Quality Scan: The Process Control Imperative
When the going gets tough, the tough get their processes under control. Lean manufacturing can take you only so far, and lean assumes a machining process that is predictable, repeatable, and capable of producing conforming parts. At a time when it's imperative to reduce operating costs, but you are fenced in financially and can't afford new machines, a low-cost, four-step approach to increase process capability can yield substantial improvements in labor, scrap, inspection, and overall equipment efficiency.
Humans can be their own worst enemies in manufacturing. Operator intervention, wherever it occurs, is a frequent source of failure and nonconformance. Removing manual processes is not the entire answer, though. Process control also requires attention to the operating environment, the machine itself, setup, and in-process controls.The key to predictable productivity is elimination of variation at its source, approaching it from the bottom up in four steps: (1) operating conditions and the environment, (2) process setting, (3) in-process control, and (4) postprocess monitoring.
Components to examine or implement in step one include design for manufacture, based on a thorough understanding of current capabilities and reliance on best practices rather than reinvention of the wheel. Engineering around standard features, tooling, and processes reduces variation and allows engineers to make far-reaching improvements when new best practices are identified. Other components of step one include controls and standardization in cutter geometry, clamping forces, part programs, workpiece preparation, ambient temperature, process-generated heat, machine/fixture cleanliness, tool-life management, machine calibration, necessary refurbishment, and routine condition monitoring. Consistent conditions at the start of the process set the foundation for consistent results.
Step two, process setting, tackles errors in setup of the machine, part, tool, and probe that are always present to a degree, and must be correct to achieve first part/good part. Building on step one, process-setting controls help eliminate human error by automating manual processes. Machine setting involves establishing relationships between key moving parts, such as spindle/bed. Uncorrected machine errors can be a dominant factor in process nonconformance, possibly causing extended setup times because the effects can be confused with other sources of process variation. Probes must be datumed to accurately measure on the machine. This involves measuring the size and position of the stylus with a datum sphere or ring gage. A tool arbor can be used to establish the position of a stylus or laser beam for toolsetting devices. Weekly probe calibration ensures other measurements are reliable. Part setting with a probe—which eliminates human intervention—establishes location and orientation of the workpiece to align the machining program. The final element is toolsetting, where length and diameter of tools are established and stored in the CNC.
Step three, in-process control, assumes steps one and two are in place. This step addresses obvious variables, such as temperature and tool wear, using probes. If only a few tools are used in the process, there's no need to probe every part feature. Check a critical feature produced by each tool using a touch probe, and update the tool offset. Give attention to the work of roughing tools during probe cycles. If a rough feature is inconsistent, the finishing cut may be as well. Redatum the spindle position, rotary table centerline, or pivot points on mill-turn machines regularly, but especially before finishing operations. Check delicate tools after each cycle. Put logic in the program to let the machine use the probe feedback to call up new tools or repeat passes. Log in-process measurements and offset updates for traceability.
Step four, postprocess monitoring, is performed by most shops using machine tool probes, handheld gages, articulating arms, and CMMs. At this stage, these are reactive controls, performed too late to influence the component being measured, unless rework is possible. Postprocess measurement with the part in the fixture makes the most sense for large, high-value parts. CMM-style checks, including GD&T, can also be performed on the machine, but will be less precise and traceable than those done on a CMM, where measurement cycles may be shorter, too, due to new technologies such as five-axis scanning. CMMs equipped for five-axis scanning will also be faster and more comprehensive in measuring complex shapes commonly produced with five-axis machine tools. CMM measurement results and point data are easily stored for long-term traceability.
Without making any fundamental changes to the machining process, these four steps can yield substantial recurring cost savings. The investment required is relatively low, with a payback of just a few months. Step one, alone, is an excellent foundation that any up-to-date shop should have in place.
This article was first published in the January 2010 edition of Manufacturing Engineering magazine.