Confused over which CMM to buy? Here are a few tips for the first-time buyer.

DMIS-based CMM software that uses native CAD files to create inspection routines will reduce programming time, especially when dealing with complex parts. Photo: LK Metrology

CMM accuracy is one of the most important questions that needs to be answered. The answer may be found in GR&R tests. Photo: Carl Zeiss IMT

Without question, there are several, perhaps dozens, of coordinate measuring machines (CMMs) available on the market that will be a great gage for any company, no matter its size. In fact, some CMMs may do too much. Too much?

Too much if the CMM is meant for parts bigger than a company will ever produce, or will reach down to tolerances that a company will never have to meet or have software that will not be needed until after the software package has been eclipsed by newer software tools.

But, with the plethora of CMMs available, how does a manufacturer, especially a small company with little extra investment capital, wade through the voluminous options to find the correct tool? The best place to start is within the company to answer some basic questions.

Why do you want it?

Indeed, determining if a company really needs a CMM is probably the most important decision. There are many different reasons to invest in a CMM. Some companies, according to Gerrit deGlee, area sales manager for Carl Zeiss IMT (Maple Grove, MN), want to digitize unknown parts to reverse engineer products. Still other companies will use the machine as a selling point. "Some customers sell the parts they produce with an inspection report as evidence of quality," he says. "It does become a selling point; you actually see job shops feature their inspection gage in their advertisements."

Other companies have customers that require them to have a CMM, says Jeff Walker, marketing manager for LK Metrology Systems Inc. (Brighton, MI). "Their customers are pushing quality down to them, and for the customer to be comfortable with what the supplier is doing with quality, the supplier has to have this type of equipment to inspect the product."

Another reason, and hopefully the best reason, Walker says, is that the company has recognized a gap in their quality system and wants to answer the most basic question, "Are parts meeting specifications?"

"They might not be getting good results," Walker says, "or it might be that they are not utilizing their employees efficiently enough. Somewhere in their quality system there is a gap and they feel that a CMM can fill it because it is a programmable device and it is flexible."

Perhaps closing that gap means improving productivity, Walker adds. Perhaps closing it means reducing costs from scrap and rework, says deGlee.

Many companies need it because they have recognized a need to contain bad parts. This may mean checking outsourced parts to ascertain if the incoming parts meet specifications. Or, companies can use a CMM for "production process control," says deGlee, "measuring workpieces that their machine tools produce, and then trying to monitor that process and intercede before more bad parts are produced."

Then what?

If any of these scenarios strike a chord, and a CMM seems to be the answer, it may be tempting to start shopping, but at this point it is important to figure out what is needed in the CMM. The right CMM is not necessarily the most expensive tool on the market, and most likely it is not the least expensive. The right CMM fits the production requirements of today and perhaps tomorrow.

The first question, agree both Walker and deGlee, is what accuracy of machine is needed. In general, the more accurate the CMM, the higher the cost. Although, many experts say that many of today's machines are as accurate at the low end as they were at the high end just a few years ago.

Companies need to look at their production tolerances and the type of geometry that needs to be measured. One way to do this is to conduct a gage repeatability and reproducibility (GR&R) test. Essentially if there is a tolerance to manufacture the part, the company wants to use as little of that manufacturing tolerance in the gaging. "A rule of thumb is when monitoring a manufacturing process, you want a GR&R in the area of 10%, that takes into consideration the restaging of the part," deGlee says. "To achieve a 10% GR&R, a gage, any gage, has to repeat within about 4% of part tolerance. In other words, the average repeatability of the part has to be within 4% of part tolerance to achieve a 10% GR&R."

Another aspect to consider is the size of the machine needed. To answer this, a company must know exact sizes of all the parts that it makes. This may look like a no-brainer-get the CMM with the biggest workspace-but simply buying bigger for bigger's sake will mean the company will forsake valuable floor space and spend more money than necessary on a machine that is not right for their product or their shop.

Walker says that that can be done in several ways. The company can look at what they produce and get a general feel for the volume, or they can use the machine tool that they are making them on as a template. He adds that companies should categorize their components into certain size families or size groups, and then evaluate the inspection requirements for those sizes.

"If they have different groups of parts and one part falls within a 40-inch cube, and they make 25 of them, and they also make two components that need a machine that can do a 60-inch cube, then they need to determine if it is worth it to get the bigger machine," he says. "If 85% or some large percentage of components that they are making will fit in a 40-inch cube, do they really need to buy a machine that is capable of doing the 60 inches and spending that additional money? Or is there another alternative inspection route that they can use on their components?"

The only caveat, Walker says, is if the bigger components are critical to their business. "They may only inspect 10 of these a year, but those 10 are critical to what they do and they have to be right," he says. Then, it may be worth getting the bigger machine. Until the company knows exactly what it produces, it will not know if the extra expense is worthwhile.

Part geometry

The geometry of the part produced is another key factor to consider because that will determine what type of sensor is needed on the CMM.

"Knowing what type of sensor is needed to measure the part is critical," says deGlee. "For example, a company may want to look at form geometry, and in that case they may want to look at a scanning type of sensor. For thin material or very small details, an optical sensor may be beneficial. Different CMMS have different types of sensors available, and those questions have to be considered when looking an a CMM."

