Introduction: Repair Using 3D Printing: 2b CAD Modelling

This guide is part of a series: Repair using 3D printing. This series of guides describes the process of reproducing a broken part by 3D printing a viable substitute. Please refer to the series’ main guide to follow the complete process. This guide includes a step-by-step explanation of the particular sub-process within Repair using 3D printing. First time readers are advised to read the whole guide, experienced readers can use the quicklists in each step to guide and speed up their next attempts. Skip to step 2 to get going!

Step 1: Background Info

When you followed the decomposition guide and found that the part features mostly measurable, geometric shapes, you probably ended up here: CAD modeling the part.

CAD modeling is a strategy for creating the 3D model that will serve as the input for the 3D printer, and represents the new spare part. In CAD modeling, various approaches exist, of which parametric modeling is the most used and also most useful in the context of repair. In parametric modeling, dimensions and other boundary conditions constrain the modeler from creating ‘impossible’ geometry, essentially only building fully defined, solid geometry that is reproducible through manufacturing (for instance in a 3D printer).

Basic parametric modeling, as commonly used in various CAD programs often starts with dimensioned, 2D line drawings called ‘sketches’, that are turned into a 3D solid using a feature. The 2D sketches are drawn on a plane, which is a defined, flat surface in the cartesian coordinate system. A basic 3D modeling space based on this system consists of three axis (X, Y, and Z), intersecting in the Origin. Often, three base planes are already active (XY, YZ and XZ) and are a convenient starting point for many 3D models. A 2D sketch is created, drawing the contours of the model to create a closed boundary profile, that can be converted into a 3D shape. Extruding the inside of this sketch contour for instance, creates a 3D shape by ‘pulling’ the surface from its plane, creating a second, parallel surfaces and a ‘wall’ between the two that follows the contour. Revolving the sketch around an axis creates a circular 3D shape that is infinitely symmetrical around that axis. Sweeping a sketch profile along a line in another sketch (the path) results in a guided extrusion.

By starting with basic shapes, building shape-upon-shape and using the same techniques to create solids that can be ‘subtracted’ from previous shapes, the 3D model takes shape.

Step 2: What You Need

The following tools are used in this step:

  • A decent computer, Windows (x64 only!) or Mac. Autodesk recommends at least 4GB of RAM and a dedicated graphics card with at least 512MB GDDR memory.
  • Autodesk Fusion 360, a licence for Students, Hobbyists and Start-up is free! Download at Autodesk Fusion 360 and upgrade to a free one-year license.
  • Camera: any digital camera or smartphone
  • Caliper: preferably a digital one Other measuring tools, such as a protractor, ruler etc.
  • (Optional) Adobe Illustrator or similar vector drawing software, capable of tracing a photo into a vector image

As an Industrial Design student, I learned to work with the CAD modeling software SolidWorks, which is a professional tool. This includes professional pricing, from 2000 euros upwards (fortunately supplied by the university!). Luckily for you, Autodesk recently launched a complete and intuitive CAD modeling program called Fusion 360, which is free for Students, Hobbyists and Start-ups. Due to this, I have started using Fusion 360 during my project to role-play a hobbyist that lacks a professional option like SolidWorks, and I loved it. Plenty of features and an intuitive interface make it ideal for beginners, but a very interesting option for advanced CAD modelers as well. This guide will therefore be focused on Fusion 360, but in general works for any parametric CAD modeling software that you might already be used to.

Step 3: Taking Measurements

The first thing you have to do is to take as much measurements as possible! Try to dimension every shape you find, measuring point-to-point from to references using the caliper. Any sharp edge, corner, between two parallel surfaces and diameters of cylinders are to be measured.

In case you do not know how a caliper works, have a non-digital (Vernier) one or want a quick refresh, read this excellent short guide on Instructables: How to read a Vernier caliper. You can use the main jaws for outside dimensions between to points, the jaws on the top for inside dimensions and the depth probe on the bottom to measure the depth of holes and cavities. Make sure you measure to an accuracy of at least 0.1 mm, as many plastic parts are designed with even higher tolerances (to fit perfectly onto other parts), but 3D printing accuracy is generally limited to about this tenth of a millimeter.

Make quick sketches of your part and note any corresponding measurements you take. Indicate the exact points you measured from, as you will be dimensioning your 3D model similar to the way you took the measurements.

