Online Education: CNC Coordintes

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Contouring

Not all cutting operations take place in either a vertical or horizontal direction. A CNC machine will often be required to cut along a diagonal line. This movement is called linear interpolation. To make this cut, a CNC machine moves on two axes simultaneously. Figure 1 shows a diagonal cut along the X- and Y-axes.

Now imagine that the same diagonal cut needed to be made, except that the cutting tool must gradually lift off the table as well. This cutting operation would require movement on the X-, Y-, and Z-axes simultaneously. Figure 2 shows another type of simultaneous movement along three axes.

Sophisticated CNC machines can move a tool along two or more axes at once. This movement allows a machine to perform contouring. Contour movements can produce curves, circles, and even cones, like those shapes in Figure 3. However, very few CNC systems factor all six axes at once during their operations, and many only address two or three axes at the same time.

Explain Part program

Part program:-

A part program consists of a set of machining instructions that the operator wants to execute.

A part program uses alphabetic text for its instructions and numeric information as the target values for those instructions. In this way, a CNC program can be developed as a series of instructions, each of which performs a machining operation. Complex machining tasks can be accomplished by combining machining operations.

v G Codes

v M Codes

v Letter Address

v G00, G01, G02, G03, G04

v Canned Cycles

v Multiple Repetitive Canned Turning Cycle

v Canned Cycle For Drilling

v Canned Grinding Cycle (Grinding Machine Only)

v Circular Interpolation

v Polar Coordinate Interpolation (G12.1, G13.1)

v Cylindrical Interpolation (G07.1)

v Constant Lead Threading (G32)

v Variable–Lead Thread Cutting (G34)

v Continuous Thread Cutting

v Multiple–Thread Cutting

v Skip Function (G31)

v Multistage Skip

v Torque Limit Skip (G31 P99)

v Inch/Metric Conversion (G20, G21)

v Diameter And Radius Programming

v Constant Surface Speed Control (G96, G97)

v Programmable Parameter Entry (G10)

v Polygonal Turning

Coordinates for the Turning Center

Milling machines are normally used to machine cubic work pieces. However, manufacturers will use turning centers (shown in Figure 1) to shape the dimensions of cylindrical work pieces.

During turning operations, the work piece is held and rotated in a spindle. A non-rotating cutting tool is moved against the rotating part to remove material. Because the Z-axis is always parallel to the spindle of the machine, it no longer describes up and down motion of the cutting tool. Instead, the Z-axis now describes the back and forth motion of the tool along the length of the work piece, parallel to the spindle. The X-axis specifies the distance of the cutting tool from the center of the work piece. Figure 2 shows both the X- and Z-axes.

The X-axis position of the cutting tool determines the diameter of the cylindrical work piece. On the typical turning center, the Y-axis is not programmable. In theory, the Y-axis defines the tool height. However, because the tip of the tool must cut on the centerline of the part, the tool is almost always fixed at the same Y-axis location.

It is true that the axes of the vertical milling machine perfectly match the right-hand rule. However, not all CNC machines follow the same setup. Horizontal milling machines require a shift in the axes because the spindle of these machines is in a different location.

Figure 1 shows the basic setup of a horizontal milling machine. As you can see, the spindle is located on the side. Because the Z-axis must always be parallel to the spindle of the machine, the Z-axis is tilted to the side, as shown in Figure 2. The X-axis still describes motion to the left and right. However, the Y-axis now describes motion that is up and down.

It may seem that the coordinates of this machine no longer match the right-hand rule. However, if you were to tilt your hand to match the worktable, you will see that the axes still line up appropriately.

Coordinates for the Vertical Milling Machine

The most recognizable coordinates can be found on the vertical milling machine. Imagine that you are standing in front of the machine and facing it. The coordinates of this machine follow the right-hand rule shown in Figure 1. As you can see in Figure 2, the X-axis describes left and right motion of the cutting tool, the Y-axis describes back and forth motion of the tool, and the Z-axis describes up and down motion.

