Double Your Spindles, DOUBLE YOUR FUN

Manufacturing Engineering, Sep 2005 by Kunick, Andy

Twin-spindle machining centers offer high-volume production without consuming lots of floor space or costing you a bundle

InEurope, 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 singlespindle 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 dualspindle 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 dualspindle 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-tohigh 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.

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 90° 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 dualspindle 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, twinspindle 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 180°. 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.