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Numerical control Numerical control of machine tools may be defined as a method of automation in which various functions of machine tools are controlled by letters, numbers and symbols. Basically a NC machine runs on a program fed to it. The program consists of precise instructions about the manufacturing methodology as well as the movements. Fox example, what tool is to be used, at what speed, at what feed and to move from which point to which point in what path, all these instructions are given. Since the program is the controlling point for product manufacture, the machine becomes versatile and can be used for any part. All the functions of an NC machine tool are therefore controlled electronically, hydraulically or pneumatically. In NC machine tools one or more of the following functions may be automatic: (i) Starting and stopping of the machine tool spindle. (ii) Controlling the spindle speed. (iii) Positioning the tool tip at desired locations and guiding it along. Desired paths by automatic control of the motion of slides. (iv) Controlling the rate of movement of tool tip. (v) Changing of tools in the spindle. Initially the need of NC machines was felt for machining complex shaped small batch components as those belonging to an aircraft. However, this spectrum currently encompasses practically all activities of manufacturing, in particular capital goods and white goods. Thus the range covered is very wide. Besides machining with which we are concerned, NC has been used in a variety of manufacturing situations. The majority of applications of NC are in metal cutting machine tools such as milling machines, lathes, drilling machines, grinding machines and gear generating machines. Besides a number of metal forming machine tools such as presses, flame cutting machines, pipe bending and forming machines, folding and shearing machines also use NC for their program control. The inspection machines called Co-ordinate Measuring (CMM) are also based on NC. Lastly the robots basically may be material handling units, but their control principles are very close to the NC. Besides these applications listed for manufacturing, other applications such as filament winding or assembly machines based on the NC principles can also be seen in the industry. NC machines have been found suitable for the following: (i) Parts having complex contours, that cannot be manufactured by conventional machine tools. (ii) Small lot production, often for even single (one off) job production, such as for prototyping, tool manufacturing, etc. (iii) Jobs requiring very high accuracy and repeatability. (iv) Jobs requiring many set-ups and/or when the set-ups are expensive. (v) Parts that are subjected to frequent design changes and consequently require more expensive manufacturing methods. (vi) The inspection cost, which is a significant portion of the total manufacturing cost. One or more of the above considerations would justify the processing of a part by an NC machine tool. Numerical Control is superior to conventional manufacturing in a number of ways. The superiority comes because of the programmability. These are as follows: (i) Parts can be produced in less time and therefore are likely to be less expensive. The idle (non-cutting) time is reduced to minimum. This of course depends on the way the part program for the part is written. The endeavour of the machine tool builder is to provide a facility whereby the non-cutting time can be brought to the minimum. It is possible to reduce the non-productive time in NC machine tools in the following ways: (a) by reducing the number of set-ups (b) by reducing set-up time (c) by reducing workpiece-handling time (d) by reducing tool-changing time. These make machines highly productive. (ii) Parts can be produced more accurately even for smaller batches. In conventional machine tools, precision is largely determined by human skill, NC machines, because of automation and the absence of interrelated human factors, provide much higher precision and thereby promise a product of consistent quality for the entire