Improving machining efficiency

Posted: February 11, 2013 - 3:13pm | Y-12 Report | Volume 9, Issue 2 | 2013

Machinists are held to exacting standards. After a part is produced, each angle, cut or groove must be measured to see if it meets the design drawing's specifications. However, combined factors such as fluctuating background temperatures and the whims of vintage equipment can cause just enough variation to disturb the precise shapes desired. Still, the goal is to have every part pass dimensional inspection the first time through.

Y‑12's lithium machining area has nearly accomplished that goal, improving significantly from a few years ago. The acceptance rate increased from about 80 percent in 2008 to 98.4 percent in 2011.

A big factor in the improvement is the ability to identify trends. To do that, the Engineering and Quality Assurance organizations collected and analyzed machining data on critical dimensions for each part. That kind of analysis can reveal trends toward the high or low end of tolerances and, therefore, the potential for deviations. “We plot the machining results and provide control charts that show any shifts in data,” the Quality Assurance statistician said. Those charts show a centerline, or process average; control limits, which define the expected variation range; and upper and lower tolerances, which coincide with the design specification.

Depending on where data points fall, the charts can also suggest a variation's cause. With such control charts, trends are easy to spot. “We don't want data to fall predominantly on either the high or low side,” the process engineer explained.

In response to trends, changes can be made to a process or machine. “For instance, one mill unknowingly had a 'sloppy' z-axis, which sometimes led to imprecise cutting depths,” said the process engineer. Since repair of the equipment was not feasible, he collaborated with the numerical control programmer and machinists to devise a fix — making two shallower cutting passes instead of one. After the first pass, the machinist measured the dimension of the part; on the basis of that measurement, he refined the final cut to produce the correct cutting depth.

For any successful operational change, the machinists are essential. “They are sensitive to producing part features within the drawing tolerances, but they also realize how important it is to machine features to the centerline value,” the process engineer said. “The closer they get to the centerline, the easier it is for the next person in the assembly process.” Accurate setups, careful scrutiny of the first machined part and measurement of certain features during the process help machinists get as close to perfect as possible.

Using the statistical analysis tool is now an integral part of machining, and the work of taking data and checking all dimensions is the norm. “Continuously monitoring the process allows us to be proactive and correct any issues before they become major problems,” the Quality Assurance statistician said. With the tool, the lithium machining area has improved the part acceptance rate while making good use of old equipment. With fewer part rejections, the crew avoids costly and time-consuming rework, saving an estimated $45,000 a year.

“This is what we do,” the process engineer said. “We come up against unique problems, find creative solutions and are able to show good value to our customer.”