CMM Inspection for CNC Machined Parts: A Complete Guide

For CNC machined parts, precision is a very important factor. Components from multiple industries including aerospace, automotive, industrial equipment, and medical devices all have high requirements for precision. When it comes to inspecting the accuracy of parts, we must mention the Coordinate Measuring Machine (CMM).

It can be said that CMM inspection is currently one of the most effective quality control methods. It helps you perform high-precision measurements on parts, thereby ensuring dimensional accuracy and batch consistency. This guide will detail the principles, types, etc. of CMM inspection, allowing you to gain a deeper understanding of it.

What is CMM Inspection?

Coordinate measuring machine (CMM) inspection is a high-precision metrology technique used to verify the dimensions and geometry of manufactured parts. During inspection, the CMM uses contact probes, scanning probes, or optical sensors to record measurement points in the X, Y, and Z directions, collecting data points from the surface of the part. The measurement software will use these measurement point coordinates to construct the geometric features of parts in a three-dimensional coordinate system, such as planes, holes, cylinders, curved surfaces, and profiles. Finally, you can compare the actual test data with the specifications in the engineering drawings and CAD models.

In addition, the CMM inspection report issued by the inspector typically includes nominal values, measured values, deviations, tolerance ranges, and compliance status. Therefore, in the manufacturing industry, CMM inspection is also commonly used for first article inspection, incoming material inspection, process verification, and final quality inspection.

What Can a CMM Measure?

• Dimensions: diameter, distance, depth, wall thickness
• Shape: flatness, roundness, cylindricity
• Orientation: parallelism, perpendicularity, tilt
• Position: position, coaxiality
• Contour: line contour, surface contour

How Does a Coordinate Measuring Machine Inspect a Part?

In fact, CMM inspections typically begin with a review of engineering drawings and CAD models. Inspectors first need to confirm part revisions, units of measurement, datum systems, critical dimensions, and GD&T tolerances. Then they can develop a reasonable inspection plan based on the part’s structure, including the clamping method, probe configuration, number of sampling points, and measurement sequence.

Please note that before formal measurement, you must ensure that the parts are clean and at a stable temperature. This is to avoid oil stains, burrs, thermal expansion and contraction, or excessive clamping from affecting the final inspection results.

After the part is fixed on the measuring platform, the operator can use the standard sphere to calibrate the probe, and then establish the part coordinate system and write a measurement program according to the datum specified in the drawing. The CMM’s probe contacts or scans the part’s surface, while the machine’s linear scales and encoders record the X, Y, and Z coordinates of each measurement point. At this time, the measurement software will construct geometric features such as planes, circles, holes, cylinders, and profiles of the part, and calculate size, position, direction, and shape errors.

After data collection is complete, the software compares the actual measurement results with the nominal values in the CAD model or the tolerances specified in the engineering drawings, and generates a detailed report.

Coordinate Measuring Machine Components and Operating Principles

A coordinate measuring machine consists of a mechanical structure, a position feedback system, a probe, a motion controller, and measurement software. Each component has its own specific function. Next, I will explain their functions in detail.

Mechanical Structure and XYZ Axes

The mechanical structure of a CMM enables the probe to move in the X, Y, and Z directions. Let’s take the most common bridge CMM as an example. The crossbeam moves along the measuring platform, the slide moves along the crossbeam, and the spindle is responsible for vertical movement.

The measurement platforms typically use dimensionally stable, high-rigidity materials to provide reliable part support and measurement reference.

Linear Scales and Encoders

Each motion axis of the CMM is equipped with a position measurement system, typically consisting of precision linear scales and encoders. When the probe moves along the X, Y, and Z axes, the encoder continuously reads the position of each axis.

What you should note is that CMM measurement does not directly read the dimensions of parts; it establishes the positions of multiple spatial points and then calculates distances, angles, diameters, and geometric errors based on these points.

Probe and Stylus System

The probe system can be said to be a very important component of CMM equipment, which is responsible for sensing part surfaces. Common probe system tools we encounter include touch-trigger probes, continuous scanning probes, and non-contact optical sensors.

