Introduction to CNC Machines 201

“Intro to CNC Machines” provides a comprehensive introduction to computer numerical control (CNC), which uses numerical data to control a machine. CNC machines rely on a system of three linear and three rotational axes in order to calculate the motion and position of machine components and workpieces. A machine control unit controls and guides the movements of the machine tool. This class also describes PTP positioning, which moves to the end position before the tool begins to cut, and continuous path systems that can move a tool along two or more axes at once and cut during the movement. Additionally, closed-loop systems provide feedback, while open-loop systems do not.

CNC machines are used to make a variety of products using a number of different processes. With proper training, a human operator can use CNC machines to make accurate parts with decreased risk of error. After taking this class users should be able to describe common components of CNC machine tools and controls.

Class Details

Class Name:
Introduction to CNC Machines 201
Description:
“Intro to CNC Machines” provides a comprehensive introduction to computer numerical control (CNC), which uses numerical data to control a machine. CNC machines rely on a system of three linear and three rotational axes in order to calculate the motion and position of machine components and workpieces. A machine control unit controls and guides the movements of the machine tool. This class also describes PTP positioning, which moves to the end position before the tool begins to cut, and continuous path systems that can move a tool along two or more axes at once and cut during the movement. Additionally, closed-loop systems provide feedback, while open-loop systems do not.

CNC machines are used to make a variety of products using a number of different processes. With proper training, a human operator can use CNC machines to make accurate parts with decreased risk of error. After taking this class users should be able to describe common components of CNC machine tools and controls.
Version:
2.0
Difficulty:
Intermediate
Number of Lessons:
20
Related 1.0 Class:
Mechanics of CNC 110

Class Outline

  • CNC for Modern Manufacturing
  • History of CNC
  • CNC Machine Movement
  • CNC Machine Variety
  • CNC Coordinates
  • Positive and Negative Directions
  • Rotational Axes
  • Contouring
  • CNC Machines
  • Machine Control Unit
  • Control Systems
  • PTP and Continuous Path Systems
  • Open-Loop and Closed-Loop Systems
  • Control System Categorization
  • Servomotors
  • Feedback Devices
  • Closed-Loop Error Correction
  • Input Methods
  • Control and Operation Features
  • CNC Machine Components

Objectives

  • Describe how CNC machines have benefited modern manufacturing.
  • Describe the origin of CNC machines.
  • Identify common methods of CNC movement.
  • Identify different varieties of CNC machines.
  • Explain the role of axes in the Cartesian coordinate system.
  • Identify positive and negative movement along machine axes.
  • Describe rotational axes and movement.
  • Describe contouring movements.
  • Describe the role of the machine control unit.
  • Distinguish between point-to-point positioning and continuous path movement.
  • Contrast open-loop and closed-loop systems.
  • Describe the role of servomotors.
  • Distinguish between different feedback devices.
  • Describe various input methods.
  • Describe different control and operation features available to the CNC machine.

