Added Dimensions

One of the most common attachments to a vertical machining center is a rotary table. It allows you to do more complex work by adding a 4th—and even a 5th—axis of motion to the X, Y and Z axes that originally came with your VMC.

Figure 1: One type of 5th-axis rotary table is an integrated package that combines the movement of both the 4th and 5th axes into a single unit.

There are three primary types of rotary tables: indexer, “semitrue” and true. What mainly distinguishes one from another is how it moves and positions.

A rotary table designed exclusively to work as an indexer will only position to discrete points along a rotational axis. Incremental positions of 5° are common, and some vendors offer indexers that move in 1° increments. The clamping system on an indexer-only rotary table is usually a curvic coupling with a face gear that is hydraulically secured for rigid machining operations.

The semitrue rotary table is essentially an indexer with much finer positioning capabilities. A total of 360,000 discrete positions can be located along the rotational axis. It also is capable of executing true interpolated movement to a resolution of 0.001°. And instead of a curvic coupling, a mechanical braking system (disc or drum) is used to stop and secure the table while actual machining occurs. The semitrue is either linked directly to the machine’s CNC or has its own control.

The true rotary table utilizes the same hardware as the semitrue style and offers the same high-accuracy positioning. However, unlike the semitrue, the true rotary table is always linked directly to the VMC’s controller just like the machine’s X, Y and Z axes. A semi must have this type of interface for it to contour-machine—a task an indexer can’t perform. 

Figure 2: One method used to achieve a 5-axis index or contour motion is to plug the 5-axis rotary table into the 4-axis unit.

A rotary table can locate a part or fixture at a discrete point along a rotational 4th or 5th axis of motion. It also makes a VMC much more productive by allowing multiple parts to be loaded onto a tombstone. Another big advantage of a rotary table is that it lets the operator position a part so that a feature can be machined in the 4th or 5th axis without refixturing.

The 5th axis of motion is a rotational axis that is perpendicular to the 4th axis. There are two common types of 5th-axis rotary tables. The first is an integrated package that combines the movement of both the 4th and 5th axes into a single rotary table (Figure 1). The second is modular and involves plugging the 5th-axis table into a 4th-axis table (Figure 2).

While rigidity is sometimes an issue with plug-in units, the manufacturers of modular tables offer end supports that provide the rigidity needed for 5-axis machining.

Servo-Type Predominates
Early indexers commonly moved in 5° increments, according to Bob Burrows, product manager at Haas Automation Inc., Chatsworth, Calif. A face-gear/curvic-coupling arrangement rigidly clamped the indexer in the desired position. Accuracy was determined by the number of gear teeth and how precisely they were ground. A pneumatic or hydraulic clamping system held the meshed gears in place and allowed moderately aggressive cuts to be made.

While face-gear-style indexers are still available today, servomotor-and-feedback-control systems are more common, according to Bill Meo, president and owner of Koma Precision Inc., East Windsor, Conn. (Koma is the exclusive North American distributor of Tsudakoma rotary tables.)

The servomotor/feedback system allows true-axis positioning at 360,000 discrete points, or the interpolated motion of a true rotary table to a resolution of 0.001° along the rotational axis. In order to accomplish the latter, though, the rotary table must be connected to a full-axis interface installed in the CNC.

Ken Harrison, product manager of the machining center group at SMW Systems Inc., Santa Fe Springs, Calif., said that the majority of modern rotary tables are stopped and held in position by a disc-brake-and-rotor arrangement, similar to what you would find on an automobile. The drawbacks to these systems, he said, are that their braking torque is only one-fourth that supplied by the face-gear tables and they’re significantly less rigid.

This is important to know if you expect to do anything beyond light machining, said Harrison. Because of this, SMW continues to offer face-gear tables that clamp at 5° intervals.

Meo pointed out, though, that many manufacturers, including Tsudakoma, have replaced the disc brakes on their rotary tables with 360° brake drums that provide clamping power and rigidity equal to face-gear systems. The improved holding power, coupled with the ability to position to a 0.001° resolution, allow these tables to perform as an indexer able to handle heavy machining or turn a 3-axis VMC into a full-contouring, 5-axis machining center.

It’s All About the Interface
The servomotors used in today’s rotary tables are controlled one of three ways. The indexer usually has its own control unit, which incorporates a resident program, or an RS-232 interface that allows the CNC’s program to send individual commands to the indexer. The interface for a true rotary table consists of a servodrive, interface card and software. This interface integrates the table’s motion with the X-, Y- and Z-axis motions controlled by the CNC.