• When the probe ball contacts the workpiece surface, the touch-trigger probe will emit a trigger signal, and the CMM system will immediately record the current coordinates.
• Scanning probes can move continuously along the part surface, collecting a large number of coordinate points in a short time.
• Optical sensors are suitable for soft, delicate, or features that are not suitable for contact measurement.

Motion Controller and Drive System

The motion control system is responsible for driving each coordinate axis of the CMM and controlling the movement direction, speed, and acceleration of the probe. The equipment can be manually controlled by operators using a joystick or automatically operated according to pre-written programs.

Measurement Software

Measurement software is responsible for converting coordinate data into understandable dimensional and geometric results. It first establishes the part coordinate system based on the measured datum features.

After that, the software will utilize the collected coordinate points to construct geometric features of parts such as planes, circles, spheres, cylinders, cones, and free-form surfaces in the coordinate system. Meanwhile, the software will also calculate diameter, distance, angle, position and form errors.

You should know that different fitting algorithms, filtering methods and point sampling strategies may yield different results.

How the Operating Principle

Actually, I believe you have gained a general understanding of the working principle of CMM detection through the detailed workflow introduction above. Here, I will summarize the basic working principle of CMM into five steps:

1. We first need to establish a part coordinate system using reference features.
2. The probe contacts or scans the part surface.
3. Encoders record the X, Y, Z coordinates of each measurement point.
4. Software constructs geometric features based on coordinates and calculates dimensions.
5. Actual measurement results are compared with CAD nominal values or drawing tolerances.

Advantages of CMM Inspection

In today’s manufacturing industry, the accuracy of parts is related to product performance, reliability and customer satisfaction. CMM testing can provide manufacturers with accurate measurement data to ensure product quality.

Dimensional and Geometric Inspection

CMM can measure the length, hole diameter, hole spacing, angle, depth and spatial relationships between multiple features of parts in a single setup, and can also evaluate flatness, roundness, perpendicularity, parallelism, position and profile.

Improve Measurement Accuracy and Repeatability

CMM measurement systems can help you reduce human influence and systematic errors, thereby keeping part measurement data consistent across multiple measurements. This repeatability is important to verify tight geometric dimensions and tolerances (GD&T), ensuring part reliability and performance.

Reduce Waste and Rework

Once errors occur in parts during the manufacturing process, the batch of parts will be reworked or directly turned into scrap. CMM inspection can help you discover part size deviations in time, so you can take corrective measures before the parts enter the production process. This undoubtedly saves a lot of material waste and rework costs.

Ability to Inspect Complex Geometries

Parts such as complex shells, blades, and multi-axis CNC machined components all feature complex geometric structures and curved surfaces, which cannot be measured by traditional measuring tools. However, CMM can establish a three-dimensional coordinate system based on CAD models and engineering drawings, and collect surface coordinates through contact probes, scanning probes, or optical sensors.

Automated and Efficient Inspection

For repetitively produced parts, CMM inspection programs can be saved and called repeatedly. And , CMM equipment can automatically inspect various features of a part from multiple directions. This is particularly advantageous for complex parts containing a large number of dimensional and GD&T requirements.

In addition, the CMM system can automatically calculate results and generate reports once the measurement is complete.

Digital Reports and Traceability

CMM software is powerful and can record inspection conditions, measurement strategies, and measurement results. CMM inspection reports typically contain data such as nominal values, measured values, deviations, tolerance ranges, and compliance status.

These digital records are helpful for customer audits and are also very beneficial for first article inspection, batch traceability, and quality system management.

Types of CMM Inspection Machine and How to Choose One

Different types of parts require different CMM equipment. CMMs can be mainly divided into five categories based on their mechanical structure: bridge type, cantilever type, gantry type, horizontal arm type and portable type.

When you are selecting suitable CMM equipment, in addition to considering the measurement range, you also need to simultaneously consider factors such as part weight, tolerances, feature structure, and operating environment.

Bridge CMM

Bridge CMM is the most common coordinate measuring machine in precision manufacturing and is the most frequently used equipment by many manufacturers. It typically consists of a granite platform, movable bridge, lateral carriage, and vertical spindle, while the probe can move along the X, Y, and Z directions.

