The purpose of this study was to investigate the effects of changes in the helical angle of the router bit on the acoustic emission (AE) signal for various workpiece grain angles. The helical angle varied from 0° to 50° at 10° increments. The workpiece grains were oriented from 0 degrees (cutting parallel to the grain), through 90° (cutting end of the grain), to 165° at 15° increments. The AE signals and machined surface roughness were measured in an attempt to clarify the relations between them. The results were summarized as follows: (1) The AE signals were lowest in the “with the grain” cutting zones and slightly increased in the “against the grain” cutting zones; they rapidly reached the highest values at the 135°–165° grain angles. The greater the helical angle of the router bit, the smaller were the AE count rates for each grain angle investigated. There was no significant change in AE generation for helical angles of 0° and 10°. Moreover, the greater the feed rate, the greater was the AE count rate for every cutting condition investigated. (2) The surface roughness, similar to the AE count rate, had the lowest values in the “with the grain” cutting zones, slightly increased until the 120° grain angle, and then rapidly become extremely rough, reaching a maximum at the grain angles of 135°–150°. There was no remarkable change in the machined surface roughness while routing “with the grain” using the router bit of greater helical angle. However, when routing “against the grain,” the greater the helical angle the smoother was the machined surface. (3) There were correlations between the AE count rate and the machined surface roughness for each helical angle investigated. Therefore, acoustic emission has shown promise for monitoring and controlling the routing operation, including various grain angles and helical angles of the router bit.