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How CNC Machining Accuracy is Determined?

How CNC Machining Accuracy is Determined?

Table of Content

The user anticipates a machine tool to produce geometrically and dimensionally defined workpieces. Machine errors cause deviations from the intended dimensions and geometry. As a result, tests and tests on the qualities and settings of machine tools that may detect them were required. These checks are carried out on every new machine tool.

 Standards and regulations specify and propose how these tests should be performed and evaluated. When it comes to testing the properties of machines, it’s not just about understanding how to measure them; it’s also about knowing how to evaluate and use the data.

Is a mechanical intervention of the machine required, or would software compensation be enough on its own? Let’s explore this in detail.

What is the Accuracy of CNC Machining?

The user expects a machine tool to produce geometrically and dimensionally defined workpieces. Machine errors cause deviations from the intended dimensions and geometry. As a result, trials and tests of machine tool qualities and parameters were required to detect these inaccuracies. So, what exactly is Accuracy?

CNC Machine accuracy – this refers to a machine’s ability to make parts within the required dimensions and geometric tolerance specifications. To put it another way, accuracy is the difference between the real measurement and the measurement taken by your system. The measurement’s accuracy is regarded to be high when the difference between the two is incredibly small. The degree of conformance is another name for it. Consider the following illustration to better understand. If the instruments are designed to do so, the tool will precisely cut a metal piece with a length of 30 mm. If the equipment is as exact as the service provider claims, it will reach closer to 30 mm without making any mistakes.

 Now, We hope you  get the clear idea of accuracy.  

Well, CNC machining accuracy is the most important aspect impacting the sample quality. It has a direct impact on the machine’s life and performance.  Standards and regulations specify and propose how these tests should be performed and evaluated. When testing machine properties, it’s important to understand not only how to measure machines (what equipment to use, what method and procedure to employ), but also how to interpret and use the results in the future.

Now, you got it what is accuracy and how much it is important for product quality? Let’s proceed ahead to explore more details.

Machine diagnostics is a set of skills that the inspector must possess in addition to knowledge of the measurement method.

  •  The first understands the measuring equipment and its management, which includes monitoring its qualities, and accuracy, and ensuring that it is calibrated on a regular basis (if necessary).
  •  The next step is to understand how to use these devices (procedures), as well as what standards and rules apply to the measured quantity, the device, and the instrument itself.
  •  However, knowing the measuring machine is also necessary; without it, we cannot properly run diagnostics or advise appropriate solutions to improve the accuracy of the machine.

Types of CNC Accuracy

You might visualize numerous qualities of the machine when you hear the word machine tool accuracy. The understanding of CNC Machine Tools’ accuracy differs from that of designers and metrologists. Accuracy in metrology refers to how closely the measurement result corresponds to the actual quantity. We can talk about different sorts of accuracy in the field of machine tools, and the accuracy of the determination is just qualitative (small, medium, high). Geometric accuracy, working accuracy, and production accuracy are the three main types of accuracy. Each precision has its own set of causes.

Other types of accuracy, such as positioning accuracy, interpolation accuracy, volumetric accuracy, and thermal expansion, are added to these three basic accuracies of CNC machine tools.

Geometry Accuracy  

Geometric accuracy refers to a machine tool’s geometric structure, which can be used to assess the qualities of functional pieces that determine its working accuracy. It also describes the machine’s unloaded assembly quality and production quality. The tests are performed on machines with no load or under machining finishing conditions.

The standard ČSN ISO 230-1 specifies the geometric accuracy of axes, as well as their measurement and evaluation. Only accuracy tests are covered in this section. It does not cover the machine’s functional tests (vibrations, jerky components movements, etc.) or the measurement of characteristic parameters (revolutions, feeds), as these tests must be completed before the accuracy tests.

Geometric tests validate component dimensions, forms, and locations, as well as their relative alignment. All actions that influence a machine part, such as planeness, alignment, axes intersection, parallelism, the squareness of straight lines, or planar surfaces, are included. They are only concerned with dimensions, forms, locations, and relative motions that may affect the machine’s accuracy.

There are six geometric errors in linear (ČSN ISO 230 – 1) and rotary (ČSN ISO 230 – 7) axes, according to the standard: three translational mistakes—positioning error, horizontal and vertical straightness error, and three angular errors. Three translation faults and three angular errors are found in a typical three-axis CNC machine tool. The linear axis’ relative squareness errors amplify these inaccuracies. All of these errors can have a negative impact on the machine’s overall positioning accuracy, as well as the accuracy of the manufactured parts. When the real location differs from the position displayed on the machine control unit, errors arise. With dynamic effects originating from axis interpolation, errors rise.

