Choosing the right CMM
- March 27, 2002
By Jeff Bourque
Assessing the CMM that will work best for your shop
As many manufacturers know, a coordinate measuring machine (or CMM) is a three dimensional device for measuring the physical characteristics of an object.
CMMs can be manually operated or computer controlled. A probe attached to the third moving axis of this machine defines measurements. Probes may be mechanical or "touch" , optical, or laser. Size is also a variable as the size of the CMM must be large enough to measure or scan the part, which also influences the CMM design. These designs include Bridge CMMs, Gantry CMMs, Horizontal Arm CMMs and Portable CMMs.
With all of these options, how do you know which CMM is best for your application? Knowing how to select the best CMM can save your company a great deal of time and money. By choosing the right CMM, you will be contributing to the efficiency of your organization, and ensuring that the process meets your lean initiatives.
A bridge CMM is composed of three axes, an X, Y and Z in a typical three dimensional coordinate system. Each axis has a scale system that indicates the location of that axis. When scanning, the machine reads the input from the probe or scanner, as directed by the operator. The machine then uses the X, Y, and Z coordinates of each of these points to determine size and position. These CMMs are appropriate for industrial measuring tasks where accurate, repeatable results are required. Entry-level models may require the operator to move the bridge and scanner, while more advanced models are fully automated. Since these CMMs have a table where the part is measured, they are ideally for use with small and medium part sizes.
Portable CMMs,typically handheld units, are best where
there is limited workspace for measurement.
Gantry CMMs are ideal for applications where inspection of extremely large, long or heavy parts is needed. While bridge CMMs are typically used in a quality control lab or area, gantry CMMS are often located on the production floor or cell. They may be standalone, or even located inline as a station. They typically are used in larger scale applications such a measuring a complete car, the aerospace industry, and defense and maritime applications.
Horizontal Arm CMM
Horizontal arm CMMs utilize an open structure that provides a unique access to the measuring envelope, and can be installed in a number of configurations such as floor level, sub floor, or part of an FMS cell. Instead of being mounted on a bridge, the measuring probe is attached to an arm that can be extended into an enclosed area for measurement. Important applications include automotive vehicle bodies, airframes, and high performance production environments.
With portable CMMs, the measuring device is usually handheld and depending on the technology, either attached to an articulated arm or fully handheld with an optical system. These units are used where there is limited workspace for measurement and directly on the shop floor, providing a "measure anywhere" capability, and are truly portable, requiring some configuration when moved. These CMMs offer free movement of the probe, and the ability to rotate the head offers measuring with the ability to move over, under and around the part being measured.
The optical CMM units measure the position of infrared LEDs, by means of linear CCD cameras. Through triangulation, the 3D position of each LED is calculated.
The optical measuring CMMs are available in a portable and a mobile configuration. The larger of these units can measure up to about a 17 m3 envelope that is ideal for applications such as:
- Fixture verification Single part and assembly inspection (body and trim parts)
- Guided assembly of prototypes
- Body closure enhancement
- Human comfort analyzing
- Body & chassis development
- Degradation analysis
One must also take into account the measuring device used to take the measurements. They range from a simple touch probe through advanced multi-stripe laser scanners. The great majority of measurement devices fall into one of two categories â€“ touch probes and laser scanners. Touch probes can vary from a manual, fixed stylus up through a motorized probe head with an extension and styli. Laser scanners also offer a great deal of variation in both the speed of scanning (stripes/second) and the size of the individual scan (50 mm stripe, 100 mm stripe, etc.). For example, if you are using your CMM to check a few spots on a machined piece to make sure that it is within tolerance levels, a touch probe on an arm or CMM will suffice. However, if you are reverse engineering a part or completing a CAD model for rapid prototyping, then a high-speed scanner on an automated CMM is the system that you require.
As the requirements of your measured part increase, so do the requirements for CMM technology. If you application is simply scanning a number of points on a part to ensure they are within tolerance, and you are only sampling the parts on an nth number basis, a simple CMM with a touch probe and stylus is sufficient. However, as the size of the part and the number of data points grow, so will your CMM requirements. If you need to create a point-cloud to compare Part-to-CAD, you will need a laser scanner to do that quickly and efficiently. If it can be done in a lab, you may want an automated CMM with a laser scanner. SMT
Jeff Bourque, manager marketing communications, the Americas, for Nikon Metrology Inc., Brighton, MI www.nikonmetrology.com