Rethinking Indexers And Rotary Tables

Many plant managers and shop owners dream of having the latest horizontal machining center (HMC) with all its features, benefits and sophisticated capability. While typical HMC features such as an automatic pallet changer and a 100+ cutting tool magazine are valuable, perhaps the most valuable characteristic is the HMC’s ability to machine on more than one side of the workpiece due to a built-in indexer or full fourth axis.

On complex workpieces that require machining on surfaces not 90 or 180 degrees from each other, indexing or fourth-axis rotation is almost essential to produce the piece. Even when rectangular workpieces with all surfaces 90 or 180 degrees from each other are put on a tombstone, the HMC’s built-in fourth axis of rotation creates a productivity advantage. This is true even if machining on more than one side of the part is not essential.

Any time you can increase the “run cycle,” do more cutting in one operation and avoid handling the workpiece, productivity goes up. Workpiece accuracy also improves. Unclamping and refixturing a workpiece to present a different surface to the cutting tool is always going to introduce some error.

Low Investment

The high cost of horizontal machining centers compared to the incredible values available today in vertical machining centers puts horizontals out of reach for many shops. Fortunately, today there are several suppliers of quality accessories that allow the VMC shop to equip its verticals with indexers, fourth axes and tombstones. These add-ons really work and give many of the benefits of an HMC at a fraction of the price.

Earlier rotary tables and indexers didn’t have the accuracy, rigidity or control flexibility of today’s models. Many shops that tried using indexers in the past had been disappointed in the performance of the older models and abandoned their use in favor of multiple operations, multiple holding fixtures and multiple handlings of the workpiece. They decided that the manual, multiple-operation process was better than trying to use ineffective early model indexers and rotary tables. Today, the situation is different. Manufacturers now offer units that are very accurate, very rigid and have a variety of control and interface options.

What’s Right For You?

The best indexer and control system for you depend on the work you need to do. As with most things, different designs compromise certain capabilities to gain others. Unless you understand these trade-offs, you are at risk of selecting something other than the best system for your requirements. Let’s see what’s available, review the differing capabilities and discuss the advantages and disadvantages of each design. Once you understand the options, you can evaluate them against your requirements and then consider prices and suppliers.

First of all, it is clear that there is no one model perfect for all applications. The ideal “fits all applications” system would:

• have the highest accuracy of absolute index positioning,
• be infinite positioning (360,000 positions),
• have very fast indexing speed,
• have maximum holding power when in position to resist cutting forces,
• be easy to retrofit to any machine and CNC control,
• be easily portable from one VMC to another,
• require no extra programming,
• be impossible for the machine operator to allow a “crash,” and
• be the lowest price from the supplier that gives the best service and support.

Of course, such a system does not exist. Add the “lowest price from the supplier that gives the best service and support” component and it probably never will exist.

Specifications Can Be Misleading

Terminology in the area of indexers is not standard. Terms such as fourth axis, indexer, rotary table and so on are used interchangeably by different machine tool and accessory companies. So, when selecting and buying, you must ask a few questions before assuming you know what you’re going to get. Also, beware of terms such as “precision,” “high precision,” “accurate,” and “rigid.” Is the “brake torque” specification some absolute break away spec or the torque at which some “unacceptable” amount of rotary deflection occurs? Is the “ten arc seconds” accuracy specification certified every one degree, or is it inspected only every 15 degrees? There are no industry standards for specifications and testing. So ask questions and deal with a supplier in which you have confidence, or buy with a guarantee of performance to make your parts.

We’ll start with the mechanical hardware and discuss the electronic control options later. There are at least three common mechanical indexer/rotary table types.

Wormgear Type Rotary Tables

These tables provide infinite positioning as well as the possibility of rotary cutting. A servomotor controlled directly either by the CNC or by a secondary servocontrol rotates a wormscrew, which drives a wormwheel on the rotary table spindle.

The absolute position accuracy of these systems is a function of the quality (precision and accuracy) of the wormgear set (wormscrew and wormwheel), the accuracy and resolution of the servosystem, and the means of servoposition feedback. Most of these servosystems utilize an encoder to monitor the position of the motor rather than the rotary spindle directly. To eliminate any inaccuracies in the wormgears and servo system, some high-end systems use a glass scale or other encoder directly on the rotary spindle to monitor actual rotary spindle position. Figure 1 (at right) shows a typical wormgear rotary table cross section.

