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.

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Coordinate Standards for Machines

A turning center is used to machine cylindrical parts

A machining center is used to machine flat or angled surfaces

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.
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Rotational Axes

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.

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Absolute Coordinates

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.

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Incremental Coordinates

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.

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Coordinates in Blueprints

                                               

                                    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)

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Positive and Negative Directions

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.

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