A modular frame structure (10, 10', 10") for a machining center (100', 100"), which comprises at least one modular unit, said modular unit comprising on its turn a first and a second individual module, each individual module comprising: - two mutually parallel vertical columns, each one extending parallel to a vertical direction; - a pair of first horizontal cross-members, each one extending parallel to a first horizontal direction, each horizontal cross-member of the pair of first cross-members being joined to both columns; said modular unit further comprising two pairs of second horizontal cross-members, each horizontal cross-member parallel to a second horizontal direction that is perpendicular to the first horizontal direction each of said vertical columns of said first individual module being connected with a respective vertical column of said second individual module by means of at least one pair of said second horizontal cross-members, said modular frame structure being characterised by at least one of the second horizontal cross-members including a tubular element passing through the respective columns of said first and second individual module extending on opposite sides of each column, said at least one tubular element of the second cross-member comprising flanged ending portions configured for hydraulically connecting in a sealed way the tubular element of the second cross-member for providing a main distribution pipeline of a process fluid, said main distribution pipeline extending parallel to said second horizontal direction. each individual module comprising: - two mutually parallel vertical columns, each one extending parallel to a vertical direction; - a pair of first horizontal cross-members, each one extending parallel to a first horizontal direction, each horizontal cross-member of the pair of first cross-members being joined to both columns; - two mutually parallel vertical columns, each one extending parallel to a vertical direction; - a pair of first horizontal cross-members, each one extending parallel to a first horizontal direction, each horizontal cross-member of the pair of first cross-members being joined to both columns; said modular unit further comprising two pairs of second horizontal cross-members, each horizontal cross-member parallel to a second horizontal direction that is perpendicular to the first horizontal direction each of said vertical columns of said first individual module being connected with a respective vertical column of said second individual module by means of at least one pair of said second horizontal cross-members, said modular frame structure being characterised by at least one of the second horizontal cross-members including a tubular element passing through the respective columns of said first and second individual module extending on opposite sides of each column, said at least one tubular element of the second cross-member comprising flanged ending portions configured for hydraulically connecting in a sealed way the tubular element of the second cross-member for providing a main distribution pipeline of a process fluid, said main distribution pipeline extending parallel to said second horizontal direction.