Learn How to Check CNC Machine Accuracy with Ease

TL;DR

• The accuracy of CNC machines can’t be overstated because the slightest deviation can lead to significant material waste and compromise the product’s integrity.
• Several factors affect the accuracy of CNC machines, including machine condition, tooling, fixturing, and deformation errors.
• Using specific methods and equipment, such as test cuts, part inspection, dial indicator tests, and laser interferometry, allows for a comprehensive assessment of CNC machine accuracy.
• Keeping CNC machines accurate involves regular testing, understanding their operational limitations, using high-quality equipment, and timely maintenance.
• Maintaining CNC machine accuracy makes the machining process reliable and increases efficiency.

The manufacturing industry relies on CNC machines for their accuracy, directly impacting the quality and consistency of the products they create. However, even the most reliable CNC machine may lose its accuracy, leading to a decrease in the precision of the machined product.

That is when checking a CNC machine’s accuracy becomes vital. When a CNC machine’s accuracy dips, it doesn’t just affect a single component. It can compromise the integrity of an entire production run. Maintaining the accuracy and repeatability of CNC machines keeps the high standard of your finished goods.
This post will show you how to check CNC machine accuracy so that every piece it produces follows strict quality standards.

What is CNC Machine Accuracy And Why Does It Matter

The level of precision of CNC machines determines how closely the produced parts match the original designs. In manufacturing, accuracy means that every produced part is consistent with the others, maintaining the quality and functionality of the final product.

For a machine operator, achieving high CNC machining accuracy means less waste, higher efficiency, and a better end product. You can’t learn how to operate a CNC machine properly without knowing how to calibrate its accuracy. The accuracy of CNC milling machines, for instance, is measured in terms of their ability to reach and maintain specific dimensions, angles, and surface finishes throughout producing multiple parts.

Why is this important in manufacturing? Higher accuracy means higher quality. In industries where you can’t compromise precision, the accuracy of the produced parts can have significant implications for safety and performance. Errors, even those as small as a few thousandths of an inch, can lead to part failure and have catastrophic outcomes.

The accuracy of CNC machines is not just about achieving perfection in manufacturing. It guarantees the reliability and safety of the final products. For manufacturers, investing in high-accuracy CNC machines and ensuring their operators are skilled and knowledgeable is a direct investment in the quality and trustworthiness of their products.

If you’re interested in CNC machining services that consistently produce highly accurate and precise projects, take a look at what JetCrafted can offer you. We use the latest CNC technology to make parts and end pieces to our client’s exact specifications.

Are CNC Accuracy and Precision The Same

You often hear terms like precision and accuracy mentioned together, but they are different concepts. CNC machining accuracy refers to how close a machined part’s measurements are to the intended dimensions specified in the design.

For example, if you want to create a part that measures 100 mm in width and your CNC machine produces it at 100.01 mm, your accuracy is high. On the other hand, precision in CNC machining talks about the consistency with which a machine can produce identical parts. Your CNC machine is precise if it makes parts that measure 100.01 mm in width every single time.

The difference is that a machine can be precise (consistently producing parts with identical dimensions) without being accurate (those dimensions might not be what you intended). A machine might be accurate (producing parts close to the intended dimensions) but not precise if there’s a lot of variability in the parts’ sizes.

What Are CNC Tolerances and Repeatability

CNC tolerances and repeatability also come up frequently alongside CNC accuracy and precision. Tolerances in CNC machining define the limit of variation in a part’s dimensions – basically, how much a part’s measurements can deviate from the specifications and still be considered acceptable.

Repeatability refers to the CNC machine’s ability to produce identical dimensions every time, under the same operating conditions. While tolerances set the stage for what’s considered an acceptable part, repeatability means that once the machine is set up correctly, it can consistently produce parts within those set tolerances. Below, we’ve prepared a table showing the differences between the four terms.

