CONVENTIONAL NC & MODERN CNC

Conventional NC and its limitations: 

Part Programming Mistakes: In punched tape preparation mistakes are common. In case of an error the tape needs to be discarded or new tape has to be punched. Lesser choice of speed and feed:  The speed and feed cannot be changed during machining process. 

• Fragileness of tape.

• Breakdown problems due to faults in tape reader. 

• Hard Wired Controller unit in which the program/ instructions/features cannot be altered easily. 

• Management Information:  Conventional NC system does not provide timely information on operational performance to management. Such information might include piece counts, machine breakdowns and tool changes.

Functions of CNC: 

• Machine Tool Control

• In Process Compensation

• Improved programming and operating features.

• Diagnostics

Machine tool control: Main advantage of CNC is its ability to incorporate variety of control features into soft-wired controller unit. Certain control functions like circular interpolation can be accomplished more efficiently with hard-wired circuits. Thus has lead to development of two alternative controller designs of CNC. 

i)                    Hybrid CNC

ii)                   Straight CNC

 Hybrid CNC consists of controller with both soft wired as well as hard wired logic circuits. Hard-wired part performs certain functions where as the remaining functions are taken by care of by soft-wired part.

Straight CNC consists of soft wired logic circuits only. The computer performs all functions including interpolation and tool position feed back. 

In Process compensation: It involves dynamic correction of machine tool motions for changes or errors. 

Improved Programming and Operating Features:  Editing of part program permits correction or optimization of program. The system has provision to display tool path to verify tape. Complicated interpolation and use of special subroutines, manual data input and storage of more than one part program are some of the improved features of CNC. 

Diagnostics: NC tools are expensive and complex. Special attention has to be taken to avoid downtime and failures. CNC machines are equipped with diagnostic capability to assist in maintaining and repairing the system. Such system will fins the reason of failure or downtime so that the maintenance personnel can do repair work quickly. Diagnostic system can give alert signals to indicate imminent failure of certain component. This can help in preventive maintenance.One advanced function that can be performed by diagnostics and control system is activation of redundant component when a component fails thus helping the repairs during normal operations. 

Advantages of CNC: 

• Part program tape and tape reader are used only once to enter the program into the computer memory. This reduces use of tape reader, which is unreliable and susceptible to faults.

• Editing Facilities

• Greater Flexibility

• User written programs: Users can generate specialized programs and these become macro subroutines that can be stored in the memory. They can be called by part program to execute frequently repeated cutting sequences.

• Total Manufacturing System: CNC system can cover various aspects of manufacturing. It can be used to regulate and control several sections and machines of an industry. By interconnecting the machines in accordance to sequence of operations, an Integrated Manufacturing System can be set up.

Adaptive Control Machining (ACM)  System : 

A control system that measures certain output process variables and uses them to control speed / feed. Such variables can be spindle deflection force, torque, cutting temperature, vibration amplitude, and horsepower.  This increases efficiency of the process. 

Sources of variability on machining: 

i)                    Variable Geometry of Cut

ii)                   Variable work piece hardness and machinability

iii)                 Variable work piece rigidity

iv)                 Tool wear

v)                  Air gaps during cutting 

Types of adaptive control:

1.      Adaptive Control Optimization (ACO)

2.      Adaptive Control Constraint      (ACC) 

Adaptive Control Optimization: In this, index of performance is specified for system. Performance index is measured of overall process performance such as production rate or cost per volume of metal removed. The objective of adaptive controller is to optimize the index of performance by manipulating speed/feed in operation.

Index of Performance  = f (MRR/ TWR)]

Here, MRR = Material Removal Rate, TWR is Tool Wear Rate. 

Adaptive Control Constraint: This utilizes constraint limits imposed on certain measured process variables. The objective of the system is to manipulate feed /speed to maintain measured process variables below constraint limit. 

Benefits of Adaptive Control Machining:  

1.      Increased Production Rate

2.      Increased Tool Life

3.      Greater Part Protection

4.      Less Operator Intervention

5.      Easier part programming