Using tolerance interval method as an alternate approach for monitoring process performance (process capability) of surface roughness of gear tooth flanks to avoid grinding burns

Abstract

The process capability measures most commonly referred to as process capability indices (PCIs), Cp, Cpk, Cpm evaluate process yield, process consistency and statistical variation compared to the specification. This paper explores an alternate approach for monitoring the process performance of tooth flank surface roughness in gear tooth grinding to avoid grinding burn. The gear tooth flanks are susceptible to microstructural damage due to high thermal energy generated during the process of grind operation. The thermal damage in ground gears can cause catastrophic failure in gears' intended life. To avoid grinding burns the process parameters (feed, speed and depth of cut) are selected such that thermal damage should not occur, however, setting up such parameters could affect the process capability of surface roughness (finish) at a desired level. The surface roughness is commonly specified in design as unilateral tolerance and considered as a critical or key characteristic, therefore, most of the automotive or component manufacturing industry requires it to exhibit a process capability index (Cpk) of 1.33 or above. Achieving and maintaining process capability of surface roughness is difficult due to the nature of process, moreover, the thermal damage of the ground feature can impair the product life. In this paper, for gears that are made with ferrous materials (plain carbon steels or alloy steels), tolerance interval method is proposed for monitoring the process performance of roughness in gear tooth grinding process as compared to the conventional process capability.