Types of CNC Accuracy

Several types of accuracy exist, from positional accuracy of CNC machines to volumetric accuracy, each affecting whether the final product meets your exact specifications. Understanding these different types of accuracy can help you better grasp how CNC machines operate and what makes them so reliable for precision manufacturing.

Geometric Accuracy

Geometric accuracy refers to a CNC machine’s ability to move its tools or parts along the intended geometric path. This type of accuracy directly impacts the shape and dimensions of the final product.

Imagine you’re creating a complex component with specific curves and angles. Geometric accuracy means the machine follows the exact path to produce those features. Without high geometric accuracy, the final product might not fit with other components or function as intended.

Positioning Accuracy

The positional accuracy of CNC machines determines how precisely a machine can move its tools to a specific location within its working space. Machines that perform operations where the tool stops at several points to create features or change directions need high positioning accuracy.

High positioning accuracy and repeatability of CNC machines mean that every part produced will closely match the original design, with minimal variation between parts.

Interpolation Accuracy

That is the machine’s capability to move along a defined path accurately, especially when that path involves more complex movements like curves or diagonal lines.

Interpolation accuracy affects the surface’s smoothness and the fidelity of the part to the intended design. There are two common types of interpolation accuracy: 

• Linear interpolation accuracy is how precisely the machine can move in a straight line
• Circular interpolation accuracy is the machine’s ability to move along a circular path accurately

Volumetric Accuracy

Volumetric accuracy assesses the machine’s ability to maintain accuracy across its entire working volume. That means that no matter where within its operating space the machine is working, it can produce parts with consistent precision.

This type of accuracy ensures that no matter where within the machine’s working envelope the part is made, it will meet the required specifications. Your parts will have uniform quality, regardless of their size or the complexity of their features.

Production Accuracy

Production accuracy focuses on the machine’s ability to produce parts that meet the design specifications over a production run. That includes wear and thermal changes that may affect the machine’s performance over time.

Mass production relies on production accuracy because thousands or even millions of parts must look and function identically. It means every part coming off the production line will meet your standards, reducing waste and keeping the project on schedule.

Working Accuracy

Sometimes overlooked, this accuracy is a machine’s ability to maintain accuracy over time and under various operating conditions. That includes wear and tear, temperature changes, and material properties. High working accuracy means a CNC machine can consistently produce parts that meet specifications, even as it ages or environmental conditions shift.

What Affects CNC Machine Accuracy

Several factors during the CNC machining process can lead to inaccuracies in your final product. If you don’t resolve these, you can produce parts that don’t meet specifications, which can lead to wasted materials, time, and money.

Machine Errors

These problems originate from the CNC machine and can happen for many reasons, ranging from wear and tear to improper machine setup. The most common machine errors include:

• Misalignment of slides occurs when the linear guides that control the movement of the parts of a CNC machine are not perfectly parallel. This leads to inaccuracies in the movement and positioning of the tool or workpiece. It can result in parts that are not dimensionally accurate or have poor surface finishes.

• Spindle runout – The situation where the spindle does not rotate on its axis perfectly, causing the tool to wobble slightly. This wobble can lead to uneven cutting and affect the quality of the finished product.

• Backlash in axes – The lost motion between the drive mechanism and the actual movement of the axis. It can also cause positioning errors, leading to inaccuracies in the produced parts.

Tooling Errors

Tooling errors are issues that arise from the tools used in the CNC machining process. These errors are often due to wear and tear on the CNC machine tools or imperfections in the cutting surfaces. The most common tooling errors are:

• Tool wear – Over time, the cutting edges of tools can become dull, leading to decreased accuracy and quality in the parts produced. This wear can cause the tool to cut less precisely, resulting in parts with incorrect dimensions or subpar surface finishes.
  
• Imperfections in the cutting surface – This can happen due to manufacturing defects or damage to the machine tool. These imperfections can cause the tool to cut unevenly, leading to inaccuracies in the finished parts.

Fixturing Errors

Fixturing errors relate to how the CNC machine holds the workpiece in place during the machining process. A frequent fixturing error is improper fixturing, which can occur if you don’t secure or position the workpiece correctly.

