Simultaneous engineering has become the means to ensure quality through incorporation of manufacturing. This proves difficult without quantitative support tools. We present here a modeling based approach to simultaneously design a product and its production process. We demonstrate an approach of examining sensitivities of output to all inputs of a system, from product design specifications, process variables, and material specifications. We further develop mathematics of estimating the effects of in-line process control changes to increase quality. We demonstrate a method to choose noise sources to measure and process variables to control in-line based upon these measurements, and estimate the error reduction that such process control changes will provide. The tool allows simultaneous engineering of the product and process to increase quality.