Inspecting with CMMs 361

“Inspecting with CMMs” provides a comprehensive overview of the functions and mechanics of the coordinate measuring machine, or CMM. A CMM’s probe contacts the various features on a workpiece and records their Cartesian coordinate locations with software. CMMs measure using either contact or noncontact methods and can be used in a lab or on the production floor. CMMs use either manual operation, joystick, or DCC to guide components.

As long as the operator is trained in its use, the CMM provides high accuracy measurements with minimum human influence in a very short amount of time. This allows the operator to respond to machining errors and reduce scrap. After this class, users should be able to describe best practices for using the CMM to inspect parts.

Class Details

Class Name:
Inspecting with CMMs 361
Description:
“Inspecting with CMMs” provides a comprehensive overview of the functions and mechanics of the coordinate measuring machine, or CMM. A CMM’s probe contacts the various features on a workpiece and records their Cartesian coordinate locations with software. CMMs measure using either contact or noncontact methods and can be used in a lab or on the production floor. CMMs use either manual operation, joystick, or DCC to guide components.

As long as the operator is trained in its use, the CMM provides high accuracy measurements with minimum human influence in a very short amount of time. This allows the operator to respond to machining errors and reduce scrap. After this class, users should be able to describe best practices for using the CMM to inspect parts.
Version:
2.0
Difficulty:
Advanced
Number of Lessons:
22
Related 1.0 Classes:
Basics of the CMM 120, Inspecting with CMMs 220

Class Outline

  • Coordinate Measuring Machines
  • CMM Components
  • Measured and Constructed Features
  • What is a Datum?
  • Parts of the CMM Review
  • Machine and Part Coordinates
  • Alignment
  • CMM Software
  • CMM Programming
  • Environmental Factors
  • Bearings
  • Types of CMMs
  • CMM Inspection
  • The Probe
  • Manual Controls
  • Direct Computer Controls
  • Scales and Encoders
  • Contact Probes
  • Noncontact Probes
  • Measuring with Noncontact Probes
  • CMM Applications and Advantages
  • Operator Involvement

Objectives

  • Define the CMM and identify its main components.
  • Distinguish between measured and constructed features.
  • Define a datum.
  • Distinguish between the machine coordinate system and the part coordinate system.
  • Describe the purpose of alignment.
  • Identify types of software used on a coordinate measuring machine and describe their purpose.
  • Describe different methods for programming a coordinate measuring machine.
  • Describe how coordinate measuring machines prevent or compensate for environmental influences.
  • Compare the characteristics of air bearings, magnetic bearings, and mechanical bearings.
  • Identify the common types of CMMs.
  • Describe the probe.
  • Compare the characteristics of manual and joystick controls.
  • Describe the operation of a direct computer control.
  • Describe the interaction of the scale with the encoder.
  • Describe how a contact probe inspects a part.
  • Describe how a noncontact probe inspects a part.
  • Describe ways to incorporate CMMs into production processes and its advantages.

