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CUTTING TOOLS
Overcoming process uncertainty to reduce scrap and rework Product quality is a key performance indicator for manufacturing businesses. Many workshops believe that achieving quality-standard certifications such as ISO, NADCAP and API affirms the quality of their work. In reality, the standards do not fully focus on how to make acceptable finished workpieces, but rather concentrate on establishing procedures for rejecting bad parts. By Patrick de Vos, Corporate Technical Education Manager, Seco Tools. A true picture of a manufacturer’s product quality comes from a look into the shop’s quality control area, especially where there are parts with stickers or tags calling for “re-inspection.” The tags indicate that for some reason the workpieces were rejected or must be returned to production for rework. Scrap and rework result when manufacturing processes do not meet expectations. Some processes, especially machining, are based on the interaction of many variables and thereby are subject to uncertainty and some degree of unreliability. It is essential that manufacturers recognise the sources of uncertainty, understand how they arise, and find ways to resolve them. In many cases, however, the sources of process uncertainty are not recognised or are ignored and accepted as standard practice. In the end, overcoming uncertainty and limiting scrap and rework comes down to the awareness, knowledge and motivation of those on the shop floor.
Sources of uncertainty Competitive pressures prompt manufacturers to perform costreduction analyses. The analyses include examination of spending for materials, tooling and machining time. Seeking out and eliminating the many instances of lost time offers significant opportunities for controlling costs. Methods used in cost analyses include overall equipment effectiveness (OEE) and Lean programs. Such analyses, however, often are based on a view of the manufacturing process as a kind of self-contained, simple box. A shop puts raw material or a semi-finished product into the box, machining processes occur, then a certain number of workpieces exit the box, either as finished products or as semi-finished workpieces destined for another box. What is often missing in the analyses is recognition that in the real world, it is not totally certain that every product that leaves the box
AMT DEC/JAN 2021
will be acceptable. Unacceptable products must be reworked or scrapped. The number of unacceptable workpieces produced depends on how closely the variables in the manufacturing process are controlled. Processes such as forging, for example, are not extremely complex and do not require an extreme level of precision. These processes therefore are relatively controllable, reliable and capable of consistently delivering the expected number of acceptable workpieces. In contrast, other manufacturing processes involve multiple variables that can reduce the predictability of the outcome. The number of acceptable workpieces coming out of the box is not reliably predictable. It is an exaggeration to call all machining processes unreliable, but describing them as not entirely predictable is quite accurate. Machining involves many variables. It subjects cutting tools and workpiece materials to strong cutting forces and high temperatures. The combination of forces and temperatures creates a complex and fluid situation. The hardness of a tool determines its ability to cut effectively, and hardness will vary between room temperature and the 800˚C temperatures generated in machining. Heat also affects the machining characteristics of the workpiece material. The effects of the variables are unpredictable. Even if machining parameters are specified for a certain application, the exact values that exist during operation will be unknown. Varying results in surface finish or dimensions can make a part a candidate for rework or the scrap bin. When parts are unacceptable, two areas need to be examined. One area is technological: a manufacturer needs to know what is actually happening, recognise which variables caused the unexpected result, and decide how to resolve the problem. The other area for examination involves determining the financial effects of producing
