Reverse engineering is often used for fasteners, machine parts and assemblies, and other mechanical forms. This method is used for the design and quality control of various industrial products.
In this article in the "Reverse Engineering for Beginners" cycle, we will look at how to reverse engineer a mechanical mold yourself in FreeCAD. FreeCAD is an open source CAD system popular among enthusiasts in the field of 3D scanning and 3D printing.

The FreeCAD program has:

• a complete geometric core based on OpenCascade, which allows for three-dimensional operations on complex types of shapes;
• a fully parametric data model, thanks to which the shape of an object can be set through parameters or other objects.;
• a modular architecture that allows you to extend the functionality of the program using modules (plug-ins): complex in C++ or simple Python scripts and macros.;
• modules for creating traditional drawings with export to .svg and .pdf.

In this article we will tell you:

• how to import scan data into FreeCAD;
• how to perform reverse engineering in FreeCAD.

We will cover the following issues in detail:

• the Works with Meshes module;
• checking the dimensions of the part;
• alignment of a part along the axes of the coordinate system;
• building the contour of a part and obtaining a solid body;
• thread construction.

We illustrate the whole process step by step with screenshots.

As an object for demonstration of work in FreeCAD, we took a small nozzle and scanned it. Mechanical shapes often turn out to be simpler than organic ones: our nozzle is symmetrical and quite simple geometrically.:

Let’s see how to use FreeCAD to refine such a "soapy" 3D scan to a full-fledged model.

1. Upload the scan data to the Works with Meshes module:

2. Import the digitization file in .stl format:

3. Check the dimensions of the part at two points on the mesh:

4. Orient the mesh according to the working coordinate system. To do this, set the view along the ZX plane and move the mesh using numeric values.:

5. Align the axis of the part with the axis of the Y coordinate system:

6. Move the mesh along the Y axis:

7. We will make a copy of the mesh, which we will then dissect without changing the original one.:

8. To distinguish the meshes, change the color of the copy:

9. Let’s start trimming the mesh copy. Let’s construct a plane passing through the central point of the coordinate system. Let’s set its location and dimensions:

10. Specify the plane, the grid, and perform the clipping:

11. For the remaining part, we will build the outline of the part.:

12. To build the outer contour, go to the PartDesign module.:

13. Specify the sketch construction plane:

14. For convenience in construction, you can use a custom grid.:

15. Let's divide the contour construction into 2 parts for a subsequent demonstration of Boolean unification. Building a contour with a set of dimensional relationships and constraints:

16. Creating a sketch is easy, because the algorithm of automatic bindings is implemented.:

17. Create the first part of a solid body by rotating the contour around the central axis of the part:

18. We also build the second part of the profile (body):

19. Combine both parts of the part into one:

20. We build a cutting rigid body with a hexagonal profile:

21. Performing pruning:

22. We build an inner conical hole and cut it out from the general model.:

23. We build the threaded part of the part. For the team to work, a cross-section profile and a guiding thread path are required.:

24. Let's do the merge:

25. Let's build the radii of the fillets, focusing on the coincidence with the mesh:

26. The result of the constructions can be saved in CAD formats .igs or .stp:

As a result, we have obtained an exact 3D copy of the real nozzle. The shape and dimensions match reliably, because we built the model from a 3D scan. Now the finished 3D model can be used to create a new part or for further design work. And now we know how to reverse engineer in FreeCAD, and we can repeat the process with a different mechanical shape.

Other articles in the series:
Geomagic Design X: Grid alignment and editing
SolidWorks: reverse engineering of organic forms