Everything You Need To Know To Find The Best Three-Axis Inspection Machine

What is a CMM machine?

A coordinate measuring machine, also known as a CMM, is a piece of equipment that measures the geometries of physical objects. CMMs using a probing system to detect discreet points on the surfaces of objects.

The very first CMM made its appearance in the early 60s. Originally developed by Ferranti Company in Scotland in the 50s, this 2-axis CMM used a 3D tracing device with a simple digital readout that displayed XYZ positions. Ferranti used its CMM to measure precision components for their military products. Three-axis models were developed in the later 60s.

CMMs are most often used to test a part or assemble to determine whether or not it respects the original design intent. CMMs are integrated within quality assurance or quality control workflows to check the dimensions of manufactured components to prevent or resolve quality issues.

The advantages of using CMMs over manual inspections or checks performed with conventional metrology instruments, such as micrometers and height gauges, are: accuracy, speed and the reduction of human error.

There are several different types of CMMs. Typically, CMMs are categorized based on their structures. Each structure has its pros and cons. Let’s take a look at different CMM types in more detail.

What are the different CMM types?

Bridge CMM

Bridge CMMs feature a probing system that moves along three axes: X, Y and Z; these axes are orthogonal to each other in a Cartesian coordinate system. Each axis has a sensor that monitors the probe’s position (in micrometres) as it moves along an object and detects points on the object’s surface. These points form what is called a point cloud, which “illustrates” the surface area users are interested in inspecting. Bridge CMMs can be divided into two CMM sub-types: moveable-table and moveable-bridge CMMs.


The pros of bridge CMMs

• One of the most accurate types of CMMs
• Ideal to measure machined parts with high tolerances
• Perfect for small- to medium-sized components
• Enabled for multi-sensor measurements, such as probing and scanning
  
  

The cons of bridge CMMs

• Can be expensive
• Have a fixed measurement volume
• Lack of portability; you need to bring the part to the system or use machinery to move them around
• Sensitive to vibrations and must be used in a metrology lab
• Require rigid setups for each inspected part
• Complex to operate and needs skilled workers to program the device
  
  

Gantry CMM

Gantry CMMs are somewhat like bridge CMMs; however, they are usually much larger. Because they are designed to eliminate the need to lift a part onto a table and offer similar accuracy levels as bridge CMMs, Gantry CMMs are regularly used for very heavy or large parts. Gantry CMMs must be mounted on a solid foundation, directly on the floor.

The pros of gantry CMMs

• Highly accurate
• Large measurement volume, which facilitates inspections of large/heavy parts
• Easier to load and unload components than a bridge CMM
  

The cons of gantry CMMs

• Can be expensive
• Have a fixed measurement volume
• Lack of portability; you need to bring the part to the system or carry out significant assembly/disassembly to move the CMM
• Takes up a lot of floor space
• Sensitive to vibrations and must be used in a metrology lab
• Require rigid setups for each inspected part
• Complex to operate and needs skilled workers to program the device

Cantilever CMM

A cantilever CMM differs from a bridge CMMS as the measuring head is only attached to one side of a rigid base. Cantilever CMMs provide open access to inspection technicians on all three sides for ease of operation

The pros of cantilever CMMs

• Highly accurate
• Suitable for smaller parts
• Access to three sides makes it easier to manually or automatically load and unload components
  
  

The cons of cantilever CMMs

• Can be expensive
• Have a fixed measurement volume
• Lack of portability; you need to bring the part to the system
• Sensitive to vibrations and must be used in a metrology lab
• Require rigid setups for each inspected part
• Complex to operate and needs skilled workers to program the device
  
  

Horizontal Arm CMM
Horizontal arm CMMs, as their name implies, have horizontally mounted probes as opposed to vertically mounted probes like other CMMs. They are designed to measure long and thin objects that could not be inspected with vertical CMMs, like sheet metal. Horizontal arm CMMs are also often used to inspect geometries that are difficult to reach. There are two types of horizontal arm CMMs: plate-mounted and runway-mounted.

The pros of horizontal arm CMMs

• Long measurement volume (long and thin parts)
• Good for parts requiring low tolerances
• Does not require a significant foundation system
• Quick and easy installation
• Smaller footprint
• Requires less ceiling height than other types of CMMs
• Cost-effective
  
  

The cons of horizontal arm CMMs

• Less accurate than other CMMs
• Have a fixed measurement volume
• Lack of portability; you need to bring the part to the system
• Sensitive to vibrations and must be used in a metrology lab
• Require rigid setups for each inspected part
• Complex to operate and needs skilled workers to program the device
  
  

Portable measuring arm CMM

Portable measuring arm CMMs are coordinate measuring machines that can take measurements of parts right on shop floors, allowing for quick results and real-time analysis. As opposed to inspectors bringing components to a lab to be measured, technicians use an articulated arm, with either a six- or seven-axis system, to measure components wherever required; this is particularly useful to analyze parts while still integrated into their fixtures or assemblies. Portable measurement arms.

