In the geometric simulation of multi-axis milling, dexel representation solid model is often used. In this modeling method, the object shape is defined as a collection of vertical segments (dexels) based on a 2 dimensional regular square grid in the xy plane. In this paper, the authors propose quad pillars algorithm and its improvement named delta pillars algorithm for converting a dexel model to an equivalent polyhedral STL model. These algorithms define a series of vertical pillar shapes for each square cell of the grid to represent the object shape as a bundle of pillars. The final polyhedral model is obtained as a result of simplified Boolean union operation of the pillar shapes. Different from the prior methods, proposed algorithms are simple and fast, and they are guaranteed to generate the water-tight polyhedral model without holes, gaps, and T-junctions. An experimental system is implemented and some conversion tests are performed. The system can convert a dexel model based on a high resolution grid to a polyhedral model in a practical amount of time.