Five-axis milling machines come in different configurations.

1) The part can rotate in two degrees of freedom by utilizing a system of rotating tables. In this case the spindle may move axially, but not angularly.

  2) The milling tool can rotate by having the spindle mounted in a system which provides two degrees of freedom. In this case the part does not move angularly.

  3) It can be a combination—where one rotation axis is a rotating table and the second rotation axis is provided at the spindle.
Normally when purchasing a three-axis milling machine, one considers several different features such as: horsepower, spindle speed, axial feedrates, work envelope and weight limitations. In addition to these, when evaluating five-axis machines one must also consider the following: repeatability, angular velocity, angular limitations and milling machine controller options for five-axis machining.

  Repeatability
Repeatability is the ability for the five-axis milling machine to return to the same point and vector consistently. Repeatability of a five-axis machine requires accuracy of not just the axial positions, but also the angular values.

  Angular Velocity
Angular velocity can be considered in degrees of rotation per second, or you may also simply consider it in revolutions per minute (rpm). This is a function of how fast the tool can rotate in relation to the part, the faster the value, the faster the machine can cut. Many older five-axis machines have slow angular movements, which is counterproductive to high-performance machining.

  Angular velocity also is important specifically to machining molds. Many five-axis milling machines have a c-axis, which by default rotates around the z-axis. To mill a deep part with a short tool, it is often necessary to incline the tool via the a/b axis and rotate the c-axis to cut around the part. RPM performance of the c-axis is critical to success in this instance.

 Angular Limitations
Angular limitations are physical limits to the amount of rotation allowed by the milling machine; these are based on the particular machines design. If it is necessary to incline the tool 50 degrees to utilize the shortest tool or cut an undercut, and you are limited to 30 degrees of rotation, you may not be able to finish everything in that setup.
Angular limitations are very important when considering the c-axis. Many five-axis milling machines have an unlimited amount of c-axis angular movement. However, there are many that are limited in their rotational travel.

  For example, a particular mill may only be able to perform +360 and -360 degrees of rotation. Imagine using an inclined angle for the tool and machining down a steep wall, going around and around the part. The majority of the movement in this instance is being handled by the c-axis in a continuous motion. If one has a limited c-axis, it will require the machine to “unwind” at regular intervals to machine the complete part.

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