What are machining centers
The traditional method of machining parts that uses different types of machine tools has been and continues to be, a valid and efficient manufacturing method. It can be highly automated in order to increase productivity, and this in fact is the underlying principle of transfer lines (also known as dedicated manufacturing lines, or DML). Commonly used in high volume or mass production, transfer lines consist of different specific machine tools arranged in a logical sequence. The workpiece (such as a block of an automotive engine) is moved from one station to the next, with a specific processing operation at each station, after which it is transferred to the next machine for further specific machining.
Sometimes, however, transfer lines are not feasible or economical, particularly when the products to be manufactured change rapidly due to factors such as product demand or changes in product shape or style. It is a very expensive and time-consuming process to rearrange these machine tools to respond to the needs for the next production cycle. An important concept that addresses flexibility in manufacturing, developed in the late 1950s, is that of machining centers.
A machining center is an advanced computer-controlled machine that performs a variety of machining operations on different surfaces and different orientations of a workpiece without the need to remove it from your workholding device. The workpiece is generally static and the cutting tools revolve around it, and the same way they do in similar operations such as milling, drilling or honing. So while in transfer lines, the workpiece is conducted to the machine, in machining centers it is the machining operation that is performed on the piece.
When we use the word workpiece, we must emphasize that the piece in a machining center may also itself be a tool. Tooling in fact include cutting and forming tools, cutters and tool holders, tool shanks for holding tool inserts, molds for casting, male and female moulds for forming, punches for powder metallurgy and metalworking, rams for extrusion, work-holding systems and devices – all instruments that must also be manufactured. Since the geometries of these tools are often quite complex and require a variety of machining operations, these tools are commonly produced in machining centers.
Undisputedly, machining center development was made possible thanks to advances in automation and computer control of machine tools. As an example of the progress in modern lathes, the figure shows a CNC lathe (turning center) with two turrets that carry various cutting tools.
Components of a Machining Center
The workpiece in a machining center is placed on a pallet, or module, which can be moved and rotated (oriented) in various directions. Once a given machining operation, another operation begins which may require reindexing of the workpiece on its pallet.
After all machining operations have been completed, the pallet with the finished piece automatically move away and another pallet (with another workpiece) is brought into position by an automatic pallet changer. All movements are computer controlled and cycle times for pallet changing are on the order of 10-30 seconds. Pallet stations are also available with different pallets ready for machining center. The machines can also be equipped with various automatic functions, such as loading and unloading devices.
A machining center is equipped with an automatic tool changer (ATC). Depending on the particular design, up to 200 cutting tools can be stored in a tool storage. In certain special or large machining centers auxiliary tool storage may also be available. Cutting tools are automatically selected for the shortest path to the machine spindle.
The maximum size that the cutting tools can reach to work around a piece in a machining center is known as the work envelope -a term used for the first time in connection with the industrial robots.
The tool change arm shown in fig.3 is a common design.
It swings to catch a particular tool and places it in the spindle. Note that each tool has its own toolholder, thus making the transfer of cutting tools to the machine spindle transfer a highly efficient operation. Tools are identified by bar codes, encoded tags, or memory chips attached directly to their toolholders. Tool changing times are usually between 5 and 10 seconds, but can reach up to 30 seconds for tools weighing up to 110 kg. As tool changing is an unproductive operation, the current trend is to reduce the times more and more.
Machining centres can be equipped with a tool-control and/or a control of the workpiece that provides information to the machine control system so that it can compensate for any changes in the tool settings or caused by wear. Touch probes can be installed into a toolholder to determine the workpiece reference surfaces for the selection of tool settings and online checking of workpieces being machined. Note in fig. 25.6 that many surfaces can be equipped with sensors and their locations determined and stored in the database of the computer software. The data is then used for programming tool paths and to compensate for the length of the tool, its diameter and even its wear in more advanced software tools.
Types of Machining Centers
There are several designs for machining centers. The two basic types are vertical spindle and horizontal spindle, but many machines are capable of operating along both axes.
Vertical-spindle Machining Centers
These are also called vertical machining centers (VMC), and are capable of performing various machining operations, especially on parts with deep cavities, as in mold and die making. A vertical-spindle machining center (similar to a vertical-spindle milling machine) is shown in Fig.4.
The tool storage is on the left, and all operations and movements are directed and assisted through the computer control panel shown on the right. Because the thrust forces in these type of machining are directed downward, such machines have high stiffness and can produce parts with good dimensional accuracy. These machines generally are less expensive than horizontal-spindle machines.
Horizontal-spindle Machining Centers
These are also called horizontal machining centers (HMC), and are suitable for large as well as tall workpieces that require machining on multiple surfaces. The pallet can be swiveled on different axes to various angular positions. Another category of horizontal-spindle machines is turning centers, which are computer-controlled lathes with several features. A three-turret turning center is shown in Fig.5. It is constructed with two horizontal spindles and three turrets equipped with a variety of cutting tools used to perform several operations on a rotating workpiece.
There are also so-called Universal machining centers, which are equipped with both vertical and horizontal spindles. With a variety of features, they are capable of machining all of the surfaces of a workpiece (i.e., vertical and horizontal and at a wide range of angles).
Serope Kalpakjian & Steven Schmid: Manufacturing Engineering & Technology