3+2-axis control milling is effectively performed on 5-axis controlled machine tools. However, it is difficult to determine the effective tool posture automatically. In this study, a fast method to detect material removal volume is proposed for tool posture decision. In the proposed method, workpiece, product and tooling shapes are modeled based on the voxel representation for calculating the discrete cross correlation function. The offset surface is generated on the boundary of the existence of the similarity between the product and tooling shapes. Tooling swept area is calculated as the convolution of the tooling shape. Consequently, the material removal volume and the unmachined volume are quickly obtained and can be simply evaluated. Moreover, the range of tool postures to calculate the material removal volume is limited by effectively using the feature of voxel models. Material removal volume is calculated at only the obtained candidates of the effective tool posture. In order to verify the usefulness of the proposed method, several case studies are conducted. It is confirmed that relatively large volume becomes machinable at the tool posture decided by using the proposed method.