assigning program zero

Keep in mind that the CNC control must be told the 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 to the starting position of the machine. This is commonly done with a G92 (or G50) 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 manufacturers 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.

Other points about axis motion

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 type 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. 

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 program 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 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 specifying 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?" 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 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. 

cnc motion control

The most basic function of any CNC machine is automatic, precise, and consistent motion control. 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, called 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 conventional 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 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 ball screw revolutions have 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 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 the 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 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, 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 most popular of these 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, 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 graph, this graph has two base lines. Each base line is used to represent something. What the base line represents is broken into increments. 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 productivity. 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 January, a point is plotted. This would then 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 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 into 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 the metric mode, the smallest increment is 0.001 millimeter. (By the way, for rotary axes the increment is 0.001 degrees.)

Just like the graph, each axis within the CNC machine's coordinate system must start somewhere. With the graph, the horizontal baseline started at January and the vertical base line started 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 called the program zero point (also called work zero, part zero, and program origin).

For this example, the two axes we happen to be showing are labeled as X and Y but keep in mine that program zero can be applied to any axis. Though the names of each axes 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 axis 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 from the print.

With this technique, if 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 automatically determine how many times to rotate each axis drive motor and ball screw to make the axis reach the commanded 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 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 minus. . 

fundamentals of cnc

While the specific intention and application for CNC machines vary from one 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 machines 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 run 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 be running under program control, the skill level required of the 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. Today'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 flexibility. 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 benefit, 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

basic definations of cnc

code ...........To create programmable sets of instructions for a CNC machine.

computer numerical control ..........A type of programmable automation, directed by mathematical data, which uses microcomputers to carry out various machining operations.

downtime ...............Unproductive blocks of time during which operations cease to function, normally due to mechanical problems or a lack of materials.

drill .............A machining tool used to penetrate the surface of a workpiece and make a round hole.

electrical circuitry .............A closed path that an electric current follows, usually through devices and wires.

end mill ...........A thin, tall mill cutter with cutting edges that wind up the sides. Both the bottom and side of the end mill are used during milling operations. End mills resemble drills.

face mill ...........A flat mill cutter with multiple cutting teeth surrounding the tool. The bottom of the face mill is primarily used during milling operations.

finishing ............Final operations performed for obtaining desired tolerance and/or surface finish.

fixed automation .........A process using mechanized machinery to perform fixed and repetitive operations in order to produce a high volume of similar parts.

grinder .............A machine that uses an abrasive to wear away at the surface of a workpiece.

hardware ..........The physical components of a CNC machine.

interface ..........The control panel and displays with which the operator interacts with the machine.

job changeover ........The time it takes to switch from one part to another.

lathe .........A tool commonly used to machine cylindrical forms. It is generally considered the backbone of the machine shop.

machine control unit .........A small, powerful computer that controls and operates a CNC machine.

magazine ...........A arrangement of multiple tools that allows a CNC machine to rapidly change from one machining operation to the next.

mill ..........A machining tool used to either horizontally or vertically remove metal from the surface of a workpiece.

mylar tape .............A thin, yet strong polyester film that was used to transmit programs to numerically controlled machines.

paper tape .........A way of transmitting programs to numerically controlled machines. This method is all but extinct.

part program ...........A series of numerical instructions used by a CNC machine to perform the necessary sequence of operations to machine a specific workpiece.

prototype ..........The original test model of a product.

punch presses ..........A machine that uses force to either cut or form a workpiece.

punching ...........Using force to cut or form a workpiece.

repeatable ........The ability to position workpieces in the same place part after part.

stamping ..........Forming metal with the use of dies and punches.

three-axis curvature data .........Information that describes the motion and position of an object using three-dimensional data.

turret press .........A CNC punch press that contains several tools.

welder .........A device used to join two pieces of metal together through the application of heat.

cnc introduction

Today, computer numerical control (CNC) machines are found almost everywhere, from small job shops in rural communities to Fortune 500 companies in large urban areas. Truly, there is hardly a facet of manufacturing that is not in some way touched by what these innovative machine tools can do.

Everyone involved in the manufacturing environment should be well aware of what is possible with these sophisticated machine tools. The design engineer, for example, must possess enough knowledge of CNC to perfect dimensioning and tolerancing techniques for workpieces to be machined on CNC machines. The tool engineer must understand CNC in order to design fixtures and cutting tools for use with CNC machines. Quality control people should understand the CNC machine tools used within their company in order to plan quality control and statistical process control accordingly. Production control personnel should be abreast of their company's CNC technology in order to make realistic production schedules. Managers, foremen, and team leaders should understand CNC well enough to communicate intelligently with fellow workers. And, it goes without saying that CNC programmers, setup people, operators, and others working directly with the CNC equipment must have an extremely good understanding of CNC.

principle of working of steam turbine

“A steam turbine is a prime mover that derives its energy of rotation due to conversion of the heat energy of steam into kinetic energy as it expands through a series of nozzles mounted on the casing or the fixed blades.”
Water is converted to steam by application of heat in the boiler, which makes the steam at specified pressure and temperature. To convert the steam’s energy into work, it must go through a thermodynamic cycle that combines expansion compression, heat input, and heat rejection. The most efficient thermodynamic cycle for an ideal fluid is Carnot cycle. It consists of an isothermal heat input, isentropic expansion, isothermal heat rejection, and an isentropic compression. Regardless of the combination, the efficiency of the cycle, assuming constant mass flow is based on the difference in the enthalpy and between the beginning and end of the cycle.

• 1 to 2: Isentropic expansion
• 2 to 3: Isothermal heat rejection
• 3 to 4: Isentropic compression
• 4 to 1: Isothermal heat supply

Steam can be used as the working fluid in the Carnot Cycle. But its properties adversely impact its usefulness. In this case the steam expansion process takes place completely in the moisture region. This requires compression of a vapour/moisture mixture to return to the cycle’s starting point. Moisture is an expansion process imposes large mechanical efficiency losses. Also, vapour compression is inefficient and consumes relatively large amounts of power.
To avoid a two-phase vapour compression process, turbines are based on the Rankine cycle. It is similar to the Carnot Cycle, except that the initial pressure of the steam is raised and the condensation process that accompanies heat rejection continues until the liquid saturation point is reached. At the end of the cycle, then, condensate is simply pumped back to the boiler to begin the cycle. The role of the steam turbine is to expand the steam from high pressure and temperature to lower pressure and temperature.


Rankine cycle is a heat engine with vapor power cycle. The common working fluid is water. The cycle consists of four processes:

1 to 2: Isentropic expansion
(Steam turbine)
2 to 3: Isobaric heat rejection
(Condenser)
3 to 4: Isentropic
compression (Pump)
4 to 1: Isobaric heat supply
(Boiler)

Several things can be done to steam to improve the Rankine Cycle efficiency. Raise initial steam condition and reduce the amount of moisture near the end of expansion stage. The first is accomplished by superheating the steam before it does any work. The second involves re- heating steam to near initial-conditions after it is partially expanded by directing it back to the heat source, then completing the expansion. In converting the thermal energy of steam into mechanical energy turbines takes advantage of this facts- as it expands or drops in pressure, through a small nozzle or opening, it accelerates and forms a high-speed jet. Directing this momentum in a rotating blade provides mechanical energy.