Job Roles

Certifications

Glossary

Vocabulary Term Definition
air bearings A device that, through the release of compressed air, lifts one part slightly off another so that the parts may glide past each other. Air bearings are used in some CMMs to lift each axis member off its linear guide way surface by a few microns.
alignment The arrangement of components so that they are lined up. For a CMM, alignment occurs when the part coordinate system matches the machine coordinate system.
axis member The moving part of a CMM that corresponds with an imaginary line formed by the Cartesian coordinate system. Bearings allow an axis member to move along its corresponding guide way.
bearings A component of a machine with moving parts that allows smooth, low-friction motion between two surfaces. A CMM can have either air or mechanical bearings.
bridge A machine part that provides support and allows movement. On a CMM, the bridge is a horizontal beam that supports the Z axis ram and provides a stable area for movement of the probe.
bridge supports A vertical column that holds up the bridge. A bridge-type CMM has two bridge supports that slide back and forth on the staging table.
bridge-type CMM A type of CMM with two vertical supports and a horizontal beam holding the probe. The bridge-type CMM is the most common type.
calibration The comparison and adjustment of a device with unknown accuracy to a device with a known, accurate standard. Calibration eliminates any variation in the device being checked.
cantilever CMM A type of CMM with a single, moveable vertical support with a suspended horizontal arm that holds the probe. Cantilever CMMs provide easy access to the staging table.
CMM Coordinate measuring machine. A sophisticated measuring instrument with a flat polished table and a suspended probe that measures parts in three-dimensional space. CMMs can measure using either contact or noncontact methods.
computer-aided design CAD. A computer software program that aids in the automated design and technical precision drawing of a part, product, process, or building. CAD software can tell a CMM how to inspect a part.
constructed feature A feature on a part that is reproduced mathematically from other existing features. Constructed features are reproduced from measured features.
contact probe A type of probe that directly contacts the surface of the part it is measuring. Contact probes are the most common type of CMM probes.
continuous analog scanning CAS. A method of gathering coordinates that requires the stylus to stay in constant contact with the part's surface. CAS is especially useful for performing reverse engineering.
coordinate measuring machine CMM. A sophisticated measuring instrument with a flat polished table and a suspended probe that measures parts in three-dimensional space. CMMs can measure using either contact or noncontact methods.
datum A point of reference for machine tools, programs, and fixtures from which measurements are taken. A datum can be a hole, line, or any three-dimensional shape.
direct computer controls DCC. Completely hands-free machine operation that is directed by a software program. It requires greater operator training but reduces the risk of operator error in a measurement.
encoders A device that translates mechanical motion into a digital signal. Also known as a reader head, the encoder is an opto-electronic device on a CMM that detects the incremental lines on a scale to determine positioning.
feature recognition Software that, when given a small amount of data, like a set of coordinates, can recognize the part in question. When the probe of a CMM touches the part at several points, this software allows the computer to decide which shape best matches the arrangement of those coordinates.
gantry-type CMM A type of CMM similar to the bridge type but much larger. Gantry CMMs can measure parts the size of a car.
guide rail A component perpendicular to the bridge on a CMM. It guides the movement of the bridge supports.
guide way The stationary part of a CMM that corresponds with an imaginary line formed by an axis in the Cartesian coordinate system. Bearings help each axis member move along their respective guide ways.
hard probe A solid contact probe consisting of a precision ball or tapered shape that is mounted to a shaft. Operators use hard probes to manually inspect the surface of a part.
horizontal CMM A type of CMM with a horizontal arm supporting the probe. The horizontal CMM makes it easy to load large, heavy parts.
joystick control A vertical handle or lever that allows remote, but hands-on, operation of a machine. These controls are very useful for measuring large components and subassemblies.
laser probe A measuring device that uses a narrow beam of light to take measurements. On a CMM, the laser projects onto the part's surface, and a lens on the probe then reads the position of the beam.
machine coordinate system An unchangeable coordinate system that has machine zero as its origin. On a CMM, the X, Y, and Z axes are mapped specifically to the components of the CMM, and these axes remain the same even if the part changes.
macros A single computer command that executes a series of commands or actions. Also called canned routines, macros are templates for inspecting basic parts on a CMM.
magnetic bearings A device that uses a magnetic field to lift one part slightly off another so that the parts may glide past each other. Magnetic bearings are used in some CMMs to lift each axis member off its linear guide way surface.
manual control A control that requires complete hands-on operation. Manual controls require the most operator involvement and thus, allow the greatest risk of operator error.
measured feature A feature on a part that is measured by making contact with various points on its surface. Measured features can be used to calculate constructed features.
measuring axis Linear axis that represents the motion and position of the probe. This is also referred to as the Z axis.