If the workpiece moves during the machining process, it causes significant inaccuracies in the finished part. With proper fixturing, the workpiece remains in the correct position throughout the machining process.

Deformation errors 

These occur when the machine or workpiece material changes shape during machining. The two most common types of deformation errors are force deformation and thermal deformation.

• Force deformation – This issue happens because of the mechanical forces applied during the cutting process. They can cause the machine part or the workpiece to bend, stretch, or compress. The result is a finished product that deviates from the intended design, which can be particularly problematic for projects requiring high precision.

• Thermal deformation – The workpiece and parts of the machine can expand when exposed to heat, leading to inaccuracies. For instance, if a machine part expands, the distance between the tool and the workpiece might change, affecting the dimensions of the finished product. Similarly, if the workpiece expands due to heat, it might not fit in the final assembly.

Other Errors

When we go deeper into what CNC machines are, we can see they are complicated equipment with many moving parts and components. That means errors affecting its accuracy can be caused by multiple factors. Some other common errors affecting CNC machine accuracy include:

• Positioning errors. They happen when the machine’s moving parts don’t align precisely with the coordinates specified in the CNC program. This misalignment can lead to cuts that are off by critical fractions of an inch, compromising the integrity of the component.
  
• Measurement errors. When the tools used to measure the machine’s alignment are inaccurate or not calibrated correctly, they can disrupt the intended and actual dimensions of the machined part.
  
• Programming errors. These are mistakes in the code that dictate the machine’s movements. A single misplaced command can result in the wrong cut, affecting the whole project.
  
• Internal stresses in the machined material. It can lead to warping or distortion during or after the machining process, which can cause the final product to deviate from its intended dimensions or shape.

How to Measure the Accuracy of CNC Machines

Measuring the accuracy of CNC milling machines means the parts you produce meet the required standards and specifications. The process involves several methods, each assessing different aspects of the machine’s performance and condition. These methods will help you maintain the integrity of your production process and the quality of your machine parts. Let’s see how to check CNC machine accuracy.

Test Cuts

With this method, you create a specific part solely to evaluate the machine’s accuracy. For instance, you might have the CNC machine cut a standard geometric shape out of a metal block. After the cut, you would measure the shape’s dimensions to see if they match the programmed dimensions.

If there’s a discrepancy, it indicates an issue with the machine’s accuracy. This practical approach helps you understand how your machine behaves under working conditions, allowing you to make adjustments before producing actual parts.

Part Inspection

After you make a machined part, it undergoes a thorough inspection process where its dimensions and surface finish are measured and compared against the design specifications. 

For example, you can use a coordinate measuring machine (CMM) to inspect a machined part and highlight even the minutest deviations from the intended design.

You’ll know if the CNC machine is producing parts within acceptable tolerance levels and helps identify potential issues that could affect the quality of the parts.

Dial Indicator Tests

A dial indicator is a precision tool that measures small distances or changes in position. In practice, you might set up a dial indicator to measure a spindle’s axial and radial runout. By rotating the spindle and observing the dial indicator’s readings, you can determine if there’s any wobble or misalignment in the spindle, which could affect the accuracy of the produced machine part.

Laser Interferometer

This method involves using a laser beam to measure the position and movement of the machine components. You can use a laser interferometer to measure the linear displacement of the machine table. The data collected can reveal any inaccuracies in the machine’s movement, such as backlash or linear errors, which can then be corrected to improve the machine’s performance.

Ballbar Test

This test checks a CNC machine’s geometric accuracy and performance by measuring how well it can move in a circular path. You attach a precision measurement device, known as a ballbar, between two points of the machine, such as the spindle and the table.

As the machine executes a programmed circular motion, the ballbar measures deviations from the perfect circle. It highlights issues like backlash, servo mismatch, or squareness errors, providing insight into areas of the machine that may need adjustment or maintenance.

