Usually, aggregates for tool changers are stationary housings with a tool interface, which include a gearbox to convert the vertical rotary motion of the main spindle, mostly into a horizontal rotary motion. In order to use change-over units, it is therefore absolutely necessary to apply an additional torque support, to prevent rotation of the entire housing.
A change-over unit with a fixed torque support can only be used in one predefined angle of incidence. By the means of a C-axis, an adjustment of the horizontal angle is possible.
There is a rare version, using 2 or more staggered torque supports on the machine. In combination with appropriate different holding devices in the tool changer, the aggregate can be used at different predefined angles.
Most frequently, change-over units are used with angular gearboxes for horizontal milling or drilling as well as for holding saw blades. However, there are also aggregates for processing from below (underfloor milling aggregate) and for non-cutting applications such as workpiece scanning, workpiece cleaning or polishing.
Change-over units can either be designed as aggregates for the use of a single tool or be equipped with several tool holders (e.g. star drill head, milling machine on one side/ saw on the other, etc.).
The aggregates are also available in versions with manually adjustable swing- and rotation angles.
Some manufacturers also offer machines with interfaces for pneumatic and power supply on the main spindle (specially for the use of exchangeable edgebanding aggregates on CNC-machining centers). These aggregates are often named adapter aggregates to highlight the difference to standard aggregates/ change-over units.
In the late 2000s, several manufacturers developed aggregates which provide a 5th positioning axis by using the drive of the C-axis (e.g. BENZ for HOMAG Flex5, Atemag for IMA/HOLZ-HER vario NC as well as the BIESSE group with its HSD Mechatronic Division).