This paper proposes a sub-aperture grinding tool for loose abrasive computer controlled surfacing, which is designed to perform epicyclic motion and rotate around its centre at a rapid rate, whilst the entire mechanism revolves around a secondary centre at a slower rate. In actual process, the wear of the tool could affect the material removal function, and make the process unstable, thus in fact, it is difficult to make a deterministic manufacturing. The focus of the present paper is on wearing characteristics of sub-aperture tools and the wear evenness as the main objectives. To make a further study, material removal function of the tool is firstly established through theoretically modelling, next, a correlative function with weighted factors is built, which is suitable for specifying the wearing degree of the tool. Finally, to discover the relationship between the material removal rate and the tool wearing characteristics, and to optimize the grinding process, analysis and experiments are then carried out on a K9 glass specimen by means of three kinds of tool materials, i.e., polyurethane pad, aluminum plate and pitch based on the proposed technique and model. The results indicated that the required high efficiency and precision could be achieved by choosing proper processes.