It is impossible to sell production parts into the aerospace, medical or automotive markets without the ability to assure the quality of those parts are manufactured to standards set by customers, such as the Production Part Approval Process or regulatory authorities including the FDA, FAA or NADCAP. Today’s manufacturers rely on methods and tools proven to industry standards using gauge repeatability and reproducibility testing.
Despite this reality, I have met many people making or hoping to make production parts for these industries using additive manufacturing (AM) methods, but who are unaware of these long-established quality requirements until confronted by their customers. How can this be? Quality assurance control systems have been around for centuries, so let’s start at the beginning.
Measurement standards were originally developed to address the need for consistent components to build solid structures. The first instance would be bricks or stones used to build houses. Churches and monuments dating back to ancient Egypt provide evidence that dimensional standards were applied to ensure individual bricks and stones fit together on the building sites. The basic unit of measurement may have been the distance between the tip of the foreman’s nose and the tip of his outstretched fingers. While this approach was sufficient for a small building, projects became much larger—requiring more distributed production of bricks and stones over a longer period of time. Clearly, there was need for a universal standard. One example is the standardization of the yard by England’s King Edgar in the ninth century, which literally paved the way for magnificent structures such as Lincoln Cathedral, which took over 150 years to complete.
The manufacture of agricultural tools, musical instruments, printing presses and firearms evolved similarly, moving beyond the domain of individual craftsmen. The demand for interchangeability of parts over distances and time drove the need for detailed definitions of measurement systems. In the 19th century, mass production of parts for the armaments industry in the United States and Europe powered another refinement of the existing systems of standardized dimensioning, limits and fits. The result was the Geometric Dimensioning and Tolerancing system introduced in the 20th century, which communicates the accuracy and precision required for controlled part features.
When it comes to part quality, the method of production might have changed, but the rules still apply. Conforming to quality assurance regulations using additive manufacturing methods is more difficult when compared to subtractive processes. This should be no surprise on its face, as the rules were developed around subtractive manufacturing.
There are three main reasons why adhering to the rules is more challenging using AM methods. First, additive manufacturing gives designers the ability to produce geometries that are impossible to create using conventional processes. Secondly, unlike subtractive manufacturing methods, the base material of the part is created during the AM shaping process. Thirdly, the process takes place in a single machine in one uninterrupted process.
It is generally possible to physically reach and inspect all the surfaces of a finished part produced via conventional manufacturing means. Consider the example of a block of material shaped by machining. The measuring device, whether it is tactile like a coordinate measuring machine (CMM) or an optical non-contact scanner, can access and measure a part surface the same way a cutting tool came into contact with it. Similarly, if the part is molded, all the surfaces formed by the mold must be accessible or the part could not be removed from the mold.
The only exceptions to this rule are parts like plastic bottle caps with collapsing molds for part removal and castings such as hollow turbine blades with cores that are chemically dissolved. These parts share the same problem as AM parts, but they are the exception. If additive manufacturing is to be used to its full capability, the challenge of how to measure internal and hidden features in complex geometry must be overcome.
Conforming to quality assurance rules is more challenging for AM as the part’s material is being created as the shaping process is underway. In a normal subtractive process, the manufacturing of the part starts with a block of material, maybe steel bar-stock or aluminum billet. The material is typically sourced from a credentialed material supplier who issues a certificate of quality and traceability for the material along with the bill of sale. The manufacturer does not need to spend a moment checking the quality and integrity of that material but need only be concerned that the shaping process cuts the material to the right size and shape. There are some exceptions, as the part material may come in a granular or powder form and will be melted before being injected into molds. When this is the case, there is the possibility of porosity and weld lines where flows of molten materials meet in the mold.
While injection molding is a subcategory of conventional processes, the potential for internal material defects due to manufacturing is present in 100 percent of additive manufactured parts. Whether the base material for an AM process is a powder or a wire, it is transformed into a part due to a process with multiple variable factors that can behave differently depending on the shape of the part, the thickness of the material, where the part is located on the bed, ambient conditions and other factors that can influence material integrity.
An additive manufactured part starts at zero. Its shape is created layer by layer with a base material, with the possibility of defects within and between each layer. If AM is used to produce customized and one-off parts, there is no opportunity to fine-tune the process. Compare this process to injection molding, in which each part is exactly the same as the one before. Here the opportunity exists to run trials and optimize process parameters to ensure repeatable material quality.
Finally, the third issue that makes producing consistently good parts more challenging for AM is that the part’s geometry is created in one machine in a single uninterrupted process. The obvious problem is there is no equivalent to inspection between discrete machining operations. Furthermore, if the AM machine is installed or set up incorrectly, there can be a ripple effect. For example, if the bed is twisted slightly along its length, every part it makes will be twisted along its length. There are very few AM machines built as substantially as a machine tool or molding machine. Great care must be used in their setup and calibration in order to generate parts that are as accurate and repeatable when produced by conventional manufacturing methods.
The nightmare scenario for an AM parts maker is to have a box of parts returned by a customer for being out of spec and not having the tools to understand why.
This is where metrology, the science of measurement, steps into the picture. Metrology is a highly evolved industry in the manufacturing ecosystem. There are many advanced measurement and inspection technologies that can be applied to AM, from laser scanners to CMMs, structured light systems to computed tomography scanners. Some combination of these metrology systems, along with automation capabilities, can readily address the part verification needs of AM users.
Quality assurance remains a roadblock for the AM industry in its evolution to become a serious contender in the precision parts arena. The manufacturers’ adage “What you can’t measure, you can’t sell” clearly applies to additive when it comes to producing and validating precision, mission-critical parts for OEMs.
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