What Equipment Do You Need to Measure CNC Machine Accuracy

CNC machine accuracy testing isn’t something you can eyeball or leave to guesswork. It’s a meticulous evaluation to see if your machine operates at its peak performance. To carry out this testing effectively, you need specialized equipment. Each tool shows you the CNC’s machine condition and the areas for improvement.

Digital dial indicators

They measure the displacement between two points, giving precise readings of any deviation or error in the machine’s output. You attach the digital dial indicator to a part of the CNC machine, and as the machine operates, the indicator provides a digital readout of the movement. You can detect even the slightest inaccuracies in the machine’s movement, which could affect the final product’s quality.

To use a digital dial indicator, secure it to the machine, zero it out before starting, and monitor the readings as the machine operates. Secure the indicator so it’s stable and unmoved throughout the testing process for accurate measurements.

Test Indicator Bases 

Without a stable base, your accuracy testing could have misleading results. These bases attach to the CNC machine or another stable platform and hold the indicator in place. The stability a test indicator base gives is crucial because any movement can skew the readings.

When setting up for accuracy testing to measure the machine’s movements, you adjust the base so the digital dial indicator is in the correct position. Finding the right spot involves trial and error but will produce reliable and correct readings.

The CNC Machine Accuracy Testing Procedure

Testing the accuracy of your CNC machine involves a series of steps you can easily follow. It starts with the initial setup and zeroing of the meter. This step sets the baseline for your measurements. The meter must be correctly calibrated and ready to record the slightest deviations.

Next, you move on to running the test across the X and Y axes. That involves moving the machine’s spindle or tool holder in a controlled manner along these axes and using the meter to record any variations from the expected path. Do this slowly and methodically, so your readings are as accurate as possible. During this phase, you also want to test the Z axis, depending on the work you plan to do with the machine.

After you’ve completed the tests, you must analyze the results. Compare the recorded measurements against the expected outcomes. Your CNC machine might have specific tolerances, and this is the time to see how well it meets those standards. Look for patterns in the deviations that might indicate a particular problem with the machine.

Reading interpretation is a critical part of this process. You need to know the acceptable deviation for your particular CNC machine. Each machine and application may have different standards, and understanding these will help you make sense of your results. If your readings show deviations, but they’re within the acceptable range for your work, your machine is considered accurate enough.

Comparing results with different meters can provide additional insights. If you get consistent results across different meters, you can be more confident in the accuracy of your CNC machine.

What is the Benefit of Measuring CNC Machine Accuracy

In manufacturing, every produced piece must match the specifications exactly, or you risk compromising the integrity of the entire project. The precision of CNC machines ensures that each component fits perfectly, and you’re not wasting material or time. Some other benefits of maintaining CNC machine accuracy are:

• Reducing scrap and rework – By measuring the accuracy of the CNC machines regularly, you can significantly reduce the amount of scrap and rework required. Accurate machines produce parts right the first time, eliminating the need for do-overs.
  
• Increase efficiency – Machines that consistently produce precise parts can run longer without interruption, speeding up production times and allowing you to meet deadlines. You won’t find yourself constantly stopping to check if parts meet quality standards or having to reconfigure machines to correct errors.
  
• Better process control – With accurate machines, you can predict outcomes more reliably and make real-time adjustments to the manufacturing process. Control over the process reduces variability in production so that each part is as close to perfect as possible.
  
• Improved safety – Accurate machining reduces the risk of producing parts that could fail under stress or wear out prematurely. Keeping your CNC machines accurate helps ensure the parts you produce perform reliably in their intended applications.

Pinion And Rack Wear

This issue arises from the regular use of the CNC machine, where the teeth of the pinion and the rack wear down due to friction. This wear leads to inaccuracies in the movement of the machine, affecting the precision of your cuts.

To fix this, regularly inspect these components. If you notice significant damage, replace the worn parts to restore accuracy. Proper lubrication can also slow the wear process, maintaining CNC rack and pinion accuracy for longer.

Dust or metal shavings can also accumulate on these parts, causing jerky movements and inaccuracies in your work. The solution is regular cleaning.

