Turning Q&A

September 1, 2010

Eccentric turning occurs when the workpiece and spindle axis do not share the same center axis. This process poses a set of unique challenges, so Canadian Industrial Machinery asked Marc Jagoda, Mazak’s assistant national applications manager, for his advice

Q: What is an example of eccentric turning?

A: Eccentric turning can cover a lot of different parts, but a crankshaft is a typical, difficult-to-machine example.

Q: How are these parts typically created?

A: Although quite often turning centers and special machines are used to create these parts, multitasking machines also can be used. A machine with a turning spindle, milling spindle, and B axis capability for full five-axis movement can be used, depending on the part’s configuration.

Q: What is the current market like for these parts?

A: The crankshaft market for large equipment stays pretty steady, but the market for crankshafts used in the oil drilling industry for directional drilling is very cyclical in nature. If companies are most familiar with automotive production work and they are used to 30-second cycle times, then they will need to get used to the new production cycle.

Q: How do long shafts complicate the machining process?

A: As the shaft gets longer, support along its length becomes much more difficult when there are eccentric diameters. Multitasking machines can help combat this because it can be done on the centerline and then machined off-center, if possible based upon the part’s configuration.

Q: Is this the same in short-shaft work?

A: Speed becomes an issue in these cases. Some manufacturers make chucks for eccentric work, kind of like an index chuck, so it offsets in one direction and you can return the part. Balance also plays a role here as well. Parts are not easily balanced after the eccentricity is machined into it. To combat this, you must reduce your speed or choose a multitasking system.

Q: How does diameter affect machining?

A: Diameter also plays into the balance and the speed. In parts with a larger diameter, speed is not as much of a factor, but the weight imbalance gets worse. Conversely, the smaller the part is, then speed becomes more of an issue and the balance problems tend to disappear.

Q: How should a part be held?

A: The length of the part is the most difficult aspect for machinists to counter because support or supports along the length will be required.

Q: How can companies choose the correct technology?

A: Picking the right machine is very important in eccentric turning work. Having the ability to modify the machine to be able to hold what you are trying to machine is important. Workholding may play more of a role than the machine tool, however. We have done some crankshafts that are very large and have 8 inches of movement. They become very difficult to hold and balance inside the machine.

Q: What roles do the control and programming play?

A: In the multitasking environment these play large roles. Programmers need to know how to apply the tools correctly. This is more than just programming, but also proper metal cutting practices. The dynamics of how the tool is going to make a round diameter as it spins eccentrically must be understood.

Q: How does tooling choice affect the process?

A: Tooling choice depends on how deep you need to reach. On a crankshaft, for example, it would have to be longer than normal. But if you are not cutting too far away from the centerline, the tools don’t necessarily have to be long. You do, however, have to have a Y axis to correctly shape a round part with a round cutter.

For more information, visit www.mazak.com.