An Engineer's Guide to CNC Turning Centers

(Left) Depiction of an ancient Egyptian lathe. (Right) a modern CNC horizontal turning center.

The lathe is one of the oldest manufacturing technologies on Earth. The earliest versions date all the way back to the ancient Egyptians, who invented a two-person, bow-driven lathe around 1300 BCE. Yet despite this venerable history, turning technology has been far from static, as evidenced by the sophisticated CNC turning centers of today.

Read on for an overview of the components, types, operations and applications of CNC turning centers.

Turning Center Basics

Lathes vs. Turning Centers: What’s the Difference?

You’ve probably seen the terms ‘CNC lathe’ and ‘CNC turning center’ used interchangeably.

“[They’re] basically the same thing in my book,” said David Fischer, lathe product specialist at Okuma America Corporation.

Nevertheless, while there is no formal distinction between lathes and turning centers, the former term is often used to refer exclusively to simpler machines—those designed for turning operations alone. In contrast, the term ‘turning center’ usually denotes machines which integrate milling or drilling capabilities, or those with sub-spindles for performing secondary operations.

“In my opinion, a CNC lathe just strictly does turning; it’s a 2-axis lathe with X and Z axes and typically only one chuck,” said Rick Bramstedt, product manager for Mazak’s Advantec division. “A CNC turning center has milling capability, or a second spindle plus milling capability, and so it might have a Y-axis as well. We also call those Multi-Tasking Machines. That’s how I see turning centers: they offer more than just turning.”

Marlow Knabach, Chief Technology Officer for DMG MORI USA, agreed:

“I see it as the evolution of the lathe,” he said. “Most people called it a lathe in the past, but as CNC became more elaborate and with the addition of milling and sub-spindles, it evolved into a CNC turning center.”

Whether you’re working on a lathe or a turning center, the basic parts are the same.

Headstock

The headstock houses the main spindle as well as the speed- and gear-changing mechanisms. The main spindle end often includes a Morse taper. In the early days of industrial lathes, the spindle was driven directly via a flat belt pulley. These days, it’s driven by an electric motor.

Bed

The lathe bed is a base connected to the headstock such that the carriage and tailstock move in parallel with the spindle access. This movement is facilitated by bedways, which restrain the carriage and tailstock in a set track.

Feedscrews and Leadscrews

The feedscrew is a long driveshaft that connects to a series of gears in the apron in order to drive the carriage along the Z-axis. The leadscrew has the same function but operates orthogonally to the feedscrew, moving the carriage along the X-axis.

Feedscrews and leadscrews are manufactured to either imperial or metric standards, which can cause compatibility issues between workpieces made on different lathes.

Carriage

The carriage holds the cutting tool and moves it longitudinally to the workpiece for turning operations or perpendicularly for facing operations. The carriage is composed of two castings: the top, or saddle, and the side, or apron.

Tailstock

The tailstock refers to the center mount which is positioned opposite to the headstock. In contrast to the headstock, the spindle in the tailstock—which can include a taper to hold drill bits, centers or other tooling—does not rotate. Instead, it travels longitudinally under the action of a leadscrew.

Turning Center Operations

There are many operations that can be performed on a lathe, and even more that can be performed on a turning center. Here are some of the most common:

Facing

Facing operations are used to produce flat surfaces on the end of a part.

Threading

Threading operations produce external or internal threads on a part.

Knurling

Knurling operations are used to produce a regularly shaped roughness on cylindrical surfaces.

Drilling

One of the most basic operations, drilling is used to generate holes in workpieces.

Boring

Boring involves enlarging a hole or cavity to produce circular, internal grooves.

Reaming

Reaming operations involve sizing and finishing existing holes.

Taper Turning

In taper turning, the diameter of the workpiece is gradually reduced over the length of the part.

Turning Center Configurations

“You have essentially two different types of CNC machining centers: the traditional, horizontal type that’s been around for quite some time, and then you have the vertical type, which spins the part like a top instead of spinning it like a car tire,” said James Petiprin, key account manager for EMAG, LLC.

“Horizontal probably makes up 60 or 70 percent of the market because it’s been around longer—every machinist learned on a horizontal lathe.”

