Dare to Repair—Phase 1: Decomposition
This is one in a series of guides used in our Dare to Repair 3D-Printed Repair Parts Contest. This guide describes the first phase of the project: Decomposition. First-time readers are advised to read the whole guide; experienced readers can use the quicklist in each step to guide.
For more information—including rules, submission guidelines, and prizes— check out our Dare to Repair contest announcement on our blog. Don't forget, your submissions are due on Monday, May 14.
For easy reference, here's a complete list of guides in this series:
- Phase 1: Decomposition
- Phase 2a: Remodeling with 3D Scanning
- Phase 2b: Remodeling with CAD
- Phase 3: Reproduction
Step 1: Background ¶
This first guide in the series is about the decomposition of the original part. Some aspects of a part need to be reproduced exactly like the original while others are less important and can be left out entirely. If you know what to look for in a part, you can easily determine its critical features.
There are three main factors to consider in reproducing a viable substitute part:
- Function and Context
First, you will look at the function and context of the part. This will tell you a lot about its intention and relation to the rest of the product. These intentions and relations translate to requirements and critical features (important aspects of the part that were set initially by the part’s designer). This phase will help you to identify what’s really important to get right, what can be simplified, and what can be left out or less accurately reproduced.
Next, we will address the geometric aspects of the part. The composition of shapes, surfaces, features, and details of the part are broken down to reveal the complexity and type of shapes used. This will help you to understand how it was initially built by the designer. Your work in this phase should give a good indication of the difficulty of the job and what modeling strategy should work best for your project.
For example, a flat gear is a simple, familiar shape: a flat cylinder with a cylindrical hole in the center, and regular teeth around the perimeter. The handle of an electric drill, however, is more complicated. It consists of double-curved shapes, and multiple screw holes—making it more difficult to reproduce for 3D printing.
Lastly, you will take a quick look at the material the part is made from. Often, the original material choice tells you a lot about the part’s intentions. The material properties of the part should fit the requirements set upfront by the designer. However, you are going to make your own material choice—as the selection of 3D printable materials at the moment is quite limited. We’ve made a selection of important material properties to take into account, and developed a selection tool to advise you which material suits your case best.
Step 2: Getting Started ¶
Before you start, make sure you have read the disclaimer in the main guide. Working with electronics and tinkering with warrantied consumer products is done at your own risk. Make sure you are working in a designated area (such as a ventilated workspace), wear appropriate safety gear when needed, and unplug your device before getting started.
You will probably need to disassemble the product you’re working on to remove the part you want to reproduce. Some products include disassembly steps in their user manual—but it’s rare these days. Try to find disassembly instructions online by searching for your particular product on iFixit or elsewhere.
Keep in mind that you might need special screwdrivers, prying tools, and heating guns to get the product apart. If you can’t figure out how to take the product apart or you don’t have the tools you need, consider bringing your product to a Repair Café if one is nearby. Often, they have the right tools and knowledge to open just about any product.
Pro tip: When taking something apart, take pictures of every step! Keep the components and screws organized. And write down what you did, too—so you don’t forget how to put it back together.
Step 3: Function Decomposition ¶
Figure out the function of the part you’ve chosen. The part can be mechanical, a force-bearing component, a solely aesthetic decoration, or a structural component connecting other parts together. Examine the part by looking at its function in relation to other parts in the product. This should help you to understand what the part should be able to do and can be translated to special requirements for the new part. Take as many notes of your findings as possible.
A broad set of requirements can be derived from the function of the part. This includes requirements for the material choice and the level of accuracy required for the shape.
The most important requirements are:
- Contact with water, heat, UV light: Does the part come in contact with either of these? Some materials are weaker to moisture/light, or they weaken quickly under raised temperatures.
- Food contact: Does the part come in contact with any food related substances? Some materials can be toxic and are not ‘clean’ enough to be used in combination with food. BTW, food-grade materials for 3D printing exist.
- Stiffness or flexibility: Does the part need to be very stiff, or bend to function properly?
- Strength and impact resistance: Does the part have to be very strong? Does it need to resist impacts (from falling or hitting objects) or be strong enough to resist constant or frequent forces.
- Wear and abrasion: Does the part have to resist wear and abrasion from rubbing against other parts? The durability differs greatly across materials. (Gears, for example, have to be resistant to wear.)
Note which of these requirements apply to your part and product. You will be taking these requirements into account when you make a spare part later on.
Critical Features ¶
Next, identify the critical features in your part. It’s important to reproduce exactly to spec as the original, or it might not fit or function properly. Look for the following features:
- Parting lines: the edges where the part touches other parts, but does not interrupt the overall shape. (Think of the door of a car.)
- Contours: similar to parting lines, but circumfere or enclose another part. Any fasteners or fixtures, including screw holes, snap fits, connecting ledges and hooks.
- Special shapes with an additional function: such as the grip of a pair of scissors (that should fit your hand comfortably), which need to be reproduced exactly after the original.
- Other surfaces or details you think are important to replicate accurately.
Critical features are the most important details to get right. Try to identify as much as possible. You might be able to leave out or simplify any non-critical features to speed up the process. This includes:
- Curved shapes that are only aesthetic, can be simplified to simple, flat faces
- Structural ribs and supports, that are added for strength whilst maintaining a constant wall thickness in many plastic parts. As we are 3D printing, you might as well make the model solid—as long as your modifications do not block other parts in the assembly.
- Other details that are not important to the function of the part.
Step 4: Geometry Decomposition ¶
Not all shapes can be scanned properly, and not all shapes are particularly easy to CAD model either. So determining the overall shape, the amount of ‘features’ attached to it, and the complexity of these aspects will help you choose between modeling or 3D scanning.
