High atop the pyramid of gages we mentioned in our post about gage selection… above the pins and blocks and calipers, above the micrometers and the air gages... sits the Maharajah of measurement, the Pasha of precision: the Coordinate Measuring Machine (CMM), the ultimate device for accurate and repeatable measurements.
A CMM is a computerized, programmable machine used to measure the size of a sample in three dimensions. It looks like an oversized granite table with a robot arm mounted to a bridge or a gantry that quietly floats around, touching the tip of a slender touch probe to the sample sitting in the middle. And then it beeps. And then it beeps again, sounding eerily like the TV-show version of a hospital operating room. Each beep represents a point noted on the part sample. The attached computer creates a 3-D map of every point it locates and plots those points against an internal CAD model of where those points should be versus where those points are. Then it tells you if your part is good or bad.
CMMs are capable of measuring complex parts with speed, precision, and repeatability. But they can also be finicky and demanding, requiring their own air conditioned rooms and highly trained programmers to achieve those results. Devotees swear by CMMs as the be all and end all of measurement tech. Others will suggest that while it has its uses, much of what a CMM does can be done by a skilled layout tech on a surface plate. As with most things, the truth lies somewhere in the middle.
So, given all the hype about CMMs, how do you know if you need one?
Properly maintained, calibrated, programmed, and operated, your CMM can give you accurate and highly repeatable measurement results on even the most challenging and complex parts. But that repeatability and reliability still have their limitations, and they come at a cost which may or may not justify the investment for every organization. So let’s take a general look at what you may need to spend to get it.
How much money do you have in your quality budget? CMMs are not cheap. A small CMM with modest capabilities can easily set you back somewhere in the $30,000 range. Bigger is usually better, up to a point, and prices can easily climb to above $100,000 or even $250,000. Other costs to consider:
Programming, Training and Maintenance
Once you have completed site preparation and installed and calibrated your new CMM, the operating expenses will kick in, including:
Some training is provided with your CMM, but that will only cover the basics.
The quality of a CMM’s measurements is directly related to the ability of the programmer. Experienced programmers understand the programming algorithms CMMs use to calculate where the points they measure actually are. They understand how many points to take for various features under different conditions in order to assure accurate results. They know the machine’s strengths and weaknesses. And since not every CMM uses the same programming and operating systems, your programmer will need to know the language of your particular machine. What this all means is that programmers are in demand and they are not cheap.
CMMs typically require an annual fee for maintenance, programming updates, etc., often called a seat license, which may cost several thousand dollars a year, plus you’ll need to fork over several thousand dollars each year for calibration. The most inexpensively priced parts of a CMM are often the probe tips themselves, which are typically under $100 each, depending on size and type. Just remember that you might need 20 or 30 to be able to measure all the different samples that come your way.
Inspect Parts Faster
Inspection times can be up to 10 times faster than manual measurements, cutting labor costs drastically. Further, that speed can reduce the amount of times machines lay idle while they are awaiting first-piece approval. For example: a complex part with a hundred characteristics might take an entire day to check on a surface plate with hand tools, but a CMM can do it in 15 minutes.
So if you assign a cost of $50 per hour for labor and overhead to inspection time, a roughly seven-hour reduction in inspection time just saved you $350. Multiply that by just one inspection per day and it adds up to more than $90,000 in just one year. Add in the revenue generated by having a machine cutting chips seven more hours per day because it isn’t sitting and waiting for the go-ahead from quality, and the CMM starts to look less like a capital expense and more like a tool that can help reduce costs.
Meet Demanding Customer Requirements
A CMM can help you meet increasingly demanding customer gaging requirements. Imagine you are in a machine shop machining castings into finished parts with tight tolerances and lots of critical features, maybe for a medical or a military customer application. Chances are good your gaging will have to meet Gage R&R or Cpk requirements, or both.
Assuming your CMM is properly calibrated, operated and maintained, the program is well-written and you have a well-thought-out fixturing program, you should have no problem meeting a 10% Gage R&R requirement, and you should be able to trust the Cpk numbers you are getting. Sure, you can do that with hand gages, too, but rarely as quickly and easily as you can with a CMM, which by its computer-controlled nature, is inherently more repeatable.
Produce Better Parts
A CMM can help rescue scrap and rework. This one is simple. Accurate and repeatable measurements help you produce more good parts and fewer bad ones. That leads to reduced scrap and rework, EVERY DOLLAR of which goes right to the bottom line. It’s impossible to assign a general number to how much your company could save, but the logic here is bullet proof.
Set Your Company Apart
CMM technology is a strong selling point for your company. Let’s be honest here. Take a few minutes and look at the websites of serious contract manufacturers. Check out the “Quality” section. Chances are almost all of them will have at least one CMM and will feature it in their equipment list. Why? Because customers and prospects like to see that they have a CMM.