A three-axis kinematic model yields 21 error parameters: 18 translational axes errors and three squareness errors between them. The following figure depicts these errors, including spindle errors, for a three-axis vertical milling machine. W (Workpiece)-X-Y-Z-T is the kinematic chain of the three-axis machine tool shown below.

                               Three-axis milling machine geometric errors

Deflection (bent component) or misalignment in the machine guides can be detected using the straightness measurement. Wear, an accident or poor machine foundations might all lead to the axis or the entire machine falling.

Position Accuracy

This parameter represents the accuracy and repeatability of placement in numerically controlled linear and rotational axes. ISO 230-2/6 (ISO 230-2 Test code) is an international standard for machine tools—Determination of accuracy and repeatability of numerically controlled axes. The ISO 230-6 test code for machine tools specifies how to determine the accuracy and consistency of numerically controlled axes. But, the directive VDI/DGQ 3441 is also frequently used.

The most common type of measurement conducted with a laser interferometer is positioning accuracy (Figure below). By comparing the position displayed on the machine to the actual position measured by the laser system, the laser system determines linear positioning accuracy and reliability.

                                        Measurement of positioning accuracy

The Laser Tracker is a more advanced technology for measuring machine placement accuracy since it provides for instantaneous examination of x, y, and z deviations. For an already completed and activated machine, the geometric accuracy of the machine and the precision of placement can be examined simultaneously (Figure below). As a consequence, the inaccuracies stated above are commonly categorized together.

                 Geometric and positioning accuracy measurement  using laser tracker

Interpolation Accuracy  

In theory, if CNC machines were 100% accurate, the machine’s circular path would match the planned circular path exactly. In practice, any of the defects (measurement error, straightness, clearance, reversal error, and so on) will cause the circle’s radius to depart from the designed circle. We can get a scale of machine tool accuracy if we can correctly measure the real circular path and compare it to the designed (nominal) path.

The measurement and evaluation of circular interpolation accuracy is addressed by the standard ČSN ISO 230-4 . The test’s goal is to provide a method for estimating the properties of numerically controlled machine tools’ contour shaping properties.

The geometric mistakes and dynamic behavior of the machine at the feed used have an impact on these inaccuracies. If the diameter and feed are the same for both machining and interpolation testing, the results are evident on machined parts under ideal machining conditions.

Volumetric Accuracy

The measurement of volumetric accuracy and subsequent adjustment is an advanced and extremely progressive procedure. These advanced compensations are designed to reduce tool center point (TCP) deviation at any location in the machine’s measured workspace. The sum of partial deviations in the different axes is the TCP volumetric deviation.

A vector map of error deviations in the workspace represents the volumetric accuracy of machine tools. The concept of volumetric accuracy for a three-axis center is defined in ISO 230-1 as the maximum range of relative deviations between the actual and ideal position in the X, Y, and Z directions. As well as, the maximum range of deviations orientation for directions of A, B, and C axes for motions in X, Y, and Z axes in the specified volume.  Where the deviations are the relative deviations between the tool and the workpiece on the machine tool for specified alignment of the workpiece.

The LaserTRACER measuring instrument (Figure below) is primarily used to determine volumetric accuracy and compensate for it. The LaserTRACER measurement principle is based on measuring beam lengths (HeNe laser wavelengths, 632.8 nm) and using the sequential multilateration method to calculate the measured point in the workspace.

 It is required to measure gradually from many points on the machine with this method (at least four LaserTRACER positions are recommended). The method is described as a GPS system analogy.

                               Volumetric accuracy measurement by Laser tracer 

Working Accuracy

This is a machine tool property that describes the quality and productivity of a future workpiece. The manufacture of a test workpiece or a succession of test workpieces demonstrates working accuracy. The relative tool path accuracy has an impact on the machine’s working accuracy.

Production accuracy

Production accuracy refers to the accuracy of the manufacturing process as measured on the workpiece. Geometrical precision, positioning accuracy, and working accuracy are all influenced by machine operator faults (incorrectly adjusted tool, inadequately clamped workpiece), as well as variations in ambient circumstances. Variations in the dimensions of test workpieces during the manufacturing process provide direct feedback on production accuracy.

