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Common Inspection Tools and Methods for CNC Parts: Ensuring Quality and Precision

In the context of the modern processing industry, where production efficiency is constantly escalating, the demands for quality control have been progressively heightening. After the completion of parts processing, the crucial question emerges: How exactly should we go about detecting various dimensions? Today, let's delve into the common inspection methods and tools utilized in CNC machining. There is a plethora of common inspection tools available, including calipers, micrometers, plug gauges, projectors, and coordinate measuring machines, to name just a few. Now, let's embark on a detailed introduction of these one by one.

Common Inspection Tools for CNC Parts

1. Calipers

Calipers indisputably stand as one of the most prevalently used and highly convenient inspection tools. Their presence can be witnessed at every stage, starting from the initial phase of material selection, through program debugging and processing inspection, and to the final stage of finished product inspection.

To provide a more intuitive understanding of the evolutionary journey of calipers, we have painstakingly crafted a table (Table 1) for a comprehensive comparison. This table showcases the advancements and refinements that calipers have undergone over time, highlighting their increasing precision and functionality. The adaptability and ease of use of calipers make them an indispensable asset in the inspection process, enabling accurate measurements and ensuring that the parts conform to the requisite standards. Their significance cannot be overstated, as they play a vital role in maintaining quality control and guaranteeing the reliability and performance of the CNC parts.

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Advantages and disadvantages

Difficulty level

Vernier caliper


Advantages: Cheap and durable

Disadvantages: Purely mechanical, requires learning to read and lacks accuracy


Dial caliper


Advantages: Easy to use, moderate price, and acceptable accuracy

Disadvantages: Afraid of falling, water, and oil


Digital caliper


Advantages: Simple reading and high accuracy

Disadvantages: Expensive and afraid of water and oil


Table 1


The above are photos and some characteristics of three common types of outside micrometers. Next, we will introduce their usage. Now the popularity of dial calipers is very high, so we mainly use the dial caliper as an example to introduce its usage method and how to read the scale (See Table 2).

Measurement position


Measurement steps

Reading steps

Outer diameter measurement



1. Use the caliper jaws to clamp the two sides of the object.


2. Keep the caliper flat and make it try to maintain a 90-degree angle with the side of the measured object as much as possible.


3. Measure several times to check if there is any difference.

1. No matter which dimension is measured, the caliper needs to be calibrated first (for example, when measuring the outer diameter, first close the caliper and observe whether there is a gap at the lower jaw and whether the dial is at the 0 position).

2. When the caliper is clamped to the specified position, it is not allowed to shake.

3. Read the measured data, that is, the size on the left plus the data on the dial. Commonly used dials have one grid of 0.02 and one grid of 0.01. Read according to the specifications of your caliper. As shown in the figure below, the measured size is 5.5 mm.


Inner diameter measurement



1. Extend the upper caliper jaw outward.

2. Keep the caliper as parallel as possible to the measured object to ensure that both sides of the jaw are in full contact with the object, rather than just touching two points.

3. Rotate the workpiece, measure several times, and read and check the difference.

Step measurement



1. Use the top step of the caliper to abut against the position to be measured.

2. The step surface of the caliper is in full contact with the measured surface, and the top of the caliper is used as the reference surface (measurement platform, the step of the part itself).

3. Slightly move and measure several positions.

Depth measurement



1. Extend the tail of the caliper to the position where the depth needs to be measured.

2. The tail step of the caliper needs to be in full contact with the end face of the product or other reference surface.

3. Adjust the position and repeat the measurement.


 Table 2

The above table precisely presents the detailed measurement steps and reading methods of the dial caliper. The measurement methods of vernier calipers and digital calipers are consistent with that of the dial caliper, so we won't introduce them here again. In terms of reading, accurate reading of the vernier caliper requires professional training, and due to the high difficulty in measurement, we won't explain it here any further. The reading of the digital caliper is extremely simple as it directly displays the measured dimension on the display screen, which is very straightforward. In addition to the outer diameter caliper, there is also a dedicated depth-measuring caliper. The measurement and reading methods are completely the same as the operation of the outer diameter caliper. If everyone has the opportunity to come into contact with it, they can also try to measure by themselves.

