IE Computer Integrated Manufacturing and Automation U. MAHİR YILDIRIM Computer Numerical Control II

Computer Numerical Control Sections: 1.Fundamentals of NC Technology 2.Computers and Numerical Control 3.Applications of NC 4.Analysis of Positioning Systems 5.NC Part Programming 2IE 420

Analysis of Positioning Systems Open-Loop Positioning Systems Closed-Loop Positioning Systems Precision in Positioning Systems 1.Fundamentals of NC Technology 2.Computers and Numerical Control 3.Applications of NC 4.Analysis of Positioning Systems 5.NC Part Programming 3IE 420

The velocity of the worktable, which corresponds to the feed rate in a machining operation, is determined by the rotational speed of the screw. An NC positioning system converts the coordinate axis values in the NC part program into relative positions of the tool and work part during processing. NC Positioning System 1.Fundamentals of NC Technology 2.Computers and Numerical Control 3.Applications of NC 4.Analysis of Positioning Systems 5.NC Part Programming 4IE 420 A simple positioning system

Analysis of Positioning NC Systems Two types of NC positioning systems: 1.Open-loop - no feedback to verify that the actual position achieved is the desired position 2.Closed-loop - uses feedback measurements to confirm that the final position is the specified position Closed-loop systems cost more than open-loop systems When are they appropriate? 1.Fundamentals of NC Technology 2.Computers and Numerical Control 3.Applications of NC 4.Analysis of Positioning Systems 5.NC Part Programming 5IE 420

Motion Control Systems Open loop Closed loop 1.Fundamentals of NC Technology 2.Computers and Numerical Control 3.Applications of NC 4.Analysis of Positioning Systems 5.NC Part Programming 6IE 420

Forces in Metal Cutting IE Fundamentals of NC Technology 2.Computers and Numerical Control 3.Applications of NC 4.Analysis of Positioning Systems 5.NC Part Programming

Open-Loop Systems vs Closed-Loop Systems Open-loop systems cost less than closed-loop systems and are appropriate when the force resisting the actuating motion is minimal. Closed-loop systems are normally specified for machines that perform continuous path operations such as milling or turning, in which there are significant forces resisting the forward motion of the cutting tool. IE Fundamentals of NC Technology 2.Computers and Numerical Control 3.Applications of NC 4.Analysis of Positioning Systems 5.NC Part Programming

Analysis of Positioning NC Systems Closed-Loop Systems A closed-loop NC system, uses servomotors and feedback measurements to ensure that the worktable is moved to the desired position. A common feedback sensor used for NC (and also for industrial robots) is an optical encoder, which is a device for measuring rotational speed that consists of a light source and a photodetector on either side of a disk. The disk contains slots uniformly spaced around the outside of its face. These slots allow the light source to shine through and energize the photodetector. The disk is connected to a rotating shaft whose angular position and velocity are to be measured. As the shaft rotates, the slots cause the light source to be seen by the photocell as a series of flashes. The flashes are converted into an equal number of electrical pulses. The optical encoder is connected directly to the leadscrew or ball screw, which drives the worktable. By counting the pulses and computing the frequency of the pulse train, the worktable position and velocity can be determined. IE Fundamentals of NC Technology 2.Computers and Numerical Control 3.Applications of NC 4.Analysis of Positioning Systems 5.NC Part Programming

Closed-Loop Systems Optical Encoder Device for measuring rotational position and speed: (a) apparatus and (b) series of pulses to measure rotation Common feedback sensor for closed-loop NC control 1.Fundamentals of NC Technology 2.Computers and Numerical Control 3.Applications of NC 4.Analysis of Positioning Systems 5.NC Part Programming 10IE 420

Example Optical Encoder 1.Fundamentals of NC Technology 2.Computers and Numerical Control 3.Applications of NC 4.Analysis of Positioning Systems 5.NC Part Programming 11IE 420 An NC worktable operates by closed-loop positioning. The system consists of a servomotor, ball screw, and optical encoder. The screw has a pitch of 6.0 mm and is coupled to the motor shaft with a gear ratio of 5:1. The optical encoder generates 48 pulses/rev of its output shaft. The table has been programmed to move a distance of 250 mm at a feed rate = 500 mm/min. Determine a)how many pulses should be received by the control system to verify that the table has moved exactly 250 mm, and b)the drive motor speed (rev/min) that corresponds to the specified feed rate.

