PART II Build, Test, Redesign, Repeat, Document
9.5 Manufacturability
9.5.3 Mistake Proofing
Mistakes can be costly to a product or process especially when they result in failure Mistakes can happen during the design phase, during assembly or execution, or by the end-user of the product or process Mistake proofing or poka-yoke (Japanese equivalent) is any method that prevents an error from occurring or makes it obvious when the mistake has occurred Mistakes occur because of human involvement in a process When an error happens, it may result in an apparent failure which may create a dangerous situation and cause harm to people or assets A cause-and-effect analysis can help the designers identify the root cause of the failure caused by the error
Mistake proofing starts with an analysis of the process by creating a process flow- chart Each step in the process is reviewed to identify the potential for human errors During the analysis, it is essential to think outside the box and expand the thinking to all possible ways the process user may take an unanticipated action and cause a malfunction or failure For each possible error discovered through testing and analy- sis, create a feature in the design to prevent that error If it is not possible or too costly to change the design to prevent the error, create user instruction or training to mitigate user error in operation Alternatively, create user instructions or training to self-check their work and prevent the error Automation may be used in some cases to bypass the error-prone step
An example of mistake-proofing is the data cable connectors for computers and cell phones The universal serial bus (USB) connectors type A was designed with a block tab to allow the connector to be inserted into a USB plug only in one orientation to assure the correct electrical connection for data and power The USB A design is an example of a poka-yoke design The user can only insert the USB plug only in one orientation This would prevent failure if the user were allowed to insert the USB plug the wrong way incorrectly A plastic block tab prevents the error However, commonly, the user attempts to put the USB plug in one way, and 50% of the time, they cannot insert the USB plug because of the tab They have to flip the USB plug and insert it the correct way An improved design of the USB-A plug connector was introduced as USB-C The electrical contacts are replicated on both sides in USB-C, so it does not matter if the user flips the connector The new design is a poka-yoke design, and it improves on the previous USB-A design by eliminating the 50% user failure to insert the plug (Figures 99–912)
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FIGURE 9.9 Poka-yoke example USB-A connector
FIGURE 9.10 Poka-yoke example USB-micro connector
FIGURE 9.11 Poka-yoke example USB-C connector
FIGURE 9.12 Poka-yoke example USB-C connector
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9.6 3 D PRINTING
3D printing has become popular over the past decade because of critical technolo- gies that have reduced prices and increased accuracy and usability 3D printing has become an effective way to prototype and create working models of many different varieties of designs The design created in the form of CAD must be specially created to allow for efficient printing and optimize the use of materials and printable geom- etries Some geometries cannot be printed on a 3D printer, such as when the walls are too thin or print features that result in internal geometries that are too small Printing usable 3D parts also require knowledge of the materials and best practices to achieve a successful and usable print
Objects are created layer by layer on a 3D printer Standard lower-priced printers extrude a melted plastic filament sintering to the top of the previous layer of plastic 3D printing is also called additive manufacturing instead of machining, where the material is removed (subtractive manufacturing) The print head in a 3D printer typi- cally moves in the x-y plane, and the glass build plate moves in the z-direction
The fused deposition modeling (FDM) technology was patented in 1989 by S Scott Crump and Lisa Crump They later co-founded Stratasys, Ltd Their tech- nology was based on feeding a plastic filament into a heated extruder layering the material by moving in the x-y direction The fundamental patents expired in 2005, opening the field for many other companies manufacturing FDM printers
Most standard 3D printers use fused filament deposition manufacturing (FFDM) technology The filament is typically 175 mm (or 285 mm), with the print-head noz- zle being 04 mm The user’s layer height can be controlled in the slicing software but should not exceed half the nozzle diameter Most printers print in a Cartesian coordinate system Some printers are non-Cartesian
3D printers use a microcontroller to control the movement of the print heads and the print bed An Arduino controller is sufficient as the microcontroller because the computational power needed for motion calculations is relatively light The microcon- troller software is called firmware A standard open-source firmware is called Marlin The process flow for 3D printing is shown in Figure 913 The designed part in CAD must be exported in a portable file format known as stereolithography (STL) The slicing software choice depends on the type of printer Simplify3D is a manufacturer of independent software that supports many different types of printers Many printer vendors provide slicer software specific to their printers free of charge Check with the staff responsible for the 3D printers at your school or organization for the needed slicer software The slicer software can read the STL file and require the user to enter param- eters to customize the print for the specific printer and materials used for the print
Standard 3D printer slicing software requires the model file to be in stereolithog- raphy (STL) format The STL format is old and inefficient, but it is broadly accepted as the norm The CAD software can export the design model in several different for- mats, including STL The STL file is an approximate representation of the detailed geometry in the CAD model The approximation is achieved by converting the model surfaces to a triangle mesh and outputting the triangle vertices coordinates and the normal vector to the triangle plane to the STL file in 3D Cartesian coordinates Therefore, the STL files are large and compute-intensive to process The user can set
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a system of units and precision intervals to adjust the resolution and size of the STL file For 3D printers, it is best to use the highest resolution model allowed by the CAD software (Figure 914)
Because of converting a precise geometry into an STL approximation, issues may arise in the STL file, such as small triangles, super long triangles, the surface not being watertight, and incorrectly oriented surface normals Most slicing software includes options to repair the model or repair the mesh If the mesh is problematic beyond the capabilities of the slicing software, another software program may be used to repair the mesh Meshlab (wwwmeshlabnet) is an example of a software pro- gram that can manipulate, repair, and optimize meshes in an STL file Simplify3D is another example of software that is efficient in mesh repair and slicing STL files for many different types of printers (Figure 915)
3D printers need software instruction sets, called gcode, to execute a print The slicer software creates gcode for the specific printer from the STL model file and user input for printer and print parameters The slicing software needs user input for printer characteristics such as the print bed dimensions and z travel distance, nozzle size installed, and temperatures for the nozzle and the print bed (Figure 916)
We discuss the usage and settings using Simplify3D as a typical slicer for 3D printing Other slicers have similar features, but the details of menus and wording options are unique to each software Simplify3D has the ability to write the output g-code file to an SD memory card which can then be loaded onto the 3D printer Another option is to connect the computer running Simplify3D to the 3D printer using a USB cable If connected by the USB cable, the computer running Simplify3D software controls the 3Dprinter for set up and motion for each step
Under the tools menu, the machine control panel is used to set up the serial port for communication to display and control all aspects of the 3D printer (Figure 917) FIGURE 9.13 3D printing process
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FIGURE 9.14 STL file loaded in the Simplify3D software
FIGURE 9.15 STL file mesh view (line drawing) in the Simplify3D software
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FIGURE 9.16 STL file statistics in the Simplify3D software
FIGURE 9.17 Process settings (Extruder) in Simplify3D software
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FIGURE 9.18 Process settings (layer height) in Simplify3D software
FIGURE 9.19 Process settings (Additions) in Simplify3D software
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