The double-point downhill secant method (the DPDS method) is proposed to solve the nonlinear equations to determine the curvature interference limit points for modified hourglass worm drives. Thereupon, the whole curvature interference limit line can be obtained by interpolation. Based on this, the undercutting feature of the corrected worm gear can be investigated. The DPDS method has two main merits in principle. The first is the avoidance of the computation of the Jacobi matrix of the system of nonlinear equations. The second is that the sensitivity to the guess value can be decreased evidently owing to adopt the technique of the norm reduction. The effectiveness of the DPDS method is inspected and verified by a numerical example.