Depending on the machine, either the cutting tool or the worktable will move during machining operations. The positive and negative directions on each axis always describe the motion of the cutting tool in relation to the worktable. If the cutting tool is the movable part, the positive directions follow the right-hand rule. However, if the worktable moves, the positive and negative directions are reversed. As shown in Figure 3, table movement to the left is actually a positive motion because it shifts the position of the cutting tool to the right of the table.

The right-hand rule matches a vertical machining center

Directions of positive and negative spindle movement on a basic mill.

. As a tool moves from rest (A) to the right (B), it travels in a positive direction along the X-axis. Table movement to the left (C) also moves the tool in a positive direction along the X-axis.

Standard Axes Locations

The following are general guidelines that determine the location of coordinate axes on any given CNC machine:

The Z-axis is always parallel to the spindle of the machine.

The X- and Y-axes are always perpendicular to the spindle of the machine.

The X-axis normally describes the longer direction of travel on the machine, and the Y-axis describes the shorter direction of travel.

Keep in mind that the spindle is different for the machining and turning center. On the machining center, the spindle is the device that holds the rotating cutting tool, as shown in Figure 1. On the turning center, the spindle holds the rotating workpiece, as shown in Figure 2. The important thing to remember is that the spindle involves rotational movement. In the next few lessons, you will learn how these guidelines apply to each particular machine.

The spindle holds a cutting tool on a machining center.

The spindle holds a workpiece on the turning center

Coordinate Standards for Machines

Without a doubt, the two most common CNC machines are the machining center and the turning center. A machining center is used to machine flat or angled surfaces, and a turning center is used to machine cylindrical parts. Figures 1 and 2 illustrate both of these machines.

CNC machines rely on the coordinate system to machine incredibly accurate dimensions. This requires a mapping of the coordinate system onto the dimensions of the machine. Imagine applying the right hand rule to a machine. Every CNC machine is programmed to recognize where the X-, Y-, and Z-axes are located on the machine itself.

The location of axes on any CNC machine is not a haphazard process. Each machine relies on Electronics Industries Association (EIA) standards that dictate the location of these axes.

A turning center is used to machine cylindrical parts

A machining center is used to machine flat or angled surfaces.

A turning center is used to machine cylindrical parts

A machining center is used to machine flat or angled surfaces

In addition to the movements on the linear X-, Y-, and Z-axes, tools and workpieces can also move along rotational axes. These rotational axes describe how a part tilts or rotates around a line defined by at least two points. The three rotational axes are the A-axis, B-axis, and C-axis, as shown in Figure 1. Each rotational axis corresponds to one of the linear axes. The A-axis rotates around the X-axis, the B-axis rotates around the Y-axis, and the C-axis rotates around the Z-axis.

Rotational movement can describe a workpiece that is turned to expose different areas of its surface for machining. Rotational movement can also describe the angle of the cutting tool. For complex parts, a cutting tool that is angled may allow it to machine hard-to-reach areas. Figure 2 shows a part that required tool movement on a rotational axis.

Though incremental coordinates are used in certain operations, absolute coordinates dictate the movements of both tool and workpiece for most CNC machines.

With absolute coordinates, the origin is always in a fixed position. Each new location is calculated from this fixed origin instead of the previous location. Even if there is an error while reaching the current location, that error is corrected once the tool or workpiece moves to the next location. Figures 1 and 2 compare two blueprints that use incremental and absolute calculations.

To return to the driving example, absolute coordinates are similar to driving according to street names instead of counting street by street. The street names act as fixed reference points that prevent you from perpetuating a driving mistake if you miss a turning point.

As a CNC machine works on a part, the machine guides either the cutting tool or the workpiece from one location to the next. Depending on the operation, a CNC machine will use either incremental coordinates or absolute coordinates to determine this movement.

FIgure 1 shows a blueprint with dimensions that reflect incremental coordinates. With incremental coordinates, a new location is calculated from the current position. Once the tool reaches the new location, it becomes the base for the next position. In other words, the current position always acts as the origin for the next position. If you were driving down the street, incremental directions would be similar to traveling three streets north, two streets east, one street north, etc.

