Although automatic impact hammers are available for use in dynamics and vibration research, they have seen little application in both well-supported labs and resource-poor settings, such as underprivileged communities and the parts of the developing world. The primary reason is that they are cost prohibitive as commercially available automatic modal hammers typically cost more than $10,000 USD, and the prevailing perspective is that testing performed using modal hammers is unrepeatable and uncontrollable. Consequently, this technology has seen limited use throughout academia and industry. An inexpensive automatic modal hammer system that is both controllable and repeatable could thus prove useful in both research and educational labs while promoting a do-it-yourself perspective in future scientists. Such a hammer would also provide immense benefit in the testing of strongly nonlinear systems whose dynamics depend greatly on the applied excitation. This research focuses on the creation and use of an open-source, scalable, repeatable, low-cost automatic modal hammer for investigating the dynamics of strongly nonlinear systems. The proposed design consists of a commercial modal hammer mounted on a stepper motor, commonly used in small 3D printers, that is equipped with an encoder and a servo driver controlled using an Arduino, enabling micro-stepping and precise control of the hammer. The capabilities of the proposed automatic modal hammer are demonstrated using a linear cantilever beam, a two-story building model with two nonlinear vibration absorbers, and a model airplane wing with a non-smooth nonlinear vibration absorber installed on its tip.