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The Case for Multiple-Spindle HMCs


Are multiple spindles the next step in the evolution of the horizontal machining center?

By Mark Witkorski
Manager Corporate Development
Al Bingeman
Chief Engineer
Liberty Precision Industries
Rochester, NY

For more than a decade, soaring productivity combined with almost no price increases have made machining centers, in particular horizontal machining centers (HMCs), a wise investment for manufacturers in a wide range of industries. By aggressively reducing machine idle time, sometimes called non-cut time or parasitic time, machine-tool builders have been able to dramatically boost the number of parts a machining center can produce.

In recent years, however, attempts to raise the part output of these HMCs have stumbled. Machine modifications that squeeze one tenth of a second from tool change time or add 10 ipm (250 mm/min) to rapid rates have only added cost and reliability risk without delivering higher part production. Machine-tool builders are facing a diminishing-returns dilemma. What will break this cycle? We believe Multiple Spindle Machining Centers (MSMCs) will provide the next great leap in productivity, while retaining the flexibility inherent to CNC machining centers.

In the 1980s, a new machine design revolutionized prismatic machining. By adding toolchangers and workpiece shuttle systems, machine builders were able to double the output of traditional milling machines. All the idle time wasted by changing a tool manually or stopping a milling cycle to unload a finished part and load a new part was eliminated. Automatic toolchangers (ATCs) and automatic workchangers (AWCs) enabled part output to rise dramatically.

By the early 1990s builders were designing a new generation of horizontal machining centers that tightly integrated faster ATCs and AWCs into the machines, rather than bolting them onto older milling machine designs. By moving the Z-axis motion under the workpiece, called by some builders a T configuration, they were able to manufacture a HMC with a one-piece base that increased machine rigidity. In addition, these second-generation HMCs were fully enclosed, preventing chips and coolant from spilling out onto the factory floor. Second-generation HMCs also featured motorized spindles and linear guideways. Spindle acceleration and deceleration times were reduced, and feed rates were boosted. The second-generation HMCs once more raised part output.

As the 1990s ended, the productivity of single-spindle HMCs had risen to where they were encroaching on part applications that previously had been the realm of dial machines and transfer lines. The opening of applications in industries such as automotive and appliances encouraged builders to further enhance HMC productivity. They attacked other non-cut times such as table indexing and AWC shuttle times. Sales into these industries raised the volume of HMCs purchased, increasing HMC production and keeping manufacturing costs low. Average selling prices throughout the 1990s remained flat, while machine productivity soared.

The late 1990s saw another technological attempt to raise HMC productivity. The introduction of HMCs equipped with linear motors began at trade shows like IMTS, EMO and JIMTOF. Unfortunately, the additional manufacturing cost and reliability risks associated with this new technology did not translate into large productivity improvements. The dilemma of diminishing returns had taken hold. Victims of their own success, the builders had reduced non-cut time to a mere fraction of overall part-cycle time. That 20% faster rapid rate or 10% faster tool-change time just didn't deliver the benefit in part output.

Is innovation with these machines finished? No. There is an emerging trend that could double the output of these machines just as toolchangers did in the 1980s, and linear guideways and motorized spindles did in the 1990s. By equipping HMCs with multiple spindles it becomes realistic to double or even quadruple part output compared to a single-spindle HMC.

Multiple-spindle machines have been popular in certain niche markets for many years. Large three-spindle profilers are an industry standard in the aerospace industry. The key benefit driving these designs was production of multiple parts simultaneously where there was a long part-cycle time compared to non-cut time. This high ratio of cut time vs. non-cut time is now found in machining center applications. That is why this benefit of simultaneous machining of multiple parts can be brought from profiling shops to production shops.

A trunnion system simplifies loading and unloading of work in a multiple-spindle HMC.

In this quad-spindle machine, one spindle is the master and the other three follow it.

What characterizes these new MSMCs? As we've stated, they retain typical machining center characteristics such as ATCs, AWCs, and high-speed spindles that provide four and five-axis machining. The two differentiating characteristics of this new generation are spindle configurations and workpiece-changer designs. By adding a second spindle, the MSMC immediately gains the productivity of two parts machined simultaneously. MSMCs can come in different configurations. They can be built with the two spindles fixed in relation to one another, or they can move independently in the X, Y, and Z axes. In addition, MSMCs can be made in both vertical and horizontal configurations.

Workpiece exchanging or shuttling on MSMCs is another area that differentiates them from single-spindle HMCs. Builders of MSMCs have moved in the direction of horizontal trunnions to swap workpieces in and out of the machining zone. The horizontal trunnion provides two benefits over traditional vertical pallet swappers.

  • When chips are machined from the workpiece they are free to fall directly into the chip collection system, and 
  • Workpieces are presented at the load/unload station to the machine operator in an ergonomically friendly orientation.

The final distinguishing feature found with MSMCs is their rigid machine design. These designs deliver high machine stiffness and minimal thermal growth. The SW BA 400 series machine, for example, has what SW calls a monobloc design. It's patented, and consists of a one-piece weldment that is designed to allow optimum force flow between the three-axis machining unit and the workpiece. It has a static stiffness of 424,188 lb/in (7582 kg/mm).

