When more is better: Drilling Performance

Like ice cream, multiple-spindle machine tools come in an assortment of flavors. The underlining ingredients, however, are enhanced throughput, reduced energy consumption and maximized floor space.

In the minds of some metalworking professionals, multiple-spindle machining conjures up images of massive transfer lines with 40 or more spindles on a head, or 6-spindle automatic screw machines for making huge quantities of the same parts—a capability that continues to diminish in demand. Six-spindle technologies are great at pounding out a lot of parts, but don’t have the flexibility to produce less-than-huge lot sizes and can’t adapt to the pace of part design changes seen in today’s production environment, according to David Fischer, product specialist for Okuma America Corp., Charlotte, N.C.

Okuma offers an array of multiple-spindle alternatives that enable more flexibility. These include twin-spindle vertical and horizontal turning centers with side-by-side spindles and horizontal turning centers with two opposing spindles. “You’re basically machining on one side and then passing it off and machining it on the other side,” Fischer said about the latter. “It may have some oddball secondary operations, but for the most part, you can go to finished part without any operator intervention with dual-spindle capability.”

VIDEO

It’s critical is to properly capacitize a multispindle machine, such as this Chiron DZ08 FX dual-spindle machine. Click here to view video courtesy of Chiron America.

He added that with the side-by-side verticals, most customers run the first operation on one spindle and the second operation on the other after a robot or operator transfers the part. In contrast, the side-by-side horizontal-type lathes come packaged with a parts loader and users either simultaneously run the same operation on both spindles or run separate operations on each spindle. “There’s a lot of flexibility and that’s the whole purpose,” Fischer said. “The customer can decide what suits him best for his production application.”

Although the builders of multispindle machine tools typically max out at 4-spindle offerings, Chiron America Inc. did produce a special-purpose 7-spindle configuration for machining injector bodies, explained Norm Holtzhauer, engineering manager for the Charlotte, N.C., company. The tool in the first spindle is for centering, the next two perform predrilling and the remainder simultaneously deep-hole drill four parts.

Automated part load and unload enhances flexibility by enabling users to teach the robot location points for a specific part and then load that set of “teach points” when a job repeats, Fischer noted. Nonetheless, the twin-spindle machines are for relatively high volumes. “There is a lot of flexibility, but you’re not going to run it for onesies or twosies or 10 or 20 parts.”

Suitable Scenarios

SW North America Inc., for example, targets its 2-, 3- and 4-spindle horizontals for production volumes starting at around 250,000 parts per year, according to Matthew Raleigh, application engineer for the machine tool builder. However, he noted if a part has a long cycle time, say 20 minutes, an annual volume of 50,000 parts can be appropriate.

Because of the need for high part volumes, the company targets Tier 1, 2 and even 3 auto suppliers, added James Campbell, vice president of sales for SW North America. The German machine builder opened its Canton, Mich., facility this year to expand business throughout the North American market.

Again, what’s critical is to properly capacitize a multispindle machine. “We have aerospace and medical companies making parts that take 12 to 15 minutes on twin-spindle machines,” Holtzhauer said. “Their volumes are only 20,000 to 30,000 parts, but it takes a whole year to make them.”

A twin-spindle machine with two part nests per spindle is the “sweet spot” and is twice as fast as a single-spindle machine with two nests, he explained. “We call that 2+2. That is the fastest you’re going to be,” he said, adding that Chiron has conducted tests with four to 12 nests. “You can add more parts, depending on the size of your machine and size of your part, but you reach a breakeven point where having to build all the extra nests doesn’t pay for the extra volume you get.”

The four nests are aligned in pairs, and two can be fixed while the other two are movable in the X, Y and Z axes, Holtzhauer added. To enable high part accuracy, the company uses ground shims to adjust the nests and lock them in within microns of each other, with two in the X-axis, one in the Y-axis and three in the Z-axis.

A 4-spindle machine becomes an option when part volume or cycle time dictate that multiple machines would otherwise be required to meet production demands, according to Holtzhauer. This machine would reduce floor space while achieving production goals. Possible applications include deep-hole drilling injector bodies and fuel pumps. “The deep-hole drilling process is slow, so we can have four spindles doing the same process,” he said.