Example Optical Encoder 1.Fundamentals of NC Technology 2.Computers and Numerical Control 3.Applications of NC 4.Analysis of Positioning Systems 5.NC Part Programming 12IE 420 Pitch A gear ratio of 5:1 → five turns of the drive motor for each turn of the screw

Solution Optical Encoder 1.Fundamentals of NC Technology 2.Computers and Numerical Control 3.Applications of NC 4.Analysis of Positioning Systems 5.NC Part Programming 13IE 420 a)The pulse count can be used to determine the distance moved by the worktable along the x-axis (or y-axis) or vica versa. Note that in a single revolution of the screw, the table moves 6.0 mm. b)Motor speed = table velocity (feed rate) divided by screw pitch, corrected for gear ratio:

To accurately machine or otherwise process a work part, an NC positioning system must possess a high degree of precision. Three measures of precision can be defined for an NC positioning system: 1.Control resolution 2.Accuracy 3.Repeatability Precision in NC Positioning 1.Fundamentals of NC Technology 2.Computers and Numerical Control 3.Applications of NC 4.Analysis of Positioning Systems 5.NC Part Programming 14IE 420

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Two-sigma limits and three-sigma limits are indicators of measurement accuracy used in control charts. When comparing 2 sigma vs 3 sigma control charts, 3 sigma control charts help ensure process stability whereas 2 sigma control charts are used to detect small shifts in the project or process. 16IE 420

Sigma Performance Levels Sigma LevelDefects Per Million Opportunities (DPMO) 1690, , ,807 46, Situation/ExampleIn 1 Sigma World In 3 Sigma World In 6 Sigma World Pieces of your mail lost per year [1,600 opportunities per year]1,106107Less than 1 Number of empty coffee pots at work (who didn’t fill the coffee pot again?) [680 opportunities per year] 47045Less than 1 Number of telephone disconnections [7,000 talk minutes]4, Erroneous business orders [250,000 opportunities per year]172,92416, IE 420

Precision in NC Positioning Control Resolution, Accuracy, Repeatability 1.Fundamentals of NC Technology 2.Computers and Numerical Control 3.Applications of NC 4.Analysis of Positioning Systems 5.NC Part Programming 18IE 420

Precision in NC Positioning Control Resolution 1.Fundamentals of NC Technology 2.Computers and Numerical Control 3.Applications of NC 4.Analysis of Positioning Systems 5.NC Part Programming 19IE 420

Accuracy - maximum possible error that can occur between the desired target point and the actual position taken by the system Accuracies in machine tools are generally expressed for a certain range of table travel, e.g mm for 250 mm of table travel Precision in NC Positioning Accuracy 1.Fundamentals of NC Technology 2.Computers and Numerical Control 3.Applications of NC 4.Analysis of Positioning Systems 5.NC Part Programming 20IE 420

Precision in NC Positioning Repeatability 1.Fundamentals of NC Technology 2.Computers and Numerical Control 3.Applications of NC 4.Analysis of Positioning Systems 5.NC Part Programming 21IE 420

NC Part Programming NC part programming consists of planning and documenting the sequence of processing steps to be performed by an NC machine. The part programmer must have a knowledge of machining (or other processing technology for which the NC machine is designed), as well as geometry and trigonometry. Part programming can be accomplished using a variety of procedures ranging from highly manual to highly automated methods. 1.Manual Part Programming 2.Computer-Assisted Part Programming 3.CAD/CAM Part Programming 4.Manual Data Input 1.Fundamentals of NC Technology 2.Computers and Numerical Control 3.Applications of NC 4.Analysis of Positioning Systems 5.NC Part Programming 22IE 420

NC Part Programming What is G-Code? How to convert piece Solidworks to (Gcode) CNC Machine 23IE 420