Geometric Dimensioning and Tolerancing (GD&T) is a key part of the reverse-engineering process. It gives manufacturers and engineers a common language to communicate the function-relevant qualities of a part, from its basic dimensions to feature sizes. But, it can be an arcane concept for some. What’s more, it’s easy to overdo the analytical process and drive up manufacturing costs if a manufacturer doesn’t show restraint and wisdom.
This article will open by giving a detailed introduction to Geometric Dimensioning and Tolerancing and its relation to engineering tolerances. From there, it will explain the importance of allowable variances before going into further detail about the technicalities of GD&T. Afterwards, we will discuss the use of GD&T in the process of reverse engineering, outline how these measurements are made, then explore the conflict between saving customers money and producing a functional part.
Geometric Dimensioning and Tolerancing (or, GD&T)
Geometric Dimensioning and Tolerancing (hereafter referred to as “GD&T”), is a system used in a variety of engineering-based industries to define and communicate engineering tolerances in engineering drawings and models. As such, to understand it properly, you need to understand the concept of engineering tolerances.
An Engineering Tolerance is a permissible limit/limit of variation in a quality of a part or a machine. The qualities measured include: physical dimensions, feature sizes a value of a physical property of a material, things like temperature and humidity, and others. For our purposes, we’ll mostly be focusing on dimensions and physical properties, as they’re the most important in the field of manufacturing.
Now, why is it important to quantify all of these qualities?
Well, the fact of the matter is that it’s impossible to craft something “perfectly” or produce a “flawless” replica of a part. While these days most parts are originally designed in a CAD (computer-aided design) program like Solidworks with theoretically exact measurements, you can’t carry that perfection over to the real-world part. Everything will have some degree of variance away from is theoretically perfect measurements. An engineering tolerance is an objective value used to measure how much variance a part can have while still maintaining proper or ideal function.
Allow us to make a quick interruption: The primary concern when outlining engineering tolerances is figuring out how wide a tolerance can be without affecting other factors or the part’s intended process. Keep this in mind as we move forward, as it’s crucial to understanding how cost-cutting considerations factor into the use of GD&T.
As we said before, GD&T is a system used to define and communicate these engineering tolerances. It functions as its own contained language used to describe the allowable variation from the nominal geometry shown in CAD models. It can be also be used to describe the theoretically perfect geometry of these parts, but for our purposes, we’ll focus on its use to outline allowable variations.
Before we go into the system itself, it’s necessary to note that there are myriad standards used to describe the symbols and rules used in GD&T. For the purposes of this article, we’ll focus on the American Society of Mechanical Engineers (ASME) standard.
The ASME outlines some fundamental rules that must be applied when using GD&T. It would be pointless to list them all in detail, here, but we will give a brief description of those relevant to this article.
1. All dimensions must have a tolerance given
2. These tolerances define the requirements of finished parts.
3. Dimensions should be applied to features in a way that represents their function, and not be subject to more than one interpretation.
4. Dimensions and tolerances are valid at 20 C unless explicitly stated otherwise. Likewise, the dimensions and tolerances are only considered valid when the item is in a free state.
Just a brief glance at those rules reveals that this system is designed to be thorough, repeatable, and function-focused.
When used properly, GD&T will ensure that a part in question has the desired form and fit, and will function within the largest possible tolerances. Again, take note that we are looking for the largest possible tolerances, meaning as far off the nominal mark as the part can get while still functioning properly. We are not looking to get the part as close to “perfect” as possible. We’ll discuss this further later in the article.
Now, GD&T measures fourteen geometric characteristics (called part features, each with their own feature symbol) grouped into five tolerance types.
Form types denote the “shape” of surfaces. Qualities measured are:
Orientation types denote the orientation (tilt) of surfaces. Qualities measured are:
Location denotes center points, axes, and median planes, and locates surfaces. It also controls orientation, size, and form in some circumstances. Qualities measured are:
– Profile of a Surface
– Profile of a Line
Runout controls surface coaxiality (the sharing of a common axis). Qualities measured are:
– Total Runout
– Circular Runout
Profile controls location derived median points. Qualities measured are:
Used properly, GD&T will determine the part features most important for a part’s function and give a number that defines their maximum allowable variation from their theoretically exact model. Which brings us to an important question: How, exactly, is GD&T done?