Advantages: 

• Infinite positioning and the flexibility to handle any job that might come up make these the choice of many job shops.
• If controlled directly by the machine tool’s CNC, they are most commonly referred to as a “full fourth axis.” A full fourth axis has the advantages of having only one CNC program, no programming required by the operator on the shop floor, minimum chance of a crash due to operator error, and the ability to make simultaneous rotary and X, Y or Z moves to do true helical milling operations as required by some more exotic workpieces.
• Many models and sizes are available from many manufacturers.

Disadvantages: 

• Claims of position accuracy are often misleading since there are no industry standards. Although some manufacturers test and certify absolute position accuracy every one degree, most do not state exactly what their specification means.For all except those few expensive systems with glass scales directly on the rotary spindle, any accuracy specification is for a new table before it has been subjected to any “crashes,” which are not uncommon. Even seemingly small crashes can damage wormgear sets.
• Typical infinite positioning wormgear systems utilize a friction brake to hold position against cutting forces. When cutting forces are applied directly on the rotary spindle centerline, friction brakes are generally adequate for most work. However, when cutting forces are applied to workpieces far off centerline, such as on the edge of a part on a tombstone fixture, the resulting torque on the rotary spindle can cause it to deflect. This result is especially likely when heavy cuts produce high thrust forces.

Face Gear Type Indexers

These indexers offer discrete positioning only. Depending on the number of teeth on the face gear, the minimum increment of index might be 15 degrees, 5 degrees or 1 degree. Whatever the minimum increment, only workpieces with angles representing some multiple of the minimum can possibly be machined.

Face gear mechanisms used in indexers are similar to those most commonly found in the turrets of CNC lathes, which by function must index very accurately and very rigidly to withstand the high cutting forces the lathe turret encounters. Face gear mechanisms generally fall into two categories, the two-piece and the three-piece design. Two-piece designs require the face plate of the indexer to “lift” to disengage the face gears. Three-piece designs maintain the same accuracy and rigidity of a two-piece without the need to “lift” the faceplate. In Figure 2 (at right), note the massive face gear that locks the indexer spindle in position.

Advantages: 

• Assuming it’s a quality face gear set, absolute position accuracy is superb and is maintained for the life of the indexer almost in spite of any “crashes” that might occur. Units with true absolute angular position accuracy of 5 arc seconds or less are available. These units are ideal for the highest precision work such as line boring half way from one side, then indexing 180 degrees and line boring half way from the other side.
• Rigidity is extremely high. A 72-tooth facegear (5-degree minimum index increment) is as rigid as any lathe turret.

Disadvantages:

• It only indexes in increments of 5, 15 degrees or whatever the minimum increment is for that particular model.
• A two-piece face gear system can be used with a tailstock or tombstone with outboard support only if the outboard support includes a sliding joint.
• Some face gear systems use a servodrive to achieve approximate position and then rely on the face gear for final accurate positioning. These systems are bi-directional and fast. Any random move can be programmed with one simple command. Some other systems use a pneumatic piston to rotate to the approximate position. Typically, these systems rotate only in one direction. All moves must be equal and may require a pause to utilize more then one M-code signal to achieve position. These work but can be tedious to program, set up and operate. They are more prone to crash due to operator error then servodriven units.

Shot Pin Indexers

These indexers are becoming a thing of the past. They have all the disadvantages of the pneumatic piston driven incremental face gear indexers. Plus, compared to face gear units, they are neither particularly accurate nor rigid. Index positions are usually limited to 15-degree increments. Position is controlled by a pin in a hole or more often by a dog in a notch on the outside of a ring.

Control Issues

Whether you select an infinite positioning wormgear rotary system or a facegear system as the best mechanical design for your work, your next decision involves how you will control the rotary axis.

With a pneumatic incremental indexer, you probably will have no choice. Your machine’s CNC will control the indexer by communicating with a special indexer control via an M-code.

If you select a system with a servodrive, you have three choices: 1.) direct “full fourth axis” using only the machine’s CNC, 2.) an M-code command from the CNC to a separate rotary control, or 3.) RS-232 communication between the machine’s CNC and a separate rotary control. Each of these choices has advantages and disadvantages.

