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Numerical Control and Computer Numerical Control
Published in Helmi Youssef, Hassan El-Hofy, Traditional Machining Technology, 2020
The part program is a computer program containing a number of lines, instructions, or statements called NC blocks that describe the detailed plan of machining instructions proposed for the part. It is written using vocabulary understood by the MCU in terms of standard words, codes, and symbols. Part programs can also be written in higher languages, such as automatically programmed tools (APT), adaptation of APT(ADAPT), extended subset of APT(EXAPT), and so on. These programs can be converted into the machine-tool language with the help of processors. Programming in APT is mostly processed with the help of computers and is thus known as computer-assisted part programming. Part programs can also be directly developed using computer-aided design/manufacturing (CAD/CAM) systems such as Unigraphics and ProEngineer or CAM systems such as Master CAM, Surf CAM, and others.
Accuracy and Error Compensation of CNC Machining Systems
Published in David A. Stephenson, John S. Agapiou, Metal Cutting Theory and Practice, 2018
David A. Stephenson, John S. Agapiou
The part program is generated by computer-aided part programming (CAD/CAM) systems. The post tool path is converted into CNC commands (as DNC operations) through processing of the CAM programs. This transfers the tool path that is originally described in the workpiece coordinate system (WCS) to the machine coordinate system (MCS) through inverse kinematic transformations. The CNC machine control unit (MCU) issues commands in the form of numeric data to the motors of the machine linear or rotary axis that positions the slides and tool according to the workpiece location in relation to the MCS. The computer-aided machining process may result in approximations because the vast majority of machines are set to use four decimal places for motion control. In addition, each machine axis is subject to following errors, leading to orientation and position errors of the tool [1], and the machine structure joints and motion errors in its quasi-static or dynamic state result in position and orientation errors of the tool [2, 3]. Moreover, the thermal variation of the machine is another source of error [4]. It is well established that the thermal deformation of the workpiece and the machine tool structure has a significant effect on machining accuracy and may contribute to more than 50%–70% of the machining error, especially when parts are made of materials of low thermal conductivity and thermal capacity or high thermal expansion coefficient. Finally, the forces from the cutting process result in tool or workpiece deflection or distortion, causing another source of machined surface location error [5].
Computer-Aided Manufacturing (CAD/CAM)
Published in Zainul Huda, Manufacturing, 2018
Working Principles: A CNC system typically comprises the following main elements: (1) part program, (2) machine control unit (MCU), and (3) processing equipment (see Figure 17.3). A part program is a set of coded instructions to be followed by the processing equipment to produce a part. The part program is generally written by using a CAM software package (e.g., NX CAM, CAM EXPRESS, etc.). The MCU is the computer hardware that stores and executes the part program by issuing pulses and by implementing the required motions in the processing unit. The MCU issues a set of pulses that act as speed and feed axis motion signals to the amplifier circuits for driving the axis mechanisms. It also implements auxiliary control functions (e.g., coolant or spindle ON/OFF) and tool change (see the 14 tools turret facility in the CNC machine: Figure 17.1). The processing equipment (e.g., automatic lathe, automatic milling machine) performs the sequence of operations by using a drive system to transform the workpiece into a completed part. A drive system consists of amplifier circuits, stepping motors or servomotors, and ball screws. In an open-loop system, steeping motors are used, whereas the closed-loop system involves the use of servomotors (see Figure 17.3).
Automation of tool path generation in multi-process micromachine tool for micromachining of prismatic and rotational parts
Published in International Journal of Computer Integrated Manufacturing, 2018
Part program is a block of instruction required to generate a tool path. Multi-process micromachine tool is controlled by means of NC code. NC code is a structured block of instruction used for the fabrication of a given part, described in terms of Geometric (G) and Miscellaneous (M) codes. ‘G’ code controls the tool movement such as linear, rapid movement, circular; ‘M’ code performs auxiliary functions such as coolant on, coolant off, spindle stop, program stop, in order to ensure proper machining. The combination of ‘G’ and ‘M’ codes has been used to create a part program. It possess the information related to interpolation type, positional information with respect to X, Y and Z coordinates, tool type, rotational speed, feed and various miscellaneous functions. All these variables are considered whilst developing an algorithm for part program generation. Pseudo code for the execution of block of instruction is shown in Figure 4.