Inverting the spindle on a vertical turret lathe can make a great lathe even better

The vertical turret lathe is known for its ability to efficiently produce large, turned parts like mag wheels, bearing hubs and gear blanks. VTLs offer many advantages over their horizontal cousins: Part loading is easier, and even very large parts can be accommodated without increasing the machine footprint.

However, VTLs have a significant drawback, one familiar to anyone operating a vertical machining center—the chips fall onto the workpiece, where they are subject to recutting.

Several machine tool builders offer VTLs that avoid this problem. By inverting the spindle, chip problems are virtually eliminated. This approach also provides nearly free automation. By using the spindle as you would a robot—picking up parts from an onboard conveyor or carousel and unloading them—throughput is increased without the hassle of robot integration, programming and gripper management.

‘Drehen Fräsen’

Jakob Petker, an applications engineer who handles technical sales at DMG Mori USA, Hoffman Estates, Ill., said the machine builder offers several models of VTLs, each with an inverted spindle and live tools. “We have three different sizes, ranging from our CTV 160 to our CTV 315. There’s also a ‘df’ version of our CTV 250—this stands for drehen fräsen, German for turn/mill. It is designed specifically for the homokinetic joints [CV joints] used in automobile drivetrains.”

Image courtesy of EMAG.

Petker said the need for efficient production of these and other automotive parts has been the primary driver behind machine development, adding that VTLs are also suitable for hydraulic components, chain wheels, bearing housings and other short, squat parts. Because of the nearly free automation that comes with an inverted spindle, these machines are typically used for automotive-level production volumes. By using flexible, quick-change workholding and clever job scheduling, even lower quantities—in the 100- to 1,000-piece range—can be cost-effectively machined.

“We also have a lot of customers that connect two machines together on the same conveyor,” Petker said. “This gives them the ability to perform Op. 10 on one spindle, flip the workpiece and then pass it to the second machine for finishing. Because milling functionality is also available, parts can often leave the cell complete.”

Kirk Stewart, director of sales for EMAG LLC, Farmington Hills, Mich., agreed. He said a traditional “right-side up” VTL is geared toward very large, heavy components that would be challenging to run on a horizontal spindle lathe. However, the sweet spot for inverted machines is from 3″ (76.2mm) to about 14″ (355.6mm) in diameter.

“Big parts are much easier to run on a VTL because you have gravity working in your favor,” he said. “You can just set the part down on the chuck and clamp it. But inverted machines compete nicely in the horizontal lathe market—especially at the higher volumes seen in the automotive market—on transmission parts, for example, or any round part that’s roughly as tall as it is wide.”

Top-Down Precision

Stewart cited the various benefits of an inverted VTL machine structure: First, because chips don’t fall onto the workpiece, there’s no chip re-cutting as there is on a traditional VTL. In addition, the machine’s scales, motors and drive components are above the work zone. Therefore, coolant and its associated contaminants are not “going into the expensive part of the machine.” Also, the ability to automatically self-load and unload parts is a given on any inverted VTL.

High-capacity carousels equipped with flip stations for secondary operations provide automated handling between machine tools. Image courtesy of DMG Mori.

In EMAG’s case, this loading and unloading is done with a racetrack-style carousel equipped with V-shaped, self-centering workpiece-blank locators. “In the case of an inverted VTL, such as our VL2 and VL2 DUO,” Stewart said, “the cutting tool remains static on the indexing turret. Meanwhile, the spindle, together with the workpiece, is moving in the X and Z axes above it.” As a result, Stewart added, the user has the ability to take the workpiece to an unload location, index the carousel or shuttle to pick up a raw part and then take it back to the turret for machining. “It’s this difference in machine architecture that allows these machines to be self-loading.”

To pick up a part without mashing it into the spindle, a gimbal plate with four heavy-duty die springs is mounted in the loading position. When the spindle comes down to grab a new blank, the springs compress enough so any Z-axis deviation is absorbed, preventing damage to the spindle and chuck. The mechanism is also equipped with sensors to detect any misloading or misalignment issues and stop the machine in the event of a problem.