MECHANICS
OVERCOMING PROCESS UNCERTAINTY TO REDUCE SCRAP AND REWORK
Figure 1: Time Spending In Machining Production Not Optimised.
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.
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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.
26 | Industry EMEA | September 2020
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Sources of Uncertainty Competitive pressures prompt manufacturers to perform cost-reduction 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 (see figure 1). Methods used in cost analyses include OEE and Lean programs. Such analyses, however, often are based on a view of the manufacturing process as a kind of selfcontained, simple box. A shop puts raw material or a semifinished 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. recognise Machining Variables What often is missing in the analyses is recognition that in the real world, it is not totally certain that every product that leaves the box will be acceptable. Unacceptable products must be reworked or scrapped.