The pros of measuring arm CMMs

• Portable and lightweight: you can bring the CMM to the part
• Extendable measurement volume (leapfrog)
• Enabled for multi-sensor measurements, such as probing and scanning
• Relatively inexpensive
• Easy to operate (no programming)
  
  

The cons of measuring arm CMMs

• Less accurate than other types of CMMs
• Sensitive to environmental vibrations
• Requires rigid setups
  
  

Optical CMM

Optical CMMs are portable non-contact devices. These CMMs use an arm-free system with optical triangulation methods to scan and acquire 3D measurements of objects. Thanks to sophisticated image processing technology, optical CMMs are ultra-fast and guarantee metrology-grade accuracy. Optical CMM scanners are particularly conducive Industry 4.0 manufacturing.

While optical CMMs have a slightly lower level of accuracy, they are nevertheless accurate for a wide range of applications. In fact, optical CMMs are used in conjunction with traditional CMMs in order to free up production bottlenecks. Therefore, parts that require the critical level of accuracy are inspected with a conventional CMM. All other components can be assessed using a more cost-effective optical CMM, which provides satisfactory accuracy—but also portability, flexibility and speed.

The pros of optical CMMs

• Portable and lightweight: you can bring the CMM to the part
• Extendable measurement volume (leapfrog)
• Enabled for multi-sensor measurements, such as probing and scanning
• Very fast acquisition times
• Relatively inexpensive
• Easy to operate (no programming)
• No rigid setups required
  
  

The cons of optical CMMs

• Somewhat less accurate than conventional CMMs, depending upon the application

Why are we talking about CMM speed all the time?

Today’s manufacturers are under more pressure to increase throughput, offer just-in-time delivery schedules, and accelerate their time to market—all while significantly reducing costs to a minimum. When bottlenecks at the CMM occur, inspection procedures extend cycle times and ultimately increase non-value-added quality costs. CMM speed and efficiency is therefore critical.

As previously mentioned, gridlocks at the CMM are often caused by the sheer volume of work that has to be carried out by a limited number of qualified metrologists. CMM programming times also significantly lengthen inspections as the CMM has to be configured for each type of component or sub-assembly to be assessed.

Conventional CMMs that are equipped with CMM probes are slow and not suitable for efficiently measuring complex shapes. Other CMMs, which have CMM sensors, tend to speed up inspection processes; however, they still need to be operated by experts.

Manufacturers are therefore increasingly looking for inspection technologies, like innovative optical CMMs, that can keep up with the breakneck pace required in demanding production environments and stringent quality assurance and quality control standards.

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Interested in taking advantage of both the accuracy and speed of cutting-edge optical CMM scanners? Looking to implement an optical CMM in your upcoming automated quality control projects? Learn more about the R-Series Need specific information? Related Content

Do you know how dimensions and tolerances for complex parts are measured?  Often, a coordinate measurement machine or CMM is used.  In this article, we will look at different types of CMM’s and discuss how they work and when they are used.  

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What is a Coordinate Measuring Machine?

A coordinate measuring machine is an instrument that is used to collect measurements of three-dimensional objects.  A CMM consists of a structure that moves in three dimensions, a probe attached to the structure, and a computer control and/or recording system.  All coordinate measuring machines operate in the same way: the probe touches points on the object to be measured, and the position of the structure is recorded.  Each measurement point is described by X, Y, and Z coordinates.  All the measured points for a single object are combined into a 3D CAD file known as a point cloud.  The point cloud can be compared to a 3D design drawing to determine if the object has been manufactured to the correct dimensions.   

Types of CMM’s

CMM’s come in all shapes and sizes, depending on the objects to be measured.  The smallest CMM’s can be portable, but the largest CMM’s require a custom concrete foundation.  Articulating arm, horizontal arm, bridge, and gantry are some of the common CMM configurations.  Below, we will discuss general characteristics of each configuration. 

Articulating Arm

These compact, versatile CMM’s consist of a vertical post with multiple articulating segments and up to 8 axes of movement.  A probe is mounted to an articulating joint at the end of the arm, and they often have a laser scanning apparatus allowing for the use of either or both at the same time.  The articulating joints allow the probe or scanner to be angled to collect measurements in hard to reach areas.  These machines are manipulated by hand to place the probe or scanner at the point to be measured.  The photos below show articulating arm type CMMs in use.   

