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N/C Machine Tool Elements N/C machine tool elements consist of dimensioning systems, control systems, servomechanisms and open-or close-loop systems. It is important to understand each element prior to actual programming of a numerically controlled part. The team measuring system in N/C refers to the method a machine tool uses to move a part from a reference point to a target point. A target point may be a certain location for drilling a slot, or other machining operation. The two measuring systems used on N/C machines are the absolute and incremental. The absolute (also called coordinate) measuring system uses a fixed reference point (origin). It is on this point that all positional information is based. In other words, all the locations to which a part will be moved must be given dimensions relating to that original fixed reference point. The incremental measuring system (also called delta) has a floating coordinating system. With the inference point each time the part is moved .Notice that with this system, each new location bases its values in X and Y from the preceding location. One disadvantage to this system is that any errors made will be repeated throughout the entire program, if not detected and corrected. There are two types of control systems commonly used on N/C equipment: point-to-point and continuous path. A point-to-point controlled N/C machine tool, sometimes referred to as a positioning control type, has the capability of moving only along a straight line. However, when two axes are programmed simultaneously with equal values a 45°angle will be generated .Point-to-point systems are generally found on drilling and simple milling machine where hole location and straight milling jobs are performed. Point-to-point systems can be utilized to generate arcs and angles by programming the machine to move in a series of small steps. Using this technique, however, the actual path machined is slightly different from the cutting path specified. Machine tools that have the capability of moving simultaneously in two or more axes are classified as continuous-path or contouring. These machines are used for machining arcs, radii, circles, and angles of any size in two or three dimensions. Continuous-path machines are more expensive than point-to-point systems and generally require a computer to aid programming when machining complex contours. N/C servomechanisms are devices used for producing accurate movement of a table or slide along an axis. Two types of servos are commonly used on N/C equipment: electric stepping motors and hydraulic motors. Stepping motor servos are frequently used on less expensive N/C equipment. These motors are generally high-torque power servos and mounted directly to a lead screw of a table or tool slide. Most stepping motors are actuated by magnetic pulses from the stator and rotor assemblies. The net result of this action is that one rotation of the motor shaft produces 200 steps. Connecting the motor shaft to a 10-pitch lead screw allows 0.0005-in, movements to be made (1/200×1/10=0.0005 in.).Hydraulic servos produce a fluid pressure that flows through gears or pistons to effect shaft rotation. Mechanical motion of lead screws and slides is accomplished through various valves and controls from these hydraulic motors. Hydraulic servos produce more torque than stepping motors. However, they are more expensive and very noisy. Larger N/C machines use hydraulic servos. N/C machines that use an open-loop system contain no-feedback signal to ensure that a machine axis has traveled the required distance. That is, if the input received was to move a particular table 1.000 in, the servo unit generally moves the table 1.000 in. There is no means for comparing the actual table movement with the input signal, however. The only assurance that has actually moved 1.000 in is the reliability of the servo system used. Open- loop systems are, of course, less expensive than close-loop systems. A close-loop system compares the actual output (the table movement of 1.000 in.) with the input signal and compensates for errors. A feedback unit actually compares the amount the table has been moved with the input signal. Some feedback units on close-loop systems are transducers, electrical or magnetic scales, and synchros. Close-loop systems greatly increase the reliability of N/C machines. 數控機床的組成部分 數控機床的組成部分包括測量系統(tǒng)，控制系統(tǒng)，伺服系統(tǒng)及開環(huán)或閉環(huán)系統(tǒng)，在對數控零件進行程序設計之前，了解各組成部分是重要的。 數控中，測量系統(tǒng)這一術語指的是機床將一個零件從基準點移動到目標點的方法。目標點可以是鉆一個孔，銑一個槽或其它加工操作的一個確定的位置。用于數控機床的兩種測量系統(tǒng)是絕對測量系統(tǒng)和增量測量系統(tǒng)。絕對測量系統(tǒng)（亦稱坐標測量系統(tǒng)）采用固定基準點（原點），所有位置信息正是以這一點為基準。換句話說，必須給出一個零件運動的所有位置相對于原始固定基準點的尺寸關系。增量測量系統(tǒng)有一個移動的坐標系統(tǒng)。運用增量系統(tǒng)時，零件每移動一次，機床就建立一個新的原點（基準點）。注意，使用這個系統(tǒng)時，每個新的位置在X和Y軸上的值都是建立在前一個位置之上的。這種系統(tǒng)的一個缺點是，如果產生的任何錯誤沒有被發(fā)現與校正，則錯誤會在整個過程中反復存在。 用于數控設備的控制系統(tǒng)通常有兩類，即點位控制系統(tǒng)和連續(xù)控制系統(tǒng)。點位控制數控機床（有時稱為位置控制型數控機床）只有沿直線運動的能力。然而，當沿兩軸以等值同時編程時，會形成45°斜線。點位控制系統(tǒng)常用于需確定孔位的鉆床和需進行直線銑削加工的簡單銑床上。點位控制系統(tǒng)可通過程序控制機床，以一系列小步運動形成弧線和斜線。然而，用這種方法時，實際加工軌跡與規(guī)定的切削軌跡略有不同。 具有在兩個或多個坐標軸方向同時運動的能力的機床，歸屬連續(xù)軌跡控制或輪廓類機床。這些機床用于加工兩維或三維空間中各種不同大小的弧形，圓角圓及斜角。連續(xù)軌跡控制的數控機床比點位控制的機床貴得多，在加工復雜輪廓時，一般需要計算機輔助程序設計。 數控伺服機構是使工作臺或滑座沿坐標軸準確運動的裝置。用于數控設備的伺服機構通常有兩種：步進電機和液壓馬達。步進電機伺服機構常用于不太貴重的數控設備上。這些電機通常是大轉矩的伺服機構，直接安裝在工作臺或刀座的絲杠上。大多數步進電機是來自定子和轉子組件的磁力脈沖驅動的，這種作用的結果是電機軸轉一轉產生200步距。把電機軸接在10扣/英寸的絲杠上，每步能產生0.0005英寸的移動（1/200×1/10=0.0005英寸）。液壓伺服馬達使壓力液體流過齒輪或柱塞，從而使軸轉動。絲杠和滑座的機械運動是通過各種閥和液壓馬達的控制來實現的。液壓伺服馬達產生比步進電機更大的轉矩，但比步進電機貴，且噪音很大。大多數大型數控機床使用液壓伺服機構。 使用開環(huán)系統(tǒng)的數控機床，沒有反饋信號來確保機床的坐標軸是否運動了所需的距離。即，如果接收的輸入信號是使一特定坐標軸移動1.000英寸，伺服裝置通常使工作臺運動1.000英寸，但無法將工作臺的實際運動與輸入信號加以比較。使工作臺實際了1.000英寸的唯一保證是所用的伺服機構的準確性。當然，開環(huán)系統(tǒng)比閉環(huán)系統(tǒng)便宜。閉環(huán)系統(tǒng)能將實際輸出（工作臺一英寸的運動量）與輸入信號加以比較，并對任何誤差進行補償。反饋裝置真實地將工作臺已運動的量與輸入信號進行了比較。用于閉環(huán)系統(tǒng)的一些反饋裝置是傳感器，電尺或磁尺以及同步器。閉環(huán)系統(tǒng)大大增加了數控機床的準確性。