The use of these calipers is of great significance in various industrial fields. They provide an accurate and convenient way to measure different dimensions, ensuring the quality and precision of products. Skilled operation and correct reading of calipers are crucial for obtaining reliable measurement results. Whether in manufacturing, quality control, or research and development, these tools play an indispensable role. Their application helps to detect and control the quality of parts, guaranteeing that they meet the required standards and specifications. Moreover, continuous innovation and improvement in caliper technology are also promoting the development and progress of the industry. We should always pay attention to the latest technological advancements and continuously improve our skills and knowledge to better utilize these valuable tools in our work and studies.

2. Micrometer

The caliper is the most commonly used measuring tool. However, the caliper generally can only measure dimensions with not very high precision. When we need to measure dimensions with relatively high precision, we need to rely on the micrometer to help us achieve it. The micrometer (that is, the screw micrometer), there are two common types of micrometers, namely the outside diameter micrometer and the inside diameter micrometer. It is generally used to measure the thickness or outside diameter of CNC parts and is not suitable for products with relatively large sizes. The common specifications are 0-25 (mm) and 25-50 (mm).

3. Gauge

The gauge is also a very commonly used measuring tool in daily production. Commonly used gauges include gauge blocks, pin gauges, feeler gauges, and thread gauges, which are respectively used to check the size of grooves, the size of holes, the flatness size, and the pass/fail condition of threads. Hereinafter, we will introduce them respectively:

  • Gauge blocks

Normally, every CNC machining workshop and quality inspection room will have gauge blocks. On the one hand, it is used to calibrate the accuracy of other inspection tools, and on the other hand, it is used to detect whether the size of the product is within the target tolerance. Gauge blocks are generally made of materials such as chromium carbide, tungsten carbide, alloy steel, stainless steel, and quartzite because they have the advantages of being wear-resistant, having good stability, being inexpensive, and being easy to process.

  • Pin gauge

The pin gauge is a cylindrical type and is a measuring tool used to measure whether the size and roundness of a hole can meet the standard. When dealing with some inner holes with relatively high precision, we all need to use the pin gauge for corresponding measurements. In CNC machining, the main purposes of designing precision holes can be roughly divided into two types: one is for high-precision positioning, and the other is to install other precision parts such as bearings. When measuring precision holes, generally two pin gauges will be prepared, a lower limit pin gauge and an upper limit pin gauge. When the lower limit pin gauge can pass through the precision hole and the upper limit pin gauge cannot pass through the precision hole, it indicates that the size of the precision hole we processed is OK. On the contrary, it means that the hole is too large or too small, or the roundness is not enough.


  • Feeler gauge

The feeler gauge is generally used to detect relatively narrow grooves or the flatness of products. This tool is composed of many very thin feeler pieces, and the general specification of the feeler pieces is 0.01-2.00 (mm). They are freely combined into the size we need, and then we use it to measure the CNC parts we need to measure.


  • Thread gauge

The thread gauge is divided into thread plug gauge and thread ring gauge, which are respectively used to measure internal threads and external threads. Due to the particularity of the thread, each specification of the thread corresponds to a certain type of thread gauge and is not universal. At the same time, one type of thread gauge is also divided into a thread go gauge (T) and a thread no-go gauge (Z). When using the thread gauge to measure the thread, first we need to use the thread go gauge to measure whether the thread can be rotated easily and smoothly and then use the thread no-go gauge to detect whether it can be prevented from rotating. The purpose of the no-go gauge is to make the thread more standard and more durable.



Thread Plug Gauge



Thread Ring Gauge

4. Projector

The measuring projector is also known as the optical projection inspection instrument or optical projection comparator. It uses the principle of optical projection to project the contour of the workpiece being measured onto the observation screen and then conducts measurement or comparison, which can efficiently detect the contour and surface shape of various workpieces with complex shapes. On the one hand, it is used to measure the contour dimensions and surface conditions of complex workpieces, such as various tools, cutters, and parts like cams, threads, gears, and forming cutters.



