Implementing Process Capability Index (Cpk) for Effective Improvement Stability the Case of a Pressing Shop of an Automobile Company

Abstract; Sheet metal forming, a sheet metal that has a basic shape, is formed between dies to obtain a part with relatively complex geometry with targeted measurement result (tolerance & dimensions). Sheet metal forming produces scraps and generates the final part product in fast paced production, usually in one stroke or a few strokes of press between upper and lower dies (1). Sheet forming process can offer potential savings in energy and materials in mass production system. Sheet metal forming is in press production shop (stamping) in our company. There are 3 lines of press shop 2000-ton, 1200 ton, and 500 ton. Each produces about 1000 parts per day. The dimension of the finished product should be in the desired tolerance and measurement. Inconsistent product dimensions will result in inaccurate results of the whole product at welding and assembly shop. Capability of process (e.g. Cp, Cpk, and CPM) calculating dimension variability to drawing, with Cpk used to focus on centering the process mean with the specification / drawing center (1, 2, 3). In this paper we analyze quality stabilization of improvement part by using Cpk to analyze whether improvement process data are accurate and stable. Process capability (Cpk) calculates and compared before and after adjusting the parameters for selected dimensions within part for measurement (based on drawing), employing an image measuring instrument for collecting 30 units data before improvement and 30 units data after improvement and makes the conclusion based on the Cpk result.