This iron has had parts of its missing bits filled in with clay
Review the part for completeness, geometry class, and composition complexity. Incomplete parts are hard to scan or measure—but you can always restore missing geometry by using modeling clay to ‘heal’ the missing bits. Find symmetry and references from the context to try reconstructing the missing pieces. Searching for images of the part on internet can help as well.
Geometry Class ¶
Next, look at the type of shapes in the part. The geometry class can be broken down into two main types: Geometric and Organic, with a few classes in between.
If the part is mainly built up out of basic, geometrical shapes (like cubes, cylinders, cones and polygons), it contains mainly geometric shapes. If the surfaces are mainly curved or even double-curved, the part is mainly organic. (You see this primarily in many plastic parts.)
If the part is organic, we’d recommend a 3D scan of the part, since accurately reconstructing curvature is very hard in CAD modeling. A 3D scan is much more shape accurate—because you scan the exact geometry.
If a part is geometrical or flat, it can be quicker and more accurate to start from scratch and completely model the part. Large flat surfaces, sharp edges, and parts that require dimensional accuracy can be difficult to scan perfectly, and we recommend CAD modeling.
Lastly, review the complexity of the part. Look at the composition of shapes categorized in the body and features. All parts consist of a main body—the largest volume making up the base shape of the part. You might also encounter additional bodies—large volumes with their own purpose within the part. For example, think of the housing of a TV remote: it’s a hollow shell supporting the electronics and holding the batteries. The overall shape of the remote is the main body. The box-shaped battery compartment can be seen as an additional body, as it serves its own purpose. Take a look at the included photo. Anything other than these main shapes—such as ridges, ribs, holes, bosses, hooks, cavities and any other alterations to these main shapes— we’re calling features.
The more complex a part is, the more difficult it will be to reproduce—whether you’re CAD modeling or 3D scanning. In CAD modeling, we recommend that you first create the main shape and then model additional features onto it. If your part is complex, it might be easier to model only the elements you’ve identified as being critical to the part’s function.
More complex parts generally contain intricate details, which are more difficult to capture accurately in 3D scanning. You probably will need to make more CAD modeling alterations afterwards. Pay close attention to cavities when modeling, as they are especially hard to scan.
Step 5: Material ¶
Next, take a quick look at the material the part is made of. A Resin Identification Code might be stamped onto the part somewhere (you’re looking for a little recycling icon with a number in it). If you’re lucky, you might also find an abbreviation identifying exactly what material is used in the part. If not as lucky, you’ll find a recycling symbol with a number ‘7’ in it, which is a designation for the “Other” category. If you’re not lucky at all, you won’t find any Resin Identification Code. So, you’ll have to do a little extra sleuthing to figure out the material used in the part.
Let’s talk more about materials. Common materials in domestic appliances include ABS, PC, (HI)PS and PP. The latter two are general use plastics, which can come into contact with food, water, and sunlight. ABS and PC are engineering plastics, and they’re mainly used in mechanical applications where impact resistance, strength, and stiffness are required.
At this point, you should be starting to think about what material you want to use for your replacement part. Be sure to take all the elements you identified in the decomposition stage into account when making your decision. Because of the limited kinds of materials available for 3D printing, you might not be able to use the material in the original part. So, here’s a handy-dandy tool to help you determine the best substitute 3D printing material (PLA, ABS, PET, Nylon or PC) for your project:
Step 6: Difficulty ¶
Besides your possible experience with CAD modeling and/or scanning already, the information gleaned in this phase can help you estimate the difficulty—in terms of complexity, time and effort, and amount of design work needed. Here’s a few things to consider:
- Completeness: is the part intact? The more geometry is missing, the more difficult it will be to remodel, as your reference is missing.
- Geometry class: does the part contain curved, double-curved or organic shapes? Modeling these accurately becomes increasingly difficult! Simplification or rough estimates might be needed to replicate them, but 3D scanning is preferred.
- Complexity: the feature count indicates difficulty, time and effort. More features means more CAD modeling work or increased difficulty in 3D scanning.
From the part’s function, you have derived special requirements. Naturally, the more requirements you have to take into account, the more difficult it will be to reproduce the part. Especially because you’re limited to a single, or small selection of materials when 3D printing.
Also, check the measurability of the part in question: can you reach all important features with a caliper or other measurement tools? Easy-to-measure parts are easier to CAD model accurately. On the other hand, immeasurable details, like cavities, are usually more difficult to scan accurately.
Step 7: Decomposition Checklist ¶
To speed things up and keep you on track as you start this project, here’s a decomposition checklist. It will remind you of important aspects in the part’s composition and help you assess your case’s difficulty.
Download and print: Decomposition checklist
Not sure whether you’re better off scanning or modeling the part? Here’s a series of questions that will help you figure out which strategy is best for your project:
In general, we’ve found that CAD modeling is good for dimensional accuracy, and 3D scanning is good for shape accuracy. So, if you need to replicate exact dimensions, and are able to measure those from the original part, CAD modeling is probably the way to go.
If your part is more complex in shape or mostly curved—these shapes are generally difficult to model accurately. 3D scanning might be a better strategy here. Unfortunately, intricate details and exact dimensions are lost in the process!
Hybrid workflows also exist: CAD modeling with photos or a 3D scan as visual reference, or modeling alterations onto a 3D scan afterwards. You will, however, choose one as a starting point based on your decomposition, and work your way along the process. The next phases include the additional steps and tips to improve your results!