5. Coordinate Measuring Machine (CMM)

The coordinate measuring machine refers to an instrument that can demonstrate measurement capabilities such as geometric shapes, lengths, and circular graduations within a hexahedral space range. This instrument can also be defined as: "A detector that can move in three directions and can move on three mutually perpendicular guide rails. The secondary detector transmits signals in a contact or non-contact manner. The displacement measurement system of the three axes (such as a grating scale) is calculated by a data processor or computer, etc. to obtain the points (x, y, z) of the workpiece and various functional measurement instruments." The measurement functions of the coordinate measuring machine include dimensional accuracy, positioning accuracy, geometric accuracy, contour accuracy, etc.



The usage methods mainly include the following steps:

  • Startup steps:

The first step is to open the air source to ensure that the air flowing to the air filtration system of the CMM is strictly dried and filtered.

The second step is to turn on the computer. The air flotation system of the CMM will be ventilated only after the computer is powered on.

The third step is to open the air valve of the CMM and supply air.

The fourth step is to start the control software.

Finally, zero the measuring machine set the zero position of the measuring machine at the lower left corner of the machine table, move the measuring machine to the zero position, and click the OK button.

  • Scanning methods:

Open Linear Scan: Applicable in the absence of a CAD model, involving inputting boundary points, direction points, and step lengths.

Closed Linear Scan: Applicable for scanning inner or outer surfaces, requiring starting points and direction points.

  • Operation process:

Cleaning work: Use absolute alcohol and dust-free cloth to clean the equipment platform.

Placing fixtures: Use flat-jaw pliers or hot melt adhesive to fix the workpiece.

Starting the equipment: Open the air pressure valve, check the air pressure value, and turn on the power supply and emergency stop button of the control cabinet.

Starting the operation: Initialize the coordinate system in the AC-DMIS software, select the probe for measurement, and record the dimensional data.

Completing the measurement: Ensure that the three axes return to their original positions and are locked, and close the air source and computer.

  • Precautions:

First, it is important to pay attention to keeping the temperature and humidity in the measurement room within a specific range.

Second, it is important to regularly clean the guide rails and wipe the operation tabletop with a dust-free cloth.

Third, it is important to carry out temperature constant temperature and verification of the measuring machine after long-term shutdown.


Through the above steps, the coordinate measuring machine can be effectively used for precision measurement.

Solutions for Using Detection Tools in Design

The above is an introduction to the common measuring tools in CNC machining and their usage methods for everyone. Of course, there are also other special detection tools. Due to factors such as usage frequency, they are not introduced one by one here. For example, there are hardness testers for professional hardness measurement, spectrometers for measuring material composition, dynamic balance detectors, and so on.

If your design requires the use of these detection tools, but you cannot especially purchase professional detections just for a certain product, what should you do? First of all, we can consider professional third-party testing agencies, and they will conduct corresponding measurements on your product and issue a test report. In addition, you can also choose a supplier with detection capabilities to process your product when processing. They can not only control the quality of your product during processing but also issue a test report for the corresponding product through a series of detection equipment after processing is completed. For example, choose Richconn to help you process. Richconn has always adhered to the principle of service first and quality first and has always been dedicated to serving customers and quality.

Richconn: Your Trusted Partner for Product Quality Assurance

To ensure the quality of the products, Richconn has to go through the following several steps of testing from processing to shipping, and the products will only be released after all are qualified.

  • Richconn will regularly maintain, service, and calibrate the testing equipment to ensure the effectiveness of the measuring tools. It will also regularly invite professional quality engineers to conduct professional technical training for the quality inspection personnel to enhance the quality ability and quality awareness.

  • At the raw material end, professional quality inspection personnel in Richconn will test the raw materials according to the drawings and customer requirements, including material performance, whether there are surface defects, shape defects, etc.

  • When Richconn is processing, skilled inspection personnel will conduct dimension and appearance inspections on the products in processing. When defects are found, they will promptly notify the technicians to make timely adjustments.

  • After the CNC parts are processed, Richconn will also conduct a comprehensive inspection of all the dimensions and appearances of the products and issue corresponding test reports to ensure that every product given to the customers is qualified.


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