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CNC MACHINING CENTERS
HISTORY OF CNC MACHINES
CNC refers to a computer that is joined to the NC machine to make the machine more versatile. Therefore, it is obvious that CNC came into existence after its predecessor NC (Numerically Controlled) machines, which were hard wired. In the early 1950’s, the first numerically controlled machine was developed by the Parsons to manufacture quickly complex jet aircraft parts for the US Air Material Command. With the availability of microprocessors, the controller technology made a tremendous progress with the support of which, in the years 1970 - 1972 first Computer Numeric Control machines were developed. Punched tape continued to be used as a medium for transferring G-codes into the controller for many decades till the 1960’s, and then it was eventually superseded by RS232 cables, floppy disks, and now it is directly tied into plant networks. Modern CNC machines use software programs to provide the instructions necessary to control various operating parameters hence they are often termed as soft wired numerical control. The introduction of CNC machines drastically changed the manufacturing industry with its ability to machine curves, intricate shapes and complex 3-D structures so easily with the number of machining steps that required human effort radically reduced.
WHAT ARE CNC MACHINING CENTERS?
In conventional machine tools the motion is caused by turning cranks and handwheels, while in the case of CNC machines almost all the motions are performed through programmed commands. The motion type (rapid, linear, and circular), the axes to move, the amount of motion and the motion rate (feed rate) are programmable with almost all CNC machine tools. The machining center, developed in the late 50’s is a machine tool capable of performing machining operations simultaneously on a work part in one setup under NC program control which was later facilitated by the addition of a computer to monitor and control the entire unit. CNC Milling machines are termed as CNC machining centers and they are often retro fitted with CNC technology and some special features by removing all the mechanisms built into the machine which requires a large amount of human intervention such as hand wheels etc. Nowadays there are many milling machines which are built specifically for CNC as opposed to being retrofitted at a later stage. Some of these machines are absolutely massive and have built in tool changers, auto-feed mechanisms for loading in material and various electrical sensors for safe monitored cutting.
CLASSIFICATION OF CNC MACHINING CENTERS
Machining centers can be classified as vertical, horizontal, or universal based upon the orientation of the machine spindle: A vertical machining center has its spindle on a vertical axis relative to the work table. A vertical machining center (VMC) is typically used for flat work pieces that require tool access from the top. E.g. mould and die cavities, large components of aircraft A horizontal machining center (HMC) is used for cube shaped parts where tool access can be best achieved from the sides of the cube. A universal machining center (UMC) has a work head that swivels its spindle axis to any angle between horizontal and vertical making it a very flexible and versatile machine tool. E.g.: aerofoil shapes, curvilinear geometries.
FEATURES OF CNC MACHINING CENTERS
machining centers are usually designed in such a way with the following features so that the non-productive time is reduced to the maximum extent which make them the most preferred machines. • Automatic tool changer: The tools are contained in a storage unit that is integrated with the machine tool. When a cutter needs to be changed, the tool drum rotates to the proper position and an automatic tool changer (ATC) operating under program control, exchanges the tool in the spindle for the tool in the tool storage unit. Capacities of tool storage unit commonly range from 16 to 80 cutting tools. • Automatic work part positioner: Many horizontal and vertical machining centers have the capability to orient the work part relative to the spindle. This is accomplished by means of a rotary table on which work part is fixed. The table can be oriented at any angle about a vertical axis to permit the cutting tool to access almost the entire surface of the part in a single setup. • Automatic pallet changer: Machining centers are often equipped with two (or more) separate pallets that can be presented to the cutting tool using an automatic pallet changer. While machining is performed with one pallet in position at the machine, the other pallet is in a safe location away from the spindle. In this location, the operator can unload the finished part and then work on the raw work piece for the next cycle.
ADVANTAGES AND LIMITATIONS OF CNC
When compared to the conventional CNC machine tools, the CNC machining centers are advantageous in the following ways: Reduced handling time of the parts Fewer number of steps are involved resulting in increased productivity Increased accuracy and repeatability because the parts utilize the same fixture throughout their processing The CNC machining centers are also affected with the same disadvantages of the conventional CNC machine tools like high amount of initial investment but which can be recovered very soon in the long run during mass production. The machining center should be also be well maintained as they are expensive to repair with high costs accounted for maintenance also. It is also restricted with the basic requirement of skilled CNC programmers.
APPLICATIONS OF CNC MACHINES
machines are widely used in the manufacturing industries mainly for the production of following kind of parts: Parts with complicated contours Parts requiring close tolerance and or good repeatability Parts which require complex and expensive jigs and fixtures when produced on conventional machines Parts which are to be manufactured without any room for human errors. Parts which have changing requirements like the ones which are in the development stages. Parts which are required urgently. Small batches or production lots.
CNC Machining Center Manufacturers
A few of the established suppliers of CNC machining centers in the market are: Makino, EMAG, HAAS Automation, GROB-WERKE, DMG MORI SEIKI, Mecanumeric, STAMA, OKUMA, HELLER, CHIRON, EMCO, Sodick, MAG and Hardinge Bridgeport.