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Turning: Spindles are Important, but so is the System

Okuma's Advanced Center for Manufacturing shares its strategy for turning lean.

"In the automotive world," Ted Driggs, says, "people want a process package." Driggs is the project manager for Automotive Sales/Application Engineering at Okuma America (Charlotte, NC), and he understands the lean trend in automotive production. As production volumes fall below the 100,000-200,000 range, he suggests, it becomes cost effective to optimize a group of "off-the-shelf" turning machines rather than investing in dedicated production equipment. Late engineering changes further encourage the use of flexible machines as exact design specifications may not be finalized until after machinery and tooling have been purchased and installed and process designed. But, as Driggs says, if you're used to buying a transfer line (and just providing a supplier with part specs and a desired capacity), you don't really want to have to become a system integrator. That's where people like Bob Lewis come in. As the senior manager for Applications Engineering & Training, Lewis helps automotive customers understand how they can optimize groups of turning machines to produce the fastest and least-expensive solution to meet their production requirements.

According to Lewis, the first consideration with turning machines is the orientation of the spindle. There are three basic choices:

  1. Horizontal. A traditional turning machine usually has the spindle on the left side of the work area and cutting tools moving in from the top or side.
  2. Vertical. Flip a horizontal machine on its side so the spindle is on the bottom of the work area and the cutting tools move in from the right or left. A basic advantage of this design is that the machine takes up less floor space. This is good because floor space costs money. It also permits putting machines close together so manual load/unload are simplified. (More on this later.) Vertical turning machines also have a gravitational advantage when it comes to fixturing parts. An operator can more accurately place a part in the chuck since gravity is pulling the part down into place. The great disadvantage of vertical turning machines is in chip control. Dished parts, like wheels, tend to accumulate chips as they're machined.
  3. Inverted vertical. Take the vertical machine and turn it upside down. Now the spindle is at the top of the work area. This solves the problem with chip control, as chips no longer collect inside the part.

The second thing Lewis looks at in designing a turning production line is how the machine will be loaded, either by a robot or a human. Loading involves both the consideration of automation outside of the machine (the transfer mechanism) and inside the machine (part orientation). The key here is ergonomics, as human operators need to be able to perform operations consistently without hurting themselves or tiring, lest they slow production. While a fully automated line with robotic load/unload removes the operator entirely, this can be a costly and less flexible strategy than using a human operator. Especially in developing markets, where capital investments need to be kept low and there is more fluctuation in production volumes, human operators can make good transfer mechanisms at a lower cost than replicating human sensory and cognitive abilities with a robot. Humans, however, can still require some automation to help them. A part unloader within the turning machine, for instance, can remove the workpiece from the chuck and rotate it so that it can be moved to the next machine. This means that the operator doesn't have to orient the part. (Rotational movements like this can cause carpal tunnel syndrome.) Of course, for parts too large or heavy for manual loading, robots are a necessity.

Lewis' third consideration with turning machines is maintenance and reliability. He states that Okuma's machines have a projected life of 7-10 years before needing to be rebuilt, with 90-95% machine availability. The great advantage of building a production line with a combination of these turning machines is that when one machine is down, production doesn't completely stop. That's why having an optimized production system is crucially important. Both parallel and serial manufacturing operations can be designed such that preventive maintenance and other unscheduled downtime only slow production, rather than stop it altogether.

The fourth point of consideration is floor space. Of course, floor space costs money, but that's not the only reason why it's important. Lewis recommends that the layout of a production line can usually be changed to increase flow, which increases capacity. For example, by putting small assembly stations at the end of turning operations, sub-assemblies can be created that reduce material handling (or turning cells can be configured as spurs to an assembly line).

As these ideas for designing a turning operation imply, there are a lot more important considerations than the machines themselves. Automotive customers may still have to get used to buying off-the-shelf turning machines, but system integrators will continue to show that it can be done, and still achieve production goals. Off-the-shelf machines save on capital equipment and allow for the purchase of lower increments of capacity. And much like the human worker who's easily retrained to load a different part, a turning machine can be easily reprogrammed.

 

Okuma's VTL-28 features an 8-10" chuck and is popular in emerging markets because of its small footprint and easy loading access for an operator.