Full Four-Axis Control

A single, four-axis CNC is the easiest to use and provides the most control. Four-axis CNC is best for certain kinds of workpieces. Full four-axis control systems are usually ordered for delivery with a new machine. Systems can be retrofitted; however, retrofitting is complicated and expensive. The advantages of a single four-axis control are numerous, and the disadvantages are primarily related to cost.

Advantages: 

• All programming is handled in the one CNC. No shopfloor programming of a separate rotary table control is required.
• The single CNC constantly tracks all three linear axes (X,Y,Z) and the rotary axis. This provides the ability to do precise helical cutting with simultaneous rotary and X, Y or Z moves.
• If a machining cycle is interrupted and restarted in the middle of the cycle, chances are low that a crash will occur due to operator error.

Disadvantages: 

• While a few machine builders offer a full four-axis control with rotary table for about 10 percent of the base price of the machine, most charge more than 20 percent.
• Very few machine builders make it easy to retrofit a full four-axis rotary table. For most builders, retrofitting is a complicated process, and the cost typically exceeds 30 percent of a base machine price.
• The motor for the rotary axis must be matched to the servodrive of the CNC. Because cable connections are not standard from one machine builder to another, rotary tables can not generally be used on more than one machine.
• Some applications may require the accuracy and rigidity of a face gear system. However, many machine builders don’t offer face gear systems with a full four-axis control, although such systems are feasible.

M-Code Actuated System

An M-code actuated system provides a fourth axis of motion by combining a standard three-axis CNC with a rotary table or face gear indexer that has its own separate rotary servocontrol. The rotary program is entered and stored in the separate rotary servocontrol. The CNC communicates with the rotary control via an M-code. When the rotary control receives the M-code signal, it executes the next rotary move stored in its memory, then sends a signal back to the CNC, telling it that the move has been completed.

Typically, the rotary program includes many separate rotary moves. One move might be a simple index to position at full rapid speed. Another might be a slower rotary move to machine a groove or other feature on the workpiece. Figure 3 (at right) shows a typical rotary servocontrol system.

Advantages: 

• High quality M-code controlled systems are available from several suppliers for a price of about 10 percent of a base machine price. (For example, a 5C rotary system at $6,000; a 6-inch faceplate system at $7,000; a 9-inch system at $10,000; and so on).
• Requiring only one M-code, 110V power and an air line for operation, these systems can be retrofitted to almost any CNC machine, typically with less than a day of downtime.
• Systems can be moved from one machine to another as long as the next machine can issue M-codes. A shop with multiple machines and multiple rotary systems can select the best system for each job regardless of the machine. For example, a small indexer can be used for small parts to avoid cutting tool interference problems and to minimize indexing times. A big indexer can be used for big parts. A face gear indexer can be used when the maximum in accuracy and rigidity are needed and the work can be accommodated by multiples of 5 degrees of index.

Disadvantages: 

• The machine operator needs to enter the rotary program into the rotary servocontrol, or select the right program if it’s already stored in the rotary control’s memory. This takes some time, and there is the chance of an error.
• If the machining cycle is ever interrupted in mid-cycle, such as to inspect a workpiece feature or replace a worn cutting tool, the operator must be sure to back up the rotary program and the CNC program to a point that keeps the two programs in sync. This step can be confusing, and any error can result in a “crash,” with a cutting tool coming down to a workpiece rotated to the wrong position.
• Although it is possible to perform simultaneous rotary and X, Y or Z moves, they are not recommended. If you have patience and can afford to scrap a few parts, you can use trial and error to find the right rotary speed to match the linear move and determine starting points that match.

RS-232 Control

Recently developed, RS-232 communication between a three-axis CNC and a rotary servocontrol offers advantages of full four-axis and M-code operation. RS-232 is the commonly used, standard electrical interface for connecting peripheral devices to a computer. Personal computers often use the RS-232 communication protocol to send information to a printer. Another common use for RS-232 communications is connecting a PC to an external modem.

Nearly all CNC units have an RS-232 port, and it is commonly used to exchange CNC programs between a computer system and the CNC. More recently, RS-232 connections have been used by CNCs to communicate with robots and rotary tables. To communicate with the rotary table’s control, a special line of code is inserted into the CNC program. This line of code sends a string of numbers and letters through the RS-232 port to the rotary table control, which translates the string of code into rotary moves.