batch. (iii) The operator involvement in part manufacture is reduced to a minimum and as a result less scrap is generated due to operator errors. No operator skill is needed, except in setting up of the tools and the work. Even here, the set-up has been simplified to a great extent. (iv) Since the part program takes care of the geometry generated, the need for expensive jigs and fixtures is reduced or eliminated, depending upon the part geometry. Even when a fixture is to be used, it is very simple compared to a conventional machine tool. It is far easier to make and store part program (tapes). (v) Inspection time is reduced, since all the parts in a batch are identical, provided proper care is taken about tool compensations and tool wear in part program preparation and operation. With the use of inspection probes in the case of some advanced CNC controllers, the measurement function also becomes part of the program. (vi) The need for certain types of form tools is completely eliminated in NC machines. This is because the profile generated can be programmed, even if it involves three dimensions. (vii) Lead times needed before the job can be put on the machine tool are reduced to a great extent, depending upon the complexity of the job. More complex jobs may require fixtures or templates if they are to be machined in conventional machine tools, which can be reduced to a large extent. (viii) CNC machining centers can perform a variety of machining operations that have to be carried out on several conventional machine tools, thus reducing the number of the machine tools on the shop floor. This would save floor space and result in less lead-time in manufacture. This would also result in an overall reduction in production costs. (ix) The set-up times are reduced, since the set-up involves simple location of the datum surface and position. Further, the number of the set-ups needed can also be reduced. All this translates into lower processing times. A component can be fully machined in a single machining center or turning center, each of which having wider machining capabilities. In conventional manufacture if the part has to be processed through a number of machine tools which are located in different departments, the time involved in completion and the resultant in process inventory would be large. This would be greatly eliminated by the use of NC machine tools. 數字控制 機床數字控制是一種由數字和符號控制完成機床各種功能的自動化方法?；旧螻C機床是通過程序控制來工作的。程序包含了制造工藝運動的精確命令，比如說，用什么樣的刀具，什么樣的切削速度，什么樣的進給量，從這點移動到那點走過什么樣的路徑，所有這些都已給出指示命令。自從加工產品的程序被控制運用，這種機器變得用途廣泛的。NC機床的所有功能也因此由電力，液壓或者是氣壓帶動實現(xiàn)的。 在NC機床下列功能中由一種或者更多的可能是自動的： （一） 機床主軸的啟動和停止。 （二） 控制主軸轉速。 （三） 確定刀尖位置和導向，自動控制滑臺的運動路徑。 （四） 控制刀具的運動速率。 （五） 刀具的轉換。 最初NC機床是用來探索飛機一小批機制合成物成分的。但是這種光譜涵蓋目前幾乎所有制造業(yè)的活動，尤其是資本貨物和白色家電。因此這個范圍是非常廣泛的。除了我們所關心的機制以外，NC被用在很多制造情形中。NV被多數應用在金屬切削機床中，比如銑床，車床，鉆床，磨床和插齒機。此外一些金屬成型機床像壓床，火焰切斷機，彎管成型機，折疊式剪切機都用NC的程序控制。統(tǒng)籌測量機也是基于NC而工作的。最后機器人基本上具有物料裝卸裝置，但是他們控制的原則是非常接近NC的。除了這些申請上市的制造業(yè)以外，其他如纏繞或裝備機器基于數控原則也是經?？梢钥吹降?。 數控機床有以下用途： (一) 部分有復雜的輪廓線，這是傳統(tǒng)機床不能制造的。 (二) 小批量生產，往往連單次（一次性）生產，如原型制作，刀具制作等。 (三) 準確度和可重復性要求很高的加工。 (四) 要求很多設備或者設備很貴的加工。 (五) 零部件經常需要更改設計的，因此需要更昂貴的制造方法。 (六) 檢測費用，占總制造成本很大比例。 一個或更多的上述因素足以證明數控機床可以處理很多問題。 數控機床在一系列的加工方式中明顯優(yōu)于傳統(tǒng)的制造業(yè)。優(yōu)勢在于數控程序可控的。優(yōu)勢如下： (一) 零件加工時間短，因此可能會很便宜。非切削時間減少到最低。這當然取決于加工零件的程序。機床設計者努力的方向是要提供一個非切削時間最低的設備。以下方式有可能減少數控機床的非加工時間： (a) 減少裝配次數 (b) 減少裝配時間 (c) 減少工件處理時間 (d) 減少更換刀具時間 這些使機器具有高生產力。 (二) 即使較小批量也可以生產的更準確。傳統(tǒng)機床，精度在很大程度上取決于人的技巧，而數控機床，由于自動化和無關聯(lián)的認為因素，提供更高的精度，從而保證了整批產品質量的一致。 (三) 要把制造階段操作者的參與減至最低并且要減少由于操作錯誤產生的廢料。除刀具和工作的設置，沒有操作技能的要求。甚至，簡化設置也有極大的影響。 (四) 由于該程序涉及了幾何學，減少或消除昂貴的工裝需要，取決于幾何結構部分。即使使用夾具，相對于傳統(tǒng)機床這也是很簡單的。制作和保存部分程序是更容易的。 (五) 檢查時間減少，因為所有的一批零件是相同的。在部分程序的制作與運作中給予刀具補償和刀具磨損以適當的照料。在使用探針檢查的情況下，測量也是一些先進數控控制器的主要功能部分。 (六) 在數控機床中徹底消除對某些成型刀具的需要。這是因為輪廓的生成是可控的，即使它涉及三個層面。 (七) 放在機床上加工之前所需的提前期可以很大程度的減少，這取決于加工的復雜性。更復雜的工件如果在傳統(tǒng)機床上加工需要設備或模具，而這些設備或模具可以很大程度上的減少。 (八) 加工中心可以進行各種加工操作而傳統(tǒng)機床是很難實現(xiàn)的，從而減少了車間的機床數量。這將節(jié)省空間和減少制造的準備時間。這也能全面降低成產成本。 (九) 裝配時間減少，因為裝配涉及了材料表面和形態(tài)的簡單位置。此外，大量的裝配需求也要減少。所有這些轉化為較低的加工時間。組件完全可以在每一個具有廣泛加工能力的單一加工中心或車削中心機加工。在傳統(tǒng)制造業(yè)中如果零件通過一系列位于不同車間的機床加工，那么完成的時間和存貨將會很多。使用數控機床將會極大地消除這些缺點。