3D Contours with Ball End Mills

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3D contours are surfaces that have smooth transitions between different depths rather than vertical walls and flat surfaces. An analogy would be hills and valleys instead of office buildings. This kind of surface is one of the most challenging to machine because, well, no tool can be entirely perpendicular to all the various angles in a 3D surface. However, a ball end mill is the next best thing and is the ideal tool for finishing 3D contours. This guide goes into detail about using ball end mills (usually in conjunction with flat end mills) to produce beautiful 3D contours.

Why should I use a ball end mill instead of a flat end mill for finishing 3D contours?

When you’re machining anything, you want the cutting surface of the tool to be parallel to the surface of the material. This is really easy when you’re using a flat end mill to machine geometry with vertical walls and flat surfaces because a flat end mill is flat on the bottom and sides, which are both cutting surfaces. The cutting surfaces and the geometry match.

The surfaces in a contour, on the other hand, are of many varying angles. The reason a ball end mill is useful is because the rounded end of the tool is the closest approximation to the various angles of that surface (much closer than the corner of a flat end mill). By using a low stepover (making many small parallel passes over the surface), you can approximate the surface enough that it looks like you want it to. The lower the stepover (i.e. the more passes), the nicer it will look. Unfortunately, the more passes you do, the longer it will take, so another aspect of machining contours is knowing how to find a happy medium where the final product looks as good as you need it to but takes the minimum amount of time.

Use a flat end mill for roughing and a ball end mill for finishing.

To minimize the time it takes to mill a contour, it’s standard practice to do a roughing pass with a flat end mill and a finishing pass with a ball end mill. The roughing pass removes the bulk of the material quickly, without much regard to accuracy. The finishing pass removes the final bits of material slowly and carefully. Here’s an example:

3d2 A roughing pass with a flat end mill.

3d3 A finishing pass with a ball end mill. The blue ribbon is actually lots of tiny lines (toolpaths).

Which toolpaths should I use?

That’s a tough one because there’s no one toolpath or formula that will cover all the different kinds of contours you may want to mill. If you’re using Fusion 360, all the toolpaths come with recommendations and visual examples, which helps a lot. For deep contours, a good rule of thumb is to start with a roughing toolpath such as 3D Adaptive Clearing or 3D Pocket Clearing, using a flat end mill. Then finish with any of the other 3D toolpaths using a ball end mill. For shallow contours, it’s sometimes possible to use only finishing toolpaths, which is simpler.

Try to use the largest tools possible. Larger tools remove material faster, and larger ball end mills also allow you to use a larger stepover. A larger stepover means fewer passes, which takes less time.

To get a more in-depth understanding, we also recommend going through a few of Fusion 360’s extensive tutorials, such as this one.

What are some examples of projects that have 3D contours?

The OtherYo 3d4

Breakfast Heroes 3d5

Kevin’s Wax Head 3d6