Mumbai University > Mechanical Engineering > Sem 7 > CAD CAM CAE

Marks: 5 Marks

Year: Dec 2016

Machining centers are multifunctional CNC machines which are capable of performing multiple operations such as turning, milling, drilling, remaining, tapping and boring on a single work piece in one setup without stopping the machine. The machining operation involves loading the work piece on a fixture which is mounted on a pallet. The pallet is then automatically taken in the machining area. The tool loaded on the machine starts operation as per the defined process.

Machining centers typically have an Automated Tool Changer (ATC). On completion of the first process, the tool is automatically loaded by the ATC and the next sequence of operation begins, till the work piece machining is complete. When the machining is underway, the pallet prepares for the loading of the next work piece, thereby preventing any downtime. After the completion of the job, the work piece is automatically unloaded on a pallet changer and the next work piece is taken up for machining. Machining centers with ATC and work loaders make them highly versatile for multiple operations. They can be either horizontal axis or vertical axis.

A horizontal machining center has its spindle on the horizontal axis and a vertical machining center has its axis vertical, relative to the worktable. These variations allow different types of operations on individual machines. A vertical machining center is generally used for flat work that requires machining access from the top, whereas a horizontal machining center is used for cube shaped parts where tool access can best be achieved on the sides of the cube. Machining centers are highly automated and versatile and their use is highly beneficial in manufacturing of complex parts at high production rates. Some of the important components of machining centers are described below.

a) Automatic Tool Changers for Machining Centers:-

The jobs done on NC and CNC machine tools have complex operations requiring number of different tools. The changing of tools is time consuming and requires resetting. For machining and turning centers, the tooling can be automatically changed by using an Automatic Tool Changer (ATC). This feature of a machining center makes it highly attractive as a complete job can be machined in one go, on one machine with one program written for it, thereby increasing productivity and repeatability of manufacturing. Tools are usually stored in on a drum type magazine or in a chain magazine.

The drums and chains are indexed in a manner so as to provide the desired tool to the ATC mechanism. Machining centers having 16/24 tools are common but versions with larger capacities are also available. The transfer of the tool from the magazine is done by first indexing and getting the desired tool at the changing position. A tool transfer arm then transfers the old tool with the new tool. The old tool can be placed at the same position from where it was taken or it can be placed at any other available position. In the latter case, coding to identify the tool held at any position would be necessary for reuse.

A type of a 1800 rotation tool changer mechanism. On receiving the tool change command, the MCU sends the spindle to its fixed tool change coordinates. At the same time, the tool magazine is indexed to the proper position. The tool changer then rotates and engages the tool in the spindle and the tool in the magazine at the same time. The tool changer then removes both the tools from their places and rotates by 1800 to swap the two tools. Thus the old tool is transferred to the magazine and the new tool is fixed on the spindle. This completes the tool change operation and the tool changer then moves to a neutral or parked position and remains there till it is again required.

b) Automatic Work Loaders for Machining Canters:-

Machining centers have an extremely high machine time cost. Keeping these machines idle for settings and work loading is extremely costly. To reduced the work loading time an automated work changer is necessary. A schematic arrangement of a typical kind of a device is the rotary pallet system.

When a work piece in station 1 is being machined, the rotary pallet table allows the work to be loaded and unloaded at the other positions. When the machining at station 1 completed, the pallet table indexes and station 2 is available for machining. The completed job at station 1 can then be indexed and can be unloaded at station 4 and a new work piece can be loaded for subsequent use.