 Factors that Affect CNC Machining Accuracy

A lot of factors influence the CNC machine tool’s operation. This allows us to comprehend not only the impact of the environment in which it is located but also the influence of the operator on the machine and its effects on the environment. These factors have an impact on the attributes that all machine tool users desire, such as run stability, repeatable machining accuracy, and trouble-free operation. Machine tools must be evaluated in a complete, hierarchical, and structured manner. The machine tool is affected by the workshop environment where it is installed by: vibrations, impurities, and heat.

Three key factors impacting the machine tool and the manufacturing process produce workpiece dimensional differences:

  • influence of temperature
  • the machine-tool-workpiece system’s static stiffness;
  • Machine-tool-workpiece system dynamic compliance

During operation and in sleep mode, every CNC machine tool is exposed to temperature impacts, both uniform and uneven. Temperature deformations occur as a result of this temperature effect, resulting in a change in the position of the workpiece relative to the tool and hence inaccuracies.

Our goal is that, despite the machine’s thermally stabilized state. The temperature variations and their manifestations of thermal deformation affect the tool’s location relative to the workpiece and  the machining accuracy as small as possible by:

  • selecting a machine design that is thermo-symmetrical;
  • Increasing the efficiency of all nodes and elements, resulting in fewer heat losses;
  • locating heat sources efficiently so that they do not interfere with the machine’s design;
  • dispersing heat through cooling, chip removal, or surface dimensioning for effective heat dissipation;
  • adjusting the machine’s settings;
  • examining the airflow and temperature, or protecting against external thermal emission

   Possible  Main Error in CNC Machining Accuracy

The degree of compliance between the actual geometric parameters (size, shape, and location) of the processed parts and the ideal geometric parameters is referred to as machining accuracy. Although errors are unavoidable in processing, they must be within the acceptable range. Master the underlying rule of its change through error analysis. So, that you may take appropriate steps to eliminate processing errors and increase processing accuracy.

Then there will be errors  and explanations for them, roughly as follows:

  • Error in Spindle Rotation
  • Internal stress error
  • Measurement error
  • Error with a guide rail.
  • Error in the transmission chain.
  • The Tool’s Geometric error.
  • Error in positioning.
  • Errors due to force and deformation of the processing system.
  • Errors due to thermal Deformation of the Process System.

 Ways for Enhancing the Accuracy of CNC machine

It is vital to improving the accuracy of the machine tool in order to improve the accuracy of machined parts. But, Why most of the CNC machining accuracy is below the standard level and how can improve it? Well, The geometric inaccuracy of the body construction, backlash, distortion due to temperature variations, humidity, servo-control error, and error motion of the spindle axis rotation all contribute to machine tool accuracy.

 Furthermore, many numerically controlled machine tools perform below the manufacturer’s advertised accuracy and do not work within the prescribed accuracy. This is because machine tool accuracy varies greatly based on set-up, environment, and previous usage. Static loads, machining loads, and heat generated by the previous machining are all part of the operational history. As a result, CNC machine tools that demand high precision machining should be calibrated on a regular basis.

 Benefits of improving accuracy

Thus, After enhancing the CNC machining accuracy It is well worth to know what are the benefits of it? Alright,  the method of enhancing accuracy  can provide numerous substantial advantages. For example; the higher accuracy in measuring and machining, the workload and setup time for the computerized maintenance management system (CMMs), as well as the rework cost, can be minimized.

You have several advantages as you update your equipment to improve accuracy. The benefits that the adjustments bring to your operation will vary depending on your present workload and the upgrades you make to your existing equipment. As technology and demand both push for more precise products from machining, upgrading today will keep your organization from falling behind.

The time saved on your projects is one of the key reasons for improving the positioning accuracy of CNC machines. More precise cutting eliminates the need for your company to redo work.  Well, Is it improving the quality of your work?

Of course,  Furthermore, automating several components of machining, such as tool placement, minimizes the time required for each process. More efficient processes will shorten the time you have to wait for your completed project. if you use the parts generated by machining.

When you control for product variability, accuracy measuring of machined parts takes less time. Variability can influence a part’s initial accuracy and capacity to produce precise reproductions. Changing your equipment to eliminate these variables can ensure that your results are accurate and consistent. Furthermore, you should not be required to measure each completed component.

