by Kip Hanson
The benefits of vertical multi-tasking machines for large part machining
Say the words “vertical turret lathe” (VTL) and many seasoned machinists immediately think of an ancient Bullard, or a big clunky King, squat monsters able to turn an ungainly chunk of steel large enough for a locomotive wheel. Yet our grandfather’s VTLs have evolved over the years into highly efficient and accurate production centres that serve a variety of high tech industries–disks used in gas turbine engines, automotive tire moulds and wheels, bearing races, flanges for oil and gas plumbing…the list goes on.
Some of these machines utilize an inverted spindle design, perfect for automatic pickup of workpieces from a load/unload station, while others are multi-tasking devices more machining centre than lathe. And like their forbearers, many VTLs are huge, able to machine workpieces several metres across and carry table loads best measured in tonnes. Simply put, vertical turret lathes are an important part of today’s manufacturing world.
Newton would be proud
Tipping a horizontal lathe on its side, so to speak, creates several advantages. The same force that makes the tide roll in each day and keeps humans from floating off into space works in one’s favour when machining large parts, especially those weighing more than an automobile. “With a VTL, gravity is on your side,” says David Fischer, lathe product specialist at Okuma America, Charlotte, NC. “This
makes part loading easier, and allows for workpieces with longer length to diameter ratios than on a horizontal lathe.”
Because gravity is your friend on a VTL, less clamping force is needed to grip parts. This makes them ideal for large rings and other thin-walled components. And considering the part diameters being turned on one, the footprint of a vertical turret lathe is substantially smaller–roughly half the size in many cases–compared to its horizontal cousins.
“VTLs are very good at what they do,” Fischer explains. “Granted, they’re more limited in some ways than horizontal lathes, primarily due to a VTL’s relatively short Z axis travel, but for shops that have the right work, they’re an awesome addition.”
Nowhere is this more true than with large workpieces. To serve this market, Okuma offers a number of machines, the largest of which is a double-column, ram-style machine capable of workpieces up to 3.5 metres in diameter, as well as the company’s VTM2000-YB, a five axis VTL. “We’ve added significantly more spindle horsepower compared to previous models, and have also moved to roller-bearing guideways,” says Fischer.
To those box-way aficionados, Fischer says roller guideways are often better for machining complex parts. “You don’t have to deal with the stick-slip issues that can occur with box-way construction. Manufacturers want machines that respond extremely quickly, with very high traverse speeds; these are two things that roller guideways are known for.”
The thin grey line
Another thing manufacturers look for is flexibility. Rick Ware, vice president of sales and marketing at Mazak Corp., Florence, KY, points to the company’s MegaTurn series for those shops dealing with low volume, complex machining. “All of the MegaTurns can be equipped with milling capability, as well as an optional toolchanger. Both serve to reduce part handling and eliminate secondary operations.”
Throughout the industry, machine tools are increasingly capable of multiple machining operations in a single part handling. Vertical turret lathes are no exception. This makes the line between machining centre and lathe a bit fuzzier with each new machine model. Aside from milling capability, Ware says many of Mazak’s VTLs are also equipped with pallet changers, a perfect option for job shops that need to accommodate a wide range of difficult-to-handle workpieces.
“More and more, shops are running small to medium lot sizes with quick turnaround times,” Ware explains. If they can get a part “done in one,” it’s a big benefit, especially when an overhead crane is needed to maneuver workpieces to secondary operations. And pallet changers add flexibility in terms of setup. When a customer calls with a rush order for a huge diesel engine component, the emergency job can be run on one pallet without disturbing the setup on the other. “This is where multi-tasking can really help them.”
There’s no argument from Ware on the increasing hybridization of machine tools. “For a machine tool builder, it’s about having the right solution for a particular application. In many cases, there may be more than one option, so it’s important to look at the customer’s needs and work with them on the best way to machine the part while working within their budget.”
In the case of a 1 m+ VTL with live tool capability, dual pallets and enough axes to form a five-letter word, the budget is going to be just as large as the machine. Randall Harland, executive vice president for DMG MORI USA, Hoffman Estates, IL, says the company’s NVL 1350MC with C axis and milling capability starts at $900,000, and their DMU 160 FD duoBlock, a five axis machine labeled as a universal milling machine, sells at a $1.5 million base price.
You say tomato
But wait–what’s this talk about milling machines? This article was supposed to be about vertical turret lathes. Don’t be alarmed. As mentioned previously, many of today’s machine tools offer little distinction between milling machine and turning centre, and DMG MORI’s duoBlock is a perfect example of this trend. “We’re approaching vertical turning in a different manner these days,” says Harland. “While we’re still selling traditional two axis vertical lathes, we’re now applying more of our efforts towards five axis, multi-function machines.”
Even if you’re looking at a whiz-bang multi-tasker, many of the same size and rigidity considerations apply—after all, machining parts larger than manhole covers requires a machine tool equipped with high horsepower and plenty of torque. But what if there’s a bolt-hole pattern on the outside diameter, or the customer wants the company logo engraved on the top? In that case, there’s simply no better machine than one with a spindle that can spin the part like a lathe, then switch gears and mill it like a machining centre.
While you’re switching gears, why not cut some as well? Because of their five axis capabilities, many VTLs can switch hit as gear hobbing machines. DMG MORI is one machine tool builder that can utilize Sandvik Coromant’s InvoMill gear milling process, but has also added its own proprietary gear making capabilities. “Our gearMill software allows for a simplified process of nearly any gear shape,” Harland says. “For example, we’re able to interpolate spiral bevel gears using an endmill, or use a disk mill to machine spur and helical gears.”
Another unique capability found on these and other mill-turn VTLs is the option to place turning tools at whatever angle you want–enter a line of G-code and the machine will tip the combination milling spindle/tool head, giving an 80° diamond a 45° (for example) lead angle. “The ability to adjust tool geometry mid-cycle improves tool life and eliminates potential interference problems,” Harland explains. “The flexibility is incredible.”
Maybe your large part machining needs are simple, and a basic twin axis VTL similar to that old Bullard you saw at the used machinery dealer’s will do. Or you might need mind-boggling machining capabilities and have your eye on a state-of-the-art multi-tasker equipped with a pallet changer and multi-axis milling. Whether you make flanges for an offshore oil platform or spur gears for an aircraft carrier, VTLs can be a versatile addition to a shop’s machine tool arsenal. Maybe it’s time to take your turning vertical. SMT