Bridge-type CMMs have strong structural characteristics and are very suitable for measuring small to medium-sized precision parts. For example, CNC machined housings, bearing seats, mold components, aerospace structural parts, and medical parts can all be inspected using bridge-type CMM detection.

There is just one limitation. The measurement range of a bridge-type CMM is relatively fixed, and the part must be able to fit inside the machine and be securely clamped.

When using bridge CMMs, in addition to confirming part dimensions, you also need to reserve space for fixtures, probe length, rotating probe heads, and motion paths.

Cantilever CMM

The beam of a cantilevered CMM is supported on only one side, and the measurement area usually remains open in three directions.You can also more conveniently disassemble parts. This equipment is commonly used for parts that are small, lightweight, and require quick setup, such as precision mechanical components, electronic components, and small mold parts.

Additionally, due to its single-sided support structure, the structural rigidity and effective measurement range of cantilever CMMs are significantly inferior to bridge-type or gantry-type CMMs.

Gantry CMM

For large parts that need to be inspected, you definitely need to use a gantry CMM. The beam of this equipment is mounted on columns or foundation structures. The measuring machine body moves above the large workpiece, and the parts are placed on a ground foundation or dedicated platform.

Gantry CMM is suitable for measuring large and heavy workpieces, such as large molds, engine components, aerospace structural parts, and heavy machinery parts, etc. In addition, the equipment, foundation, and constant temperature environment costs of gantry CMM are relatively high.

Horizontal-Arm CMM

As the name suggests, the measuring spindle of the horizontal arm CMM extends from the horizontal direction of the column, allowing access to large thin-walled parts or assemblies from the side. It usually has two configurations: single arm and double arm.

The equipment is ideal for inspecting sheet metal parts, automotive bodies, large molds, instrument panel brackets and aerospace thin-walled structural parts. The open measurement area makes it convenient for you to disassemble various large parts.

However, one point you need to pay attention to is that the measuring arm of the Horizontal-Arm CMM is easily affected by structural deflection and environmental changes. Therefore, this equipment often has difficulty achieving the accuracy level of bridge CMMs in metrology rooms.

Portable Articulated-Arm CMM

Portable articulated arm CMM consists of multiple rotating joints, angle encoders and a contact probe. You can bring the equipment to where the parts are located and manually move the probe to complete measurements. It is well suited for inspecting large fixtures, welded assemblies, machine tools, molds, and heavy parts.

However, the measurement results of articulated arms are more susceptible to influences from multiple aspects such as operators, equipment placement, and joint postures. Therefore, their accuracy and repeatability are usually lower than bridge CMMs

CMM Type Comparison Table

Below, I have also compiled a comparison table of various CMM types for you.

Accuracy Standards and Industry Specifications

To determine whether the inspection of a coordinate measuring machine (CMM) is reliable, you cannot only focus on the equipment’s accuracy, but you must also verify which standards the equipment follows, how measurement errors are expressed, and whether the inspection results meet the customer’s quality requirements.

ISO 10360 Series Standards

ISO 10360 is the core series of international standards for acceptance and re-inspection testing of coordinate measuring machines (CMMs). It specifies how to evaluate the length measurement errors, probing errors, and performance of various sensing systems of CMMs under controlled conditions.

Note:

ISO 10360 is mainly used to evaluate equipment performance. It does not directly specify whether a particular part is qualified. Whether your parts can ultimately be accepted depends on drawing tolerances, measurement procedures, clamping methods, and other factors.

ASME B89.4.1

ASME B89.4.1 is a representative standard in the American coordinate measuring machine performance evaluation system. It is mainly used to evaluate the length measurement capability and related performance of CMM under specified conditions. Its basic logic is similar to ISO 10360.

In US customer projects and aerospace projects, you may still see ASME B89.4.1. However, some projects may have already adopted the ASME B89 standard or a standard harmonized with ISO 10360. Therefore, you should confirm the customer’s requirements before conducting inspections:

• Standard number and version;
• Test items and measurement directions
• Environmental temperature and equipment status
• Acceptance limits and criteria;
• Calibration Certificate
• Traceability Requirements

MPE（Maximum Permissible Error）

MPE refers to the maximum permissible error that equipment is allowed to have under specified measurement conditions. The length measurement MPE of CMM is usually expressed as:

$$ MPE = \pm(A + L/K)\ \mu m $$

or

$$ MPE = \pm(A + B \times L)\ \mu m $$

Where:

• A: Basic error independent of measured length
• L: Length being measured;
• K or B: Coefficient indicating how the error increases with the measured length;
• ±: Indicates the allowable error range in positive and negative directions.

