Double Your Spindles, Double Your Fun
Twin-spindle machining centers offer high-volume production without consuming lots of floor space or costing you a bundle
By Andy Kunick
Chiron America Inc.
In Europe, dual-spindle machining centers are widely used in automotive production, and automotive component manufacturers continue to be the biggest market for this equipment. This situation is especially true for suppliers who experience quick turnover from job-to-job. As vehicle design times fall from decades to months, part suppliers must be able to accommodate the OEM's design changes. The flexibility of dual-spindle machines permits a supplier to switch quickly from one design to the next, and produce parts in considerable volume.
There are certainly advantages to using twin-spindle units in general manufacturing. For example, they can double cutting productivity when compared to single-spindle machines, and do it in the same footprint as a single-spindle machine. In addition, dual-spindle centers can reduce utility costs, enable a reduction in required support services, and improve manpower utilization.
A twin-spindle machining center requires fewer components and systems than two single-spindle machines. For example, instead of two coolant systems, there's one; instead of two CNC controllers, one. Therefore, a dual-spindle machine can be less expensive to maintain.
And what about cost, you say? Do we double machine cost in order to double machine output? The short answer is no. A twin-spindle machine costs 35 - 40% more than a single-spindle machine. But at our company we've found that a complete turnkey dual-spindle machining center, including peripheral items such as machine options, fixturing, tooling, etc., can be as much as 60% cheaper than two comparably equipped single-spindle machines that produce the same part volume as the dual-spindle unit.
Twin-spindle machines were originally designed for high-volume, high-speed machining. The two spindles of a dual-spindle machine are tied together in all axes. What one spindle does, the other will do. Dual-spindle machines have since been adapted to allow their use in medium-to-high volume applications. We've found that by machining two parts simultaneously, dial-machine-level volumes can be reached with dual-spindle machining centers.
There are, of course, some steps that need to be taken to optimize the performance of a dual-spindle machining center. Let's make it clear that workholding is of utmost importance on a twin-spindle machine.
Accurate fixtures are important when using dual-spindle machines. If a fixture is built with a Z-axis height variation, one part will be smaller than the other (top). If an NC table is used to provide fourth-axis capability, the Z-axis variation becomes a Y-axis variation when the fixture is rotated 90º about the X-axis (right).
Any misalignment on any axis can cause significant positional problems, and reduce the quality of the machined part. It's not difficult, however, to avoid misalignment by adding a few basic ideas into the fixture design.
Consider a fixture for two parts that's built with a variation in the Z-axis height. Clearly, that variation will cause the parts to be machined to different levels. One of the parts will be smaller than the other by a value equal to the variation. If the fixture is mounted to an NC table to achieve fourth-axis capability, the Z-axis problem in the home position becomes a Y-axis problem when the fixture is rotated 90º about the X axis (take a look at the illustrations if this explanation seems a bit cloudy). Fixturing must be accurate in all axes and workpiece orientations.
Also, because the centerline between the machine's two spindles is fixed, any offsetting adjustments to position parts properly in the fixture must be made in the fixture. You cannot correct relative offsets by programming the CNC to compensate for them. This isn't a limitation or defect, it's simply a consequence of the machine architecture. Workholding for a twin-spindle machine needs to be built more precisely than workholding devices intended for a single-spindle machine.
At our company, we build limited part-location adjustment into our workholding systems. This allows us to align a workpiece precisely to the spindle spacing. But on the Z axis, the fixtures must be dead-accurate.
It's true that a higher-accuracy fixture costs more than one made to less demanding specifications. But the dual-spindle machine can double production and reduce costs in other areas--as mentioned earlier. Further, once a fixture is set up correctly and run off, it's locked in place with pins. Unless an operator crashes the machine, the fixture is very unlikely to move. It becomes a reusable asset in a high-volume production system, and can pay for itself quickly.
Automation is compatible with dual-spindle technology--in fact, automation works as well with these machines as with single-spindle units. Actually, twin-spindle machines provide improved utilization rates for automation. Anyone who decides to automate operations is facing a significant investment. That investment can add thousands or hundreds of thousands to the cost of a machine. Automation that sits idle is very expensive. Twin-spindle machines produce parts at a faster rate than single-spindle centers, and thereby increase the utilization of automation.
To maximize throughput, we build twin-spindle machines with a system we call a workpiece changer--a worktable that can rotate 180º. This table has guarding that bisects it, enabling the operator or robot to load parts on one side while the machine's spindles are cutting on the other side. Of course this approach requires you to invest in fixturing for at least four parts, but the payoff comes in part throughput and spindle utilization.
Shown here is a Chiron twin-spindle machining system set up to produce automotive parts.
What about accuracy? Does increased productivity mean sloppy parts? Well, that depends on the accuracies you need to achieve. When both spindles are cutting parts, a dual-spindle machine can achieve true positional accuracy to 0.05 mm, and maintain a Cpk level of 1.67. When machining a feature using circular interpolation, tool-grind quality can become critical. For crucial operations, however, the programmer can change one tool into one spindle, and operate the machine as a single-spindle unit. This permits the CNC to provide individual spindle offset capability. Think about the case of a valve-bore machining job that requires a positional tolerance of 0.025 mm. On a dual-spindle machine, the bores can be roughed using the twin-spindle operating mode to reduce cycle time. Finishing can be done individually on each spindle to permit positional compensation. As another example, consider the case of an OD thread that can be milled. Because variation occurs from one tool to another, the diameter can be roughed in twin-spindle mode, then thread-milled using a single spindle to permit adjustment of the cutter diam that influences thread quality.
Twin-spindle machines are made in a range of sizes. We provide spindle centerlines with 250, 320, 400, and 600-mm center distances to accommodate different part sizes. Our machines are designed to fabricate parts that fit into a cube ranging in size from 1/8 to 16" (3.2 to 406 mm). Within this work envelope, machines made by us can hold tolerances to ten-thousandths of an inch (a few microns) on a regular basis. Linear motors allow 90 m/min rapids and 2-g acceleration, and permit a machine to position to sub-micron levels for additional productivity.
It's perfectly possible to employ a fourth or fifth axis on dual-spindle machines, making five-axis work possible. Programming for these axes is tied into the CNC--in our case, we use either a Siemens 840D or a Fanuc 18i controller.
Useful features found on single-spindle machines also can be secured on dual-spindle equipment. Tool magazine storage, for example, is certainly available. We offer machines with tool magazines that carry 8 - 36 tools per spindle to provide redundancy for tool breakage, and for tool replacement made necessary by tool wear in lights-out production or untended third-shift operations.
Worldwide, twin-spindle technology has been well accepted. Machines that use this architecture enable a single operator to be more productive, and this enhancement in personnel utilization is attractive almost everywhere. To be sure, regions where labor is relatively costly are most inclined to investigate dual-spindle technology, as a means of competing with lower wages in the formerly communist nations of Central and Eastern Europe, and Asian countries like China. The countries of the European Union, where labor costs are really quite high, are where twin-spindle technology has been most widely applied. But two-spindle machines are a flexible means of improving the productivity of any company, anywhere, that's involved in medium-to-high volume machining of parts.
This article was first published in the September 2005 edition of Manufacturing Engineering magazine.