An indexer under its own control executes M codes. The program governs feeds, speeds and the duration of rotational motions. But this information is not fed back to the machine tool’s CNC. This means that the CNC will not detect any motion or programming errors that may occur during rotation.

In other words, explained Harrison, two programs—the indexer’s and the CNC’s—are running simultaneously and independently of each other. A crash is likely if an error in either the rotary table or CNC program occurs during operation.

Meo added that another potential problem can arise after a cutting tool breaks. Once the tool is replaced, the operator must realign the lines in each control’s program to ensure that they execute in sync.

In recent years, some indexer manufacturers have taken advantage of the standard RS-232 capability available with most CNCs. The RS-232 protocol allows the CNC to send motion information directly from its resident program to the indexer control that executes the commands. This eliminates the risk of a crash caused by running separate programs that do not have a full two-way communication capability.

With the RS-232 approach, an indexer appears to function like a true rotary table—but it’s not. The CNC doesn’t receive the real-time feedback provided by a full-axis interface, so it is still technically indexing.

A true rotary table interface has a servomotor that’s matched to the specific brand and model of CNC. The servodrive, interface card and control software are machine-specific and permanently mounted in the CNC’s control cabinet. This adds two axes of movement that are as functional as the VMC’s X, Y and Z axes. The real-time interpolation and feedback loop created by this upgrade can turn a 3-axis mill into a true 5-axis machining center.

However, while a true rotary table can provide contouring capability, it is not portable. Unlike the indexer equipped with its own control, the true rotary table cannot be transferred from one machine to another. Therefore, a true rotary table will likely sit idle if the parts being produced do not require indexing or contouring.

Deciding What to Buy
The price of a rotary table package ranges from $10,000 to $90,000. But before you plunk down the cash for one, you need to consider what makes the most sense for your shop. Considerations should include what job(s) you will use the table for and how often you plan to run that type of work.

Both Burrows and Harrison reported that 80 to 90 percent of all machining applications they see require only discrete positioning—not the contouring capability offered by a true rotary table.

In fact, the biggest benefit a rotary table can provide your shop is to improve simple machining tasks. For instance, an indexer might allow you to machine part features that couldn’t be reached with an X-, Y- or Z-axis motion without refixturing.

A 4-axis rotary table can be equipped with a tombstone that will allow multiple sides of parts to be machined without refixturing.

Burrows provided the following example of the benefits possible by using an indexer. Let’s say you have a prismatic part that must be machined on all four sides. Each side requires five separate operations: facemilling, spot drilling, drilling, tapping and chamfering. Without a rotary table, you would have to place the part—or set of parts—in a vise and machine the first side. This would require you to make five tool changes to machine a single side. Then you would need to rotate and reclamp the part.

If dimensional relationships needed to be maintained from side to side, you would need to either use a probe to locate the part or have special locating features in the vise or fixture.

In addition to several minutes of spindle downtime required to manually rotate the parts, you would have to make a total of 19 tool changes.

Now consider the advantages of having several parts mounted on each side of a tombstone that’s affixed to an indexer. With a single tool change, you could reach up to three sides of the part. An added benefit is that the fixturing is simple and the rotary table maintains location accuracy.

If you’re considering the purchase of a rotary table with its own control, you’ll need to buy one of two interfaces from the machine tool builder before you can use the unit as an indexer. The first would only require you to purchase the necessary M codes. The controls of most late-model VMCs already have these codes in place. Older CNCs may not have them. An M-code upgrade can range in price from a few hundred dollars to a couple of thousand dollars, depending on the machine tool builder.

The second interface option, which SMW Systems’ Harrison prefers, is the RS-232. Sometimes called the “B” protocol, it has both a software and hardware component. If your CNC already has an RS-232 hardware port and you’re thinking that you’ll just need the software, you may be in for a surprise. Even if the port can be used to transfer part programs back and forth, it’s probably not the entire interface you’ll need to operate an indexer.

The pricing and availability of RS-232 software depends, again, on the machine tool builder. Some offer a complete RS-232 interface as standard equipment while others charge $1,000 or more, according to Harrison.

The advantage of the RS-232 approach, said Harrison, is that one program—resident in the CNC—governs the movement of all axes. This eliminates the risk of a crash caused by the rotary table control and the CNC executing different programs simultaneously. In addition, if you need 5-axis rotation, which involves affixing two rotary tables to the VMC, they could be “daisy-chained” together on one RS-232 line. The CNC would directly control both rotary tables.