NOTE:

You cannot directly understand MPE as part tolerance, and MPE is not equivalent to measurement uncertainty. When selecting a CMM, you should compare the calculated MPE with the actual part tolerance, rather than only comparing the basic value A in the formula.

FAI and AS9102 in Aerospace Inspection

Finally, let me mention FAI and AS9102 in the aerospace field. In the aerospace industry, FAI is used to verify that the actual production process can manufacture parts that meet all engineering requirements. AS9102 specifies the implementation and documentation framework for aerospace first article inspection.

CMM is mainly used in FAI to verify dimensions and GD&T features. The results in the CMM report can be used to support verification of AS9102 characteristics.

CMM Inspection in CNC Manufacturing

In the CNC manufacturing industry, CMM inspection can be said to be an important link in CNC machining quality control. CMM inspection can evaluate dimensional, positional and geometric tolerances, thereby ensuring that CNC machining parts quality can meet customer standards.

First Article Inspection

After completing CNC machine setup, manufacturers typically conduct CMM inspection on the first piece. The inspection results can help engineers determine the causes of deviations.

In-Process Inspection

CMM can also be used for process validation between critical processes. Typical applications include:

Final Inspection and Traceability

In the final inspection stage, CMM programs should be established in accordance with applicable drawing versions, datum systems, and GD&T requirements. Quality inspectors need to use CMM equipment to conduct detailed inspections of CNC machined parts to ensure that all inspection data, including CNC machining tolerances, meet qualified standards.

Afterward, these finished parts can be packed and shipped from the factory.

Using CMM Data to Improve CNC Processes

Another point I want to mention is that many smart engineers know how to make good use of CMM data. Through these continuous measurement data, we can know about some changes that occur in the processing process. For example:

• If the hole diameter gradually decreases as the number of parts machined increases, this may indicate that the machine tool’s cutting tool is wearing out;
• If multiple features on a part exhibit positional deviations in the same direction, this may be related to the workpiece coordinates or fixture positioning;
• The length changes continuously over the course of machining, which may be related to the temperature of the machine tool or the workpiece.

In fact, there are many more examples like the ones above. What you need to do is integrate these measurement data for analysis. In this way, you can transform the CMM from a final inspection tool into a CNC process optimization tool.

Manufacturing Process Suitable for CMM Inspection

CMM inspection is suitable for manufactured parts with CAD geometry, dimensional tolerances, or GD&T requirements. Different processes cause errors for different reasons, so the role of CMM is not only to determine whether parts are qualified, but also to help you locate process problems through dimensional and geometric deviations.

CNC Machining

This is one of the most common processes. During CNC milling, turning, drilling and other machining processes, part accuracy can be easily affected by tool wear, clamping deviation and multiple directional machining operations.

A CMM can inspect a part’s hole diameter, hole location, flatness, perpendicularity, coaxiality, positional accuracy, and contour in a unified coordinate system, and directly compare the results with CAD models or engineering drawings. Furthermore, these measurement data can also help you well in correcting tool compensation, workpiece coordinates, and clamping schemes.

Sheet Metal Fabrication

In sheet metal cutting, stamping, metal bending and other processes, sheet metal parts may exhibit issues such as springback, hole misalignment, and bending angle errors.

A CMM can inspect the positioning of holes relative to bent edges, flange angles, contour accuracy, and flatness. The measurement data it outputs can help you determine whether the problem comes from blanking, stamping, bending compensation, or welding processes.

Forging

Forged parts are susceptible to mold wear, misalignment between the upper and lower dies, material flow, and cooling deformation.

A CMM can measure the overall contour, reference surfaces, critical cross-sections, and machining allowance distribution of forged parts, helping you determine whether a blank is suitable for subsequent machining.

Casting

Common issues with castings and die-cast parts include material shrinkage, uneven cooling, warping, and core displacement.

