Understanding Simultaneous Versus Indexical Cutting

Our 5AXISMAKER milling machines operate in 5 axes – tilt, rotate, left-to-right (the X axis), up-and-down (Y), and back-and-forth (Z).

But there are two ways to operate a milling tool in 5 axes: simultaneous and indexical modes. Each uses all 5 axes, but in different ways that have pluses and minuses.

These two cutting methods are related to the two kinds of 3D computer models that are the blueprints for milling. One is called a “surface model,” also known as NURBS (Non-Uniform Rational B-Splines), and the other is a “mesh model.”

A surface model of a new car, for instance, is built mathematically. This means that curves and intricate details can be defined and modeled at any resolution, and each surface can be separately defined, producing infinitely-smooth planes.  

So, if you want to mill a scale model of a new sports car with curved, organic surfaces and a high level of detail, for instance, a surface model is the way to go.

‘Simultaneous’ or ‘Full 5-Axis’ Cutting

To get fine resolution from a surface model in a curved, detailed physical rendition, it’s best to use a tool with a half-sphere tip that can handle a smooth, curved surface. And the best way to employ that kind of tip and make those kinds of highly controlled cutting movements is by using what’s called “full 5-axis” or “simultaneous” cutting.

A cutting tool used in simultaneous mode can move seamlessly between any of the 5 axes: tilt, rotate, left-right, up-down and back-forth. It can also do all the axes at the same time in combination – tilt and rotate together, for instance.

Any modern 5-axis machine can run major CAM software – such as SolidCAM, HyperMill and PowerMill – that supports simultaneous cutting as well as indexical.

Mesh Models

But simultaneous mode doesn’t work well in the other kind of 3D computer model used for milling, called mesh.

Unlike surface models, mesh models are defined by spatial coordinates rather than mathematical calculations. In fact, mesh models are often created from scans of physical objects that are assembled from data models or photos of actual things. These scans of actual objects can only produce a mesh model, while surface models are usually drawn by hand on a computer.

Surfaces in mesh models are composed of various-sized triangles, but have no detailed info about the surfaces themselves. No matter how small the triangles, a mesh model can’t offer the subtle feature boundaries of a surface model.

A mesh model is best handled by an end mill or cutting tool working in “indexical” or “3+2” mode. In indexical mode, the tool orientation is tilted, rotated or both, and then locked, at which point it can then move in the X, Y and Z axes. Indexical milling doesn’t employ the fluid movement between axes that is available in simultaneous mode.

When Indexical Is Best

With a mesh model, it’s very difficult to employ simultaneous cutting with a highly articulated tool tip like a ball tip. Indexical cutting is not only employed with mesh models, but it can also be used with models featuring geometrical features (circles, squares, holes, and so on), whether a mesh and a surface model.

While indexical cutting can create curved surfaces, they won’t be as finely defined as is possible in simultaneous mode. But indexical works well for a number of different applications, such as medical ones. The medical industry employs a lot of mesh models that have been created from scans.

But the automotive industry, creator of increasingly sleek vehicles, often uses surface models to develop finely detailed scale models.

Like the various cutting tools themselves, simultaneous and indexical modes of milling are part of a user’s toolkit – optimized for a given project’s needs and for the available model.