When you hear precision cnc machining, what do you think of? Isn’t all turning and milling “precision”? Not quite. With turning and milling, you can make very precise parts relatively quickly. However, it goes much further than that. In this article, we’ll discuss what precision work is, how you can verify that your parts are precise, and how you can make more precise parts in your own CNC machining processes.
What separates regular CNC work from precision CNC work?
Accurate CNC machining is sometimes assumed to mean that the parts made match the specifications/geometry of the intended design very precisely. This is not technically wrong, but a better word for this is accuracy. There is a subtle but important distinction between these two terms, which we will cover in more detail in the next section. CNC machines are extremely capable and make parts that are precise enough for many applications, but sometimes it’s not quite enough. When you add precision to the mix, you can take it to the next level. Which brings us to the real question: what is precision machining?
So what does precision really mean?
Precision and accuracy are often mixed up and used interchangeably. Both are indeed related to quality, but each has its own specific meaning. Accuracy means close to or exactly the target values (which many people think is also the definition of precision). Precision is actually the ability to consistently replicate parts over a large number of units produced. The key is in that “large number of units produced.” Precision machining refers to many or more units. You can have one accurate part, but you have to have many parts to measure and compare to call them accurate. For example, if someone asks to make a single part using precision CNC machining, this is counterintuitive because “precision” involves multiple parts. The image below is a good representation of the difference between accuracy and precision. When you throw darts on a dartboard, you might hit the same area on the board, but it might not be in the middle. This would be precise, but not accurate. This is an important point because even in machining it is possible to make many parts the same, but not within your intended tolerances! It is necessary to check for both accuracy and precision.
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Now let’s talk about precision as it relates specifically to CNC machining. When you order precision CNC-machined parts, what should you expect? Suppliers and machine shops should have procedures in place to inspect your parts and ensure they meet certain quality standards. They measure and compare a representative sample of the parts they produce to complete these quality checks. In the next section, we look more closely at some of these quality processes.
How do you know if your process and parts are accurate?
Measurement systems! Your measurement system is an integral part of measuring and determining the quality and precision of your parts. When you want to improve a process, it is important to be able to identify and measure any issues. This can help you determine where the problem or error is coming from and how to optimize it. The first step is to make sure your measurement system is good enough. Is it measuring your parts accurately over and over again? (Accuracy and precision!) One way to check this is to use meter parts and calibration to make sure your measurement system is working properly. Meter parts are parts that are machined with extreme precision to a known value. For example, you can measure end gauges or pins and then compare the value obtained from your measurement system to the known measurement of that part. End gauges can also help calibrate your measurement system, something that should be done regularly. Some common processes used in development to find measurement errors are GR&Rs and MSAs. These are important to follow because they will help you collect good data that can be used to identify the error in your measurement system or components. MSA stands for Measurement Systems Analysis. By performing an MSA, you can identify where the variation in your measurements is coming from. Is the measurement system consistent and reliable? Are the people taking the measurements doing so consistently? Are your parts made with minimal variation? These are three sources of error that can lead to greater measurement variation. If you find that these are causing a significantly larger portion of the variation, you can address that problem and know that you are getting good measurements. GR&R stands for Gauge Repeatability and Reproducibility. This is a common type of MSA. Repeatability refers to the operator’s ability to get the same measurement over and over again (on the same part). Reproducibility is the ability of another person to get the same measurement over and over again. By identifying the variation within one person’s measurements and from person to person, you can identify the actual variation from part to part. To perform a GR&R, you need 10 parts and 3 people to measure the parts. Each person measures each part 10 times. With the resulting data, there are numerous analyses you can do to analyze the measurement system. You can calculate the components of variation as mentioned earlier: repeatability, reproducibility, operator variation and variation from part to part. You can look at an Xbar or R chart, which are graphical representations of variation by operator. In short, there are many ways to look at the data and find out if your measurements are accurate. Identifying these different sources of variation is so important to increasing the precision of your CNC machining process and parts. For example, if you find that most errors are caused by lack of reproducibility, you can focus on training operators rather than wasting time and money on improving the measurement system.
How can you improve the precision of your CNC machining process?
If we keep thinking about our hypothetical MSA, there is also the chance that the measurement system is not the problem. Maybe you get really great dimensions, but your parts are not as accurate as you need. There are many ways to improve the precision of CNC machined parts. A good first step is to think about the design from a DFM perspective. Many DFM principles will also contribute to part precision. The five design parameters to consider are: geometry, material, tolerance, part size and look and feel. The effort factors for these parameters are also tied to precision: the more effort it takes to machine these parts, the more effort it takes to machine them with precision. A particularly important aspect is tolerance. Precision machining is about consistently staying within tolerance limits. For example, the stiffness of the entire system is an important factor in tolerance and therefore precision. There is a line of compliance between the part, the workholding method, the tool and the machine itself. If there is any reduction in stiffness along this path, it can lead to deflection and chatter, which in turn causes dimensional inaccuracies. To increase the precision of the CNC machining process, make sure the system (workpiece, workpiece fixture, tool and machine) is optimized for stiffness. 
Workload is another important piece of the precision puzzle. Again, when we talk about precision, we focus on the variation from part to part. How do you ensure that each part is placed in the exact position as the part before it, so that the CNC program results in the same part? The fixture method is very important; it must fasten each part rigidly and in the exact same position. One concept you can incorporate into your fixturing method is kinematic coupling. Kinematic coupling allows you to limit all six degrees of freedom of a part without overloading it. This comes from the idea of exact constraint design, which says that the number of points of constraint must correspond to the number of degrees of freedom you want to constrain. There are two types of kinematic coupling: Maxwell coupling and Kelvin coupling. A Maxwell coupling system consists of two parts: one with three V-shaped grooves on one part, all facing the center, and one with three curved surfaces that fit into the grooves. On the other hand, a Kelvin coupling system consists of one part with a concave tetrahedron, a V-shaped groove and a flat plate, and a matching part with three convex surfaces. This is an important concept in precision engineering and can significantly improve the precision of parts and CNC machining process.
 Maxwell coupling
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 Kelvin coupling |
Why is precision important?
So far we have talked a lot about precision: what it is, how to analyze parts and the process, and how to improve the precision of CNC machined parts. Finally, I want to address why it is so important to spend valuable time and money on these steps. Precision machining can be more important in some industries than others. Some parts or products need to be extremely reliable and work every time. Take medical devices, for example. The surgical robot that the doctor controls must do exactly what it is told to do, without lag, slippage or unexpected movements. Even a “simple” daily blood pressure reading is very important for proper diagnosis and must be correct every time. There is not much room for error when it comes to people’s health. The auto industry is another that needs precision parts. After all, you want your car to run reliably every day? And consumer electronics: you want your phone to be able to turn on and make calls when needed. Each of these products comes down to the interaction of many parts that must fit and work together. This requires precise parts made the same way every time. Thinking more upstream than the customer, the people who design and build these parts and products must also care about precision. An inaccurate process is costly; it results in more rejected parts that don’t work, more rework cycles, more time and generally more money! However, it is also important to realize when precision is not important. Fancy new measurement systems are expensive and require time and attention to set up and characterize, and you can make the price unnecessarily more expensive by focusing too much on precision. Again, CNC machining is an amazingly capable process that can immediately increase the quality and efficiency of your parts and processes. With some parts, it is important to pay extra attention to the precision component: are the parts consistently within the tolerance and quality required? Consider parts you are currently working on. How important is precision in the specific application? How can you improve your design, machining and measurement processes to improve the precision of your CNC machined parts?
