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數(shù)字控制
機床數(shù)字控制是一種由數(shù)字和符號控制完成機床各種功能的自動化方法。基本上NC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(chǎn)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(zhuǎn)速。
（三） 確定刀尖位置和導(dǎo)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(zhuǎn)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(guān)心的機制以外，NC被用在很多制造情形中。NV被多數(shù)應(yīng)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(shù)控原則也是經(jīng)?？梢钥吹降?。
數(shù)控機床有以下用途：
(一) 部分有復(fù)雜的輪廓線，這是傳統(tǒng)機床不能制造的。
(二) 小批量生產(chǎn)，往往連單次（一次性）生產(chǎn)，如原型制作，刀具制作等。
(三) 準確度和可重復(fù)性要求很高的加工。
(四) 要求很多設(shè)備或者設(shè)備很貴的加工。
(五) 零部件經(jīng)常需要更改設(shè)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(shù)控機床可以處理很多問題。
數(shù)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(shù)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(shè)計者努力的方向是要提供一個非切削時間最低的設(shè)備。以下方式有可能減少數(shù)控機床的非加工時間：
(a) 減少裝配次數(shù)
(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(chǎn)力。
(二) 即使較小批量也可以生產(chǎn)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(shù)控機床，由于自動化和無關(guān)聯(lián)的認為因素，提供更高的精度，從而保證了整批產(chǎn)品質(zhì)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(chǎn)生的廢料。除刀具和工作的設(shè)置，沒有操作技能的要求。甚至，簡化設(shè)置也有極大的影響。
(四) 由于該程序涉及了幾何學(xué)，減少或消除昂貴的工裝需要，取決于幾何結(jié)構(gòu)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?shù)恼樟?。在使用探針檢查的情況下，測量也是一些先進數(shù)控控制器的主要功能部分。
(六) 在數(shù)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(fù)雜性。更復(fù)雜的工件如果在傳統(tǒng)機床上加工需要設(shè)備或模具，而這些設(shè)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(shù)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(chǎn)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(zhuǎn)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(shù)控機床將會極大地消除這些缺點。
絞盤和轉(zhuǎn)塔機床
中心機床是通用機床,加工中心的一些限制是：
(i) 加工前的設(shè)置時間是很長的。
(ii) 正常的加工過程中只能選用一把刀具。有時候有四把刀具的正方形刀架取代了傳統(tǒng)刀架。
(iii) 設(shè)置過程中的閑置時間和兩切削間隔中刀具的運動是大的。
(iv) 如果操作者不適當?shù)年P(guān)注，那么到指定地點的刀具精確運動是很難達到的。
所有的這些困難意味著中心機床將不會在生產(chǎn)中使用以消除低的生產(chǎn)率。因
此為了提高生產(chǎn)率中心機床必須加以調(diào)整改進。各種各樣改進后的機床有絞盤和轉(zhuǎn)塔車床，半自動機床和全自動機床。
在以下方面的改進基本上已經(jīng)達到了：
(a) 有效的加工方法
(b) 多個有效的刀具
(c) 全自動刀具進給
(d) 在精確位置刀具的自動停止
(e) 按照正確的操作次序自動控制
絞盤和轉(zhuǎn)塔機床的主要特征是六個方（六角）塊裝在床尾的一端取代正常尾座。這使得六個安裝刀具座中每一個都可以根據(jù)需要包含一個或多個刀具。另外兩個刀架固定在十字滑臺上，一個在前面，另一個在后面。它們中的每一個每次可容納多達四個刀具。因此，總的可以最高達到14個刀具，一個刀具安裝在一個位置。
轉(zhuǎn)塔車床包含了全直齒齒輪，具有一系列心軸轉(zhuǎn)速的重負荷軸承。轉(zhuǎn)頭安裝在滑動座架上，滑動座架可以在床身上依次滑動。在回擊期間當座架在床身上滑動時，它會自動檢索下一個刀具的位置，從而減少機器的閑置時間。
轉(zhuǎn)塔車床刀具在進料桿上設(shè)有停行系統(tǒng),可精確的控制刀具移動的實際距離.因此有可能確定和控制零件所需的刀具單獨運動。
支承裝置工作型式可以在轉(zhuǎn)塔車床中使用類似于普通車床，但是由于生產(chǎn)力要求較高和可重復(fù)性要求更大，一般的自動裝置，例如夾頭，自定心夾緊或氣動夾緊均被使用。
襯套有各種不同的設(shè)計。實際夾緊是由襯套沿著軸線方向推或拉的運動來完成的。有時襯套閉合期間棒料不是推前就是后拉。這可以防止因外管鎖住停止使軸向運動被阻止。
常常由多種轉(zhuǎn)塔車床零件是從未加工的棒料加工出來的。連續(xù)進給棒料，特殊的棒料進給安排是可行的，通過確切數(shù)據(jù)越過在一轉(zhuǎn)開始六角轉(zhuǎn)臺表面提供的障礙推進棒料。
十字滑臺刀架用的大部分刀具都非常類似于加工中心。成型刀具也通常用在十字滑臺中。為提供更大的生產(chǎn)力轉(zhuǎn)塔中使用了種類繁多的專用刀具。一箱工具通常用于車削工藝，因為刀具切割時也支持這個工作。它們有一把切削刀還有滾齒刀以提供對工作部件必要的支承。這有助于對桿的加工，但加工操作方面沒有很好的支承。盒裝刀具中也可能有至少一把切斷刀，那樣在提供工件支承的同時可以重復(fù)切削。
組合刀具讓很多復(fù)合切割刀具按規(guī)定調(diào)整以適應(yīng)加工形勢。它們可以同時執(zhí)行一個以上的切削操作，從而減少實際操作加工所需的時間。在一個刀架上有內(nèi)外切削刀具，那樣工件可以達到更高的精度。
許多轉(zhuǎn)塔車床將裝有與加工中心很相似的錐形轉(zhuǎn)向裝置來加工錐形物。小的錐形物可以用成型刀具生產(chǎn)，而內(nèi)部錐形可以用錐形鉸刀來加工。
因此可以發(fā)現(xiàn)絞盤和轉(zhuǎn)塔與通用機床之間的很多不同點：
(一) 軸承有越來越多的各級轉(zhuǎn)速允許更高的生產(chǎn)速率。
(二) 刀架是可轉(zhuǎn)位的（四把刀具）。任何一個工具都可以進入切削位置。
(三) 一個有六把刀具位置的刀架取代了刀柄尾部。
(四) 可以通過進給停止器來調(diào)節(jié)每一個刀具的進給。
(五) 安裝在同一個刀架表面的兩個或以上的刀具可以同時進行切削。
(六) 半熟練的操作要求。
(七) 以上是用于涉及更好重復(fù)性的生產(chǎn)經(jīng)營。
轉(zhuǎn)塔車床的變化是絞盤車床，其中轉(zhuǎn)臺可以在滑動座架上移動，滑動座架視工件的長度可以固定在床身的任何位置。因此刀具的行程局限于滑動座架的長度。這種類型通常用于小規(guī)模的機器。
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.
