The coast side teeth of asymmetric gear with a small number of teeth which using small pressure angle lead to undercutting.Traditional undercutting caused by rectilinear segment of the rack-cutter

called the first type of undercutting.Besides

another undercutting caused by the rack-cutter fillet

called the second type and the third type of undercutting.In order to study the boundary condition

rotational coordinate system attached to generated gear and translational coordinate system attached to rack cutter are established.The parametric equations of boundary fillets for the second and third undercutting are solved

and the maximum fillet radius coefficient of the rack-cutter is obtained.The results show that

in order to avoid the undercutting of the tooth

it is necessary to satisfy the boundary condition of the first type and the second type of undercutting simultaneously.