1.Major Functions
Machining centers are widely used in manufacturing sites.Machining centers are critical equipment for machining metal parts and components in addition to their primary purpose of die manufacture.As an example,machining centers in the automobile industry are used for efficient grinding and drilling of engine parts as well as for making dies for body components.Many other products commonly used in daily life and in industrial settings are manufactured in machining centers.
Machining centers feature a computerized automatic tool change function.In general,an operator must exchange the tools on an NC machine tool.Machining centers,however,have a change arm that automatically retrieves and exchanges tools from a tool magazine where the tools are stored during the machining of workpieces.This saves time and effort required for standard tool changeovers,and consequentially enables automated and power-saving operations while reducing costs.
In recent years,the emergence of five-axis machining centers,equipped with two rotational axes in addition to the conventional three axes,has enabled machining of shapes with even greater complexity.
2.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.2 is a common design.
Fig2
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.
3.Types and characteristics of Machining Centers
Horizontal machining centers have a blade-mounted spindle that comes out sideways,which machines workpieces in the horizontal direction.The column moves along the X axis,the saddle along the Y axis,and table along the Z axis,and this combination enables three-dimensional machining.Additionally,some models have a B axis that rotates the table horizontally,making it possible to machine materials using a total of four axes.
One advantage of horizontal types is the capability to machine four surfaces of a workpiece―when using a four-axis machining center with B axis―all at once.This eliminates the need for operators to manually switch the four sides of the workpiece,and thus also contributes to higher machining precision.Moreover,machining from the horizontal direction allows chips to fall down,which helps prevent the chips from accumulating on the workpiece and digging into the blade.
Conversely,vertical machining centers have the spindle in a vertical position,and workpieces are machined from above.Generally,the table travels horizontally on the X and Y axes,and the spindle moves vertically,enabling triaxial machining.
Compared with horizontal types where the spindle is situated to the side of the workpiece,vertical types take up less installation space,making them a popular choice.In addition,machining from above the workpiece allows operators to work while comparing the machining to the design drawings.However,machining on the top of the workpiece causes chips to accumulate on the workpiece,creating the need for a blower that uses compressed air,or rinsing with lubricant to remove the chips appropriately.
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