The Process of GD&T and Its Use in Reverse-Engineering
While complicated at first glance, the theory of GD&T is simple and almost intuitive once you take the time to think about it. But, like any methodology, it becomes far more complicated when you get to applying the process. This is doubly true when applying it to something like reverse engineering, a task faced by any aftermarket parts manufacturer.
What makes it so complicated?
The answer is the struggle to find the maximum allowable variation in a tolerance or part feature. Allow us to explain.
Any time an aftermarket manufacturer is asked to make a part, they are faced with the need to reverse engineer it. They don’t have the original design specifications, so they have to start with what they have. What’s more, they often have very little to work with.
The best-case scenario is that a part is sent to an aftermarket manufacturer with details on how it fits into other parts (or maybe shipped along with the other parts in the assembly), notes about the part’s function (which, as you’ve noticed, is the key concern with GD&T), and even a parts book to help the manufacturer with the reverse engineering process. Unfortunately, aftermarket manufacturers are rarely this lucky. More often, they’re shipped apart without context and are lucky to have a parts book to look at.
While it would be easy to make a rough copy of the part, the aftermarket manufacturer needs to ensure that the part functions properly. As a matter of fact, many aftermarket manufacturers pride themselves on producing a part that exceeds the quality of the original. How do they do that?
Well, GD&T, of course!
When a manufacturer receives a part, their first goal is to use everything at their disposal to determine its function. They want to know the purpose of the part, and everything relevant to that purpose, and they’ll look at parts books or mating parts to determine how it’s used. But, determining the part’s function is not the end goal. Rather, they want to determine which tolerances and part features have the greatest impact on the part functioning properly.
To do that, manufacturers use a variety of tools at their disposal. They’ll look at how the part was made, look at the milling marks, the turning marks, wear and tear on familiar points of contact (determined by checking it with mating parts). Alongside that, they’ll take the kind of basic measurements used in GD&T—checking angles, shapes, measuring surface roughness, smoothness, and hardness— all while keeping its intended function in mind. For instance, they can take note of a “mirror-smooth” surface finish and extrapolate that the part needs to move as smoothly as possible.
From there, they can even use 3D modeling to “craft” the part in a program like Solidworks, then run it in a simulation as a part of the entire assembly. By doing so, they can ensure that the part will work the way it’s supposed to and that all the tolerances are accurate, and get a better idea of the part’s most important features and their maximum allowable variance. At times, this process can result in the production of aftermarket parts that are head-and-shoulders above the quality of the part created by the OEM.
It’s here that things get complicated, and we see the importance of figuring out a part’s maximum allowable tolerance. You see, the manufacturer needs to hit a balance between accuracy and cost. The more accurate and fine-tuned they try to make their engineering drawings and modeling and the lower they make their tolerances, the greater the cost of reverse-engineering (and making) the part. These measurement processes take a lot of time, labor, effort, and money, and ensuring that the created part matches them doubles up on all four.
In other words—the more “perfect” you try to make a part, the more expensive the part becomes. And this growth is exponential.
Remember that aftermarket manufacturers exist for the purpose of saving potential buyers money without sacrificing quality. As such, they need to be capable of reverse engineering parts and creating a copy that exceeds the quality of the OEM, while still saving the customer time and money.
So, to accomplish this, an aftermarket manufacturer needs to determine which part features (tolerances) are most important to the part’s function, and figure out the maximum allowable variation in these tolerances. They make sure the parts work while being sure to save you money. And the best way to do this is to create a good drawing/design using GD&T principles wisely, rather than becoming obsessed with needless accuracy. This is often a matter of experience, as well-versed and well-equipped manufacturers will be more capable of figuring out which tolerances are most important to the function of a part and knowing where to draw the proverbial line between accuracy and cost.
Remember, a part never needs to be “perfect.” It just needs to function.
Obviously, this affects the savings and bottom-line of any company buying a part from an aftermarket manufacturer. They need to be certain that they are purchasing a part that will be cheap, received quickly, and function properly. Otherwise, they’ll lose money.
GD&T is a cornerstone of modern engineering and manufacturing processes, as it works to communicate and define the engineering tolerances that guide the proper functioning of the parts used in industry. It’s especially important in the process of reverse engineering, as it gives aftermarket manufacturers a toolkit to ensure that they can reproduce the part accurately and quickly, without overdimensioning and wasting money. But, it’s important to remember that GD&T is just a tool. Without proper workers, overseers, and a solid dose of wisdom about when to apply these principles and when to draw the line, it’s easy to over-do it and waste money.