Capstan And Turret Lathes
The center lathe is a general purpose machine tool,it has a number of limitations of center lathes are:
(i) The setting time for the job in terms of holding the job is large.
(ii) Only one tool can be used in the normal course. Sometimes the conventional tool post can be replaced by a square tool post with four tools.
(iii) The idle times involved in the setting and movement of tools between the cuts is large.
(iv) Precise movement of the tools to destined places is difficult to achieve if proper care is not taken by the operator.
All these difficulties mean that the center lathe cannot be used for production work in view of the low production rate. The center lathe is thus modified to improve the production rate. The various modified lathes are turret and capstan lathes, semi-automatics and automatics.
Improvements are achieved basically in the following areas:
(a) work holding methods
(b) multiple tool availability
(c) automatic feeding of the tools
(d) automatic stopping of the tools at precise locations
(e) automatic control of the proper sequence of operations.
The main characteristic feature of the capstan and turret lathes is the six sides (hexagonal) block mounted on one end of the bed replacing the normal tailstock. This allows for mounting six tool blocks, each one of which can contain one or more tools depending upon the requirement. Further on the cross slide, two tool posts are mounted, one in the front and the other in the rear. Each one of them can hold up to four tools each. Thus the total carrying capacity is a maximum of 14 tools when only one tool is mounted in each of the locations.
The turret lathe consists of an all gear, heavy duty headstock with a greater range of spindle speeds. The turret is mounted on a saddle which in turn is sliding on the bed. When the saddle moves on the bed during the return stroke it would automatically be indexed to the next tool position, thus reducing the idle time of the machine.
The tools in the turret lathe are provided with a system of stops and trips on the feed rod which can precisely control the actual distance moved by the tool. Thus it is possible to set and control the individual movements of the tools as required by the component.
The type of work holding devices that can be used with turret lathe is similar to the conventional lathes, but in view of the higher productivity demanded and greater repeatability required, generally automatic fixtures such as collets, self centering chucks or pneumatic chucks are used.
The collet chucks come in a variety of designs. The actual clamping is done by the movement of the collet tube along the axis of the spindle by either pushing or pulling .Sometimes the bar material is either pushed or pulled back during the closing of the collet. This can be prevented by having an external tubular locking stop so that the axial movement is prevented.
Often a large variety of components on a turret lathe are machined from raw material which is in a bar from. For continuous feeding of the bar special bar feeding arrangements are available which pushes the bar by a precise amount against a stop provided on the face of the hexagonal turret at the beginning of the cycle.
Most of the tools used in the cross slide tool post are very similar to those used in the center lathe. Form tools are generally used in the cross slide. A large variety of special tool holders are available for use in the turret for providing greater productivity. A box tool is generally used for long turning jobs since the tool while cutting also supports the job. They have a cutting tool and also support rollers for providing the necessary support to the workpiece. This helps in machining of bars which are not well supported during the machining operation .It is also possible to have more than one cutting tool held in a box tool such that there is an overlap of the cuts while providing support for the workpiece.
Combination tool holders allow for mounting multiple cutting tools with provisions for their adjustment to suit the machining situation. They have the ability to perform more than one cutting operation at the same time, thereby reducing the actual machining time required for the operation. They can be have both the internal and external cutting tools in a single tool holder such that the workpiece support can be taken care of so that higher accuracy can be achieved.
Many turret lathes would be fitted with taper turning attachments very similar to that used in center-lathes, for machining tapers. Small tapers can be produced by form tools from the cross slide, while internal tapers are produced by taper reamers.
Thus the various differences that can be found between capstan and turret with that of a general purpose center lathe are:
(i) The headstock has more and heavier range of speeds and to allow for higher rate of production.
(ii) The tool post is indexable (four tools). Any one tool can be brought into the cutting position.
(iii) The tail stock is replaced by a tool turret with six tool positions.
(iv) Feed of each tool can be regulated by means of feed stops.
(v) Two or more tools mounted on a single tool face can cut simultaneously.
(vi) Semi-skilled operators are required.
(vii) These are used for production operations involving better repeatability.
A variation of the turret lathe is the capstan lathe, in which the turret moves on the saddle while the saddle can itself be fixed at any position on the bed depending upon the length of the job. Thus the tool travel length is limited to the length of the saddle. This type of arrangement is normally used for small size machines.