 How to Check CNC machine Accuracy

The basic objective is to show in detail the entire process, from the measurement of error components through the verification of different kinds of accuracy after error compensation. It also proposes a generalized kinematic model for multiaxis machine tools that describes the volumetric error. The kinematic error model in a study is made up of 21 error components. We tested the error components multiple times to show the impacts of thermal distortion and operating history on the error components. The basic steps involved in this study are as follows:

            1. Volumetric error modelling.

2. Error measurement experimental procedure

3. A method for compensating tool location and path.

4. The method of verification.

The machine tool status may be fully detected using the same way, and software error compensation can improve its accuracy. When the machine tool is employed as a coordinate measurement machine, the point compensation method can be used to modify the nominal tool path and perform an on-machine inspection. A vertical 3-axis CNC machine with a laser interferometer and a ball bar methodology was used to demonstrate the accuracy of the error calibration method during the study.

 Measurement for the Calculation of CNC machine accuracy

The accuracy of a CNC milling machine is determined by how well it can follow its preset patterns. The overall accuracy of a milled item is determined by three separate measurement methods: positioning accuracy, precision or repeatability, and tolerance. What exactly is the  Position accuracy? It is  the difference between the required distance between points on a part and the actual measured distance between those locations after milling. The rolling mill’s accuracy is determined by taking several measurements and determining the statistical average of the variances at the CNC machining workshop.

The position accuracy can be measured by following the experimental method.

Error Components Measurement

Except for roll, which is evaluated with an electronic level, the 19 error components are measured with a laser interferometer. The figure below shows the experimental setup for measuring the linear and squareness errors in the Bridgeport CNC-milling machine with a laser interferometer. At three different room temperatures, the measurement was repeated three times for each of the 21 error components. The following procedures must be completed prior to measuring the error components.

                         Measuring position error of Bridgeport CNC machining 

Determine the measuring dimensions, number of points in the measuring space, travel pattern (linear cycle, quasi-pilgrim cycle, etc.), and the number of repetitions on the machine tool.

Allow enough time for the machine tool to warm up. Allow the spindle to spin at a typical speed for several hours while the table and tool traverse the entire measuring space.

Thermocouples are used to monitor the temperature of crucial machine tool elements. The temperature of the spindle, each axis’ motors, the x,y-lead-screw, table, and the hydraulic oil must all be checked.

The table displayed below shows the 21 error components measured by a laser interferometer for the three-axis, as well as an electronic level for the roll error measurement. The values of the error components for any given point can be determined by linearly interpolating the values of the error components of the two previously measured points.

Temperature variations can have a big impact on machine tool error components. As a result, we must determine the trend of error component variation as a function of temperature change. We measured the machine error components in the bi-directional positions in August and November, respectively, to determine the backlash effect. Figure 9 depicts the x-axis positioning accuracy, which was the most critical of the 21 error components. The huge change in error data may be seen in Figure . This could be due to items like ball screws and guides contracting and expanding in response to changes in ambient temperature.

Verification of positioning accuracy

We employed a ball bar method, which is commonly used to assess the dynamic properties of CNC machine machines, to measure the positioning accuracy for every point in 3D space. The distance between the centers of a fixed ball and a moving ball is measured using this approach. The trials with the RENISHAV,/I ball bar system and the arrangement of the measuring stations are shown in the following Figure. The nominal tilted angle of the ball bar in 3D length measurement was 45 degrees from the horizontal plane. To eliminate the backlash effect, the ball bar data were collected by averaging data received by travelling backward and forwards for measurement sites along the ball bar path.

                        Points and configurations that were measured

Precision

Precision is used to get the same results after every measurement. The closeness of a measurement to the true or acceptable value is called accuracy. Precision relates to how closely identical measurements of the same thing are obtained. Precision is distinct from accuracy.

Tolerance

The allowable mechanical deviation of a component from the acceptable value of CNC machining can be determined from tolerance. This is another way to verify the required accuracy of the product.

Conclusion

So, basically, how can we determine the accuracy?

There is a lot of expertise and knowledge required not only for how to measure machines  accuracy (what equipment to use, what method and procedure to employ) but also for how to interpret and use the results in the future. Standards and regulations specify and propose how these tests should be performed and evaluated. When testing machine properties, it’s important to understand all basics as well. There are different types of CNC machining accuracy that can be evaluated for accurate measurement. There are some factors as well like workshop environment, impurities, and heat that impact accuracy. It is important to consider them as well while calculating CNC machining accuracy.

We hope the read of this post will be enough to get all you need about the Accuracy of CNC machining.

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