mechanical bearings A small, highly polished metal ball that allows objects resting on it to flow smoothly and freely. Mechanical bearings are usually used in multiples.
microns The shorter term for micrometer. A micron is 0.000001 of a meter or approximately 0.000039 of an inch.
noncontact probes A type of probe that consists of a laser or scanning system that does not contact the part surface. Noncontact probes are used to inspect small, flexible parts.
operating system The software on a computer that allows files to be created and organized, manages the interaction of different programs, holds data in memory, and performs other functions. In short, the operating system runs the computer.
opto-electronic devices A mechanism that uses light to generate an electronic signal or that interprets light as an electronic signal. An encoder is one example of an opto-electronic device.
part coordinate system The coordinate system in which the X, Y, and Z axes are mapped specifically to the features of the workpiece. Each part has its own part coordinate system.
part program A string of software commands that provide a sequence of operations to be performed on a part. The part program tells a coordinate measuring machine how to inspect the part.
part program A string of software commands that tells a coordinate measuring machine how to inspect a part. Part programs are developed by the CMM operator or programmer.
pixels The smallest piece of information in an image. When joined together, pixels form a cohesive image.
portable CMM A type of CMM consisting of a moveable arm with multiple joints that holds a probe. Portable CMMs can be taken to the part for inspection.
primary datum The datum feature that first situates the part within the datum reference frame. The primary datum is the first feature to contact a fixture or surface during assembly.
probe A device that gathers measurement data from the workpiece. On a CMM, the probe is attached to the Z axis ram and uses either contact or noncontact methods to measure a part.
reader head A device that translates mechanical motion into a digital signal. Also known as an encoder, it is an opto-electronic device on a CMM that detects the incremental lines on a scale to determine positioning.
repeatability The ability of an object to deliver accurate and dependable results over a long period of time. A probe's measurements are more repeatable when the stylus is kept perpendicular to the surface of the workpiece.
resolution The fineness of detail that can be distinguished. On a CMM, the resolution of the encoder is the smallest value that can be counted.
reverse engineering A type of design in which a finished piece is examined to determine how it was made. The data collected allows the original to be reproduced.
ruby A gemstone mineral that is wear resistant. A synthetic ruby sphere is often used as the stylus.
sample A representative part from a larger group. As part of a quality process, a sample of the parts produced in a large batch are inspected to predict flaws.
scales A standard of measurement that is often displayed as a series of lines. On a CMM, the scale is located along the axis member. When read by an encoder, the scale tells the CMM the axis coordinates.
secondary datum The datum feature that situates the part within the datum reference frame after the primary datum. The secondary datum is the second feature to contact a fixture or surface during assembly.
servomotors A small motor that can move machine components in very small, precise increments. Joystick controls are guided by servomotors.
software The programs and instructions that control the computer hardware functions and operations. Operators use software on a computer to calculate measurements on a CMM.
staging table The flat surface on which a part is measured. The staging table must be stable to ensure accurate measurements.
stitch-scanning A measurement method in which a series of individual points on a part are touched and their positions recorded. The CMM then estimates the surface location between these points.
stylus The precision tip that records measurements. On a CMM, the stylus is the spherical portion of the probe that is mounted on the shaft and makes contact with the part.
tertiary datum The datum feature that situates the part within the datum reference frame after the secondary datum. This plane must be perpendicular to both the primary and secondary planes and is usually the smallest surface of the workpiece.
thermal stability The ability of a material to resist changes in physical shape or size as its temperature changes. The parts of a CMM need thermal stability to maintain accurate measurements.
touch trigger probe A type of contact probe that detects a feature on a part and generates an electronic signal to record its dimension. Touch trigger probes are the most accurate and commonly used probes on the CMM.
vision probe A measuring device, also known as a video probe, that takes a digital image of the part. The image is then broken up into tiny dots, which are measured and counted by the software.
X axis The linear axis representing side-to-side movement in a device, relative to the origin. When facing the front of the CMM, the X axis represents the motions and positions along a line parallel to the table and to the left and right of the observer.
Y axis The linear axis representing back and forth movement in a device, relative to the origin. When facing the front of the CMM, the Y axis represents the motions and positions along a line parallel to the table and toward and away from the observer.
Z axis The linear axis that represents up and down movement in a device, relative to the origin. When facing the front of the CMM, the Z axis, also referred to as the measuring axis, represents the motions and positions along a line perpendicular to the table that runs from top to bottom.
Z axis ram The device positioned on the Z axis that holds the probe. The Z axis ram can be raised or lowered.