Cnc machining nm09/2
Types of machine control units (MCU)
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- 1.10 Machine control unit (MCU) development
- 1.10.2 NC control limitations
- Non-optional speeds and feeds
- 1.10.3 Computer numerical control
- 1.10.4 Advantages of CNC
- 1.10.5 Direct numerical control
- 1.10.6 Components of a DNC system
- 1.10.7 Advantages of DNC
1.9 Types of machine control units (MCU)Over the last twenty years or so, Machine Control Units have progressively evolved from simple units which could control motion on a point basis to the sophisticated control units which can control a number of axis movements simultaneously to produce components with complex geometry. This evolution of MCU development can be summarised as: 1.9.1 Point to pointPoint to point control of motion as shown below was a feature of early NC machines where the NC control was often limited to 2 axis table motion only. Such controls were usually to drilling and tapping operations. 
Figure 1.7 Point to point control 1.9.2 Continuous pathAs multi axis control became more developed, straight line continuous path cutting ability soon made simple point to point machines obsolete. The ‘continuous path’ form of motion control as shown is referred to as ‘Linear Interpolation’ which depending on the machine, can involve all three axis being driven simultaneously. 
Figure 1.8 Continuous or linear interpolation 1. 9.3 Contour pathWith the development of computer numerical control (CNC) machines evolved the ability to cut true true geometric arcs and circles. This motion control referred to as circular interpolation allows cutting motion to blend from straight line to curves in one continuous motion. 
Figure 1.9 Contour path programming 
Figure 1.10 Path of linear interpolation Although linear interpolation is essentially a straight line motion control, circles and arcs can be cut, but programming these is a series of short straight line cuts. The accuracy of the arc is dependent on the high and low limits of size within the arc. 1.10 Machine control unit (MCU) development1.10.1 NC machinesNumerical Control is the term used to describe a process of sending program commands to the machine block by block. Put another way, the part program data is fed to the MCU via a tape reader in such a way that the MCU receives a block of information, processes it and then performs that step. Once completed, the tape is advanced and the next block of data is sent to the MCU for action. This step by step process repeats itself until all blocks have been executed. With NC machining the program punched tape is the source of program data and, as such, any changes or edits will require a new tape to be punched. 1.10.2 NC control limitationsConventional NC control units are either simple point to point or linear interpolation controls and as such have certain limitations which include: Part programming mistakesIn preparing the punched tape, part programming mistakes are common. The mistakes can be either syntax or numerical errors, and it is not uncommon for three or more passes to be required before the NC tape is correct. Although there are tape editing facilities available they are generally clumsy and for this reason any changes to a part program will normally require a new tape to be punched. Non-optional speeds and feedsIn conventional numerical control, the control system does not provide the opportunity to make changes in speeds and feeds during the cutting process. As a consequence, the programmer must set the speeds and feeds for worst-case conditions. The result is lower productivity. Tool ringingThe fact that motion is controlled as single block steps leads to cutting tools dwelling in the same spot waiting for the next command this dwell leads to tool ringing and poor finish. Punched tapeAnother problem related to programming is the tape itself. Paper tape is especially fragile, and its susceptibility to wear and tear causes it to be an unreliable NC component for repeated use in the shop. More durable tape materials, such as, mylar are utilised to help overcome this difficulty. However, these materials are relatively expensive. Tape readerThe tape reader that interprets the punched tape is generally acknowledged among NC users to be the least reliable hardware component of the machine. When a breakdown is encountered on an NC machine, the maintenance personnel usually begin their search for the problem with the tape reader. 1.10.3 Computer numerical controlComputer Numerical Control is an NC system that utilises a dedicated computer to perform some or all of the basic numerical control functions. Because of the trend toward downsizing in computers, most of the CNC systems sold today use a microcomputer-based controller unit. Over the years, minicomputers have also been used in CNC controls. The external appearance of a CNC machine is very similar to that of a conventional NC machine. Part programs are initially entered in a similar manner. Punched tape readers are still used to input the part program into the system. However, unlike conventional NC, where the punched tape is cycled through the reader for every workpiece, CNC, programs are only read once and then stored in the computer memory. Thus the tape reader is used only for the original loading of the part program and data. Compared to conventional NC, CNC offers additional flexibility and computational capability. New system options can be incorporated into the CNC controller simply by re-programming the unit. Because of this re-programming capacity, both in terms of part programs ands system control options, CNC is often referred to by the term ‘soft wired’ NC. 
Figure 1.11 CNC system configuration Computer Numerical Control has a number of advantages over conventional NC. preceding discussion: 1.10.4 Advantages of CNC
1.10.5 Direct numerical controlDirect numerical control can be defined as a manufacturing system in which a number of machines are controlled by a computer through direct connection and in real time. The tape reader is omitted in DNC, thus relieving the system of its least reliable component. Instead of using the tape reader, the part program is transmitted to the machine tool directly from the computer memory. In principle, one larger computer can be used to control more than 100 separate machines. The DNC computer is designed to provide instructions to each machine tool demand. When the machine needs control commands, they are communicated to it immediately. DNC also involves data collection and processing from the machine tool back to the computer. 1.10.6 Components of a DNC system
A basic overview of a DNC system us shown below: 
Figure 1.12 DNC system configuration In the DNC system configuration the computer calls the part program instructions from bulk storage and sends them to the individual machines as needed. It also receives data back from the machines. This two way information flow occurs in real time, which means that each machine's requests for instructions must be satisfied almost immediately. Similarly, the computer must always be ready to receive information from the machines and to respond accordingly. The remarkable feature of the DNC system is that the computer is servicing a large number of separate machine tools all in real time. Just as CNC had certain advantages over a conventional NC system, there are also advantages associated with the use of direct numerical control. The following list will recapitulate much of our previous discussion of DNC.
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