Many of today's CMMs can be outfitted with multiple sensors including video and other optics, lasers, laser scanners, as well as a variety of contact probes such as touch trigger or scanning probes. Getting only what is needed can help keep costs down. If a company only needs to inspect size and position and not form, then simple touch-trigger probes may work. If form measurement is needed, Walker says that users will likely need to go to analog probing technology. "It is a little more expensive and if they don't need it, if they only need size and position, then the touch-trigger technology will work well and help keep the capital costs down."

Knowing what accuracy and inspection needs and part geometry also helps determine the type of machine to buy.

For instance, the mechanical platform is a consideration especially if a company is starting to get into analog probing, Walker says. If an analog probe is needed, the CMM will need a more rigid structure. "The demand that the probe always be in contact with the part and always under acceleration means that the machine has to be more sound and more rigid than other types of machines," Walker says.

Software is a key consideration

Software is something that many companies will spend the most time evaluating because it will be used every day, Walker says. Just as with any other aspect of a CMM, it is important to first understand what is needed.

Some customers may want computer-aided design (CAD), for example. But, do they need it?

"There are a lot of people out there that want CAD but don't really need it," Walker says. "Do they need to pay for it when maybe they can wait two to three years to get CAD capability? In that time, a company is probably going to replace the software anyway.

"If you are not using the software today, or don't expect to in the next couple years, then there is probably not a reason to invest in it," Walker adds. "If customers are demanding it or they are creating CAD files internally, then probably they want to invest in it."

Software can be a cost of ownership issue. "Once you get to large shops," Walker says, "by five to seven years into the life of the machine, the company has probably invested more in part programs than they did in the initial investment in the machine. That is a cost of ownership issue that needs to be taken into account."

Cost of ownership goes beyond the initial costs. Other factors that need to be considered include training, servicing, installation and the ability to upgrade.

Where is it going to go?

Once upon a time, in the not too distant past, CMMs were only found in the laboratory and were run by quality assurance professionals, if not metrologists. That is changing. In fact, Walker says most of his company's sales are to the manufacturing department.

Many companies today want the machine to go as close to production as possible. That requires a number of upfront considerations-temperature being one of the most important. "Every gage has a thermal specification," says deGlee. "Parts are affected by temperature. As the temperature increases or decreases, the part shrinks and grows, and CMMs do the same."

Companies should study the temperature of their shop, including variations from hour to hour. After the thermal range is determined, it can be used when looking at a particular CMM's thermal range. This figure is often in specification sheets, but if not, buyers should ask.

"This will tell you if the CMM will work in this environment, or if the company has to build a laboratory to house the machine, which will have an impact on the cost," deGlee says.

Humidity is another factor to consider because some machines are more susceptible to this, says deGlee. "Typically, granite-based CMMs are hydroscopic, meaning that the granite acts like a sponge when humidity increases and it actually swells, which can cause error to get into the measurement. This can be accounted for. Typically, CMMs have a humidity specification."

Dust and grime are two other environmental factors, although many machines have been hardened to withstand these potential climatic problems. Still, if uncertain, it is important to question the robustness of the machine.

Another potential problem could be vibration. Most CMMs have a vibration isolation system, and CMMs will usually have a vibration specification. "For a relatively inexpensive cost," deGlee says, "you can do a vibration analysis in your factory, and the inspector will check various areas to determine the best place in the factory."

Time to shop?

After answering the questions, it may be time to match CMMs with needs. Specification sheets can help and many are available at a vendor's Web site.

Don't take them as gospel, however. Specification sheets may not necessarily reflect what the machine will do after it is installed. CMM manufacturers describe accuracy in terms of standards such as a B89 or ISO 10360, but it may be difficult to take the accuracy on the specification sheet and automatically expect to get that in the shop.

"Using B89, you can compare spec sheet to spec sheet, but you can't take B89 and compare it to your process and your requirement, because it is a different type of test," Walker says. "You have to basically figure out what you want as far as tolerancing is concerned and see if that machine can inspect to those limits."

Still, deGlee says there may be a couple things to consider. For instance, is accuracy specified over a temperature range? If third-party products such as sensors are installed on the CMM, is this reflected as a separate line item on the spec sheets? Or, was the accuracy specification generated with a sensor that a company won't use in the shop?

"You may see that accuracy is determined using a 10-millimeter stylus, but 90% of your applications require a different sized stylus," deGlee says. "What actual tooling will be needed to measure the part? You may need to add up all these different uncertainties. On some competitive sensors, as you double the length of the stylus, you double the uncertainty of the sensor."

So, specification sheets can help, but companies still need to see the CMMs run. Trade shows are good places to see them, and many vendors will arrange for tours to see them "in action." Still, customers should be wary. CMMs at a trade show will not operate the same as it will in a factory. And, the parts being measured are different. Before buying, request a GR&R test using actual parts to ensure specifications can be met. Armed with this knowledge, the potential buyer will more likely get the right CMM for their company.

Tech Tips

TECH tips

1. Before shopping, determine the required tolerances, the size and shape of components, and the location where the CMM will be installed.

2. These factors will determine the size of the machine, the sensors needed and if it will need to be enclosed.

3. Figuring out what is needed within the company will cut the field of possible CMMs to a more manageable level.