Offsets ​​and Parameters

Reference point

After switching on a CNC machine, the controller does not have any real-time positioning information about the machine's axes. Therefore, all the axes need to be referenced before proceeding with the machining work. Referencing means bringing all the axes to a specific location from where all the axis movement measurements will be carried out. Reference points of machine axes are usually specified by the machine builders and can be changed by the user if required. CNC machines and axes running with "Absolute Encoder" position feedback do not require referencing each time after switching on. An absolute encoder holds its last position value before switching off the machine (detailed information about Absolute Encoders is discussed in the 'Feedback' chapter). The zero positions of all the axes are calculated based on the reference points, and the CNC controller associates them with the coordinate system.

Machine zero

All the axis positions of a CNC machine are related to the coordinate system, and the origin or zero point of the axis measurement is called Machine Zero or Machine Home position. All the axes assume the zero position here and display the actual value as 0.000. Sometimes the reference point and machine zero position may be in the same location, and occasionally they may differ. If the axis position displays 0.000 after referencing the machine, the machine zero, and the reference point will be at the same place. Otherwise, after referencing, the axis position will display the actual range from the zero position, usually defined by machine builders.

Workpiece Zero

Refers to a point on a workpiece that is used for machining, where all the dimensions or coordinate points are assumed for programming. Based on this point, different workpiece dimensions are calculated in accordance with the drawing. In other words, workpiece zero is a temporary coordinate zero point that can be easily exchanged for error-free programming. The distance from the machine zero to the workpiece zero position is called the zero offset. The following image illustrates the reference point, machine zero point, and workpiece zero. Here, the reference and machine zero points are considered two separate points.

 

Zero Offset

Zero offset refers to the distance from the machine zero to the workpiece zero position, and in other words, it is the coordinate distance from the machine zero to the workpiece zero. Zero offset values are adjusted for different axes and change with workpieces. G54, G55, G56, etc., codes are used in the program to define different zero offset values. Offset values are usually stored in the CNC controller memory and are recalled whenever they are needed for operation. The following picture shows how the zero offset values of a workpiece are considered for X, Y, and Z-axis positions. 

The previous picture shows the distance between the workpiece zero and the machine zero position, where X = 500 mm, Y = 1000 mm, and Z = 400 mm, respectively (all in the negative direction). The zero offset values for this particular workpiece are considered as G54 and will be stored in the controller memory. By using the G54 code within a program, the coordinate zero position (0, 0) will shift to the workpiece zero position, and the X, Y, and Z axes will move 500 mm, 1000 mm, and 400 mm (in the negative direction), respectively, considering all axis positions as the coordinate zero position temporarily. Further movements of the axes will take place based on this zero position only, and to return to the machine zero or cancel the workpiece zero, the G53 code is applied in the programming.

 

Tool Offset

With CNC machines, cutting tools used for machining have different diameters and sizes. It is inconvenient to program and estimates separate lengths and radii for each tool, as the axis movements will be different for individual tool lengths. The program always assumes a standard tool length and radius, and during program execution, the actual tool length and diameter are compensated based on the standard tool. The height and diameter of each tool are stored in the controller memory in a table format as tool offset data. Sometimes, a single tool may have more than one tool offset, depending on the machining conditions. When a particular tool is called in a program for machining, the actual axis movements will be finalized by compensating the tool offset data with the standard tool data. Tool offset data is commonly represented by the letter D, and within a program, T1 D1 represents Tool T1 with an offset value of D1.

   

R-Parameter

R-parameters are used to achieve flexibility in CNC programming, and sometimes instead of using numeric values in a program, R-parameters are used to make the program shorter. With R-parameters, a complex program can be written more efficiently, and any mathematical function inside the program can be expressed effortlessly. R01, R10, R50, etc., are written instead of using numeric values, and the R-values are stored in the CNC controller memory.