Power quality considerations for CNC machines: grounding

Computer numerical control (CNC) machines are used to shape metal parts by milling, boring, cutting, drilling, and grinding. A CNC machine generally consists of a computer-controlled servo-amplifier, servo-motors, spindle motor, and various tooling. The machine can be programmed to shape a part by use of a front control panel. More sophisticated models allow a computer-aided design drawing to be uploaded to the machine. The electronic components within a CNC machine are particularly sensitive to the grounding techniques used in the electrical supply to the machine. Malfunction, degradation, and damage to the electronics can often be traced to supplemental ground rods and lightning strikes to earth. Production downtime, product loss, and expensive repair bills result. With the wide-spread use of CNC machines across the world, these problems have become a significant financial concern to many CNC machine users and their electric utility companies. This paper begins with a brief explanation of the fundamentals of service and equipment grounding. The basic design of CNC machines is also explained. Based on a survey of several CNC machine representatives, the paper will explore the common grounding techniques recommended by many CNC machine tool builders with particular emphasis on the ground-rod problem. In addition, several actual case studies that support the ground-rod problem are described. Finally, a recommended powering and grounding practice is presented to help eliminate power quality related operating problems with CNC machines while maintaining the safety requirements of electrical codes.