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1、The basics of Computer Numerical Control While the specific intention and application for CNC machines vary from machine type to another, all forms of CNC have common benefits. Though the thrust of this presentation is to teach you CNC usage, it helps to understand why these sophisticated machi

2、nes have become so popular. Here are but a few of the more important benefits offered by CNC equipment. The first benefit offered by all forms of CNC machine tools is improved automation. The operator intervention related to producing workpieces can be reduced or eliminated. Many CNC machines can r

3、un unattended during their entire machining cycle, freeing the operator to do other tasks. This gives the CNC user several side benefits including reduced operator fatigue, fewer mistakes caused by human error, and consistent and predictable machining time for each workpiece. Since the machine will

4、be running under program control, the skill level required of the a CNC operator (related to basic machining practice) is also reduced as compared to a machinist producing workpieces with conventional machine tools. The second major benefit of CNC technology is consistent and accurate workpieces. T

5、oday’s CNC machines boast almost unbelievable accuracy and repeatability specifications. This means that once a program is verified, two, ten, or one thousand identical workpieces can be easily produced with precision and consistency. A third benefit offered by most forms of CNC machine tools is fl

6、exibility. Since these machines are run from programs, running a different workpiece is almost as easy as loading a different program. Once a program has been verified and executed for one production run, it can be easily recalled the next time the workpiece is to be run. This leads to yet another b

7、enefit, fast change-overs. Since these machines are very easy to setup and run, and since programs can be easily loaded, they allow very short setup time. This is imperative with today’s Just-In-Time product requirements. 1. Motion control-the heart of CNC The most basic function of any CNC mach

8、ine is automatic, precise, and consistent motion. Rather than applying completely mechanical devices to cause motion as is required on most conventional machine tools, CNC machines allow motion control in a revolutionary manner. All forms of CNC equipment have two or more directions of motion, calle

9、d axes. These axes can be precisely and automatically positioned along their lengths of travel. The two most common axis types are linear (driven along a straight path) and rotary (driven along a circular path). Instead of causing motion by turning cranks and handwheels as is required on convention

10、al machine tools, CNC machines allow motions to be commanded through programmed commands. Generally speaking, the motion type (rapid, linear, and circular), the axes to move, the amount of motion and the motion rate (feedrate) are programmable with almost all CNC machine tools. Accurate positioning

11、 is accomplished by the operator counting the number of revolutions made on the handwheel plus the graduations on the dial. The drive motor is rotated a corresponding amount, which in turn drives the ball screw, causing linear motion of the axis. A feedback device confirms that the proper amount of

12、ball screw revolutions has occurred. A CNC command executed within the control (commonly through a program) tells the drive motor to rotate a precise number of times. The rotation of the drive motor in turn rotates the ball screw. And the ball screw causes drives the linear axis. A feedback device

13、at the opposite end of the ball screw allows the control to confirm that the commanded number of rotations has taken place. Though a rather crude analogy, the same basic linear motion can be found on a common table vise. As you rotate the vise crank, you rotate a lead screw that, in turn, drives th

14、e movable jaw on the vise. By comparison, a linear axis on a CNC machine tool is extremely precise. The number of revolutions of the axis drive motor precisely controls the amount of linear motion along the axis. How axis motion is commanded-understanding coordinate systems. It would be infeasible

15、for the CNC user to cause axis motion by trying to tell each axis drive motor how many times to rotate in order to command a given linear motion amount. (This would be like having to figure out how many turns of the handle on a table vise will cause the movable jaw to move exactly one inch!) Instead

16、, all CNC controls allow axis motion to be commanded in a much simpler and more logical way by utilizing some form of coordinate system. The two most popular coordinate systems used with CNC machines are the rectangular coordinate system and the polar coordinate system. By far, the move popular of t

17、hese two is the rectangular coordinate system, and we’ll use it for all discussions made during this presentation. One very common application for the rectangular coordinate system is graphing. Almost everyone has had to make or interpret a graph. Since the need to utilize graphs is so commonplace,

18、 and since it closely resembles what is required to cause axis motion on a CNC machine, let’s review the basics of graphing. As with any two dimensional graphs, this type of graph has two base lines. Each base line is used to represent something. What the base line represents is broken into increme

