The success of many open source software projects revealed the power of voluntary collaborative production of large/complex software systems. The research community is therefore curious about the viability of open source projects in other areas. In around 2000, open source practices started to take place in the commercial hardware realm, and so far, the phenomenon has not been fully explored. Using grounded theory, the authors studied 31 firms for an average of 2.3 years, discovering a 4-phase growth pattern of open source hardware firms, including starting the firm, identifying core competencies, business model improvement and business maturation. The firms behaviors in each stage are reported, as well as the evolution of community demography, behaviors and impact in different growth phases.