Whether they should or not, people tend to trust the results you give them if they come from a CMM. And to a certain extent, they are right. Generally speaking, CMM results are reliable and accurate. But even a CMM has limitations, and it’s good to know what they are.
CMMs are ideal for measuring:
GD&T callouts. CMMs are terrific at measuring GD&T callouts like true position, flatness, parallelism and perpendicularity, measurements which are often difficult if not impossible to make using plate techniques and hand gages.
Angles. The same applies to angles. Plate measurements using a sine bar and gage blocks can be accurate, assuming you have an inspector who knows how to use those tools . But a sine plate measurement takes a long time, and an angle you can measure on an optical comparator is limited in accuracy and repeatability, even with a vernier.
Radii and circles. Your CMM is ideal for measuring all things round: holes, slots, cones, radii, spherical radii, roundness and concentricity. The time savings and increase in accuracy the CMM will give you will be surprising. The same applies to calculating hole-to-hole center-point distances, especially if they are on different planes or axes. There are some limitations, but we’ll get to them in a bit.
Profiles. Profiles are almost impossible to measure using plate techniques, but a scanning CMM can make short work of them, checking literally thousands of points in a few seconds.
CMMs are not ideal for measuring:
Features smaller than its probe tips. A CMM can’t measure something smaller than its smallest probe tip. Many of the most frequently used CMM probes have ruby tips: very precisely machined ruby spheres of different sizes, some as small as .020”. But the CMM requires a certain amount of room for the probe tip to approach the feature being measured in addition to the size of the ball. So an .030” probe tip might not be able to probe a hole smaller than .060 or .065”. The same applies to radii. It can’t measure a radius smaller than the ball itself.
Rough surfaces. Rough surfaces like castings pose a problem for CMMs. The roughness may prevent the CMM from making clean, precise contact with the sample part.
Threaded holes. Generally, a CMM cannot be used to measure threaded holes, principally because threaded holes rarely start at the same spot every time. And since the CMM’s computerized program needs that consistency, threads are often impossible to measure.
What a difference a micron can make
But there is a caveat to everything you do with a CMM. The mechanics of how a CMM works are fairly straightforward, and use well-known and proven technologies. But you need a programmer who understands how they work and which mathematical algorithms a given CMM uses. Different languages like PC-DMIS, Open-DMIS and Calypso all work differently and cannot be programmed the same. Just remember that people trust a CMM implicitly, so you have to be sure that you get it all right: the programming, the calibration, the fixturing, the environment… everything. People often use the CMM to validate measurements from other gages. But it is often impossible to use another gage to validate the CMM.
One question to ask is: even if a CMM can measure a given feature, should it? Let’s say you need to check a dimension of 2.500” and you have a tolerance of +/- .030. Should you have the CMM check it? Maybe not. The tolerance is certainly large enough where you could use a caliper or a 2-3” micrometer instead.
While deciding to do that one measurement manually won't make a big difference on CMM productivity, hundreds of such small decisions can. Save your CMM for measurements you need it to make. When a hand gage can do the job, it might make sense to use one.
Making the decision to buy and use a CMM is not as simple as saying, “Yeah, I want more accurate and repeatable measurements.” It can require an organization to make substantial investments in money and time to take full advantage of the benefits a CMM offers.
As with any investment decision, evaluate the costs of both ownership and operation against the value provided by owning and using a CMM. Then consider the procedural and culture changes that a CMM will bring about. Add in considerations that are less tangible, like the goodwill of your customers and the increase of productivity a CMM makes possible. Are skilled programmers available and are you willing to pay them what the market says they are worth? Do you have the space to build a separate controlled room for the CMM?
Generally speaking, the tighter the tolerances and the more complex the parts you make, the more compelling the case for buying a CMM becomes.
Sooner rather than later, though, all these questions and decisions are likely to be moot because your customers will require it. Not by explicitly requiring you to have a CMM, but rather by requiring you to make measurements you just cannot make any other way.
My recommendation to you: Embrace the inevitable. Despite all the ifs, ands, or buts that precede this paragraph, a CMM can transform your business in ways you might not anticipate. Don’t think of it as just a machine. Think of it as a tool, a force multiplier, the “tip of the spear” in your ongoing battle to continually improve quality, reduce costs, and increase business.
About the Author:
Rich Silverman is a veteran of close to 20 years on the front lines of quality in both TS 16949 automotive and AS9100 aerospace environments. At various points in his career he has been responsible for incoming, in process and final inspections; gage calibration and repair, internal audit, and APQP / PPAP. Armed with "just enough knowledge to be dangerous," he admits that First Article Inspection Reports are among his favorite job responsibilities. A life-long resident of Northeast Ohio, in his spare time he takes advantage of his college degree in Journalism to work as a freelance writer.