviations of the axes of features of size like holes and cylinders and cones.

These zones are used to control location, orientation, and the extent of derived median lines used in concentricity and straightness.

 The first look at this zone is like the following graphic, which shows a cylindrical part with a tolerance zone for its datum axis and a cylindrical feature axis as it deviates in its allowable “cylinder” of volume.

The cylinder is used to apply a tolerance zone to location tolerances and the volume inside the cylinder becomes the area that the axis of the feature is allowed to roam while remaining in tolerance.

The graphic above shows an internal feature of size with a cylindrical tolerance zone applied to control its location, shown as the 3 dimensional purple zone in which the hole’s axis must remain.

Our next tolerance zone is called 2 parallel lines. This zone controls two dimensional elements in straightness applications. These controls are limited to the surfaces of cylinders and flat surfaces or derived median planes. The control of flat surfaces is limited to lines parallel to the longitude of the profile view of the callout as shown below.  

The control of the straightness of cylindrical elements is done by controlling either surface elements of the cylinder or by controlling the axis of the feature of size as shown in the graphic below; note the diameter symbol in the control frame of the second example.

Our next zone will be coaxial circles. There are several controls that use circles for tolerance zones. This type of zone is for surfaces and is 2-dimensional. Circularity, runout and possibly profile are controls that use this type of zone. There are two versions of this zone, one of which can only be used in runout.

The first type of coaxial circles tolerance zone is used in the circularity and runout controls and is identified as coaxial circles offset radially. They can be disposed on a cylinder, along its axis and control the specified zone of  circularity or runout for a 2-dimensional cross section of the cylinder at any point along its length.

The graphic below shows a cylinder toleranced with circular runout to .1 mm and illustrates the .1 mm tolerance zone after taking a cross section, perpendicular to its axis, through the surface of it. This shows the coaxial circles offset radially.

The next graphic shows the axis of datum A-B with two sets of coaxial circles offset axially along that datum, spaced at a .5 mm tolerance. These circles would represent tolerance zones for circular runout taken at two locations on a plane perpendicular to the axis of rotation of A-B.

The last zone in our discussion is known as 2 coaxial cylinders. It’s a common zone used in form, profile and runout controls. This zone is also a surface control and limits the deviations of controlled surfaces to lie within the specified value needed for the application such as is pictured in the graphic below, shown in grey.

So to recap, we have looked at the following 6 zones.

1) two parallel planes.

2) the cylinder.

3)  two parallel lines.

4) two coaxial circles, both offset radially and axially.

And 5) two coaxial cylinders.