19、nts. Also, each base line has limits. In our productivity example, the horizontal base line is being used to represent time. For this base line, the time increment is in months. Remember this base line has limits-it starts at January and end with December. The vertical base line is representing prod

20、uctivity. Productivity is broken into ten percent increments and starts at zero percent productivity and ends with one hundred percent productivity. The person making the graph would look up the company’s productivity for January of last year and at the productivity position on the graph for Januar

21、y, a point is plotted. This would when be repeated for February, March, and each month of the year. Once all points are plotted, a line or curve can be drawn through each of the points to make it more clear as to how the company did last year. Let’s take what we now know about graphs and relate it

22、to CNC axis motion. Instead of plotting theoretical points to represent conceptual ideas, the CNC programmer is going to be plotting physical end points for axis motions. Each linear axis of the machine tool can be thought of as like a base line of the graph. Like graph base lines, axes are broken i

23、nto increments. But instead of being broken into increments of conceptual ideas like time and productivity, each linear axis of a CNC machine’s rectangular coordinate system is broken into increments of measurement. In the inch mode, the smallest increment is usually 0.0001 inch (in). In the metric

24、mode, the smallest increment is 0.001 millimeter (mm). (By the way, for rotary axes the increment is 0.001). Just like the graph, each axis within the CNC machine’s coordinate system must start somewhere. With the graph, the horizontal base line started at January and the vertical base line starter

25、 at zero percent productivity. This place where the vertical and horizontal base lines come together is called the origin point of the graph. For CNC purposes, this origin point is commonly caller the program zero point (also called work zero, part zero, and program origin). For this example, the t

26、wo axes we happen to be showing are labeler as X and Y but keep in mine that program zero can be applied to any axis. Though the names of each axis will change from one CNC machine type to another (other common names include Z, A, B, C, U, V, and W), this example should work nicely to show you how a

27、xis motion can be commanded. The program zero point establishes the point of reference for motion commands in a CNC program. This allows the programmer to specify movements from a common location. If program zero is chosen wisely, usually coordinates needed for the program can be taken directly fr

28、om the print. With this technique, is the programmer wishes the tool to be sent to a position one inch to the right of the program zero point, X1.0 is commanded. If the programmer wishes the tool to move to a position one inch above the program zero point, Y1.0 is commanded. The control will automa

29、tically determine how many times to rotate each axis drive motor and ball screw to make the axis reach the commander destination point. This lets the programmer command axis motion in a very logical manner. With the examples given so far, all points happened to be up and to the right of the program

30、 zero point. This area up and to the right of the program zero point is called a quadrant (in this case, quadrant number one). It is not uncommon on CNC machines that end points needed within the program fall in other quadrants. When this happens, at least one of the coordinates must be specified as

31、 minus. 2. Understanding absolute versus incremental motion All discussions to this point assume that the absolute mode of programming is used. The most common CNC word used to designate the absolute mode is G90. In the absolute mode, the end points for all motions will be specified from the prog

32、ram zero point. For beginners, this is usually the best and easiest method of specifying end points for motion commands. However, there is another way of specifying end points for axis motion. In the incremental mode (commonly specified by G91), end points for motions are specified from the tool’s

33、current position, not from program zero. With this method of commanding motion, the programmer must always be asking “How far should I move the tool?〞 While there are times when the incremental mode can be very helpful, generally speaking, this is the more cumbersome and difficult method of specifyi

34、ng motion and beginners should concentrate on using the absolute mode. Be careful when making motion commands. Beginners have the tendency to think incrementally. If working in the absolute mode (as beginners should), the programmer should always be asking “To what position should the tool be moved

35、?〞 This position is relative to program zero, not from the tools current position. Aside from making it very easy to determine the current position for any command, another benefit of working in the absolute mode has to do with mistakes made during motion commands. In the absolute mode, if a motion

36、 mistake is made in one command of the program, only one movement will be incorrect. On the other hand, if a mistake is made during incremental movements, all motions from the point of the mistake will also be incorrect. 3. Assigning program zero Keep in mind that the CNC control must be told t

37、he location of the program zero point by one means or another. How this is done varies dramatically from one CNC machine and control to another. One (older) method is to assign program zero in the program. With this method, the programmer tells the control how far it is from the program zero point t

