Types of CMMs: Bridge, Gantry, Horizontal and More

Q: How is a Bridge-type CMM different from a Gantry-type CMM?

A Bridge CMM has a rigid bridge structure supported on both sides, with the probe or carriage moving in an XYZ configuration. It's typically more compact and very precise. A Gantry CMM is like a large version of the bridge style, built for large parts, often floor-mounted, with a span that allows measurement over big workpieces. It's less portable, requires more space/foundation, but gives capacity for large measurement volume.

Q: What are the benefits and trade-offs of a Horizontal Arm CMM?

Benefits : good for measuring large, flat or awkwardly shaped components; the arm can reach over parts or around obstacles; useful when the part cannot easily be moved into a bridge-style machine. Trade-offs : longer arms may introduce deflection and reduce accuracy; environmental factors like temperature and vibration can have more effect; speed and repeatability may be lower than more rigid fixed structures for tight tolerances.

Q: How do I choose the right type of CMM for my needs?

Key considerations include: Part size & geometry : large parts require larger measurement volume (gantry, horizontal arm), smaller precision parts may suit bridge or cantilever types. Tolerance/accuracy required : tighter tolerances favour more rigid designs (bridge, gantry) or better environmental control. Environment : shop floor vs metrology lab; temperature, vibrations, cleanliness. Throughput & usage : frequent measurement means reliability and automation may matter; occasional or field jobs may favor portable types. Budget & maintenance : initial costs, space, calibration & maintenance costs.

Coordinate Measuring Machines (CMMs) come in various sizes and types. The machines can be distinguished on various factors such as physical structure and axes of the machines, probing systems incorporated and type of control they have. Various provisions and accessories can make these machines more versatile.

In this article we will have a look at the various types of these machines. In the mean time you can learn more about their principles and operation in our Comprehensive Guide to Coordinate Measuring Machines (CMMs) →

Table Of Contents

Physical Structure and Axes Systems
Probing Technology
Frequently Asked Questions (FAQs)

Physical Structure and Axes Systems

Bridge Type

The configuration of these machines is bridge type, a beam is supported by two vertical columns that can move in X, Y, and Z directions. The Z column is supported by a counterbalance to make the movement smooth and accurate. These CMMs are the most common type of machines being used in the market. There bed sizes can vary from 500×500 to more than 2000×3000. Used for the inspection of small to medium sized parts that need high precision; machining parts; laboratory/metrology room inspections.

Gantry Type

These are like the bridge type configurations, however, these are used for larger volumes of inspection and for bigger parts. Instead of a beam structure overhead, these machines utilize a gantry (rail type) supported by two vertical columns usually mounted on floors. These are used for Large/heavy components; aerospace frames; large automotive bodies; ship parts. These are very expensive machines.

Cantilever Type

These machines have probe mounted only on one side (side of base), rather than along beam. They allow more access from three sides. It is used for comparatively simpler parts. These are less rigid than gantry type CMMs.

Horizontal Arm CMM

There is a vertical column on the machine, and a horizontal arm is attached with the probing system. These CMMs are usually used for the inspections of sheet metal parts in automotive sector where access from the top is difficult. They have lower accuracy then bridge or horizontal type machines.

Image of Horizontal Type Hexagon CMM — Horizontal Type Hexagon CMM

Portable Arm-Style Articulated CMM

These are also known and PCMMs. Recently, they are becoming more popular alternate to the fixed type CMMs as they can move around anywhere. There working envelopes are also large going up to 4000 mm. However, the accuracy of these machines is less way less than fixed type CMMs. Their articulated style allows them to be used in precision assembly and alignment tasks as well.

Image of FARO Portable Arm CMM — FARO Portable Arm CMM

Optical / Non-Contact CMMs

These are optical CMMs with non-contact sensors like camera, laser lights, distance meters, videoscopes etc. These sensors can also be purchased in addition to the conventional probing systems. These are suitable for soft materials, freeform surfaces where touching the part might be damaging. These are fast but the accuracy is low as compared to other sensors.

Image of Renishaw Non Contact Probing Head — Renishaw Non Contact Probing Head

Custom Designed / Hybrid Specialized CMMs

There are many applications in specialized industries for specific measurements. The custom built CMMs can allow for integration with robotic arms on production and assembly lines, where they can measure seamlessly. They have extremely high build costs and complexities, but they are robust to the specific task and very high accuracies.

Image of Robotic Arm CMMs — Robotic Arm CMMs

Probing Technology

The method of sensing the surface determines several CMM types, including contact and non-contact systems.

