Sizing Up Rotary Table Advantages
Smaller may be better when it comes to rotary tables that enable a three-axis VMC to do five-axis machining.
Ivo Straessle believes that when it comes to rotary tables that enable three-axis VMCs to perform five-axis work, smaller is better.
Mr. Straessle is pL Lehmann product manager at Exsys Tool. He says that while five-axis machines have become more affordable, they still can be two to three times the cost of a three-axis VMC, depending on the model. Therein lies one advantage to rotary tables, which, when installed on the bed of a three-axis VMC, combine a rotational C axis and a tilting A axis to enable the machine to perform either five-axis positioning work (aka 3+2 machining) or full five-axis contouring. Three-axis VMCs outfitted with two-axis rotary tables are commonly smaller than five-axis machines, too, saving valuable floor space. Unfortunately, they also tend to have tighter work envelopes, meaning a sizeable rotary table will be more restrictive of a machine’s movements.
A compact rotary table preserves work envelope space, providing more room for spindle and tool movement. Plus, shops have the option to use just the rotary table, mount standard vises next to it or remove the rotary table altogether, which is made simpler when the rotary table has a zero-point-type locating system to eliminate the need for alignment when it is removed and then remounted.
That said, Mr. Straessle says compact rotary tables still must possess design elements that enable them to perform effectively for a range of work. Here, he details four such features inherent to the pL Lehmann 500 series of modular-designed rotary tables, available in North America through Exsys Tool.
1. Load capacity. The compact 500 series provides load capacities that, in the past, were only available with larger rotary tables. For instance, the 3-inch rotary tables in this series are generally half the size of comparable units, but can handle workpiece loads as heavy as 400 pounds, he says.
2. Speed and torque. Rotary tables commonly use either direct-drive or gear-driven technologies, the former able to achieve high rotational speeds and the latter offering high torque. The 500 series uses the company’s pre-loaded gear drive (PGD) design, which is said to offer the best of both worlds in terms of high speed and high torque while being as much as four times as rigid as direct-drive systems. Plus, it enables the rotary tables to generate rotational speeds as fast as 160 rpm with virtually zero backlash.
3. Spindle clamping power. Once a rotary table is oriented to the desired position, an effective spindle clamping system is needed to securely hold that position during machining operations. Some rotary tables use an external intensifier or some other type of standalone hydraulic unit combined with disc braking to achieve necessary clamping forces. Conversely, the 500 series features an integral unit that converts air pressure into hydraulic holding power, using a
cylinder/piston mechanism that requires only 90 psi of air pressure to generate more than 3,000 psi of hydraulic clamping pressure to firmly clamp the spindle in place at any position around its 360 degrees of rotation. In addition, this air-over-oil clamping system can often achieve unclamp-to-clamp times of less than 0.4 second as compared to the multiple seconds external systems typically require.
4. Monitoring system. The 500 series features a self-contained, internal monitoring system used to track and record vital rotary table information, including temperature, rotational speed and clamping force. It also detects internal pressure decay.
In addition, the rotary tables have USB ports to enable users to download monitoring data to log the operational history of the table. Not only does this data highlight crashes or malfunctions, but it also facilitates more effective preventive maintenance by offering early warnings to possible issues so users can avoid unplanned downtime. Plus, the monitoring system activates a series of LED warning lights when it recognizes a potential problem. For example, a blinking orange light is an alert to a situation that can be addressed before it becomes serious, while a continuous red light means immediate attention is needed. Because these rotary tables integrate with all the common machine tool CNCs, users can have warnings displayed on the control screens, too. By connecting a laptop running software such as pL Lehmann’s TeamViewer, users can also solicit assistance via the Internet from a company technician who can log on to assist in troubleshooting.
Mr. Straessle says the company guarantees the spindles on its standard rotary tables to achieve runout of 5 microns and its high-accuracy models to achieve runout of less than 3 microns (the latter is commonly used in grinding applications). In addition, glass scales provide ±1-arcsecond positioning capability for the C axis. These rotary tables are also sealed to prevent internal contamination that could hinder performance and feature a motor housing internally pressurized with oil to achieve an IP67 rating.
The 500 series is available in four standard, single-fourth-axis models. The smallest is the EA 507, offering a 3-inch face diameter and a spindle nose load capacity of approximately 240 pounds, while the high-speed EA 511 model is said to deliver twice the speed of the standard models. Each model can be configured as a two-axis system that provides 180 degrees of A-axis tilt with full C-axis rotation, as is the case with the series 500 T1 (trunnion-style) and TF (cantilever-style) units. They can also be ganged together with multiple spindles/C axes.
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