3-D Technical Services (Franklin, Ohio) is an industrial design prototyping company. It provides its customers with assistance in product development, product engineering, sales aids, architectural models, marketing tools, litigation models and tradeshow displays. The company employs several processes in building models and prototypes, including CNC machining, stereolithography, Room Temperature Vulcanization (RTV) tooling, urethane casting and vacuum forming.
One critical aspect of the rapid prototyping business is turnaround time—the time it takes to produce a prototype from a customer design. Reliability of machine tools is critical in this competitive business.
3-D Technical Services began experiencing problems in its CNC machining operations when spindles on its Milltronics vertical machining centers failed. Dust was entering the spindle bearing race, even though the bearings were protected by a labyrinth seal, causing the bearings to wear prematurely. Each time a spindle was replaced, the machining center was out of commission for one or two days until repairs were complete.
To remedy the situation, spindles in the Milltronics VMCs were replaced by a new spindle design that included a combination air purge labyrinth style seal. This new spindle design was developed by The SETCO Group, a Cincinnati, Ohio-based manufacturer of precision spindles and slides, working in conjunction with Milltronics Manufacturing Company (Waconia, Minnesota).
The machining operation at 3-D Technical Services can be considered light duty. The spindles on the vertical machining centers are rated at 10,000 rpm but are being run in the 2,000 to 4,000 rpm range. Feed rates are typically 100 to 200 inches per minute. The tolerance for most prototype parts, such as DVD players, refrigerator components and telephone housings, is 0.005 inch.
The most common solution to keeping contaminants away from spindle bearings is a mechanical seal around the spindle shaft that acts as a barrier to prevent contaminants from entering the spindle housing, causing the bearings to seize and burn out.
To be 100 percent effective, however, the seal must protect the spindle in both the dynamic mode (when the machine tool is running and coolant pressure and volume are at their greatest) and in the static mode (when the machine tool is stopped and contaminants have a tendency to leach into the bearings). Both problems were effectively solved by the SETCO spindles.
During spindle operation, the SETCO air purge labyrinth seal produces an outward flow of clean, dry air through the seal periphery to prevent contaminants from entering the spindle and keep humidity ingress levels low.
In addition to the air seals, the spindles that SETCO designed and built in cooperation with Milltronics also include modifications in bearing spread and pre-load. Although the details of the design are proprietary, the variations in bearing placement and spread are critical to spindle performance. For example, the bearing spread has been optimized for each spindle configuration to improve stiffness.
Since the new SETCO spindles have been installed, spindle-bearing failure has been eliminated as a cause of machine downtime at 3-D Technical Services. One retrofitted machine has been in operation for nearly four years without a single spindle failure.