There are several different styles of coordinate measuring machines, each fit for a specific set of applications. Here are a few examples.
A coordinate measuring machine (CMM) provides ultra-precise verification and documentation of manufactured parts and assemblies. They are capable of measuring a number of factors, including size, form, dimensions and relationships between features. The then verify these measurements against the part and design definition established by a CAD file.
In today’s high-quality, low defect manufacturing environment, verifying features of a part—parallel, concentric, flat or at the correct angle relative to other features—is a critical function that’s difficult, time-consuming for a human worker to accomplish correctly in a repeatable way. Some measurements are impossible for a human to accurately conduct.
Three-dimensional contoured shapes cannot be verified by hand tools such as micrometers, calipers and gage pins. Many relationship measurements such parallelism and concentricity require considerable time to set up and measure with dial indicators or other manual techniques. Calipers and micrometers touch the feature being measured in only two points of contact. Measurements vary due to the inspector’s grip on the gage and workpiece, and readings could vary from one inspector to the next.
CMM Design Overview
There are several different types of CMM, each made to handle different part sizes and accuracy requirements as well as budget and other practical considerations. Here’s a look at some common options:
Bridge Style
The most common option, named for its upright carriage structure of two vertical beams that support a horizontal (bridge) beam. A servo motor moves the bridge across the table with its position is tracked by precision measuring scales—fixed, coded tape read by sensors on the carriage. Old analog systems have been replaced by digital scales using a hexadecimal data stream.
Suspended from the bridge beam is a motorized vertical beam (Z-axis) that uses a separate movement scale to bring the probe head and probe to the part being inspected. They measure parts as small as a microwave oven to parts as large as a very small car.
Bridge CMMS are accurate, readily available at moderate cost and are common enough to make service, expertise, consulting and parts easy to source. However, the bridge partially obstructs access to the inspection area, limiting the type of parts that can be inspected compared to other styles of CMM.
Cantilever Style
This CMM design has a single guideway on one side with a movable crossbeam jutting out over the inspection area (X axis). The vertical probe support beam moves along this cross beam (Y axis) and moves the up or down (Z axis) to deliver the probe head to the inspection area. This design gives better access to the inspection table compared to the bridge style, but is slightly less accurate. This design is better for smaller parts or when better part access is needed.
Cantilever CMMS offer easy access through three out of the four sides of the inspection table, which is better for automation and part change-out. They are generally less expensive and are better suited to smaller parts, but this comes with the trade-off of reduced accuracy.
Horizontal Arm Style
These are an older style of CMM but are still used today if the application calls for them. It is a lower-cost solution that has historically been used for whole-car body panel measurement where the attach points and relationship between panels must be verified. These have generally been replaced by non-contact scanners in body panel and stamped part contour analysis. A common variation of the horizontal arm setup is pairing two towers with probing heads, usually mounted to the floor instead of a granite table, giving them a large measurement range.
These CMMs are relatively inexpensive and provide unimpeded access to the measurement area, but it’s getting harder to find parts for older table-based versions. These tend to be less accurate and have mostly been replaced by laser or white light scanning technologies.
Gantry style
These are generally used for larger parts but smaller versions are available. Big versions have two large steel side structures with a crossbeam running on top-mounted guideways down the length of those structures. A vertical beam is suspended from the crossbeam, where it moves up and down to carry out the Z-axis movement. Big gantries are mounted to the floor and smaller ones use granite surface plates.
The stable, rigid design of gantry CMMs make them accurate for large format parts and are common in the shop floor environment. But they offer the least amount of access to the table and inspection area and are cost-prohibitive to use in any application that does not specifically require one. Laser trackers have generally replaced gantry units for portable CMM inspection of large parts and tooling.
Mobile and Shop Floor Styles
A typical CMM installation calls for a climate-controlled metrology lab isolated from the shop to reduce inconsistencies, temperature variations and thermal expansion found on the shop floor. However, having a CMM in a work cell allows you to add automation for efficient or lights-out operation. And bringing the CMM to the work avoids the cost involved in environmentally controlled rooms and dealing with high voltage requirements of typical CMMs. These newer CMMs account for temperature variations in the shop using specific hardware and software features.
Using shop floor models means higher throughput potential compared to processing parts through the CMM lab. Any issues identified by a shop floor CMM tend to be resolved faster. But trained CMM experts may not be as accessible on the shop floor as they are in a CMM lab. The CMMs are exposed to environmental and shop impurities that can damage them. Control, maintenance, machine accuracy, and calibration status may be more difficult to maintain.