Why Y-Axis For Turn/Mill Machines?

In recent years, most turning-center builders have added "live tooling" to their product lines. The idea started rather simply with some or all stations on the turning-center turret capable of driving a milling, drilling or tapping head attachment. Shops quickly realized the production advantages of integrating milling and turning operations.

As more shops applied driven-tool turning centers, a processing limitation became apparent. Because in most machine designs the rotating cutters are located on the turning-tool turret, they can move only in the same two axes of motion as the turning tools. This means any feature on the workpiece that's not parallel or perpendicular to the spindle center line, along the center line, can't be directly accessed to the rotating drill, mill or tap.

That's where Y-axis comes in. It solves the access problem by adding a third linear axis to the turning-center turret--enabling rotary cutters to machine across the spindle center line. To find out how Y-axis works and about some of its processing benefits and considerations, we talked to general manager of engineering Yusuf Venjara of Hitachi Seiki, USA (Congers, New York). Hitachi makes several models of Y-axis configured, turn/mill machines.

First, Why Live Tools?

In recent years, shops have experienced market changes that continue to impact how parts are manufactured. Shorter lead times, reduced lot sizes and the drive to eliminate inventory have conspired to change the manufacturing process.

In days past, a shop bought a milling machine to mill, a turning machine to turn, a drill press to make holes and so on. Each operation was an incremental step in the manufacturing process--ultimately resulting in a completely machined part.

With the advent of CNC and the application of the automatic tool-changer, it became possible (even preferable) to combine some operations that were previously independent manufacturing steps into a single setup on one machine tool DMG MORI USA, Inc. the machining center.

None of the progressive metalcutting steps went away. Metalcutting itself still means making chips, but how the work is processed has changed. On a machining center, much of the physical movement of work from one single-operation machine to the next has been eliminated along with the need to set up additional machines. Prismatic workpieces are processed completely or, nearly complete, in a single handling and single setup.

What machining centers have done for prismatic workpieces, live tooling is doing for turned work. Rather than take a turned part to a second machine tool when it needs a slot milled or a hole drilled, lathes equipped with live tools can do simple milling/drilling/tapping, right on the turning center. This capability is changing the way turning shops process their work.

How Does Y-Axis Work?

To get Y-axis motion, an extra set of ways is used to move the live tool across the face of the spindle. Several configurations are offered by different builders. Hitachi Seiki makes two sizes and configurations of Y-axis turn/mill machines. The smaller model has ten tool positions with five live-tool pockets on the turret. These tools are driven by a 5-horsepower motor at up to 3,000 rpm. The Y-axis ways are independently placed under the carriage/cross slide, on which the turret is mounted. The ten tools on the turret are augmented by a 12- or 20-tool fixed/live toolchanger.

On Hitachi's larger machine, a dedicated milling tool driver (basically a machining center spindle) is mounted to the turret face. This 7.5-horsepower motor is serviced by a 16- or 30-tool capacity automatic toolchanger.

There are other configurations that allow mounting the Y-axis ways on a slant bed, and other builders also mount an independent milling head with tool change capability.

Angularity between the three linear axes is 90 degrees, like a three-axis machining center. A servo-driven ballscrew actuates the Y-axis slide. Linear guides or box ways are the most common tracking choices.

Travel range for Y-axis on Hitachi's two machine frame sizes is 4 or 8 inches respectively. That's plus or minus two and plus or minus four from the spindle center line. Other Y-axis machine builders generally offer machines within these travel ranges.

In the Y-axis plane, tools can move in a plus or minus direction perpendicular to the Z-axis and spindle center line. Viewed from the operator's perspective, this Y-axis motion is toward or away from the door of the machine while X-axis motion is floor to ceiling. With three linear axes of motion at the tool, a machining center analogy for this setup would be a traveling column design.

C-Axis Makes Four

For live tooling to be effective, precise circular orientation of the workpiece is a necessity. This is accomplished by the spindle or rotary C-axis.

Before a turn/mill machine can perform a milling routine, the main spindle drive mechanism is disconnected. In its place an auxiliary servodrive is engaged to precisely rotate the spindle. Anti-backlash gearing also is used for accurate, bidirectional rotation.

Under CNC, the servo operates as a fourth axis and can be used for positioning using a Curvic coupling or other precision locking mechanism--usually in 0.001 degree increments (360,000 positions).

Eliminating a mechanical lockup device and using the servomotor to hold position allows the C-axis dual functionality for both positioning and contouring motion. Full contouring movement of the workpiece allows profiling cuts to be made using simultaneous X-Y-Z-axes motion with the C-axis. This makes it a true four-axis machine.

What Will Y-Axis Do For You?

Y-axis capability is an enhancement to a turn/mill machine tool. The constraint of two axes of linear motion is inescapable on some applications. While there are techniques, such as polar coordinate programming, which use axis interpolation to get three-axis like cuts, there remain some workpiece features that simply can't be cut using the two linear axes available on a standard turn/mill machine.

An example is drilling holes in the corners of a milled flat on a cylinder. You can't get to the corners of the flat without Y-axis. Adding Y-axis increases the linear motion capability of a turn/mill machine tool by 33 percent.

"It's best to think of a Y-axis equipped turn/mill center as two independent machines tools in one," says Mr. Venjara. "First there is the turning center. It operates exactly like any other 2-axis lathe. The second machine-tool-in-one is a four-axis machining center."

