Mastering Precision: Seven techniques for assessing cnc machine tool accuracy

The precision of CNC machine tools hinges on the precise movement of each component under the control of the CNC device. Unlike manual machines, where precision is influenced by human operation and reading errors, CNC machines operate based on digital program instructions. This means that the CNC system and mechanical transmission determine their accuracy. As each component moves according to the CNC's instructions, its ability to adhere to the programmed commands directly affects the accuracy of the final machined parts. Therefore, evaluating positioning accuracy is essential for assessing the performance of machine tools in tasks such as accurate machining, quality control in machining processes, detecting tool wear, machining intricate components, especially for automotive and aerospace components, and ensuring optimal tool material selection for machining operation.

1. Detection of Linear Motion Positioning Accuracy:

Measuring how accurately CNC machine tools move in a straight line is essential for getting precise results. We use laser interferometry and ball bar testing to check this accuracy. These techniques help us see if the machine can move exactly where it should along straight lines.

When we test how well the machine moves in a straight line, we do it without putting any extra weight on the machine or the workbench. National rules and ISO guidelines say we should use lasers for these tests. However, we can compare using a standard ruler and a microscope if we don't have a laser.

It's super important to ensure the ruler or the laser we use for testing is more accurate than what we're trying to measure by 1 or 2 levels. This helps us get reliable measurements that show exactly how well the machine is doing.

To ensure we don't miss any errors when the machine moves multiple times, the ISO rules say we should take five measurements and then use a particular calculation called a scatter band of -3. This helps us see the accuracy of each movement point.

2. Detection of Repeat Positioning Accuracy of Linear Motion:

Checking if a machine can consistently return to the same spot after moving around a bunch is essential. We call this "repeat positioning accuracy." We measure it using tools like dial indicators and laser projection systems. These tools help us see if the machine is ending where it should be.

Here's how we measure repeat positioning accuracy: we pick three spots near the middle and ends of each way the machine can move. Then, we move the machine quickly to each spot and do it seven times in a row under the same conditions. After each move, we checked where the machine stopped and compared it to where it was supposed to be prevented. We can determine the repeat positioning accuracy by looking at the most significant difference between the three spots. This helps us see how the machine keeps moving accurately in each direction.

3. Detection of the Return Accuracy of the Origin of Linear Motion:

Ensuring CNC machines reliably return to their initial position is crucial for consistent performance. This aspect, known as "origin return accuracy," is vital for maintaining precision in machining operations. Laser trackers and coordinate measuring machines (CMMs) are instrumental in verifying this accuracy, ensuring the machine consistently lands precisely where it should.

Origin return accuracy is akin to repeat positioning accuracy but focuses on a specific starting point on the machine. We follow a similar process to measuring repeat positioning accuracy to evaluate it. We select several points along the machine's movement range, then rapidly move the machine to each point seven times consecutively under identical conditions. After each movement, we assess how accurately the machine returns to its starting position. This method allows us to accurately gauge the machine's consistency in returning to its initial point, helping optimize spindle speed and overall machining efficiency.

4. Reverse Error Detection of Linear Motion:

Spotting errors in linear motion, whether forward or backward, is critical for the precision machining process. This is called "reverse error detection." Methods like laser interferometry and ballbar testing are great for finding these errors, helping manufacturers tweak their machines for top-notch performance.

Reverse error, also known as loss of momentum, includes factors like dead zones in drive parts (like servo motors) and mechanical deformations. The bigger the error, the lower the machine's accuracy.

We move the machine back and forth within its range to find reverse errors, using a stop position as a reference. Then, we command it to move a certain distance and measure the difference from the reference point. We repeat this a few times at different positions and calculate the average difference at each spot. The highest value we get is the reverse error.

5. Detection of Positioning Accuracy of Rotary Table:

The precise positioning of rotary tables is crucial for ensuring accurate rotational movements in CNC machines. Techniques like laser interferometry and projection systems help measure this accuracy, ensuring workpieces align perfectly.

Tools like standard turntables and circular gratings are used for measurement. The process involves rotating the table, locking it, and measuring the difference between actual and theoretical angles to determine indexing errors.

For CNC tables, targets are set every 30 degrees and positioned seven times in both directions. Deviations from these targets help calculate positioning accuracy errors.

Focusing on critical angles like 0, 90, 180, and 270 degrees is vital for enhanced precision, especially in critical applications like dry-type transformers.

6. Repeated Indexing Accuracy Detection of Rotary Table:

Ensuring a rotary table consistently hits its intended angles is vital for precise CNC machining. We use tools like dial indicators and laser trackers to check this accuracy.

To measure it, we pick three positions around the rotary table and repeat each position in both directions. We note the most significant difference between the intended and actual positions.

For CNC rotary tables, we measure every 30 degrees, five times each way. We calculate the deviation from the target and the standard deviation according to GB10931-89. The highest standard deviation shows the repeated indexing accuracy of the CNC rotary table.

7. Detection of the Return-to-Origin Accuracy of the Rotary Table:

Ensuring rotary tables accurately return to their starting points is vital for precision in CNC machining. Laser trackers and coordinate measuring machines help measure this accuracy, letting manufacturers correct any deviations from the intended positions.

To measure it, we reset the origin from 7 arbitrary positions and note their stopping points. The most significant difference observed is the accuracy of the origin reset.

Knowing that current positioning accuracy is assessed under fast conditions is crucial. Some CNC machines may show different accuracy with various feed rates. Also, measurements can vary with temperature and axis working conditions. Despite using semi-closed loop systems and position detection on drive motors, errors due to thermal elongation, around 0.01-0.02mm within a 1m stroke, can happen. Some machines use pre-stretching methods to reduce this.

The repeated positioning accuracy of each axis is vital for stable production. While CNC systems can fix some errors, random errors remain. Only fine-tuning the feed drive chain can resolve these issues. Thus, choosing a machine with high repeated positioning accuracy is wise.

In conclusion, mastering precision in CNC machining requires a comprehensive understanding of various techniques for assessing machine tool accuracy. Manufacturers can identify and correct deviations by utilizing the seven techniques above, ensuring consistent and precise machining operations.