RS-232 communication between a three-axis CNC and a rotary servocontrol provides much of the best of both worlds of full four-axis and M-code operation. Both the linear and rotary moves are stored in the CNC as part of the workpiece program. When a rotary move is required, the CNC sends the commands for that one move (rotary speed and angle of rotation) through an RS-232 line to the rotary control.
The rotary control executes that one move and sends back a signal to the CNC, indicating that this move has been completed. The CNC then commands its next linear move. The separate rotary servocontrol simply works as a slave to the CNC. The machine operator turns the rotary control on in the morning and does not need to attend to it the rest of the day. Figure 4 (at right) shows a tilting rotary table system utilizing two rotary servocontrols with RS-232, providing five-axis capability from a standard three-axis CNC.

Advantages: 

• All programming is handled in the three-axis CNC. No separate shopfloor programming or program selection is required.
• Crashes are nearly as unlikely as with a full four-axis control. The correct rotary program is always selected because it is part of the total workpiece program stored in the machine’s CNC. Note: Rotary moves should be programmed in “absolute position” so that if the machining cycle is interrupted, the operator can back up the CNC program to just in front of a rotary move, then safely resume the program.
• As with M-code operation, rotary systems can be shared among any machine that can communicate either via RS-232 or via M-code.
• Retrofitting is easy provided the machine’s CNC has an RS-232 port and appropriate communication software, which may already reside in the CNC or be available from the machine builder.
• Cost is modestly higher than an M-code-only system but provides the flexibility to operate via either RS-232 or M-code.
• With RS-232, two rotary controls can be operated by most three-axis CNCs with only one RS-232 port. Five-axis capability with a tilting rotary table setup can be retrofitted to a three-axis machine for about $25,000 (a new, full five-axis VMC option is typically priced at $95,000).

Disadvantages: 

• As with M-code control, simultaneous rotary and X,Y or Z moves are not recommended.
• CNCs without an RS-232 port and appropriate communication software must be updated. Most relatively new machines are likely to be so equipped.
• Programming will be somewhat more complicated than with a true four-axis control and a lot more complicated for five-axis work.

Making The Right Selection

Both the work you need to do and the machines you own or intend to purchase will influence what you select for a rotary axis. These guidelines summarize what you should consider.

1. When buying a new machine, get prices on everything the builder offers, no matter what kind of workpieces you’ll be machining. If the builder offers a full four-axis system with a high-quality, infinite-positioning rotary table at a price of about 10 percent off the base machine, this system will probably be your best choice.
2. If your workpieces can take advantage of the accuracy and rigidity of a face-gear system, and you can live with the 5-degree minimum increment, a face gear system controlled by RS-232 or M-code is a good choice. A few builders offer a face gear system with true four-axis control.
3. If you’re doing a variety of work that requires simultaneous rotary and linear helical moves, you’ll probably want a true four-axis system regardless of the cost. However, you should consider a more economically priced RS-232 or M-code system when you are retrofitting an existing machine and have only a couple of jobs requiring these moves, especially if these jobs are long run and you can afford some extra programming and setup time. These systems are worth considering if you simply can’t afford the price of a true fourth axis.
4. If you’re retrofitting existing machines, especially if you have several and want to do rotary work on more then one of them, check with the builder on the cost of upgrading to full four axis. You may conclude that the cost and flexibility advantages of RS-232 or M-code will make one of them the best choice.

Adding a rotary axis to a VMC is worthwhile whether you want to do full four-axis simultaneous machining of exotic workpieces, simple indexing of parts that need machining on surfaces not at 90 degrees from each other, or tombstone processing of rectangular parts that benefit from a longer unmanned machining cycle. Today, many good options exist. If you’re buying a new machine, have the builder quote the optional systems it offers. If you’re going to retrofit an existing machine, contact either the original supplier or the companies that offer complete indexer and rotary table systems. Retrofitting is highly affordable. (Systems from SMW Systems, for example, generally cost a little over $1,000 per inch of faceplate diameter, including installation and training.) MMS

About the Author: Ken Harrison, a licensed professional mechanical engineer, is product manager at SMW Systems, Inc. in Santa Fe Springs, California.