This paper proposes an outer rotor and spoke type longitudinal flux direct-drive machine with advantages such as high precision, robustness, reliability, no backlash and a simple structure for large diameter turret applications. This paper deals with the optimum design using Latin hypercube sampling (LHS) with the many design variables of the developed motor. The effective design variables are selected by screening using an analysis of means (AMOM). This paper presents an optimum design for maximum torque and efficiency with the constraints of torque ripple ratio and maximum current using response surface methodology (RSM). The simulation results are compared with the experiment, and are within a 5% deviation of each other in the second prototype. All design performances of the second prototype are verified successfully. The predicted optimum design performances are consistent with the simulation results with a maximum error of 0.283%.