Step 4: Modelling

Fusion 360 distinguishes itself from the rest by merging different modelling strategies into a single software package, as well as rendering, drawing and simulation tools. For this guide, I will stick to the basic, parametric modelling operations however and focus on modelling either simple, geometric parts or altering scans, and using references to do so.

The goal with this is to create a 3D model that represents the original, physical part as good as possible. An exact replication, as it was designed initially, is in most cases near impossible and unnecessary. Not only because of the difficulty, but also as 3D printing is a different manufacturing technique that was initially used on the original part (which brings along its own requirements). As the Decomposition concluded with the identification of ‘Critical features’, so can the non-critical features probably be simplified to speed up the process of modelling. Thus, look for these critical features and keep them in mind when building the model.

As described before, you start off with creating a 2D sketch on a plane. As you review the original part, try to think of a ‘logical’ orientation, that makes sense in the 3D modelling environment. Often, some faces, parallels or feature line out with the base planes and axis of the coordinate system. Do the same when importing a 3D scanned model, try to align the scan to the coordinate system in a way that is convenient for your modelling operations.

Start with a global sketch of the part’s Main body, providing dimensions for the lines you draw from the measurements you took from the original part using a caliper. The more accurate you do this, the more accurate the model will be. Try to dimension the 2D sketches in a similar manner to the way you took the measurement from the original part.

Try using extrusion operations for shapes with two parallel surfaces, use revolve or sweep operations for cylindrical shapes to create 3D shapes from the sketched profiles. All resulting bodies can be set to ‘add’ or ‘subtract’ geometry from previous shapes.

(sidetrack) modelling alterations onto a 3D scan

If you start with a 3D scan, as described in the 3D scanning guide, it can be imported into Fusion 360 to model further additions and alterations. To do so, open the Sculpt environment by clicking Create Form. Import the exported .OBJ ‘quads’ model (see 3D scanning guide) and convert it to T-spline by right-clicking, convert.

It is now important that the new body is orientated similar to what you would do when modelling from scratch; to match the 3D modelling environment in a convenient way. Think of the alterations you want to model, it is much easier when these can be aligned with the base planes or axis of the system. After moving and rotating until satisfactory, close the Sculpt environment to return to basic modelling and proceed with the regular Plane-Sketch-Feature operations, which can be used to either add or subtract geometry from the scan body.

Step 5: Modelling References: Support Your Modelling

When critical features include curvature, particular angles or other immeasurable geometry, you can use photo references to approximate these difficult shapes. This can be done in two ways: either take pictures of the critical pictures and import them on a plane in the modelling environment, aligned with the model, or process the photo to use directly in modelling.

Take pictures of the critical features of your part or in the context of the complete product. Include a ruler or other measurable reference in the picture, to help scaling the image in Fusion 360. Think of convenient views to do so, such as a ‘Top view’ or ‘Side view’. These picture then capture the outline of the part, viewed from a logical perspective that corresponds with the base planes in the modelling environment. For instance, the parting line on this iron’s backplate is best viewed from the side, so I took a side view picture and used it as a visual reference to recreate the curvature. In Fusion 360, under Insert > Attached Canvas, the image can be imported onto a plane and scaled/orientated to match the model you are building. To scale the image, you can either drag and adjust, or right-click the canvas in the browser (left) and use the Calibrate function to scale point-to-point (providing a fixed distance between two points).

Vector tracing the photo

Another option is to process a photo for direct use in modelling. This requires a vector drawing programme such as Adobe Illustrator to ‘trace’ the photo into a vector image. This is particularly interesting to accurately produce a curved outline. Again, providing a scaling reference is important. Instead of a photo, a 2D scan of the part, on a regular flatbed scanner, can also be used to do so, which automatically scans to true size when a standard paper size is chosen.

Import the image into Illustrator and trace it into a black and white vector image. The threshold and accuracy settings can be adjusted to create a usable result. Expand and clean up the vector from any undesired details by removing all anchor points other than the contour. Don’t forget to scale the traced image using measurable references! Use Object>Path>Simplify to smoothen out the tracing result if necessary. Export the resulting vector in a .DXF format, to insert into Fusion 360. The vector image is now imported as a sketch, creating a usable contour for further modelling.

Step 6: Continue!

Now that you have created a 3D model using CAD modelling, it is ready to be printed! In fusion, export the model in the .STL file format by clicking Make > 3D print. This creates a 3D print ready model. However, boundary conditions of 3D printing have to be taken into account to find out if the model is really printable. To find out, continue to the next guide:

3 Reproduction guide


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