Are you in the market for 4th-axis machining in the  ±3 arc-sec accuracy and ±2 arc-sec repeatability class? Are you interested in further reducing your cycle time? The following is a quick synopsis of direct-drive rotary technology as manufactured by Hardinge.

What is direct-drive rotary technology?
A direct-drive rotary table consists of a housing, bearings, an encoder and a spindle with a high-energy rare-earth permanent magnet DC torque motor rotor wrapped around the spindle. The encoder is mounted directly to the spindle to provide position feedback. Position accuracy is only limited by the resolution of the encoder and the concentricity of the encoder to the spindle and its bearings. Hardinge systems provide ±.077 arc-sec resolution at the encoder for super-precision positioning and repeatability.

System compliance, such as that found in a gear-driven system, is negligible in a direct-drive system since there are no compliant couplings between the motor and the load. Also, since there are no gears between the motor and the load, there is no backlash. Accordingly, the servo gain (stiffness) can be quite high without any concern regarding servo instability. And since there is no backlash, bi-directional movement can be made without the need to compensate for backlash error.

Positioning speed is limited only by the torque available to accelerate the inertia to be positioned. Very high accelerations and positioning speeds are demonstrated on Hardinge models. These high accelerations are the same as seen with DC linear motors that are used as axis drives on some machine tools.

The only losses that will create heat are the motor losses and the bearing losses. However, brushless permanent-magnet DC torque motors are typically 90-percent or more efficient. The Hardinge direct-drive rotary system was designed with a very predictable continuous-duty cycle at a certain torque output. There are two continuous-duty cycles. One is when the motor is operated in air without any cooling liquid applied. A continuous-duty cycle with a higher torque rating is available when cooling liquid is applied. Continuous-duty cycles defined for these tables mean 24 hours a day, 365 days a year. For all practical purposes a table will not be used in this manner, so intermediate-duty cycles that utilize much more torque than the continuous rating up to the peak torque available can be used without much concern. If cooling liquid is required for loads generated by off-center drilling or other rigorous duty cycles, you simply connect a standard coolant line to the housing, utilizing the existing machine’s coolant supply.

Another advantage of direct-drive rotary tables is that if a position move is made with the intent of cutting while stationary, it may not be necessary to clamp the spindle. If the cutting forces to be applied are within the torque available from the motor, the servo will hold position with plenty of stiffness to permit the cut since, as previously mentioned, there is negligible compliance in the direct-drive system. This is vastly different from the traditional geared system that uses a servo motor at the end of a power transmission linkage.  With the direct drive system, after the cut, since no clamp was applied, there is no time delay to wait for the clamp to release, so movement to the next position will be faster.

Why has Hardinge taken advantage of direct-drive technology?
The milling machines available on the market today generally have very fast axis travels as machine builders have attempted to reduce the time a machine is not in the cut and to increase the amount of material removed. If the machine axes travels are faster than the positioning speeds available from the rotary unit, the machine will often be waiting for the unit to position. The faster the rotary unit can move from one position to another, the less time the machine is waiting to cut and the faster the cycle time. Hardinge offers 4th axis rotary indexing speeds up to 4,200 degrees per second. The direct-drive rotary technology provides speeds that will take advantage of the faster axis travels, but it also offers the ability to do complex bi-directional machining at rates faster than can typically be done on geared systems. Hardinge rotary systems are available for use on Bridgeport and most other brands of machining centers and will take advantage of “look-ahead” programming when integrated as a Advantag4th-axis to the machine.

In many cases, it is a time-consuming matter of finding a method to properly machine the part to the required specification. For example, a standard gear-driven rotary system could not achieve the required accuracy. The super-precision positioning and repeatability of the Hardinge direct-drive system has opened the door to jobs that were previously out of reach, or jobs that were lost to competitors who could simply get the job done faster.

The direct-drive technology may not be the answer for all applications of course, but when it fits the application, it will produce outstanding results for a user and will enable the user to stay on the cutting edge of productivity. All Hardinge Rotary Systems are manufactured to strict specifications in Elmira, New York. For more information call 800-843-8801, email info@hardinge.com or visit http://www.shophardinge.com/.