Job Roles

Certifications

Glossary

Vocabulary Term Definition
A axis The rotational axis describing motion around the X axis. This movement can be in a clockwise or counterclockwise direction.
AC servo A servomechanism that is powered using an alternating current. AC servos consume less power for better reliability and performance than a DC servo.
alternating current AC. Current that flows in opposite directions at different times. Alternating current can be used to power a servomechanism.
American Standard Code for Information Interchange ASCII. The standard for information exchange between computers. ASCII text is used to communicate code in a part program.
automatic testing machines Technology that uses software to run tests without human involvement. Automatic testing machines find out whether or not a machine is functioning correctly.
axis An imaginary straight line or circle used to describe the location or movement of an object in three-dimensional space. The linear axes of the Cartesian coordinate system are the X, Y, and Z axes, and the rotational axes are the A, B, and C axes.
B axis The rotational axis describing motion around the Y axis. This movement can be in a clockwise or counterclockwise direction.
ballscrew A long, threaded device that rotates to move the worktable or cutting tool of a CNC machine. Ballscrews are powered by a motor.
C axis The rotational axis describing motion around the Z axis. This movement can be in a clockwise or counterclockwise direction.
canned cycles A predetermined machining sequence used to simplify programming. Canned cycles can be used for common operations such as drilling holes.
Cartesian coordinate system A numerical system that describes the location of an object by expressing its distance from a fixed position along three linear axes. The Cartesian coordinate system is used to program machining positions on a CNC machine.
changeover The process of switching a machine from one part setup to another. CNC machines require very little changeover.
closed-loop system A type of control system that automatically changes the output based on the difference between the feedback signal to the input signal. This allows closed-loop systems to correct errors in position.
CNC Computer numerical control. A self-contained system of computers and precision motors that executes program instructions to guide machine tool components and manufacture parts. CNC machines can be either open-loop or closed-loop systems.
CNC lathe A machine tool that uses computer numerical data to control cutting operations on cylindrical workpieces. On a CNC lathe, the cutting tool moves against the rotating workpiece.
CNC mill A machine tool that uses computer numerical data to control cutting operations on flat, square, or rectangular workpieces. On a CNC mill, the cutting tool rotates against a workpiece that is fixed to a worktable.
compact discs CD. A data storage format that reads and writes optical data. CNC machines sometimes use CDs to store part programs.
computer numerical control CNC. A self-contained system of computers and precision motors that executes program instructions to guide machine tool components and manufacture parts. CNC machines can be either open-loop or closed-loop systems.
continuous path A type of CNC control system where cutting can take place as the tool moves from one position to the next. Continuous path systems are usually closed-loop.
contouring Tool movement along two or more axes at the same time that creates a curved or angled surface. Contouring is possible only in a continuous path system.
control system A method of tool and part movement in CNC machining. Point-to-point positioning and continuous path are the two main types of control systems.
cross slide The toolholding component of a lathe that allows tools to approach a workpiece. The cross slide moves along two axes.
cutting tool A device that removes workpiece material in the form of chips. Cutting tools are available in many shapes and sizes.
DC servo A servomechanism that is powered using a direct current. A DC servo is less costly than an AC servo.
depth of cut The depth at which the cutting tool penetrates the surface of the workpiece. Depth of cut can be controlled by computer numerical control.
direct current DC. Current that flows in one direction. Direct current can be used to power a servomechanism.
drafting systems Technology that uses a computer to design a product. Computer-aided design software is a form of drafting system.
drills A metal cutting tool used to penetrate the surface of a workpiece and make a round hole equal to the drill diameter. Automated drills usually use a PTP control system.
dry run A preliminary operation to ensure that the part program will machine workpieces properly. Dry runs are performed without any parts or cutting fluid.
electrical logic circuitry A closed path, usually composed of various devices and wires, through which an electric current follows. Electrical circuitry controlled NC machines before personal computers became common.
electronic assembly systems Technoloy that uses automated machines to assemble products. Electronic assembly systems often attach parts using fasteners.
Ethernet A form of computer networking used by modern CNC machines to input part programs from a central server into the machine control unit. Ethernet networks connect computers using designated cables standardized by the EIA.
feed The rate at which the cutting tool and the workpiece move in relation to one another. Feed is typically a linear movement.
feedback A return signal that confirms the position of the cutting tool or worktable. Feedback provides information that allows the control system to compare actual position to planned position and signal the motor to move if the positions do not match.
feedback device A device that sends information back to the controller in a closed-loop system. The feedback device is what allows for slight corrections in position if errors are detected.
finishing A final metal cutting process that emphasizes tight tolerances and smooth surface finish. Grinding is a finishing technique.
floppy disk drives A computer peripheral that reads and writes magnetic data to and from a floppy disk. Older CNC machines sometimes use floppy disk drives to store part programs.
grinders A machine that uses an abrasive to wear away at the surface of a workpiece. Grinders may use computer numerical data to control machine operations.
hardware Any physical or mechanical component. CNC hardware includes the machine control unit, tape readers, and disk drives, depending on the machine vintage.
hydraulic servo A servomechanism driven by fluids that deliver power to the motor. This type of servo is not powered by electricity.
hydraulics A system that uses the force of flowing liquids to transmit power. Hydraulics can be used to power a servomechanism.
laser cutting A machining process that uses an intense, focused beam of light to cut materials. Laser cutting is generally a CNC process.