38、o the starting position of the machine. This is commonly done with a G92 (orG50) command at least at the beginning of the program and possibly at the beginning of each tool. Another, newer and better way to assign program zero is through some form of offset. Commonly machining center control manufa

39、cturers call offsets used to assign program zero fixture offsets. Turning center manufacturers commonly call offsets used to assign program zero for each tool geometry offsets. More on how program zero can be assigned will be presented during key concept number four. 4. Other points about axis moti

40、on To this point, our primary concern has been to show you how to determine the end point of each motion command. As you have seen, doing this requires an understanding of the rectangular coordinate system. However, there are other concerns about how a motion will take place. Fore example, the ty

41、pe of motion (rapid, straight line, circular, etc.), and motion rate (feedrate), will also be of concern to the programmer. We’ll discuss these other considerations during key concept number three. 5. Telling the machine what to do-the CNC program Almost all current CNC controls use a word addre

42、ss format for programming. (The only exceptions to this are certain conversational control.) By word address format, we mean that the CNC program is made up of sentence-like commands. Each command is made up of CNC words. Each CNC word has a letter address and a numerical value. The letter address (

43、X, Y, Z, etc.) tells the control the kind of word and the numerical value tells the control the value of the word. Used like words and sentences in the English language, words in a CNC command tell the CNC machine what we wish to do at the present time. One very good analogy to what happens in a CN

44、C program is found in any set of step by step instructions. Say for example, you have some visitors coming in from out of town to visit your company. You need to write down instructions to get from the local airport to your company. To do so, you must first be able to visualize the path from the air

45、port to your company. You will then, in sequential order, write down one instruction at a time. The person following your instructions will perform the first step and then go on to the next until he or she reaches your company. In similar manner, a manual CNC programmer must be able to visualize th

46、e machining operations that are to be performed during the execution of the program. Then, in step by step order, the programmer will give a set of commands that makes the machine behave accordingly. Though slight off the subject at hand, we wish to make a strong point about visualization. Just as

47、the person developing travel directions MUST be able to visualize the path taken, so MUST the CNC programmer be able to visualize the movements the CNC machine will be making BEFORE a program can be successfully developed. Without this visualization ability, the programmer will not be able to develo

48、p the movements in the program correctly. This is one reason why machinists make the best CNC users. An experienced machinist should be able to easily visualize any machining operation taking place. Just as each concise travel instruction will be made up of one sentence, so will each instruction gi

49、ven within a CNC program be made up of one command. Just as the travel instruction sentence is made up of words (in English), so is the CNC command made up of CNC words (in CNC language). The person following your set of travel instructions will execute them explicitly. If you make a mistake with y

50、our set of instructions, the person will get lost on the way to your company. In similar fashion, the CNC machine will execute a CNC program explicitly. If there is a mistake in the program, the CNC machine will not behave correctly. While the words and commands in this program probably do not make

51、 much sense to you (yet), remember that we are stressing the sequential order by which the CNC program will be executed. The control will first read, interpret and execute the very first command in the program. Only then will it go on to the next command. Read, interpret, execute. Then go to the nex

52、t command. The control will continue to execute the program in sequential order for the balance of the program. Again, notice the similarity to giving any set of step by step instructions. 6. Other notes about program makeup As stated programs are made up of commands and commands re made up of wor

53、d. Each word has a letter address and a numerical value. The letter address tells the control the word type. CNC control manufacturers do vary with regard to how they determine word names (letter addresses) and their meanings. The beginning CNC programmer must reference the control manufacturer’s pr

54、ogramming manual to determine the word names and meanings. Here is a brief list of some of the word types and their common letter address specifications. 7. M-Miscellaneous function (See below) As you can see, many of the letter addresses are chosen in a rather logical manner (T for tool, S for sp

55、indle, F for federate, etc.). A few require memorizing. There are two letter addresses (G and M) which allow special functions to be designated. The preparatory function (G) specified is commonly used to set modes. We already introduced absolute mode, specified by G90 and incremental mode, specifie

56、d by G91. There are but two of the preparatory functions used. You must reference your control manufacturer’s manual to find the list of preparatory functions, miscellaneous functions for your particular machine. Like preparatory functions, miscellaneous functions (M words) allow a variety of speci