These CMM’s are highly portable and do not require climate-controlled rooms for operation.  They can be used to collect field measurements from installed parts.  Advertised accuracy is as high as 0.”, but the setup’s environmental conditions can limit it.  Additionally, the measuring range of these machines is limited only by the arm’s reach, so they can be used to measure a wide variety of parts, making them well-suited for many commercial machine shop applications.  However, articulating arm CMM’s cannot perform an automatic measurement, and they are not typically used for measurement in high-volume manufacturing processes.   

Horizontal Arm

The basic structure of this type of CMM consists of a base with one or more articulating arms or beams.  A probe is mounted on a rotating joint at the end of the arm.  The arms can be extended or retracted from the base in any direction and will also pivot about the base, allowing for measurements in three axes.  However, for some configurations, the probe is capable of traversing perpendicular to the base on a horizontal arm, providing a third measurement axis. The photo below shows a portable, rotating, horizontal arm type CMM manufactured by zCAT.  

Horizontal arm CMM’s can be huge, and one common application for large machines is automobile body measurement.  The photo below shows a pair of horizontal arm CMM’s manufactured by Mitutoyo.   

This type of CMM is often fully computer-controlled, allowing them to measure, record, and evaluate data without operator intervention.  A joystick for manual operation can control some machines.  Accuracy is typically better than an articulating arm CMM, but it suffers somewhat from an inherent lack of rigidity in the cantilevered design.  

Bridge

Bridge machines are the most common type of CMM.  These machines are typically benchtop or floor-mounted and comprised of a granite measurement table, a “bridge” structure that moves horizontally above the table, a vertical beam that moves along the bridge structure as well as vertically, and a probe mounted to the bottom of the vertical beam via a rotary joint.  The bridge traverse movement function is typically driven from one side only and equipped with air bearings to minimize friction.   

Bridge CMM’s can be manually controlled or fully automated.  The rigid bridge structure makes this the most accurate type of CCM, and temperature compensation is often available.   

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Cantilever

A cantilever type CMM is similar to a bridge-style machine, but with only one support to allow easier access to the measuring table. 

Gantry

The largest CMM’s are gantry type.  These machines are mounted on custom foundations and installed in bespoke climate-controlled buildings.  The layout is similar to bridge machines.  Accessibility and the ability to measure very large parts are the primary advantages of a gantry CMM.   

CMM Operating Principles

Encoders

CMM’s use linear scales (sometimes called linear encoders) to measure translation movements in the X, Y, and Z axes.  These scales can operate on a variety of principles, but generally consist of a strip that encodes position information and a sensor that reads the strip.  Common types of encoding are optical, magnetic, capacitive, inductive, and eddy current.   

Rotary encoders operate on the same principles as linear encoders, and they are used to monitor the angles of articulating joints.  From the encoder angle and known member lengths, trigonometry is used to calculate the probe position. Coordinates are adjusted to provide the proper X, Y, Z coordinates for the measurement.   

Probes

CMM probes are often touch-triggered, collecting a measurement point every time they touch a surface.  A typical probe tip consists of a spherical ruby mounted to the end of a thin stylus.  When the probe touches a surface, it transfers pressure through the stylus to a sensor within the probe body, triggering a measurement.   

More sophisticated machines have scanning probes that drag across the surface to be measured, collecting measurement points at pre-programmed intervals.   

In some cases, contact probes can be replaced with optical scanning probes.  These scanning probes use reflections of light to triangulate measurement points on the object’s surface.  Recently, advances in technology have allowed optical scanners to increase in accuracy and range.  As a result, stand-alone optical scanners are replacing CMM’s for some applications.   

Use of CMM’s

CMM’s are used for various purposes, including quality inspection, field inspection, creation of drawings for existing equipment, and measurement of worn parts for refurbishment.  

CMM’s can quickly and accurately obtain measurements for many geometrical features that would otherwise be very difficult to measure.  For example, a CMM could easily check the profile of a surface to determine if a turbine blade is manufactured to the proper curvature.  Other properties commonly measured with a CMM include flatness, straightness, circularity, cylindricity, the profile of a line (of a complex curve), concentricity, symmetry, and axial location/orientation.   

Operation and programming of coordinate measurement machines are typically conducted by someone who has received specific training for that purpose.  Training materials and courses relating to CMM operation are commonly available from the equipment manufacturer.  Since CMM operators are often checking the dimensions of a part against a drawing or design, the ability to read technical drawings and knowledge of geometric dimensioning and tolerancing, or GD&T, is essential for these personnel.   

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Key Take-Aways