linear axes Each of three imaginary perpendicular lines that describe movement along a straight line. Linear axes serve as reference points in the Cartesian coordinate system.
linear scale A type of feedback device that relies on the size of an electrical current to convey the position or distance on a CNC machine. A linear scale is one of the most accurate feedback devices.
lot The number of parts made with a particular tooling setup. A lot is composed of identical or similar parts or materials.
machine control unit MCU. A small, powerful computer that controls and operates a CNC machine. The MCU is also known as the controller and interprets the numerical data in the part program to guide machine movement.
machining center A sophisticated CNC mill that can perform multiple machining operations, including milling and various hole-making operations, in the same setup with a variety of tools. Machining centers usually use a continuous path control system.
MCU Machine control unit. A small, powerful computer that controls and operates a CNC machine. The MCU interprets the numerical data in the part program to guide machine movement.
mylar tape A thin, yet strong polyester film that was used to transmit programs to numerically controlled machines. Programs executed on mylar tape had to be rewound after each use.
numerical control NC. A system that guides machine tool components and manufactures parts using electrical logic circuitry to execute program instructions punched into paper or mylar tape. NC was the precursor to CNC.
open-loop system A type of control system that uses only an input signal to actuate an output. There is no automatic feedback to adjust the process, so adjustments must be made manually by the operator.
optical encoder A type of feedback device that records light reflections during the machining process. The optical encoder then converts those reflections into feedback signals.
optical sensors A type of sensor that detects the presence of an object through the use of a beam of light. Optical sensors are used by optical encoders.
origin The fixed, central point in a coordinate system. The origin has a numerical value of zero.
paper tape A way of transmitting programs to numerically controlled machines. Program executed on paper tape had to be rewound after each use.
part program A series of alphanumerical instructions used by a CNC machine to perform the necessary sequence of operations to machine a specific workpiece. Multiple part programs can be stored in a CNC at one time.
point-to-point positioning PTP. A type of CNC control system where no cutting takes place during the movement of the tool from one position to the next. PTP systems are usually open-loop.
punch presses A machine with a stationary base and an upper arm that moves along a vertical axis to shear, bend, or form sheet metal. Punch presses often use computer numerical data to control machine movements.
punching A shearing operation that creates an open hole in sheet metal by separating an interior section. Punching can be performed using a punch press.
right-hand rule A quick reference that uses the hand to show the location of Cartesian axes. The right-hand rule extends the thumb and index finger in an "L" shape and the middle finger upwards to represent the three axes.
robots Automated technology to aid or replace humans. Robots can be used in manufacturing environments that are hazardous or inaccessible to workers.
rotary resolver A type of feedback device that records the ballscrew rotations of a CNC machine. The rotary resolver then sends electronic signals back to the CNC control to indicate position or speed.
rotational axes Each of three imaginary circles that describe rotation around a linear axis. The A axis rotates around the X axis, the B axis rotates around the Y axis, and the C axis rotates around the Z axis.
RS232 A standard serial binary cable used to transmit data between two or more hard drives. CNC controls can send and receive part programs using RS232 standard connection cables.
server The physical computer that shares information with other computers within its network. The server for a network of CNC machines would share part programs.
servomechanism Servo. A special servomotor used in closed-loop CNC machines that adjusts the position of machining based on feedback. Servos are powered using electricity or hydraulics.
servomotor A motor in a CNC machine that powers movement of the machine tool. A servomotor rotates the ballscrew, causing the table of a mill or the slides of a lathe to move.
signal A message sent electronically. An open-loop system signals the motor to move, and a closed-loop system signals the motor to start and to adjust its position based on a feedback signal.
software The machine programs and instructions that control the computer hardware functions and operations. CNC software includes the part program or a canned cycle.
speed The rate at which the machine spindle rotates. The spindle speed affects how fast the cutting tool or workpiece moves at the point of contact.
stamping A metalworking process that involves forming or separating sheet metal into parts with the use of dies and punches. Stamping can be performed using a punch press.
stepper motor A servomotor that generates steps from electronic pulses that move the tool and the worktable. Stepper motors are used in open-loop systems.
toolpath A series of program blocks that describes the movement of a single cutting tool. Canned cycle toolpaths are evaluated using dry runs.
turning center A sophisticated CNC lathe that can also perform a variety of drilling and milling operations all at the same location. Turning centers usually use a continuous path control system.
turret A lathe component that holds a number of cutting tools. Turrets rotate to place tools in cutting position.
turret punch press A CNC punch press with an open frame and a turret containing multiple punches. A turret press is used to perform a variety of punching and shearing operations.
Universal Serial Bus USB. A small, portable memory card that can be used to store data. CNC machines can use USB drives to store part programs.
words The pairing of an address and a numerical value. CNC uses words to control a machine tool's motion.
workpiece A part that is in the process of being manufactured. A workpiece may be a complete product in itself or one component of a product consisting of many parts.
worktable The component of a CNC mill that supports the workpiece and any workholding devices during machining. The worktable can remain fixed or move.
X axis The linear axis representing coordinate positions along a line parallel to the longest edge of the worktable. The X axis usually runs left-to-right.
Y axis The linear axis representing coordinate positions along a line parallel to the shortest edge of the worktable. The Y axis usually runs back and forth.
Z axis The linear axis representing coordinate positions along a line parallel to the spindle and perpendicular to the worktable. The Z axis usually runs up and down.