The theoretical tooth surface model of spiral bevel gears was built

and the causes of global form error and the error separation method were analyzed theoretically.The tooth surface topographic deviations of the spiral bevel gear was separated into three globe form deviations

that is

the 0th

the 1st

and the 2nd order form deviation.Difference surface in combination with the least square method was proposed and the diagnostic parameters of tooth surface topographic deviation were derived.A software of tooth surface topographic deviation evaluation for the spiral bevel gear was developed with VC++6.0 in combination with Matlab.The theoretical tooth surface and difference surface of the gearwheel and pinion can be drawn by the software

and the diagnostic parameters of each order form deviation can be computed rapidly according to the derived difference surface

which leads to evaluate the tooth surface topographic deviation quantificationally.