Portal machines usually have a higher stiffness than cantilever machines due to their both-sided supports and thus, are often used for processing workpieces with large dimensions, for great cutting forces or processing with especially high speeds and their resulting extreme values for acceleration and braking. Another application is the low-cost sector, in order to produce a machine with acceptable stiffness with minimal material costs.
A fixed portal holds the aggregates. One axis (usually X) is realized by moving the machine table, the other axis (usually Y) by moving the aggregates support along the portal. Rarely used are machines with fixed machine tables and a separate Y-axis on the aggregates support. Fixed portals are often used when there shall be an efficient processing on multiple tables, because on table can always be moved out of the machine and the operator is not limited by processing movements.
The portal is mounted along the fixed machine table and movable. For the "real" gantry it is driven by synchronized motors on both of its sides. For cost reasons, sometimes a central engine is also used, which drives both sides of the protal mechanically synchronized. Some very inexpensive machines, which are gantry machines, only have an engine on one side. This leads to a minimal difference between driven and non-driven side. For most of the applications in woodworking this is irrelevant.
Since the mid-2000s, even for machines which are cantilever machines, a support which is guided on prism guides is used to improve the stiffness at the end of the cantilever. This leads to the optical impression that it is a moving gantry.