A potential problem with incremental coordinates is that an error can be carried from one dimension to the next. Let’s say that, while driving, you missed a street. All the rest of your distances would be inaccurate, and you would have to correct your instructions to reach your final destination.

Every part that is made in the shop is originally designed on a computer or a blueprint drawing. In order to lay out the dimensions of a workpiece, a designer uses a coordinate system to describe the measurements of each dimension. Figure shows a blueprint drawing that contains these measurements.

Both blueprint drawings and CNC systems rely on these numerical coordinates to determine the location and size of workpiece dimensions. Every location is given a numerical value that places it within the three axes.

A CNC machine can only perform what it is programmed to do. Before a part is made on a CNC machine, a programmer takes the numerical dimensions of the blueprint and turns them into step-by-step instructions. The CNC machine then uses the coordinate system to perform these instructions, one after another, to make the part.

Blueprints Pediatrics (Blueprints Series)

Figure 1. Each axis has a positive and negative direction.

Figure 2. The positive direction of the Y-axis points away, and the positive direction of the Z-axis points upward.

Each axis line contains a range of numbers. The origin, or the center of the axis line, is always zero. Numbers are then counted as they move away from the center on each side of the axis. One direction is positive (+), and the other is negative (-).

Measurements taken to the right side of the origin along the X-axis line are generally considered to be positive, or +x, while measurements to the left of the origin along the X-axis are negative, or –x, as shown in Figure 1. The positive and negative directions work the same way for the Y- and Z-axes as well.

Figure 2 shows the box corner. The positive direction of the Y-axis points away and the positive direction of the Z-axis points upward.

An effective way to picture the three Cartesian axes shown in Figure 1 is by learning the right-hand rule. Turn your right palm up, and then extend only the thumb and forefinger. This forms an "L," with the middle finger up to the ceiling, as in Figure The thumb is the X-axis, the forefinger is the Y-axis, and the elevated center finger is the Z-axis.

The point where all three axes meet is called the origin. This occurs roughly in the palm of your hand, or the corner of the box. The origin is important because it is the central reference point for all the dimensions. You can think of the origin as the "You are here!" dot on a map. Within the Cartesian system, any specific location can be described by its place along the three axes and the distance from the origin.

Coordinate Metrology - Technology and Application

All the positions and motions of a computer numerical control (CNC) machine are understood in terms of numbers. Numbers describe the shape of the work piece, the movement of the cutting tool, the depth and speed of the cut, etc.

How exactly are numbers used in this way? The common system used to describe location is called the Cartesian coordinate system, which consists of a cubic grid of imaginary lines. The Cartesian system defines the location of a single point in three-dimensional space using three axes, which are shown in Figure 1. These axes are called the X-axis, Y-axis, and Z-axis, and each one indicate a particular direction.

An axis is an imaginary straight line. In the Cartesian system, the X-, Y-, and Z-axes meet at right angles. In other words, they join together like the corner of a box, as shown in Figure 2.

CNC Coordinates

The Cartesian coordinate system is the fundamental system used to describe the motion of the tool and workpiece within a three-dimensional space. CNC machines use numbers to locate a particular point along the X-, Y-, and Z-axes. They perform a series of instructions, one after another, to machine the workpiece and create incredibly accurate dimensions.

CNC machines use either incremental or absolute coordinates to move from one location to the next. With incremental coordinates, the current position acts as the origin for the next position. With absolute coordinates, the origin stays in a fixed location, and each new location is calculated from that fixed position. Most CNC machines can move along multiple axes at once to perform contour operations.

The axes on any CNC machine are determined by set standards. The Z-axis is always parallel to the machine spindle. On a machining center, the spindle holds the cutting tool. On a turning center, the spindle holds the rotating workpiece. Nowadays, CNC machines can create complex shapes such as circles, curves, and cones.

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Coordinates for the Horizontal Milling Machine

Coordinates for the Turning Center

Machine Zero and Program Zero

A part program consists of a set of machining instructions that the operator wants to execute.

CNC Coordinates

Coordinates for the Horizontal Milling Machine

Coordinates for the Turning Center

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