Computer Numerically Controlled (CNC) machines are common pieces of equipment in manufacturing plants. In recent years, CNC-user interfaces have been evolving from CNC command languages to menu based and direct manipulation interfaces. However, there have been few efforts to evaluate CNC-user interfaces and to identify those interface features that have major effects on the usability of CNC machines. Thus, an objective assessment of the improvement achieved from different user interface technologies is needed. Two experiments compared the effectiveness of two types of CNC-user interfaces: command-language and direct manipulation interfaces. Two command language interfaces and one object oriented interface were designed and evaluated in terms of human performance and preferences. Experiment 1 compared two command-language interfaces: Layout 1 was a simulation of the original control panel of a CNC-Dyna machine, and Layout 2 was a modified user interface based on the analysis of the Dyna control language. Four factors were manipulated: (1) users’ level of expertise, (2) layouts, (3) tasks, and (4) trials. It was found that both groups of subjects completed the tasks faster using Layout 2 than Layout 1. Modification tasks were performed faster than copy tasks. Subjects’ responses to a questionnaire indicated that Layout 2 was preferred over Layout 1 in several categories. In Experiment 2 subjects used a command-language interface and a direct-manipulation interface. Four factors were manipulated: (1) experimental sessions (two sessions with a one-week interval between them), (2) users’ level of expertise, (3) tasks, and (4) trials. The interval between sessions had a significant effect on task completion time when the command-language interface was used, but there was not a significant effect of this factor when the direct-manipulation interface was used. Subjects’ subjective responses indicated a stable preference for the direct-manipulation interface in several categories of usability. / Ph. D.