CMMs: Choosing the right one for your shop
- February 9, 2002
by Shop MT staff
Quality is critical in manufacturing and coordinate measuring machines play an important role in ensuring that quality.
But how do choose the CMM that’s right for your shop? Shop Metalworking Technology put together a short selection guide based on information from industry suppliers, including Brown & Sharpe (Hexagon Metrology) Carl Zeiss, Faro, Mahr Federal, Mitutoyo and Nikkon Metrology.
Two important questions to answer are "what are you measuring and where are you measuring," advises John Lewis, marketing maanger with Carl Zeiss Industrial Metrology LLC, Maple Grove, IL. For example, for 3D prismatic parts, you would need contract measurement with a scanning sensor and ruby-tip sylus. For a part with unique angles, "an articulating arm would provide the best access to measure these surfaces. For large parts with very deep features, a fixed head sensor is best since it can support the extensions to reach into the part and measure them accurately."
For parts with fine structures or materials that could deform with contact methods, Lewis advises optical measurement as the best solution.
Different designs for different needs
While it seems obvious, an important starting point is to consider the type of CMM that suits your quality needs. CMMs are available as vertical and horizontal fixed units - e.g. bridge style, cantilever and gantry models - and portable units - articulated arms and laser trackers. CMMs are also offered in manual and automated models too and of course as shop floor or inspection room type units.
"Choosing the right machine depends on present and future measurement needs," explains Fritz Jordaan, support service manager for Mitutoyo Canada Inc., Mississauga, ON. "Choosing between manually driven CMM’s and CNC/DCC driven machine, depends on the size of parts, the amount of throughput of parts expected, the complexity of the measurement and whether the operators workload requires them to be assigned to other tasks making them more efficient."
As a general rule, fixed CMMs offer higher precision measurements and can be linked to a CAD model. These units are typically found in a temperature controlled room, which means products must be taken off the line and brought to the CMM. In some cases, the CMM may be located near the line, but products will still need to be removed from the line and placed on the CMM. Vertical CMMs (the probe attached to the vertical axis) are available in many sizes and handle workpieces from engine blocks to large aircraft bodies. Horizontal CMMs (the probe is on the horizontal axis) are suitable for large part measuring, such as automotive bodies, according to information in an online article by Brown and Sharpe ( "Selecting Your CMM”, Manufacturing Engineering June 2003 Vol. 130 No. 6).
Portable CMMs, by their nature are more flexible, lighter weight and less expensive than their fixed cousins. According to Faro, an articulated arm - which determines and records the location of a probe in 3D space and reports results through software - is useful for dimensional analysis, CAD-based inspection, on-machine verification, first-article inspection, alignment and reverse engineering. Laser tracker applications include alignment, installation (lay out or level machine foundation), part inspection, tool building and reverse engineering.
It may be a simple and obvious point to make, but remember that the size of your CMM should be determined by the largest part you need to measure. Words of advice from Brown & Sharpe’s Gary Card, technical support services, in a Manufacturing Engineering online article, "consider choosing a machine whose X, Y and Z measuring ranges are twice the width, length and height of the largest part you need to measure.”
A critical component of a CMM is software. Most suppliers have their own software but there are some who do not. Evaluating software can be a challenge if you are not an experienced CMM operator, but there are some general rules of thumb to consider that can help assess if the software program is right for your measuring needs.
First, the software should be easy to use and should include features such as real time SPC, the ability to export to CAD, the type of platform on which the software works, service contracts (are upgrades available) and most importantly, technical support because even with easy-to-use software, there can be challenges navigating through the software and when something does go wrong, you need to ensure you have sufficient support from your supplier to minimize downtime.
Mitutoyo Canada's Fritz Jordaan, offers some guidance when it comes to selection software.
"Operating software should allow operators to do basic and complex measurements. It must enable operators to program their CMM from a CAD file. It should assist the operators to determine the fit of a part when comparing it to the CAD model. It should provide data collection for statistical analysis, and also provide both 2D and 3D measurements and most of all, the software must be easy to use."
Whether your CMM is an a controlled room or on the shop floor, the environment within which it operates will impact performance. Suppliers typically will provide the best temperature range and temperature variations over a period of time and day when a CMM performs best. Other considerations include floor vibrations, which can also negatively impact CMM performance. Manufacturers can purchase optional vibration dampening systems if this is an issue.
Most CMMs are built to handle air quality conditions and temperature changes in controlled rooms or labs but when you have a CMM on a production floor or inline, "then things change," warns Carl Zeiss' John Lewis.
"You can put a traditional machine into an enclosure on the production floor to protect it, or you can choose a CMM built for the production floor; one that has enclosed guideways, bearing and scale covers, an anti-vibrtion system and also uses materials that do not expand or contract."
The best way to assess the performance of different brands of CMMs is to study the performance standards. While there are a few out there, the most common is ISO 10360-2.
Suppliers suggest that manufacturers ensure they compare similar specifications from different CMMs to make a fair comparison on performance. L.S. Starrett’s Steve Meredith, says manufacturers should also remember accuracy when evaluating CMMs. "Accuracy requirements depend primarily on the tolerancing of the parts to be measured,” he writes in an online Quality Digest article ( "Choosing a CM: Selecting the System that’s Best for You”), adding that "linear accuracy…is an accuracy reading for each axis independently…each machine axis must be built mutually perpendicular to the other two. Therefore, volumetric accuracy better represents a CMM’s true overall accuracy.
Consider the entire package
Suppliers concur that manufacturers should look at the entire package and assess the construction of a CMM (is it durable?), consider the software (is it user friendly and offer the key functions you require?) and the probing systems (accuracies of the probe head and the probe and different stylus modules).
More importantly, consider the cost of ownership, says Mitutoyo's Fritz Jordaan.
"Owners of CMM’s need to be aware that there are many moving parts in any CMM and that cost-of-ownership plays an important role in the operation of the CMM. Dealing with a CMM supplier that is able to provide the necessary maintenance, service parts and accredited certification to the CMM, is important ensuring there is minimal down time."
Top image: Carl Zeiss's Acurra II multi-sensor capable CMM.