57、al functions. Miscellaneous functions are typically used as programmable switches (like spindle on/off, coolant on/off, and so on). They are also used to allow programming of many other programmable functions of the CNC machine tool. To a beginner, all of this may seem like CNC programming requires

58、 a great deal of memorization. But rest assured that there are only about 30-40 different words used with CNC programming. If you can think of learning CNC manual programming as like learning a foreign language that has only 40 words, it shouldn’t seem too difficult. 計算機數(shù)字控制根底 CNC 機器的特別目的和應用

59、，他們的類型是不斷地從這變化到另外的。但實際上，所有形式的 CNC 都是有共同的特點，有共同的用處。 從插口的說明指導書可以教你 CNC 的使用方法, 它可以幫助你了解為什么這些復雜的機器現(xiàn)在已經(jīng)變得這么流行。下面列舉出來的是 CNC 設備所提供出來的一些重要的用處。 第一，從所有形式的 CNC 工作母機得到的第一種好處就是經(jīng)過改造后可實現(xiàn)全自動化。從而生產(chǎn)工件的操作員將會被大大減少或者干脆不用。許多 CNC 機器能不需要操作員就可以自動地在它的機器周期里平安運轉，從而這也就解放了一些操作員，使他們有時間可以去做別的事情。這樣給CNC 的使用者帶來幾種好處：既可以降低了操作員的疲累度，又可以

60、減少由于人工所引起的錯誤，每個工件的加工時間就可以預期和統(tǒng)一。因為機器是利用程序來控制，所以CNC的使用者的技術要求水平相對傳統(tǒng)的機械師使用機械工具來生產(chǎn)工件的要求也有所下降〔相關的機器練習〕。 第二，另外的好處是CNC 技術根底是統(tǒng)一的，對工件的加工精度也比擬高。 現(xiàn)在的 CNC 機器都有幾乎難以置信的精度和重復性規(guī)格。這也就意味著，一旦一個程序經(jīng)過驗證，就會有兩個或者十個甚至上千個同樣的工件可以輕松得按高精度統(tǒng)一來生產(chǎn)。 第三，CNC 工作母機得到的第三種好處是設計具有彈性，有機動性。因為這些機器是按照程序來運行控制的，改造一個工件幾乎就像裝載一個不同的程序那么簡單。一旦有一段程序被驗

61、證成功并且用來指導生產(chǎn)工件，下次還可以調出來使用，對工件進行加工。這樣就又有另外一個好處：快速變化。因為這些機器的裝配和運轉比擬容易，程序裝載容易，所需要的時間短。這些可以滿足了當今產(chǎn)品的即時生產(chǎn)要求。 1、運動控制-CNC 的核心 大局部的 CNC 機器的最根本功能是自動化，高精確度和一致協(xié)調的運動。并非像應用完整的傳統(tǒng)機器設備來產(chǎn)生運動，CNC機器的運動使用了一個全新的方式。即所有的CNC機器設備具有兩個或兩個以上的運動方向，也就是通常所說的軸向運動。這些軸可以精確地，自動地沿著它們的長度運動。最常見的兩種軸是：一種是線性的〔沿一條直線的路徑驅動〕，另一種是旋轉式的〔沿著一條圓形的

62、路徑驅動〕。 CNC 機器設備是按照程序的要求來運動，并非由曲柄和手輪需要在傳統(tǒng)的工作母機上引起運動。一般來說, 運動類型 (快速，直線，圓形)、軸的移動方向、軸運動的數(shù)量和速率幾乎都是可以通過 CNC 工作母機編程來設計得到。 正確配置的完成也需要操作員計算手輪上面的刻度旋轉的數(shù)值，輪流驅動螺旋槳的引起沿著軸直線運動的旋轉馬達的相應數(shù)量，還有一個反應裝置用以確定螺旋槳的適當數(shù)量。 一條CNC 控制的指令〔一般是在一段程序內(nèi)〕告訴馬達在一個時段內(nèi)驅動一個精確的數(shù)量。馬達輪流驅動螺旋槳，然后螺旋槳驅動線性的軸承。在相反端的螺旋槳的反應設備允許控制以確定所要求的數(shù)量的旋轉運動的發(fā)生。 雖然

