**5. Conclusions**

In this study, a stiffness model for the R-CUBE manipulator is derived via VJM. A volumetric isotropy index was proposed and compared with the isotropy index that is used in the literature. The proposed index enables a precise evaluation of *n*-dimensional stiffness problem. The comparison of indices was achieved in a normalized domain. The isotropy distribution is illustrated in 3-dimensional figures using color mapping and by sketching iso-curves. In addition, the normal distribution is given for both indices. It was observed that the proposed isotropy index is more sensitive than the conventional one. It has a lower standard deviation. Hence, it can distinguish isotropic and non-isotropic poses for spatial stiffness matrices, precisely.

While in this paper VJM is chosen to compute the stiffness model, other stiffness model computation can be employed, and the proposed stiffness index and the other indices can be calculated by using these models as well. As a future study, an optimal design will be conducted for R-CUBE mechanism by using the volumetric isotropy index.

**Author Contributions:** All the work presented in this paper was carried out by ˙ I.G. under the supervision of M. ˙ I.C.D.

**Funding:** The study is supported by The Scientific and Technological Research Council of Turkey via gran<sup>t</sup> number 117M405.

**Conflicts of Interest:** The authors declare no conflict of interest.
