As rapid prototyping techniques evolve from sculpture and form representation into parts that are utilized in the field, the need to convey the CAD designer’s design, manufacturing, and inspection intent for a product that is manufactured using additive manufacturing (AM) techniques must also be captured in conjunction with the geometry.
I know what you’re thinking, because I think it too:
No way will I put all the dimensions and tolerances onto a drawing, if I’m going to just send my part to a 3D printer. THAT IS STUPID!
And while I agree, if a product is to be used as the production end item, you must capture important details not typically conveyed through transfer of a 3D model.
These product definition details typically sit on the drawing, such as:
- number
- description
- revision
- material acceptable manufacturing tolerances
- what product features (hole patterns or bearing bores) need inspection for acceptance of the part
However, in order to leverage Model Based Definition (MBD) strategies, we must employ 3D profile tolerances, 3D annotations, embedded attributes, and metadata.
Capturing design intent in a 3D solid model is inherently different than capturing it when a 2D drawing is the authoritative source of the design. A prevalent difference between product definition in 2D and 3D is there is no longer implied perpendicularity in a 3D model because there are no orthographic views. However, the use of MBD is predicated on an evolved set of GD&T protocols to capture the design, manufacturing, and inspection intent of the product.
A brief history of GD&T and why we still need it
GD&T was first pioneered in the 1940s and traces back to a man named Stanley Parker who spawned the concept of allowing a circular tolerance zone, rather than a square tolerance zone, as typical with a linear dimensioning scheme. It is fitting that a British inspector of submarine torpedo parts became frustrated enough with the process of:
- Measure to the best of your ability
- Compare that measurement against the dimension on the drawing (which was determined from hand drawn geometry).
Instead, he produced a tolerancing system, which over the many years has evolved into the ASME Y14.5 Dimensioning and Tolerancing standard, which was first released in 1966 by the American National Standard Institute (ANSI). Additionally, there are several ISO standards that address GD&T presentation (what it looks like to the human eye) for European-based manufacturing.
Why put GD&T in place for your organization for a 3D print, or for all your products?
The simple answer is that if you absolutely need your parts to fit together, then you need to apply math to the manufacturing allowances. I know it sounds weird, but doing a little math will ensure that two parts will always fit together if they meet specification (GD&T). Thoroughly analyzed manufacturing allowances guarantee that each part you receive will fit together because you “did the math” during engineering design and product documentation.
The short and the sweet of GD&T is that it is an unambiguous mathematical communication method of how far out from "perfect" your part can vary and still fit together with its mating part. Although AM is a superior and game-changing method to produce parts, it is not an exception to the rule of good product documentation.
3D Printing requires a change to the way we define our products
As 3D printing continues to evolve into a viable manufacturing method used not only for prototyping, but also as the production end item, so too must designers evolve our methods of design documentation. Or I suppose, if you haven’t sprouted grey hair yet, then you wonder:
I have a 3D model that I can send directly to a 3D printer that I buy from Home Depot, why do I need all these complicated hieroglyphics?
- GD&T or Geometric Dimensioning and Tolerance is necessary for any production level product. If a product made using AM processes is the end item, then it too needs proper definition and traceability.
- Because documentation in three-dimensions (3D) lends itself to a more obvious method of design and manufacturing intent, it is necessary to re-think the communication of dimensions and tolerances. Doing it the 2D drawing way is no longer valid.
In essence, 3D printing is becoming a new way to manufacture parts. Fifteen years ago, the available rapid prototyping technology offered a relatively small, fragile, and expensive prototype. However, today the technology has advanced to offer 3D printing of CAD models with choices such as: multiple materials, surface finishes, accuracies, color options, and increased size.
These improvements offer designers increased options when determining how to manufacture engineering models for marketing, form and fit checks, testing and even to use as final products. When your design matures beyond garage manufacturing into a sustainable product, proper design documentation techniques are required to capture design, manufacturing and inspection intent.
Yes, we still use GD&T, but we use it differently than we do for 2D Drawings
To explore why GD&T is still necessary, even when using AM techniques, it makes the most sense to dive into some examples.
Example 1: tree frog
Oh no! What happened to my cute little 3D printed tree frog? He was carelessly stuffed into the side pocket of my computer bag, and then, clearly his little limbs were sheared from his body. Poor little sculpture.
Example 2: train console
When the frog broke, I thought of this train console module that I designed to mount LED switches and train track switching levers onto. This 3D print is so rigid that I might be able to drive a truck over it - a result of an aerospace engineer designing a consumer part.
Both the frog and the train console parts are 3D printed using FDM (Fused Deposition Modeling). If either part had been made in other materials, limb removal or truck chalk re-purposing would not necessarily yield the same result. The frog machined from steel would always withstand casual traveler carelessness and the train console cast from rubber would collapse under any force.
As designers, the way in which we communicate our products is just as important as the material a part is made from. Identifying allowable tolerances on our designs enables manufacturing flexibility, while clearly stating the critical features of a product where more careful attention should be paid. In the case of 3D printing, these may be areas of the geometry that require post-processing.
When an AM method is used, which is simply an advanced manufacturing method, it is still necessary to communicate the design and manufacturing allowances to whoever is building the part. Because mathematics is a universal language, it is prudent to use a standard math-based method to convey design intent and manufacturing allowances.
The available and standardized math-based method that conveys manufacturing allowances is called Geometric, Dimensioning and Tolerancing (GD&T). The most commonly known standard for GD&T is ASME Y14.5, but there are others such as ASME Y14.41, and likely more on the way.
Final word
Because the geometry shape and topology is inherent in the 3D model and directly translated by 3D printing software, then time is saved by NOT dimensioning every geometric feature in the part, as is required when making 2D drawings. Additionally, accuracy is improved when directly porting the shape and topology that you already defined in your native 3D model into an AM processing software tool.
I don’t think it’s too much to ask to spend a few more minutes to get your product properly documented. Identifying the proper switch to cutout fit was simple, and applying an overall profile tolerance is straightforward, so no more excuses that hardware documentation is hard. It is necessary to complete the communication of your product.