Raleigh noted 4-spindle machines are for parts with tight feature tolerances, such as a brake caliper, but don’t require tight positional tolerances. “On more critical parts, let’s say an aluminum transmission housing, generally we always go with a 2-spindle machine,” he said. Three-spindle machines are for special applications that fall between the volume requirements for a 2- and 4-spindle one.

Step Lightly

In addition, multispindle customers tend to be concerned about floor space. An SW twin-spindle machine can replace three single-spindle machines, according to Campbell. The multispindle unit, even with its heavy monoblock design to minimize deflection, has only a slightly larger footprint than a comparable single-spindle machine while costing about 50 percent more, he added. Most multispindle machines are for new projects and customers want to see how many parts they can produce per square foot while reducing the cost per part. “Nobody wants to add brick and mortar anymore,” Campbell said.

A twin-spindle machine with two part nests per spindle is the “sweet spot” and is twice as fast as a single-spindle machine with two nests, according to Chiron America. Courtesy of Chiron America.

A multispindle machine reduces the cost per part by reducing the required labor, automation and ancillary equipment, Raleigh pointed out. “You don’t have to buy two cooling units, for example, and you don’t need two robots to load and unload it,” he said, noting that one robot or a gantry system is sufficient. “You’re machining two, three or four parts at the same time and making tool changes at the same time. You can amortize those values over two, three or four parts, depending on how many spindles you have.”

A multispindle machine also reduces energy costs compared to a single-spindle machine. For example, generating clean compressed air consumes expensive energy. According to SW, four 1-spindle machines each have a one-position encoder in the X, Y and Z axes, all 12 of which require air for sealing. A 4-spindle machine only needs four (two in the Y-axis) and, therefore, sealing requires 67 percent less compressed air.

“It doesn’t show up on a lot of spreadsheets, but there is a lower energy cost,” Campbell said.

Lightning Fast and Lights Out

To boost throughput by getting complex parts done in one, Eurotech offers double-spindle, double- or triple-turret turn/mill centers with two or three Y axes, said Larry Greenawalt, applications engineering manager for the Brooksville, Fla., machine builder. “You start with raw bar stock coming from a bar feeder through the spindle and complete everything that’s needed on that part.” This can include polygon milling, broaching, spline cutting and drilling angular holes with an articulating B-axis or angular live tools. “Can we produce this part where maybe the customer just has to clean it, put it in a bag and send it out? That’s what we look at first.”

Eurotech’s Clearshift feature enables the subspindle to be offset from the main spindle. This eliminates most interference problems between the two turrets on Trofeo series machines. Both turrets can machine on the main spindle or subspindle. Courtesy of Eurotech.

Greenawalt emphasized the ability of the upper turret to perform ID operations up to about 10 ” (254mm) long with the Clearshift subspindle on Eurotech’s Trofeo twin-spindle machines. To eliminate interference between the two turrets, the subspindle shifts down and away from the main spindle centerline instead of having the subspindle in line with the main.

The alternative is to build a machine with an extremely long Z-axis. “You don’t find that many long-bed, 2-turret machines out there,” he said. “But we have that flexibility to shift down and out of the way, so when simultaneous operations are going on you really don’t have any concern over clearance between two turrets trying to share the same space.”

A gantry loader helps the Trofeo machine perform unattended machining, Greenawalt explained. It normally comes down and catches the part coming off the subspindle, but it can also articulate 180° and catch from the main spindle, which is beneficial when handling short bar remnants. Rather than pushing a remnant into the chip pan and letting the conveyor carry it, which might cause it to jam the conveyor or need to be manually separated from the machined chips, the gantry unloader catches the remnant and brings it to the work conveyor that removes parts from the machine. While good parts come out towards the operator, an M code reverses the conveyor to deliver the remnant to the back of the machine.

A gantry loader helps the Trofeo machine from Eurotech perform unattended machining. Courtesy of Eurotech.