When a turned part has a cross section that is unequal when referenced to its centreline, it is considered to be eccentric. Camshafts often are referred to as the most common of these workpieces, but those parts typically are manufactured on a machine solely dedicated to their production. So what is a job shop to do when an order for a part comes through the door that needs eccentric turning? The answer, surprisingly, may actually start with a milling operation.
The process of eccentric turning is similar to traditional turning only with the addition of another workpiece axis, which is parallel to the centre axis. To produce the eccentric part feature, a cutting tool removes material to the required depth as the part is turned, typically at low speed.
What type of machine setup is best suited to perform an eccentric turning operation?
It all depends on whether the CNC lathe has a milling spindle.
If a multitasking machine is used, we recommend milling the eccentric part or part feature to a near-net shape before using a finish turning pass to complete the feature.
There is no real change to the machine structure or horsepower requirements if you start with a good, stable machining platform.
What makes the milling operation important?
If you mill the part feature in a multitasking machine before performing the finishing pass with a turning tool, you create several efficiencies. Some of these come from the fact that you are not moving a part from machine to machine, but others come from using a mill to remove material from the part, which is faster.
A good milling pass will set up a good finish turning pass, so it’s imperative to get it right. We are not going to turn the part feature from scratch because it takes too long [compared to milling] because of the low speed we run at.
In most cases, we are going to mill it to get close and just the final pass will be eccentric turning.
How does the eccentric turning process affect setup?
The traditional method of machining complex parts involves multiple machines, both lathes and mills, and creates more setup time than is necessary. Operators must make sure parts are properly aligned and measured to ensure that each part has the same dimensions as the previous one. But these setup and measurement steps aren’t needed when all of the machining operations are performed on a single machine.