63、類推的方法比擬粗糙，但是同樣的根本線性運動可以在普通的虎頭鉗曲柄找到。當你旋轉虎頭鉗曲柄時候，你可以引領螺絲釘，驅動虎頭鉗活動的鉗叉。通過比擬，在一個 CNC 工作母機上的一個線性的非常地精確，許多的軸旋轉來驅動，精確地控制軸上面的線性運動。 軸的運動如何，它們可以被命令和了解并接受同等的系統(tǒng)。它會讓 CNC 使用者嘗試去編程實現(xiàn)告訴引起軸如何去運動，每個軸駕駛馬達如何替換，替換多少次。以到達要命令一個給定的線運動總量. (這會像必須理解柄的旋轉在一個桌子虎頭鉗，將會導致可動的鉗叉完全地精確的移動一寸!) 相反的, 所有的 CNC 控制利用一些形式的同等的系統(tǒng)在一個比擬簡單和更多很多符合邏輯

64、方法命令讓軸運動。被 CNC 機器用的二個最流行的同等系統(tǒng)是矩形的同等系統(tǒng)和兩極的同等系統(tǒng)。顯然, 到目前為止，這兩種同等系統(tǒng)比擬流行的是矩形的。我們將會使用它作為在這發(fā)表期間做的所有的討論。 一個的普遍的通常的應用矩形的同等系統(tǒng)是曲線圖。幾乎每個人必須制造或者解釋一個曲線圖。自從以后需要利用曲線圖如此平凡, 而且自從它之后，更加接近一部 CNC 機器上的軸運動，下面讓我們復習以下曲線圖的根本。 關于任何的二個空間的曲線圖, 這類型的曲線圖有二條基線. 每個基線用來表現(xiàn)某件事或者某個量。每個基線代表的是一個突然的增量。同時，每個線都是有個極限的，在我們的生產(chǎn)例子中, 水平的基線是用來表示時

65、間。 對于這一條基線, 所有的增量都是以月來算。務必記得這一條基線是有極限，它以一月份開始，以十二月份做為結束。垂直的基線表示生產(chǎn)總量，以百分之十作為增量單位。從百分之零開始，以百分之百作為生產(chǎn)總量的結束。 制造曲線圖的人會對于去年一月份公司的生產(chǎn)總量調查清楚，在生產(chǎn)總量所表示的曲線圖上對應于一點, 這點可能會在二月，三月甚至在一年中的每個月中重復出現(xiàn), 一經(jīng)所有的點被標注后，一條由這些點組成的直線或者曲線就能通過每個點表示關于公司如何去年生產(chǎn)如何，使它變成更一目了然。 讓我們拿我們現(xiàn)在知道曲線圖而且使它和 CNC 軸上的運動產(chǎn)生關聯(lián)。理論上的點只能表現(xiàn)概念上的意思, CNC 程序師要為軸

66、運動設計實際運動的結束點。 工作母機的每個線軸能被設想到為同類曲線圖的一條基線。 相似的曲線圖以線作根底, 軸可以參加增量。 但是而非參加的增量與之前提到的增量時間和生產(chǎn)總量不同。一個 CNC 機器的矩形同等的系統(tǒng)的每個線軸所測量到的增量，在寸模態(tài)中, 最小的增量通常是 0.0001 寸。在公制的模態(tài)中, 最小的增量是 0.001 毫米。(順便一提, 旋轉的軸增量是 0.001)。 正如曲線圖, CNC 機器的里面的每個軸同等的系統(tǒng)一定從某處開始。同曲線圖, 在零的百分比生產(chǎn)力的在一月和垂直的基線起動器被開始的水平基線. 這一個地方，垂直的和水平的基線交點被稱為曲線圖的起源點。對于 CNC 的目的而言， 這起源點普遍被稱為零點 (也叫做工作零，部份零和源點)。 對于這一個例子, 二個軸我們可以分別貼標簽為 X軸 和 Y軸 ，但是在我們的意念中零點適用于任何的軸。雖然每個軸的名字將會從一個 CNC 機器類型變成另外的 (其他的通常名字包括 Z ， A ， B ， C ， U ， V 和 W等等) ，但前提條件是這一個例子應該很好地工作來表示軸運動能如何被命令。 程序的零點建立