Thermal growth between spindles is also minimized by the machine's symmetrical design and direct coupling of the spindles. When it's machining features in a common setting and surface--that is, when the parts in a common fixture and the parts don't rotate--the small thermal variation, high stiffness, and precision inherent to the design allows the MSMC to achieve accuracies that approach those achieved by the best single-spindle machines. True position tolerances of 0.05 mm or less are possible, and a process capability ratio as high as 2.0 Cpk can be reached, depending upon the feature machines, tolerance, fixturing, tooling, etc.

When multiple part clampings or part rotations are added into the process, the user needs to pay more attention to fixture setup. Making fixtures easy to adjust can minimize the impact of this additional setup on the process. Position tolerances of 0.100 mm or less, and process capability index to 2.0 Cpk can be achieved. If, however, there's a particular, tightly toleranced feature on the part, an option allows the user to program the machine to go into a single-spindle operating mode to machine the tight-tolerance feature, and then return to multiple-spindle operation.

With anything new, questions are bound to arise. Typically, there are questions concerning the programming and operation of these machines. Let us be clear right here; your current programming system will support these machines. The CNC is executing one part program. It's the mechanical design of the machine that produces the additional one or three parts.

A machine with two or more spindles may appear to be a complex machine, implying higher downtime and lower availability rates. In fact, when compared to a single-spindle HMC, the MSMC, for the equivalent part output, has 300 fewer potential points of failure (switches, wiring points, mechanical parts) than the single-spindle HMC. In addition, the second spindle acts as a redundancy that allows the machine to be run in a single-spindle mode in the event that one of the spindles fails.


Over time, attempts to raise HMC productivity by reducing non-cut time have delivered diminishing returns.

Fixturing for a MSMC is very similar to fixturing for a traditional single-spindle machining center. The primary difference is that there are two parts located side by side. When setting the machine up, a relationship is established between the first spindle and the first fixture. After the relationship is established on the first fixture, then the second fixture is adjusted in relationship to the second spindle. If a machine crash occurs, this process will need to be repeated to reset the relationships between machine spindle, fixture, and part, just as would be done on a single-spindle machine.

Some manufacturers offer software programs that assist in determining the amount of offsetting adjustments required over time. CMM data are fed into the program, and the software will determine the amount of movement required. For example, if spindle #1 is at the mean of the tolerance zone and spindle #2 is out of the tolerance zone, a positional move would take the first spindle away from its mean, and the second spindle would move within its acceptable tolerance zone. Options are also available to use a probing head to check the position of the fixture prior to machining. This system can be deployed to check fixture position as frequently as every cycle, or it can be used as little as once per shift or once a day.

Another emerging application is the integration of MSMCs into high-volume hybrid systems. The MSMC is teamed with traditional dedicated slide units to interject flexibility into transfer lines.

Multiple Spindle Machining Centers (MSMCs) are the Third Generation of machining centers. When compared to single-spindle machining centers, these machines deliver reduced floorspace, lower energy consumption, less automation, superior ergonomics and chip control, fewer points of failure and higher part output. With a large installed base in Europe, this technology is ready for prime time in North America.


A View from the Floor 

Continental Teves, Brake and Chassis Systems, (Culpepper, VA), makes antilock braking systems, electronic stability system components, and traction control systems. It's a high-production shop, according to Manufacturing Engineer Don Sanner. "Completing one job will require making a few million parts," he observes.

Sanner and his colleagues use dual-spindle and quad-spindle HMCs made by Schwabische Werkzeugmaschinen GmbH (SW), as well as single-spindle machines. They had been accustomed to working with single-spindle equipment, and found that moving to multiple-spindle machines required them to think more about process design strategy.

Single-spindle machines enable manufacturing personnel to move the spindle about to accommodate minor fixturing errors. Putting it differently, the machining operation could be tweaked. When more than one spindle is involved, says Sanner, "you step back almost to a transfer-line situation. The fixturing has to be perfect--you need to follow the fixture." Because the spindles are aligned, and you're making two to four parts in one go, each part must be held accurately to match the spindle spacing.

The machines are great for producing lots of parts. "We nearly doubled or quadrupled our machining capacity in this area," Sanner observes. "Setup is more challenging, however. There is much more setup time involved prior to machining."

In Sanner's opinion, whether someone should use the machines depends upon the complexity of the part being made. "We use these machines in a five-axis situation. If it's a simple part, the machines work well. If you have very tight product tolerances, say within 0.06 mm in five-axis work, it's difficult. Because it's hard to make fixturing that is that good when you consider the tolerance stackup on the machine out at five axes. Our tolerances are in the 0.1-mm range, which makes the machines well suited to us."

The machines use the same controllers as the single-spindle HMCs employed at Teves. "One spindle is a master," says Sanner, "and the others are slaves."

He also likes the design of the SW machines. "They're set up in three basic areas: the operator area is isolated outside the machine, there's the machining area, and the maintenance area, which is easy to get into. Access to the machine is very simple." And that's important, in Sanner's view, because the higher technology employed in the machine increases maintenance time somewhat. "You cannot misalign spindles," he warns, "they must be accurately aligned."

Don Sanner regards his company's multiple-spindle HMCs as very well-suited for a high-production or medium-volume environment. He's doubtful about their value in a job shop due to the setup time involved for this type of machine.


This article was first published in the March 2004 edition of Manufacturing Engineering magazine. 

Published Date : 3/1/2004

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