Horizontal Turning vs. Vertical Turning

CNC turning centers come in either horizontal or vertical configurations. There are also inverted vertical turning centers, which reverse the position of the spindle and the chuck. All three machine types generally consist of the same basic components (i.e., headstock, carriage, etc.), but differ in their orientation. Deciding whether to opt for a horizontal, vertical or inverted vertical lathe depends on a host of factors, but there are some rules of thumb that can help you make the decision.

“The advantage with a horizontal lathe is that gravity pulls the chips away from the part,” said Knabach. “In other words, as you’re turning, all the chips fall down into the chip conveyor or bin.”

A horizontal lathe: Mazak's QTU-200. (Image courtesy of Mazak.)

“The advantage of a vertical lathe is that gravity helps seat your part into your workholding,” he continued. “But the chips can be an issue, especially if your part is concave, since it can trap the chips internally. So you have the possibility of re-cutting your chips. The other concern with a vertical lathe is that the chips fall down into the spindle itself, so your guarding has to be extremely efficient.”

“Generally, horizontal lathes are more flexible since they can have longer bed lengths relative to spindle size,” said Fischer. “They can also use barfeeders and commonly have tailstocks, a rarity on verticals. On the other hand, if you are machining large diameter short parts, especially if they are heavy parts, the vertical lathe works well.”

A vertical lathe: Okuma's V920EX. (Image courtesy of Okuma.)

“It’s primarily part size; that’s the biggest factor that determines between the two,” said Bramstedt. “When we look at small turning applications, a lot of automotive turning applications (transmission gear blanks, brake rotors, etc.) are done vertically and typically with a twin spindle. One benefit of that is that you have gravity working for you; when you put the part in the chuck, it seats itself. Another benefit is chip flow, again thanks to gravity—all the chips tend to fall away from the part into the pan or conveyor.”

“I’ve seen 30-inch diameter parts run on a horizontal machine,” he added, “but loading it is tricky because you need to push the part into the chuck and then hold it while you’re clamping it.”

Another factor to consider when choosing between horizontal and vertical configurations is the extent to which your turning center will be automated. “Horizontal lathes are usually easier [to automate] since the spindles and/or tailstock are at opposite ends of the machine and the turret can be positioned in such a way as to present minimal clearance issues,” said Fischer.

An inverted vertical lathe: EMAG's VL 4. (Image courtesy of EMAG.)

Bramstedt offered a different opinion: “As far as automatic loading, vertical is probably the preferred method because of chip flow and because you don’t need the robot to push the part in order to seat it.”

Regarding inverted vertical lathes, Petiprin noted that, “Inverted vertical gives you a built-in automation that you don’t have on a horizontal, since the spindle comes over and picks up the part, whereas in a horizontal you need a robot or gantry to load the part.”

Is inverted vertical turning right for you? Follow the link to find out.

Turning Applications

“CNC turning centers today are used in most metal cutting environments—whether it’s automotive, aerospace, agriculture,” said Knabach. “Any component that has a high degree of round parts—any type of a gear—usually is machined on a turning center at least blank, prior to machining any of the gears.”

“You really find turning centers in all industries,” said Petiprin. “EMAG’s business is 85 percent automotive. If you break automotive out, there’s transmission and driveline since they have the most round parts, and if you’re using a lathe you’re turning round parts.”

Turret Tooling vs. Gang Tooling

On the chip-making side, a subject of frequent debate amongst manufacturing professionals when it comes to CNC turning centers concerns the choice between turret tooling and gang tooling.

Gang tooling on a CNC lathe.

Turret tooling involves mounting a set of cutting tools on a rotating, indexable toolholder. In contrast, gang tooling involves setting up a row of tools inside the lathe on a cross-slide, which is similar to the table on a milling machine.

Deciding which configuration is best for you depends—as always—on your application, but there are some rules of thumb that can help you make your decision.

“Generally, gang tooling is more useful on small lathes cutting small parts, using a limited number of tools,” said Fischer. “Gang tooling works well in these situations because the cycle time can be minimized since the turret index time is eliminated. Also, tool change-over time can be reduced to nearly zero since the tool plates can be switched out quickly for each job.”

