According to the Japan Machine Tool Builders' Association, a Machining Center refers to an NC machine which can do various jobs, such as milling, punching, boring, screwing, and etc., without changing the setting of the part. Furthermore, it has a device which can bring in many types of different tools to the working position automatically, or it has a quick tool changing function with the structure which can process at least 2 sides . Thus, an NC machine is a machine which can perform various process by changing tools automatically.
Generally speaking, the very first machining center was the machine named "Milwaukee Magic" which the company, Carney & Treker (USA) made in 1985. The definition is "Multifunctional NC machine which processes by rotating tools with automatic tool changing capability" which Machine Tools Trades' Association (USA) adopted in 1977 in USA. And according to the Japan Machine Tool Builders' Association, "a Machining Center refers to an NC machine which can do various jobs, such as milling, punching, boring, screwing, and etc., without changing the setting of the part. Futermore, it has a device which can bring in many types of different tools to the working position automatically, or it has a quick tool changing function with the structure which can process at least 2 sides."
The types of machining center are categorized into horizontal type and vertical type by the direction of the main axis (spindle). The spindle is installed horizontally for horizontal type and vertically for vertical type.
Besides the above two, there are other types, such as those having both horizontal spindle and vertical spindle; those having a spindle head revolving horizontally and vertically and processing 5 sides; cantilever types with column structure; double housing type;, and those having cutting function added.
Names of each part for machining center are as follows.
- NC device ------- Calculates NC data which is read and gives commands to the machine.
- Tool magazine ------- Stores tools used.
- Middle arm ------ Takes out a tool from the magazine.
- ATC arm ------- Changes tools
- Spindle ------- Axis attached tools and rotates tools.
- Pallet ------- A tray where material is placed and fixed.
- Table ------- Moves pallets
- Cutting oil tank ------- A tank which keeps cutting oil.
NC machine runs by reading NC data and giving commands to each device. The "brains" is the NC device. NC device calculates using NC data, read as a character string for giving commands to operate each device. With NC code, the machine give commands to rotate a spindle, move a table, and change tools. NC codes may vary depending on NC devices. The same NC code may work differently. Predetermining tool allocation used by ATC and the minimum setting unit, NC device refers to the parameters and commands the action of each part. NC device is also called a controller.
The data medium of NC varies with the types of reading device installed in NC device. For NC device that has only paper tape reader, reading NC data is done with paper tapes. Because a paper tape has only 250m length per a tape, it has limitation for the quantity of NC data. Also, because the reel size of tape reader decides the length of the paper tape, it is not appropriate for large NC data (such as 3 dimensional data). Generally, many NC devices support a floppy disk to read NC data. Also, a portable floppy device may be used to connect to the NC device. Lately reading NC data with DNC is getting popular. There is no limitation in quantity of NC data, because NC data is sent from a computer for DNC to the machine directly. Also some DNC can schedule for the availability of the machine (unoccupied time) and processing time. It is essential for unattended operation.
The tool magazine is a device which store tools supplied to the ATC device. The shape varies depending on the number of tools stored and the location. There are cylinder shape, circular cone shape, oval shape, rectangle shape, and etc. When NC device gives the command for tool change, the magazine rotates and brings the tool selected to the changing position. Normally tools are lined up in one line. However, for a large magazine, tools are drawn up in two lines or three lines. Some machines have 2 tool magazines installed.
Because the tool direction is the same as the spindle direction for the ATC device shown above, a tool can be changed at the tool changing position by the ATC arm. However, if it's not the same, the middle arm needs to take out a tool from the magazine and turn it to the spindle direction, And then a tool will be changed by the ATC arm. ATC arm refers to the arm which grabs and pulls out the tool attached to the spindle and the tool to be used next. It then rotates them 180 degrees, and change them.
The spindle is the rotating axis to which a tool is attached. It is driven by either a DC motor or AC inverter motor. There are 2 ways to change the number of rotations for the motors: multi-gear and non-gear.. For multi-gear, the number of rotations is changed by combining progressive gears. For no gear, it is changed by the electric control. The number of rotation of the spindle is defined by NC code. However, for the multi-gear way, the speed will be the closest one to the defined speed. To attach a tool to the spindle, a chuck is used. So, tools that have chucks should be stored in the magazine of ATC device.
When you place and fix material on the table, you will fix the material directly on the table or put the material on a pallet and then fix the pallet on the table. If you fix the material directly on the table, you need to change the material after the material finishes being processed. However, if you use pallets, you can change the material by pallet and save time for setup. Also, machines that have APC (Auto Pallet Changer) or APM (Auto Pallet Magazine) can change the pallets with NC code. Thus, continuous processing of unattended operation can be done.
The table moves as NC data commands either fixing a material or having a pallet which a material is fixed on. The moving directions are X direction, Y direction (Z direction for horizontal machining center) and rotation. Some machines do not have rotating tables.
Because a machine with a rotating table can process 4 sides with one setup, the processing time can be shortened. The movement of the table and the rotation is defined by NC codes. Though the movement can be made along a line or an arc, it is done by sequencing movement of the minimum moving unit. The moving speed is also defined by NC codes.
When a material is processed at a machining center, cutting oil may be used to cool tools or decrease the cutting resistance. Using cutting oil helps to extend tool life, discharge scrapings, and improve the quality of the processed surface. The cutting oil tank is a tank which saves cutting oil. Cutting oil is pumped through the nozzle, and pours onto the part being processed.
Cutting oil is categorized into non-water soluble oil (which is used for lubrication) and water soluble oil (which is used for cooling).
The procedures at the machining center are as follows:
| (1) Place and fix material on the table. (2) Attach a tool and move it to the processing origin position. (3) Wait for input of NC data. |
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| (4) Read NC data, rotate a spindle, and apply cutting oil. |  |
| (5) Read more NC data, move a table, begin cutting. (6) If there is a NC code to change a tool in NC data, a tool is changed automatically and continues processing. (7) After processing is done, it stops by a command of NC code. |
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Each axis of horizontal or vertical machining center is as follows.
However, the coordinate axis may vary between machines, because there are types where a table moves and types where a column moves.