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.
Number of Lessons 22
- Coordinate Measuring Machines
- CMM Components
- Measured and Constructed Features
- Parts of the CMM Review
- Machine and Part Coordinates
- CMM Software
- CMM Programming
- Environmental Factors
- 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
- Describe the CMM and its main components.
- Describe the CMM and its main components.
- Distinguish between measured and constructed features.
- Define 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 how a noncontact probe inspects a part.
- Describe the methods and benefits of incorporating CMMs into production processes.
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.
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.
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.
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.
A type of CMM with two vertical supports and a horizontal beam holding the probe. The bridge-type CMM is the most common type.
Computer-aided design. 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.
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.
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.
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.
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.
A feature on a part that is reproduced mathematically from other existing features. Constructed features cannot be measured, so they must be reproduced from measured features.
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 precision-ground table and a suspended probe that measures parts in three-dimensional space. Coordinate measuring machines can measure using either contact or noncontact methods.
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.
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.
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.
A type of CMM similar to the bridge type but much larger. Gantry CMMs can measure parts the size of a car.
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.
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.
A type of CMM with a horizontal arm supporting the probe. The horizontal CMM makes it easy to load large, heavy parts.
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.
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.
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.
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.
A control that requires complete hands-on operation. Manual controls require the most operator involvement and thus, allow the greatest risk of operator error.
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.
An imaginary line that represents the motion and position of the probe. The measuring axis is also referred to as the Z axis.
A small, highly polished metal ball that allows objects resting on it to flow smoothly and freely. Mechanical bearings are usually used in multiples.
The shorter term for micrometer. A micron is 0.000001 of a meter or approximately 0.000039 of an inch.
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.
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.
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.
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.
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.
The smallest piece of information in an image. When joined together, pixels form a cohesive image.
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.
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.
A CMM device that gathers measurement data from a workpiece. Probes may use either contact or noncontact methods to measure a part.
A device that translates mechanical motion into a digital signal. Also known as an encoder, the reader head is an opto-electronic device on a CMM that detects the incremental lines on a scale to determine positioning.
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.
The fineness of detail that can be distinguished. On a CMM, the resolution of the encoder is the smallest value that can be counted.
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.
A gemstone mineral that is wear resistant. A synthetic ruby sphere is often used as the stylus.
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.
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.
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.
A small motor that can move machine components in very small, precise increments. Joystick controls are guided by servomotors.
Erroneously touching the shaft of a CMM probe to a workpiece instead of the stylus. Shanking causes incorrect measurements and should be avoided.
The programs and instructions that control the computer hardware functions and operations. Operators use software on a computer to calculate measurements on a CMM.
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.
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.
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.
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.
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.
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.
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.
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.