Bramstedt agreed: “That’s the primary reason for choosing gang tooling: dedicated, high-volume parts.

Turret tooling on a CNC lathe.

“A turret can give you 12 tools, but it takes a tenth of a second or half a second to index each tool and you typically have to come off of the part to do that. That costs cycle time, but it’s also very flexible—you can keep the same 12 tools in there and just reprogram the machine to cut a different part.”

“The advantage to a turret is that you usually have a much larger work envelope, so you have less interference from tool to tool,” said Knabach, “which allows you to maintain a larger diameter with a turret, as opposed to gang tooling. Of course, it’s determined by the configuration of the machine, but generally speaking, gang tooling is much closer together and therefore you have a smaller interference zone.”

“A turret permits greater tooling clearance and generally works better for larger parts,” Fischer added. “On turrets, quick change tooling can be used on each individual station to speed up change-over, but it won’t be as fast as changing out the tool plate.”

Knabach agreed. “In today’s technology, it’s pretty short—you’re talking a second or less—but if you have a lot of tools then that can still count up, especially if you’re looking at near net shape turning,” he said.

This means that if you’re going for high-volume, it’s generally better to opt for gang tooling in order to minimize cycle times. “We offer a couple of gang machines,” said Bramstedt. “But those are typically dedicated to high-volume parts because of that half a second turret index time.”

On the other hand, if your primary concerns are changeover and flexibility, turret tooling may be the better option. “Changeover is very long on a gang machine,” commented Bramstedt, “because you have to position the tools, probe them, make a test cut, move them a little and take another test cut, etc. Whereas with a turret, you load the tools, touch off your tool setter and you go.”

Live Tooling

Many CNC turning centers can be equipped with live tooling, i.e., rotary cutting tools powered by independent electric motors. This makes it possible to drill holes in a part perpendicular to the main axis, which can be extremely useful. Does that mean live tooling is always worthwhile?

Live tooling on a CNC turning center.

“Live tooling really has revolutionized the lathe, especially at DMG MORI,” said Knabach. “We have technology where we incorporate a direct drive motor inside our turret. A conventional drive mechanism would rely on gears or a belt and pulley to drive the tool, but with ours it’s an integral spindle.”

“Live tooling is often one of those things that you don’t realize how great it is until you actually use it,” agreed Fischer. “Once customers gain some experience with it we often see them add the Y-axis as well to provide even more capability and flexibility.”

“If I’m doing a bearing race or a bearing cone—something where you don’t need to drill any holes—then I won’t need rotary tooling,” said Bramstedt, “But other than that, I can’t really think of any cases where it wouldn’t be useful.”

Fischer agreed: “There are, of course, certain situations where it is better to break out the individual processes but these cases are fewer and fewer. As lot sizes get smaller and machine capabilities get better (and faster), the need to break out processes is reduced.”

“The benefit of live tooling is that it reduces work-in-process, which is dollars in your pocket,” said Bramstedt. “If I have to turn this part on a lathe and then take it off to cut a keyway, I have to put that on a vertical machining center, so now my part is sitting on a skid while I set that up.”

Live tooling on DMG MORI's CTX beta 800 TC. (Image courtesy of DMG MORI.)

“We actually have some customers in applications where they’re not even using the lathe for turning,” said Knabach. “They’re using it as an automated production center with a bar feed, feeding the bar into the machine and perhaps never turning on the turning spindle, but just using the live tools as a machining center. They might use the turning spindle only to turn the bar long enough for the cut off operation. Then you can use your parts catcher or overhead gantry, whichever’s required, and now it’s a fully automated system from a simple lathe with live tooling and perhaps a Y-axis.”

Turning Center Automation

With the dawn of Industry 4.0, automated machining is becoming more widespread. CNC turning centers are no exception, although automating a CNC lathe depends on the configuration of the machine (Vertical vs. Horizontal vs. Inverted Vertical).

Robot unloading a horizontal lathe. (Image courtesy of Zacobria.)

“A horizontal lathe is typically loaded by a manual operator, so for automation you go with a robot or a gantry—something that would pick it up from a known location and then put it in the chuck,” explained Petiprin. “You want to utilize 80 percent of the robot’s time, so that usually means you can split two machines between one robot, though if you have a shorter cycle time then you’ll need one robot per machine.”

