Using the cutting capabilities of a superabrasive grinding wheel requires a systematic processing approach. It takes more to get good process results than just slapping a superabrasive grinding wheel on a machine tool.
To keep up with consumers' demands of greater quality, ISO 9001-certified Black & Decker U.S. Power Tool Operations (Fayetteville, North Carolina) knew it needed to seek the best technology. Goals included quieter, smoother operation for the small electric motors inside power tools and tight tolerances for surface finish and roundness of shafts and spindles for the small, high-speed electric motors.
At the heart of all the tools is the motor shaft or spindle. Typically steel, these parts are usually shaped on a screw machine and ground to finish specifications. Some are ground from barstock. Most have multiple diameters.
Black & Decker engineers knew that upgrading grinding capability could have a big impact on plant production quality and cost. Warranty and rework issues are important dollar concerns, and plantwide Cpk goal is six sigma.
The solution was to purchase two Koyo CNC precision centerless grinders with automatic parts feeders and parts handling. Previously, multiple diameters on armature shafts had been ground on a manual, between-center grinding machine with contact gaging and cam-type dressers. Production rates sometimes lagged demand, and part quality could be inconsistent. As production increased and tolerances tightened, it became clear that a multi-axis CNC infeed grinder with CNC wheel dressing capability would be needed to help the plant keep up with the production requirements of several million parts a year.
Black & Decker engineers decided that a Koyo Model 6015 CNC in-feed centerless grinder with 6-inch wide grinding and regulating wheels with multiple diameters would help them meet the challenge. "It permitted faster setups and greater throughput, and the CNC dressing eliminated cam templates, saving time and improving grinding accuracy and reliability," said Mike Hinnant, manufacturing engineer. "Wheels are pre-dressed. And with the combination of CNC and pre-dressed wheels, we can quickly dedicate the machine to any shaft or spindle our plant requires as they need them."
"On one of the shafts we produce, we cut the cycle time from 28 seconds to 13 seconds," Mr. Hinnant continued. "The machine removes 0.010 inch of stock on diameter and maintains a 2.0 Cpk while maintaining roundness and surface roughness to part print tolerances." On that shaft, the machine grinds four diameters.
The Koyo centerless grinder is equipped with nine axes for machining, including two axes for grinding wheel dressing, two axes for regulating wheel dressing, two axes of infeed, upper and lower slide, two axes for loading and one axis for automatic swivel for taper control. It also includes several sets of grippers for automatic loading of eight different part numbers.
Black & Decker 's second machine was a Koyo KC-200 CNC in-feed centerless grinder. The main need was to be able to grind longer parts than would fit on the 6015 and to provide increased capacity. For its part in the production improvement at the plant, the KC-200 cut cycle times approximately in half for a family of nine parts it grinds.
Part quality is checked automatically with a non-contact post-process laser gage. Size and taper data is automatically fed back to the CNC, which compensates by adjusting the table and swivel axes.
"Roundness on the motor shafts is a critical specification," said Mr. Hinnant. "If the shaft is not round, the motor will arc and fail. We are minimizing lobing by optimizing regulating wheel speed, number of dresses on the wheel, and blade height." With CNC, hydraulics have no part on the KC-200, and thermal effects on machine function are avoided.
In operation, several hundred parts are loaded at one time through a hopper, freeing the operator to perform other tasks. Parts cycle through the machine automatically, followed by a blow-off and the laser gaging before being deposited into a bin.
Environmentally, the machine is clean, said Steve Terry, group leader on the shaft machining line. A mist collection system keeps the air around the machine clean and free of irritating vapors. And the part blow-off at the laser gaging station is so effective that not only is gaging reliable, but the usual post-process part-cleaning operation has been eliminated, saving 55 gallons of solvent a year, yet permitting a part storage life of up to 30 days, rust-free.
"All in all, the machines have been a very positive experience—with improved part quality, 50 percent greater productivity, and noticeable environmental improvement," Mr. Hinnant said.blog comments powered by Disqus