Y-Axis And Job Shops

According to Mr. Venjara, 62 percent of turned workpieces have subsequent machining performed on them. They're all potentially turn/mill users. Of that 62 percent, 25 percent are candidates for Y-axis machining. Y-axis then, is a subset of turn/mill applications.

Critical to successful application of a Y-axis machine is changing how a shop looks at processing workpieces that require turning and machining operations. Turn/mill machines with a Y-axis are simply tools designed to help a shop streamline throughput. They are more about process integration than technology.

According to Mr. Venjara, "the job shop, especially one that's cell oriented, is an ideal environment to take advantage of process integration technology offered by these machine tools." Most of the driving factors behind development of driven-tool turning centers and, subsequently Y-axis capability, are found on the job-shop floor.

"There is generally no production contest when a single mill/machine tries to compete with a stand-alone turning center and machining center producing the same part," says Mr. Venjara. Even with a total cycle time penalty for work handling and setup, over time two spindles working simultaneously will throughput more parts than a single spindle machine. But it takes time to overcome the work handling and setup penalty. That means higher volume is needed.

For medium-to-small lot sizes, total machine time, including setup and multi-machine handling, must be factored. These time losses cannot be amortized on shorter runs. So, for short and medium runs, setup for two machines becomes a higher percentage of total lot processing time.

This is where a multi-processing turning machine can make a difference. And this is the kind of work a job shop does. Setting up only one machine and processing a workpiece complete can result in significant total time savings for small-to-medium lot sizes. Additionally, the second operation machining center can now be used to process another job better suited to a machining center's processing strengths.

Keeping Relationships

Also very high on the turn/mill justification list are benefits derived from the single handling of a workpiece. Beyond processing time savings, gripping a workpiece once simplifies machining of features that have relationships.

Here's an example. A simple flange with a major ID bore of 2 inches is surrounded by three 0.5-inch diameter bolt holes. Conventional processing calls for turning the workpiece and boring the major ID. Then the part is moved to a machining center to drill and tap the three bolt holes.

What often gets lost during the trip from the lathe to the machining center is the relationship of the bolt holes to the major ID. Very accurate fixturing is needed on the machining center to assure these holes are the correct distance from each other and from the center of the bore.

Keeping the workpiece chucked on a turn/mill machine simplifies machining these related features. Because the bore is on center, a drilling and tapping tool can cut the holes as programmed--without concern (like one has when refixturing) that the ID center line may be slightly off.

Obviously, on complex workpieces, feature relationships become even more critical. One-stop fixturing is a solution to many alignment headaches for shops.

One Machine, Two Programs?

So how do you make a turn/mill machine play your tune? It certainly seems logical, from a programming point of view, that a combination two-axis lathe and four-axis machining center is a more complicated beast than either machine tool would be by itself.

Programming for Y-axis equipped turn/mill machines is more complex than programming a standard turning center. But it's no more complex than programming a machining center. "The skill of your machining center programmer is generally sufficient for these machines," says Mr. Venjara.

The turning function and machining function on these machines are sequential, not simultaneous. The workpiece program is created as a series of turning and machining functions. The machine tool CNC switches from turning to machining mode with an M-code command. Mechanically, the switchover requires engaging the spindle servodrive motor in place of the main turning spindle drive and the rotary tool drive on the turret. The switchover takes approximately 10 to 15 seconds.

Once in milling mode, the programming commands are standard G-code, identical to a machining center program. For minimum cycle time, turning functions are performed followed by machining functions. This saves switchover time. However, there is no limit to switching between turning and milling modes if the application calls for it.

Some Considerations

Key to successful implementation of the turn/mill concept is how a shop views its turning center work. "Approaching jobs with the idea of process integration from the start, seems to work best," says Mr. Venjara.

Once the decision to go with live tooling on the turning center is reached, Y-axis capability simply adds more flexibility. Three linear axes of motion accommodate more workpiece configurations than two axes.

Before leaping headlong into turn/mill, here are some considerations:

• It's expensive. Mr. Venjara says the premium for Y-axis capability on a turn/mill machine tool is about 30 percent which is still less than a turning center and machining center together.
• Pick the right parts. At its heart, a turn/mill machine is a turning center. Driven tools on a turning machine, even machines specifically designed for both processes, are no substitute for the power and range of a stand-alone machining center when substantial amounts of heavy cutting are needed.
• Holding the work is critical. There isn't much cost savings if, in order to process a part on a turn/mill machine, you must create an expensive fixture for the chuck when a simple vise would suffice on a machining center.
• Consider cycle time. Three factors are at play here: how long for turning, how long for machining and how many. Using a 30-horsepower turning center for a turning operation that lasts 2 minutes and a milling operation that lasts 10 minutes when the milling might last only 4 minutes on a machining center may not be the best use of your assets.

All that said, there is certainly a place for this technology in manufacturing today. "It's important to match the machine to the job," says Mr. Venjara. "When applied right, these machines have produced savings of 50 to 60 percent over conventional processing. The technology works."

Will it work for your shop? If turning and then milling/drilling/tapping is big in your shop and, if small lot production and efficient changeover/setup is a key measure of profitability, a look at this technology might change the way you make parts and money.

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