Coordinate Measuring Machines (CMMs) can establish stable reference datums for castings and inspect their contour profiles, wall thickness positions, hole positions, bosses, and assembly interfaces. You can also use the inspection results to adjust mold sizes, optimize machining reference, and monitor the dimensions of castings from different production batches.

Injection Molding

The dimensions of injection-molded parts are susceptible to material shrinkage, holding pressure, mold temperature, and demolding deformation.

CMM can inspect the hole positions, flatness, contour, and assembly features of injection molded parts. It can also compare dimensional variations of injection molded components under different cavities, batches, and process parameters. This data helps you adjust mold compensation, holding pressure, and cooling conditions.

How to Read a CMM Inspection Report

Regarding how to correctly read a CMM Inspection Report, I believe quite a few people still feel confused. Here I will also make a statement.

You cannot just look at the green PASS or red FAIL when reading CMM inspection reports. First, you need to make sure the report corresponds to the parts, drawings, and measurement procedures. Then, you need to understand the meaning of each column of data in the report, and finally combine the reference standards, measurement uncertainty, and deviation trends to determine whether the results are reliable.

Verify the Report Information

Before you read the dimensional data, first check the traceability information in the report header or page header, including:

• Part Name and Part Number
• Serial Number, Lot Number, or Sample Number
• Engineering Drawings and Version
• CAD Model Version
• CMM Program Name and Version
• Units of Measurement
• Inspection Date and Operator
• CMM Equipment Number
• Probe and Probe Tip Configuration
• Part Temperature or Environmental Conditions
• Reference Points and Coordinate Alignment Methods Used

After all, if the report uses the wrong drawing version or measurement units, all subsequent compliance conclusions may be invalid.

Understand the Main Report Columns

Although the report formats of different CMM software are not completely the same, they usually include the following fields:

For standard sizes, we usually calculate the deviation as follows:

Deviation = Actual Value − Nominal Value

 Positive deviation indicates that the measured value is greater than the nominal value, while negative deviation indicates that the measured value is less than the nominal value.

Read Nominal, Actual, and Deviation

Here, I’ll give you a direct example so you can understand more clearly.

For example: a drawing requirement for an aperture is:

15.000 ± 0.020 mm

Therefore, the acceptable range for the hole diameter of this part is 14.980–15.020 mm.  If the report shows:

• Nominal：15.000 mm
• Actual：15.015mm
• Deviation：+0.015mm
• Status：PASS

It shows that the measured hole diameter is 0.015 mm larger than the nominal value, but still within the allowable range.

Check the Datum and Alignment

Additionally, for GD&T items such as position, perpendicularity, parallelism, and profile, measurement results are directly related to the part coordinate system. The report should specify which reference features were used and the order in which they were used to establish the part coordinate system.

Interpret GD&T Results

The report method for the GD&T characteristics is also different from that of standard sizes. Let me give a simple example:

• Position Actual：0.18 mm
• Position Tolerance：0.20 mm
• Status：PASS

It indicates that the measured position error is 0.18 mm, which does not exceed the allowable 0.20 mm.

If the drawing also contains Maximum Material Condition (MMC) or Least Material Condition (LMC) requirements, then this CMM report may also include bonus tolerance.

Application industries Of CMM inspection

Many industries rely heavily on the dimensional accuracy of parts to ensure the overall quality of their products. So, in which industries exactly can CMM inspection be used?

Aerospace

Aerospace components have very high precision requirements, as well as strict GD&T specifications and comprehensive quality traceability needs. CMM can effectively help verify the critical dimensions of various aerospace components, ensuring that their measured data meets quality standards.

Automotive

The automotive industry involves a large number of die-cast parts, forged parts, stamped parts, injection molded parts and CNC machined parts. Common auto parts include engine blocks, cylinder heads, transmission housings, gears, drivetrain components, etc.

CMM can be used for first article inspection, production process sampling inspection, mold verification and final dimensional audit.

Medical Devices

Medical device components are usually small in size and have precise structures. These components require stable and traceable inspection methods. Through CMM inspection, manufacturers can ensure the quality stability of medical parts.

