Bradbury-Nielsen gates (BNGs) are a standard way for gating or steering beams of charged particles in ion mobility spectrometry and time-of-flight mass spectrometry. They consist of a pair of interleaved electrodes that when at the same potential allow ions to pass through the electrodes undeflected and, when a voltage is applied, cause the ions to be deflected from their propagation axis. Previous efforts to construct such devices have relied on mechanical assembly by winding wires across an aperture. We describe a micromachining method for making monolithic BNGs using deep reactive ion etching of silicon-on-insulator wafers. This method enables the creation of electrodes with spacings ranging from 25 to 100 microm with a thickness of 20 microm, covering a 5 mm by 5 mm active area. We characterize the performance of these micromachined BNGs by ion imaging in a pseudorandom time-of-flight mass spectrometer.