CNC Turning Center – An Engineer’s Guide

Turning Center Configurations

Essentially we have two different types of CNC machining centers,  the traditional, horizontal type and the vertical type, which spins the part like a top instead of spinning it like a car tire. 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 configuration. 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, in other words, as you’re turning, all the chips fall down into the chip conveyor or bin.

The advantage of a vertical lathe is that gravity helps seat your part into your workholding, 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. 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.

It’s primarily part size; that’s the biggest factor that determines between the two, 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.

When 30-inch diameter parts run on a horizontal machine 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.

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. 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

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.

Turning Applications

CNC turning centers today are used in most metal cutting environments—whether it’s automotive, aerospace, agriculture, 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!

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. 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.

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, 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. 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.

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.

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.

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, 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, 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.

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

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.

We actually have some customers in applications where they’re not even using the lathe for turning. 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

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. 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.

Robot unloading a horizontal lathe

“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. 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.

Vertical lathes offer built-in automation

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, we still hear about people who have manual lathes in their shop. This is their first CNC and it just amazes us that they can be competitive.

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.

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, 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 the prove-out time can be minimized and the machine is protected from operator error. 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.

• Some customers want to maximize feeds and speeds, so they crank it up to run 20 percent faster. 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. Cycle times will be slower, but at the end of the month the throughput is better.

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 Robotturn for more in-depth coverage of manufacturing.