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 (Z); - a pair of first horizontal cross-members , each one extending parallel to a first horizontal direction (X), 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 (Y) that is perpendicular to the first horizontal direction (X) 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 (Y). each individual module comprising: - two mutually parallel vertical columns , each one extending parallel to a vertical direction (Z); - a pair of first horizontal cross-members , each one extending parallel to a first horizontal direction (X), 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 (Z); - a pair of first horizontal cross-members , each one extending parallel to a first horizontal direction (X), 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 (Y) that is perpendicular to the first horizontal direction (X) 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 (Y). <EOS>