Unsolicited advice for the new engineer: GD&T & design software
Learning GD&T is just as important as learning trigonometry.
After spending 20 years designing advanced hardware, I have some unsolicited advice for new engineers. Although you may be a most innovative thinker and may be able to create fantastic widgets, understanding how your part will be manufactured is just as important (perhaps moreso) than that new idea. Even if 2D drawings go away, you will still need to communicate key dimensions for inspection and allowable tolerances for manufacturing.
The wag of my finger begins with an instruction to complete your engineering degree in whatever discipline interests you most. Second, appreciate, study, and get certified (if possible) in dimensioning and tolerancing (in America we abide by ASME who has ordained tolerancing standards as “Geometric Dimensioning and Tolerancing” or GD&T). Third, if you’re still in school, take advantage of your student status and get a certification in as many CAD software design programs as possible.
Unsolicited advice #1: dimensioning and tolerancing
Whether your career intentions include traditional manufacturing or building products using additive manufacturing, it is critical to know the math behind tolerancing and commonly accepted symbology. This is what facilitates the communication of your design intent. If tolerances are applied to dimensions on a 2D drawing, or to geometric features of a 3D model, GD&T applies to both documentation schemes. Tolerances applied to the “feature” (reference ASME Y14.5-2009, 1.3.27) of your part not only convey design intent, but also instruct what machine may be used to yield the desired product. Additionally, tolerances and inspection criteria may determine how labor intensive the quality and inspection process becomes.
This GrabCAD post that references hole and cylinder fits using a variety of AM machine tools is essential information for 3D print designers to understand. This post begins to characterize allowable tolerances from a variety of 3D Printers. The hardware industry has robustly characterized the allowable tolerances of most subtractive manufacturing processes, but has yet to quantify, characterize, and repeat additive manufacturing method achievable tolerances using a breadth of robust data.
The ASME standard that you are probably most familiar with, and invoke (hopefully) in the title block of your drawings is ASME Y14.5. From the 1940s through today, ASME and GD&T have primarily focused on 2D drawing capability. Now the times are changing to accommodate Model-Based Definition (MBD), a method that uses the 3D model as the source for the geometric definition, and in some cases for the dimensions and tolerances as well.
I expect we all agree that dimensions and tolerances are needed to convey design intent of a product. However, we often disagree (young to old, inexperienced to trained GD&T experts, designers to machinists) about what the dimensions should look like and how many dimensions should be displayed.
From my very first job, to regularly building flight qualified space hardware, to experimenting with MBD for additive manufacturing (AM), there was always a learning curve for how to communicate each product's (part or assembly) design intent to the appropriate guy next in-line to receive the goods. Therefore, I am consistently cogitating about the best methods to communicate dimensions and tolerances (and all that title block info - name, number, material, next higher assembly, etc).
How to apply dimensions
Used on a drawing, they must all be there in 2D black and white. If not, the human (that’s you) cannot rebuild the shape. Given the ability to use a model as the source for the geometry, all dimensions typically read from a drawing that’s already are embedded in the 3D geometry. This means all 3D geometry is "basic" in GD&T terms, unless otherwise specified.
There are several bonuses here:
- A) You don't have to duplicate and/or display the dimensional presentation (unless you need to identify a critical feature of the part that should be explicitly inspected)
- B) Human error reentry is eliminated because the geometry is consumed by the software directly
- C) With a 3D annotation, which is digitally associated to its adjoined feature, you have now created a highly reusable data set ready to enable downstream automation techniques.
When the conversation turns to 3D printing, and when that 3D printed item is intended for production use rather than prototype, we need appropriate tolerance controls to:
- define the manufacturing tolerance allowances
- measure those tolerances of the as-built part
- repeat those tolerances with consistency throughout the manufacture of those parts to ensure quality of the product
As ASME volunteers develop a suite of standards that use 3D models, rather than 2D drawings, the GD&T evolves. Again, here’s what is important to you as a budding engineer: understand the math, symbology, and context of when, where, and how to use GD&T in either a 2D drawing or on a 3D model.
ASME has a certification exam for GD&T. This is an intense certification, but just the act of studying for the exam will give you a leg-up on other applicants. As far as learning GD&T for 2D drawings, there are a number of companies, including ASME, that offer GD&T training. Google “GD&T training” to find a variety of options.
As you move forward in your career, the widget making industry may begin to shed drawings, especially for 3D printing and complex machined parts, but the need to communicate tolerances will still remain. So don’t poo-poo the need for dimensions and tolerances, we are working hard to make them more efficient, but understanding the basics will get you ahead.
Unsolicited advice #2: design software
You are probably already keenly aware of at least one of the following CAD design software programs. For building widgets and hardware (from cars to spaceships), the most notable CAD programs are:
- SolidWorks (Dassault)
- Creo (PTC)
- NX (Seimens)
- Catia (Dassault)
- Inventor (Autodesk)
I list Autodesk last, because traditionally they live in the 2D drawing-only department, and are highly skilled at facility layout and design, but generally have lagged behind other 3D CAD systems. However, I would not count them out of 3D market just yet. Inventor is in constant improvement and because Autodesk has huge market share, I assume they have a big budget and smart people backing their CAD software development.
Many universities and community colleges offer CAD certification programs, and you would be foolish not to take advantage of the inexpensive, if not free offerings. These are the certification programs that I am aware of.
- SolidWorks offers the most robust set of certification exams that are widely used by recruiters. The certification process is progressive and ranges from associate to advanced functions. Information on certification exams can be found here: https://www.solidworks.com/sw/support/mcad-certification-programs.htm
- Creo has an academic education partner program and may offer some certification for CAD, but it is not as recognizable as the SolidWorks certification program.
- NX offers a “NX Designer Certification” which is a half-day on-site evaluation course. It’s pretty rigorous – rest assured it is thorough and certified designers will be of high caliber. Information on “NX Designer Certification” can be found here: https://training.plm.automation.siemens.com/certification/designercert.cfm
- Catia does not have a formal certification program, but there are many courses available in a very large number of advanced CAD modeling disciplines. The full course catalogue is here: http://www.3ds.com/fileadmin/Training/PDF/course-catalog-v6-oct-2015.pdf
- Autodesk also has a certification program that appears to be accompanied by a training course. More information here: http://www.autodesk.com/training-and-certification/certification/certification-options
I promote that mechanical engineers get experience with as many CAD platforms as possible. The more CAD systems you know, the better your chances are of finding your dream job.
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About the author: (Jennifer Herron)
Jennifer Herron is a practicing aerospace and defense design engineer with a unique understanding of the communication needed to span the gap between 2D detailed drawings for manufacture and the desire to maximize 3D CAD re-use. She is the CEO of Action Engineering and chairs the ASME Y14.46 subcommittee on Product Definition for Additive Manufacturing. Jennifer regularly conducts trainings and consults on model-based topics and is an expert in multiple CAD packages, such as Creo, NX, and SOLIDWORKS. She is the author of Re-Use Your CAD: The Model-Based CAD Handbook, published in 2013.
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