“Generally horizontal lathes are easier since the spindles and/or tailstock are at opposite ends of the machine and the turret can be positioned in such a way as to present minimal clearance issues,” Fischer added. “Either way, all of our machines have been automated in many different configurations.”

Although CNC turning centers are just as amenable to automation as other machine tools, there are some important differences between automating a lathe and automating a machining center.

EMAG's inverted vertical lathes offer built-in automation. (Image courtesy of EMAG.)

“The fixturing or the workholding is the biggest difference,” said Bramstedt. “We’ve automated machining centers on a regular basis, but you’re typically loading into some type of special fixture with automatic or even sequential clamping. It’s a much more complicated fixture and typically dedicated to a part. That’s opposed to a lathe, where I can load a variety of different parts with just a three jaw chuck. It’s very flexible as far as workholding goes.”

CNC Turning: Bottlenecks and Mistakes

In manufacturing, mistakes and bottlenecks in efficiency are to be avoided at all costs. This holds true even for a technology as old as the lathe, though the advent of computing in manufacturing has gone a long way toward minimizing these issues.

“Believe it or not, I still hear about people who have manual lathes in their shop,” Bramstedt commented. “This is their first CNC and it just amazes me that they can be competitive.”

Fischer offered a very different perspective:

“A seasoned veteran machinist (who also happened to be my dad) once told me that no matter how many CNC lathes a shop had, they would always keep at least one manual engine lathe and that is true here where we have two manual Okuma lathes still in service.”

Bottlenecks for CNC Turning

One of the major limitations on lathe efficiency lies in the turning operation itself. This is not an issue that can be overcome with better tooling, as Bramstedt explained:

“Machine tools are always trying to catch up to tooling capabilities. You might be able to cut at X amount of surface speed, but I can’t spin my chuck fast enough to generate that kind of surface speed. With turning, your rpm is limited because of chuck grip capabilities. The centrifugal force means that as the chuck is spinning around, the jaws want to move outward. On a machining center, the part is stationary and my tool—which is very compact—can be spun at 10, 12, 20 or even 40,000 rpm without big issues. There’s no way you can spin a chuck at 40,000 rpm; it would just come apart.”

Another bottleneck for turning efficiency is one which applies to most manufacturing processes: changeover.

“Depending on lot size the importance of change-over varies,” said Fischer. “For a shop that does short run production, change-over is critical and your workholding and tooling systems must be carefully considered. These things take away from the available production time of the machine.”

This raises the issue of program prove-out, which can be time consuming, though there are ways to reduce it. “By having good simulation software and utilizing Okuma’s Collision Avoidance System (CAS) the prove-out time can be minimized and the machine is protected from operator error,” Fischer commented. “And nothing eats into your production time like a machine crash!”

Common Mistakes in CNC Turning

Maximizing efficiency is one goal in manufacturing, but that goes hand in hand with minimizing mistakes. Bramstedt and Fischer each pointed to different sources of error when it comes to using CNC turning centers.

“Some customers want to maximize feeds and speeds, so they crank it up to run 20 percent faster,” said Bramstedt. “They can do that, but then they have to stand there and wait for something to explode, whereas if they back off a little bit they can walk away from it. That’s not to say nothing will happen, but the chances of unexpected tool failure are minimized because we’re not pushing it to the maximum. We have a lot of unattended cells here at Mazak where we do just that. Cycle times will be slower, but at the end of the month the throughput is better.”

Fischer emphasized the importance of training: “You are placing a high-tech piece of equipment in the hands of an operator/setup person/programmer. Invest in training these people so that they can perform at a high level.”

Getting the Most from Your CNC Turning Center

Lathes have been around for most of human history, and with good reason. Although the underlying technology has continued to advance, turning operations remain a vital part of many manufacturing processes.

That being said, turning is just one aspect of manufacturing among many. Keep an eye on Manufacturing 101 for more in-depth coverage of manufacturing, including:

For more information on turning centers, visit the websites for DMG MORI, EMAG, Mazak and Okuma.