Numerical and Computer Numerical Control Machine Tools: A Comprehensive Overview, Schemes and Mind Maps of Computer Aided Design (CAD)

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NUMERICAL CONTROL

(NC)

HISTORICAL DEVELOPMENT

• 15th century - machining metal.
• 18th century - industrialization, production-type machine tools.
• 20th century - F.W. Taylor - Tool metal - HSS Automated production equipment - Screw machines Transfer lines Assembly lines using cams and preset stops Programmable automation - NC PLC Robots

A Definition of NC

• N u m e r i c al Control is a system in which actions are

controlled by the direct insertion of n u m e r i c a l d a t a at

some point.

• In other words, Programmable automation in which the

mechanical actions of a ‘machine tool’ are controlled by a

program

• or
• It is defined Method of programmable automation in which

various functions of machine tools are controlled by numbers

, letters and symbols.

In NC machine numbers form of program of instructions are: 1.TO Start (or) Stop the machine tool spindle.

2. To control the spindle speed. 3.To change the tool. 4.To change the feed rate. 5.To switch the coolant on/off. 6.To position the tool at a desired position Principle of operation: 1.The program of instructions in terms of part geometry , cutting process parameters and type of tool serves as the input of machine control unit (MCU) which in turns commands the machine tool to make the product. 2.The machine axes are connected to servomotors , which work under the control of the machine control unit (MCU) 3.The servomotors control the movement of the cutting tool with respect to work piece.

1.The control of an axis in an NC machine as shown

2. It is a closed loop positioning system
3. The MCU Generates a pulse signal until the signal returned from the feed back transducer agrees with the original number of pulses requires to execute the movement.
4. The comparator compares the count of feedback pulses with the original number, and the error signal is the output until the table reaches the desired position.
5. Feed back transducers are linear/rotary encoders used to obtain the correct position or velocity feedback.

MACHINE UNIT NUMERICAL CONTROLLER NUMERICAL DATA (N umbers , letters and symbols. ) MANUFACTURING OPERATOR PROCESSED PART Drive Control

COMPONENTS OF NC MACHINES

MCU Machine Tool CLU DPU MCU - Machine control unit CLU - Control-loops unit DPU - Data processing unit Hardware Configuration of NC Machine

2. Machine Control Unit (MCU)

• NC machine tool has a main unit, which is known as Machine Control Unit.
• It consists of some electronic hardware that reads theNC programme, interprets it and conversely translates it for mechanical actions of the machine tool.
• MCU consists of two parts : Data processing unit (DPU) and
• control loops unit (CLU).
• Function of DPU: read the decode the instructions available on the tape & to provide the decoded data to the control loops unit (CLU).
• Function of (CLU): To control the drives attached to the axes and receive the feedback signals from machine tool
• CLU also prompts a signal that the previous data segment is completed and that the DPU can read the next block of the part program.

8 3. NC Machine Tool :

• Machine tool is the main components of a numerical control system, which executes the operations.
• It may consist of worktable, cutting tools, jigs and fixtures, motors for driving spindle and coolant and lubricating system.
• The latest development in the numerical control machine tool is the versatile machining center.
• This is a single machine capable of doing a number of operations such as milling, boring, drilling, reaming, and tapping by Automatic Tool Changer (ATC) under the control of tool selection instruction.

4 .NC cutting tools: 1.The ISO Procedure is adopted for the designation of NC cutting tools. 2.A power operated draw bar may be used to pull the tolling at the retention knob. 3.A typical end mill as shown as the programmer obtain the history of the tools such as tool number , cutter compensation etc.

OTHER COMPONENTS OF AN NC MACHINE TOOL

10

COORDINATE SYSTEMS

x Right hand rule y z x y z

For turning operations:

1.Two axes are normally required to control the movement of

the tool relative to the rotating work piece.

2.The z- axis is the axis of rotation of the work piece and the

x- axis radial location of the cutting tool.

For Drilling and milling operations:

1. Two axes x and y are defined in the plane of the table and

the z- axes is perpendicular to the plane and the movement in

the z- direction is controlled by the vertical motion of the

spindle.

BASIC REQUIREMENT OF NC MACHINE CONTROL a. Preparatory functions: which unit, which interpolator, absolute or incremental programming, which circular interpolation plane, cutter compensation, etc. b. Coordinates: three translational, and three rotational axes. c. Machining parameters: feed, and speed. d. Tool control: tool diameter, next tool number, tool change. e. Cycle functions: drill cycle, ream cycle, bore cycle, mill cycle, clearance plane. f. Coolant control: coolant on/off, flood, mist. g. Miscellaneous control: spindle on/off, tape rewind, spindle rotation direction, pallet change, clamps control, etc. h. Interpolators: linear, circular interpolation