Contact Probing Systems

The CMMs with contact probes have a touch and trigger probe, that senses when in it contacts the workpiece. The most popular touch and trigger probing system is the TP20 probe from Renishaw. In the past, probing systems were manufactured only by Renishaw and it was the industry leader. However, lately the CMM manufacturers have developed their own probing systems. These probing systems have single and multiple styli for taking measurements.

Non-Contact Probing Systems

The non-contact probing systems consists of optical sensors. One of the noncontact probes are OTP6M probe. It allows for noncontact measurements. It is used where parts are sensitive to touch like extra polished mirrors and alignment sensors on navigation units.

Continuous Probing Systems

The continuous probing systems can scan (open and closed loop) profiles and contours whose inspection through conventional means is a very difficult task. SP600 and SP25 both from Renishaw are popular choices in these types of probing systems. They provide highly reliable results for profile inspections.

Frequently Asked Questions (FAQs)

What does “CMM” stand for, and what is it used for?

CMM means Coordinate Measuring Machine. It’s a precision tool used in manufacturing and inspection to measure the geometry of physical parts—like the dimensions, angles, and surface shapes—by probing or scanning points on the surface. It ensures parts meet tolerance and quality specifications.

What are the main types of CMMs?

Some of the main types include:

– Bridge CMM
– Gantry CMM
– Cantilever CMM
– Horizontal Arm CMM
– Portable/Articulating Arm CMM
– Optical / Multi-sensor CMMs

How is a Bridge-type CMM different from a Gantry-type CMM?

A Bridge CMM has a rigid bridge structure supported on both sides, with the probe or carriage moving in an XYZ configuration. It’s typically more compact and very precise.

A Gantry CMM is like a large version of the bridge style, built for large parts, often floor-mounted, with a span that allows measurement over big workpieces. It’s less portable, requires more space/foundation, but gives capacity for large measurement volume.

When is a Cantilever-type CMM useful?

A Cantilever CMM is useful when you need access to the part from three sides, or when the part is easier to load/unload from one side. It offers greater accessibility and is often chosen for parts of medium size or when space is limited. However, it may be less rigid than full bridge or gantry designs, so accuracy might be somewhat lower under certain conditions.

What are the benefits and trade-offs of a Horizontal Arm CMM?

Benefits: good for measuring large, flat or awkwardly shaped components; the arm can reach over parts or around obstacles; useful when the part cannot easily be moved into a bridge-style machine.

Trade-offs: longer arms may introduce deflection and reduce accuracy; environmental factors like temperature and vibration can have more effect; speed and repeatability may be lower than more rigid fixed structures for tight tolerances.

How often should a CMM be calibrated?

Calibration frequency depends on factors such as how much the machine is used, the environment (clean room vs shop floor), the accuracy required, and any regulatory/industry standards. As general guidance:

– Once per year for machines in controlled lab or clean room environments.
– Every ~6 months for machines on the shop floor or under harsher use.
– Also, manufacturers often recommend full calibration after a defined number of operating hours (e.g. ~2000 hours) or if machine behaviour changes (after a collision, drift, etc.).

What are probe calibrations, and how often should those be done?

Probe calibration refers to verifying and adjusting the probe or stylus (or tip) so that measurements taken by the probe are accurate. Differences in probe tips (length, tip diameter), damage, or wear can affect results. Some labs calibrate probe tips weekly, bi-weekly, or every few weeks, especially if they are used heavily or in demanding environments. If a probe tip is replaced or the machine is restarted or “homed,” it’s good practice to re-qualify or calibrate the probe.

How do I choose the right type of CMM for my needs?

Key considerations include:

Part size & geometry: large parts require larger measurement volume (gantry, horizontal arm), smaller precision parts may suit bridge or cantilever types.
Tolerance/accuracy required: tighter tolerances favour more rigid designs (bridge, gantry) or better environmental control.
Environment: shop floor vs metrology lab; temperature, vibrations, cleanliness.
Throughput & usage: frequent measurement means reliability and automation may matter; occasional or field jobs may favor portable types.
Budget & maintenance: initial costs, space, calibration & maintenance costs.

What standards govern CMM measurement and calibration?

Some of the relevant standards are:

– ISO 10360 series (for CMM performance: probing error, length measurement error, etc.)
– ISO/IEC 17025 (for calibration laboratories)
– ASME B89 (in some regions, especially North America) for reference.

Types of CMMs: Bridge, Gantry, Horizontal and More