Process forces arising during Friction Stir Welding (FSW) have become of interest to investigators interested in obtaining weld quality information from recorded weld data. Successful analysis of process forces require the separation of force signals stemming from material flow mechanisms within the weld from signals influenced by the FSW machine. Three modes of FSW control were characterized for system response: Servo Position (SPC), Electronic Deflection Compensation (EDC), and Constant Load Control (CLC). The gain value of the feedback loops associated with EDC and CLC modes were altered and characterized. SPC mode response to vertical changes in the tool position was also characterized. Machine-specific force signatures associated with the motor transmission assembly and spindle resonance were also identified. Characterization of the influence of machine control modes and other machinespecific frequencies on process force signals will allow future investigators to identify segments of welds during which machine actions influenced recorded force data.