The 5th Element
According to John Raymer, president of Technitron Inc., a Cincinnati-based installer of rotary tables, 5-axis machining was a rarity just five years ago. But the growing popularity of multitasking has pushed even small shops to consider upgrading their capabilities so that they can perform 5-axis work.

Harrison agreed that interest is growing rapidly. “The reason is money, “ he explained. “A typical shop might charge $60 per hour for 3-axis VMC time, while they could charge as much as $125 per hour for 4-axis work. And 5-axis work, done properly, can command even more money.”

True contour machining requires 5-axis, simultaneously interpolated motion. Two ways are promoted to accomplish the task: with modular dual rotary tables or an integrated tilt table. The dual-rotary concept involves adding a perpendicular axis of rotation (the 5th axis) to the machine’s 4th axis.

The modular nature of the dual-table approach is attractive. After installing the 4th axis, the 5th axis can be added later. Or, if this type of 5th axis is installed with the 4th axis, it can be removed from the machine when desired.

Critics, however, say that modular units lack the rigidity of an integrated tilt table. Because of this concern, manufacturers of dual-table systems sell end-support accessories that add rigidity.

There are also two ways to establish the interface that links the 4th axis, 5th axis and CNC. The first is the same RS-232 approach discussed earlier. The CNC sends motion signals to the 4th- and 5th-axis control units via daisy-chained RS-232 cables.

The main advantage of the RS-232 approach is cost. Harrison said that installing an SMW system by this method typically would top out at about $25,000. The disadvantage of an RS-232 is the lack of feedback it provides to the CNC about the actual machining. Feedback is a key component of true contour machining.

Raymer urges people who are considering adding a 5th axis in this manner to exercise caution. The modular/ RS-232 method “may work well for light-duty applications, such as a knee-mill arrangement or for infrequent use on a VMC,” he said, “but for serious 5-axis work, I find that most customers have been disappointed with this approach.”

Raymer favors the second approach—the integrated method—which involves integrating two full axes of motion with the VMC’s existing axes. Doing this requires purchasing a tilt-type rotary table, two servomotors, two servodrives, two control cards and software. The total package can run as high as $90,000.

Both Raymer and Koma Precision’s Meo said that high-end users, such as those found in the aerospace industry, tend to favor the integrated approach. It provides full 5-axis, interpolated motion for true contour machining in an integrated 4th- and 5th-axis package.

Upgrade Caveats
The good news for those shops looking for a rotary table is that almost any modern machine tool working today can accommodate at least an indexer directed via M codes. And most RS-232-capable machines can be upgraded with the appropriate software.

However, when it comes to true 4-axis and 5-axis rotary tables, the CNC generally needs to be no more than 10 years old. The reason is that the servomotor, servodrive and control card must be available and supported by the CNC vendor.

The Fanuc 6 control is over 20 years old and parts are still available, said Raymer, but that is the exception and not the rule.

Even if you have a newer CNC, adding a true 4th and/or 5th axis will still require the purchase of an interface from the machine tool builder. A servomotor that’s compatible with your machine must be purchased to drive the rotary table. And a machine-specific servodrive, interface card and software will be needed to complete the upgrade.

For those contemplating such an upgrade, Raymer recommends purchasing a turnkey package. This transfers the risk of installation to an experienced third party and eliminates the risk of finger-pointing that can occur when the interface, machine and rotary table are purchased from different vendors and then don’t function properly together.

As for training personnel, Raymer said that a competent CNC operator will require very little training to run an index-only rotary table. Others interviewed for this article concurred.

As Burrows noted, “It (an index-only table) is kind of like a new VCR. If you have the patience to get out the manual and take some time, most people can figure out how to use it.”

Meo added that Koma technicians typically can train a competent CNC operator to run the Tsudakoma indexer in about two hours.

The programming necessary for operating a true rotary table is a different story, though. Burrows described true 4th- and 5th-axis machining as being the equivalent of transitioning from 2-D to 3-D CAD drawing—the extra dimensions significantly increase the complexity of the programming.

In the collective experience of our group, most 4- and 5-axis work is programmed by a dedicated CNC programmer, usually using a CAM system. Raymer said that the more progressive shops he works with are using offline CAM systems for 4- and 5-axis programming.

The type of rotary table a shop chooses will depend, of course, on its specific needs, machining operations and pocketbook. But given the growing demand for companies with multitask-machining capabilities, it’s a safe bet that acquiring any type of rotary table will prove to be a wise investment.