Damaged Rails

Rails guide the movement of the machine and, if damaged, can lead to deviations in the tool path. You must inspect the rails for signs of damage and align them when necessary. If you notice damage, replace or repair the rails to ensure smooth and accurate machine operation.

Backlash And Runout

Backlash refers to the loss of motion that occurs during a change of direction, while runout is the tendency of the machine’s spindle to move out of the center as it rotates. For backlash, adjusting the space between the drive components can reduce this issue. For runout, checking if the spindle and tool holder are not worn down and replacing them, can improve accuracy.

How To Achieve Optimal CNC Machine Accuracy

Regular testing is one of the most effective methods to achieve and maintain CNC machine accuracy. Periodically check the calibration of your machines and perform accuracy tests to catch any deviations early. This proactive approach identifies issues before they escalate. Keep a log of these tests to track the machine’s performance over time, making it easier to predict when maintenance or adjustments are needed.

Understanding the limitations of your CNC machines is crucial for effective troubleshooting. Each machine has capabilities that help you identify when it is pushed beyond its limits, which can lead to inaccuracies. By familiarizing yourself with the specific tolerances and capacities of your equipment, you can better diagnose issues when they arise. 

Using High-quality And Upgraded Equipment

As parts of the CNC machine wear out or become outdated, they can affect its precision. By replacing old components with newer, more advanced ones, you extend the life of your machine and improve its performance. That could mean upgrading software for more precise control, replacing cutting tools with ones of higher quality, or installing more accurate sensors.

Investing in high-quality CNC machines and components from reputable manufacturers ensures that you have a solid foundation. Quality equipment maintains high levels of precision over long periods, even under rigorous use. It’s also more likely to have reliable customer service and technical support, providing the assistance you need to keep your machines accurate.

Be Accurate Till The End

Maintaining CNC machine accuracy is vital for machining because even minor deviations can lead to significant errors in the final product. That can affect the fit, function, and aesthetics of the parts you produce. It can lead to wasted materials, time, and money.

Regular testing, maintenance, and resolving common issues are a few strategies for keeping CNC machines accurate. By adopting these practices, your CNC equipment will continue to operate at its best, consistently producing high-quality parts. Knowing how to check CNC machine accuracy maintains the reliability of your machining process and supports the overall success of your manufacturing operations.

If you want to find CNC machining with guaranteed accuracy and precision, take a look at the roster of services JetCrafted has to offer.

FAQs

1. How do I know if my CNC machine is losing accuracy?

You can tell a CNC machine is less accurate by noticing irregularities in your finished products or if the dimensions of the parts don’t match the designs. Regularly checking the alignment and measuring the parts against the original specifications can also indicate if there’s a decrease in accuracy. Look out for unusual noises or movements during operation, as these can be signs of wear affecting precision.

2. Does temperature affect CNC machine accuracy?

Extreme heat or cold can cause the metal components to expand or contract, leading to deviations in machining accuracy. Maintaining a stable and controlled environment around your CNC machine helps minimize these effects. Use temperature controls or operate your machine within the recommended temperature ranges.

3. How to clean the rack and pinion on a CNC machine?

Power off the machine and use a brush or compressed air to remove loose debris from the rack and pinion. Apply a recommended solvent to a cloth and wipe down the components to remove grease and built-up grime. Lubricate the parts according to the machine’s guidelines to keep them moving smoothly and prevent wear.

4. How often Should I test the accuracy of my CNC machine?

Testing the accuracy of your CNC machine should be a regular part of your maintenance routine. A thorough accuracy test every three to six months is sufficient for most operations. However, if your machine operates non-stop or handles high-precision tasks, you must test the accuracy more frequently.

5. What should I do if my accuracy readings are consistently high?

First, verify that your measuring tools are calibrated correctly. If the tools are accurate, inspect your CNC machine for mechanical issues, such as wear and tear on moving parts or misalignments. Consider consulting the machine’s manual or a professional for adjustments or repairs.