Precision Machining and Industrial Equipment

Precision machining companies manufacture various types of parts, and CMM is a common universal inspection equipment for such companies. Typical parts include shaft supports, valve blocks, pump bodies, custom fixtures, etc. You can use CMM to measure multiple features of parts such as holes, planes, cylinders, etc. in a single setup.

Electronics and Semiconductor Equipment

Electronic and semiconductor equipment typically contains small, thin-walled or high-precision components, such as connectors, heat sinks, optical mounts, etc. CMM can inspect the mounting holes, datum surfaces, parallelism, perpendicularity, etc. of these electronic precision parts.

Energy and Heavy Engineering

The energy industry uses a large number of parts that are large in size, heavy in weight, and complex in structure, such as turbine components, compressor casings, pump bodies, valve bodies, and so on. For these large parts, you can use gantry-type CMM and horizontal-arm CMM to inspect their dimensional features.

Mold

The mold industry is also not to be ignored. This is because the dimensions of the mold directly affect the shrinkage, profile, and assembly features of the final product.

Therefore, you can also use CMM to inspect the cavity contour, datums, hole positions, etc. of molds. Moreover, after mold repair, you can also confirm whether the correction amount is accurate through repeated measurements.

Common Causes of Incorrect CMM Results

Finally, based on some common CMM inspection problems that everyone encounters in reality, I have developed corresponding solutions, hoping to be helpful for your future work.

Conclusion

Alright, that’s all the content about CMM inspection. Only through CMM inspection can the manufacturing quality of CNC parts be guaranteed.

DSR is a professional CNC manufacturer equipped with advanced CNC equipment and CMM inspection equipment. We can manufacture various high-quality parts for you and ensure their quality through strict quality inspection processes. Our quality inspectors will follow your requirements and drawing specifications to conduct detailed inspections of part specifications. We believe that the parts delivered to you will definitely meet your usage requirements.

FAQ:

Q1: What does CMM stand for?

A: Coordinate measuring machine.

Q2: How accurate is a CMM inspection?

A: The accuracy of CMM inspection depends on factors such as the equipment class, measurement range, probe system, ambient temperature, fixture stability, and measurement procedures.

Q3: What are the 4 types of inspection?

A: Incoming inspection, in-process inspection, final inspection and pre-shipment inspection.

Q4: What is a CMM inspector?

A: CMM inspector refers to quality personnel who use CMM to perform dimensional, geometric tolerance and critical feature inspection of parts.

Q5: What is QA and QC inspection?

A: They all belong to quality management. QA is more focused on prevention and system management, while QC is more focused on detection and determination.

Q6: How long does a CMM inspection take?

 A: Dimensional inspection of simple parts may only take a few minutes to ten minutes; CNC machined parts of medium complexity typically require 30 minutes to several hours; for parts such as aerospace, automotive molds, complex housings, complete CMM inspection may take several hours or even longer.

Q7: Can a CMM inspect GD&T tolerances?

 A: Yes. CMMs can measure most GD&T requirements, such as positional tolerance, flatness, straightness, roundness, cylindricity, perpendicularity, parallelism, coaxiality, symmetry, profile, and runout.

Q8: What ISO standard is used for CMM inspection?

A: ISO 10360 series, ISO 1101, ISO 5459, and ISO/IEC 17025, etc.

Q9: Which software is used for CMM?

A: Common CMM inspection software includes: PC-DMIS, ZEISS CALYPSO, PolyWorks Inspector, MCOSMOS, CMM-Manager, MODUS, Metrolog X4, RationalDMIS, etc.

Q10: What are the disadvantages of CMM inspection?

A: The equipment cost is high, the detection speed is relatively slow, and CMM has high requirements on the environment, operators and measurement procedures.

Q11: What are the key steps in an inspection?

A: Review requirements, prepare setup, measure part, record results and judge and act.

Q12: What is the purpose of a CMM?

A: The main purpose of CMM is to perform high-precision measurement and verification of the geometric dimensions and geometric tolerances of parts, confirming whether the parts meet the requirements of drawings, CAD models, or customer specifications.

Q13: What is the difference between a CMM and a 3D scanner?

A: The main differences between CMM and 3D scanner are: