CNC West Dec jan 2019

56 www.CNC-West.com CNC WEST December 2018/January 2019 CAD/CAM/CNC Perspective By: Tim Paul Tim.Paul@Autodesk.com Instagram: OneEarTim Fundamentals that changed my 3D game… true (theoretically exact) profile. This may remind you of a “profile of a surface” GD&T boundary. N oisy toolpaths that waterfall beyond containment boundaries and over vertical edges are a byprod- uct of the toolpath finding triangulated surfaces outside of the containment boundary combined with the “Con- tact Point Boundary” option. “Contact Point Boundary” simply allows the centerline of the tool to go outside of the boundary so long as the contact point, the point at which the tool contacts the part, is still touching the boundary. On a ball mill the contact point of the tool can occur anywhere along the full radius of the tool all the way up the vertical flutes. Activating “Contact Point Y our depth of understanding of the things around you changes your perspective on everything. When I learned how to drive a manual transmission, it came to me naturally. I always assumed it was because I learned how the mechanisms worked before my legs were long enough to touch the pedals. On the contrary, I’ve always struggled with English because none of it makes sense. I could fill up this whole article with exam- ples of things I’ve excelled at and struggled with based on my understanding of how they work. H ow does this have anything to do with 3D machining? Simple: the more I understand how 3D machining works, the faster I produce good toolpaths. And like my friend Rob Lockwood says, “Good toolpaths make good parts.” This month I wanted to share some thoughts on what I’ve learned about 3D surfacing toolpaths that have changed my 3D game and have helped me make better parts more efficiently. Very quickly I realized summarizing a topic that could fill a whole book wasn’t going to add a lot of value to anyone. So, this will be the first of a multi-part series on 3D machining. In this issue I’ll cover how to work with tessellated surfaces and how tolerance, toolpath point distribution, and smoothing/filtering affects toolpaths. U nderstanding tessellated surfaces: I f you’ve ever seen what I would call a “noisy” toolpath like the one in Figure 1, then you’ve experienced one issue with tessellated surfaces that a lot of peo- ple struggle with. This generally only happens when “Contact Point Boundary” or other similar options are selected. Most people’s reaction to a noisy toolpath is to delete the toolpath and try a different one, cre- ate check surfaces, or try other random edits hoping something will work. However, once you understand what is happening in the background of the CAM, it’s easier to make an informed edit and get the toolpath you want. C AM software creates 3D toolpaths by first triangu- lating the model as shown in Figures 2 & 3 (though the mesh is likely far finer than shown in these imag- es). The tessellated mesh triangles are driven by the toolpath tolerance in the operation. The mesh is used to calculate the toolpath based on the tool size and shape. The “finer” or smaller the toolpath tolerance, the smaller the mesh triangles are. 3D toolpaths are calculation intensive (requiring more time for the CPU to process) in part because of the extensive checking required to ensure the tool doesn’t gouge the part as it moves across the surface topology. The higher the triangulation count of the mesh, the more calcula- tion intensive the toolpath will be. Figure 2 is a mesh exported at low resolution while the mesh in Figure 3 was exported at high resolution, showing the compari- son in triangle count. This example shows an increase from 7930 to 54012 triangles. I only use these images as a visual example of what is happening in the back- ground of your CAM software. The toolpath tolerance controls how closely the mesh triangulations are Figure 1 wrapped to the theoretically exact surface of the part. The tolerance is a plus/minus value, so the total cut tolerance is a uniform boundary split equally along the