Table of Contents

I. How Does a CMM Work?
II. Types of CMM Styli/Probes
III. CMM Software
IV. Principles of CMM
V. Benefits of CMM
VI. Four Most Common Problems Solved
VII. Get In Touch

I. How Does a CMM Work?

Coordinate measuring machine

The object to be measured is placed on the stage of the machine. Made of stable, rigid, and dense materials, the area is designed to keep objects stable (to ensure accuracy and precision). The probe touches the points along the X, Y, and Z axis.
The computer accurately measures the dimensions based on the signals sent by the probe. The process is repeated for each facet of the object until the dimensions of the object are measured.
If using a scanning probe, a point cloud can be created using all the measured points and compared to a 3D design drawing to check dimensional accuracy.

Modern CMMs have five main parts – a stylus; a probe; the machine structure; a controller; and a metrology software application via a connected PC.

II. Types of CMM Styli/Probes

The stylus and the probe essentially function as one unit, but there are important distinctions between them. The stylus typically consists of a metal/carbide shaft with a ruby ball, with diameters ranging from 0.2mm to 6mm or even larger. Ruby is much more affordable than diamond, but almost as hard (ruby is a 9 on the Mohs Scale of Hardness, and diamond is 10). Other use-specific shapes (cylinders, discs) and materials (steel, ceramic) are also available. These styli are then attached to the probe. The probe is one of the CMM’s most important parts. It has highly touch-sensitive sensors designed to collect data points. When the probe contacts the specimen, it triggers the system to record the XYZ value of that point. Once the probe of a CMM collects the measurement data, it is moved to the next point, where the process is repeated.

Renishaw probe for CMM

Here are some common types of CMM probes:

Touch Trigger Probe:
Touch trigger probe is the most common type of probe. As the name suggests, it touches the surface of a workpiece to generate an electrical signal. This signal is sent to the system with the point’s coordinates. Touch trigger probes are highly accurate and capable of measuring the dimensions of a wide variety of objects. You can use a CMM with a touch trigger probe to measure the geometry of anything rigid enough to withstand the very low force of the probing.

Scanning Probes
Scanning probes measure the surface of the object in a similar manner to touch probes, but instead of physically taking discrete points, the scanning probe is dragged along the surface. These probes can generate hundreds or even thousands of points per inch. The density of the points can be adjusted accordingly. Scanning probes are usually used to measure complex contours and gather detailed surface information.  

Non-contact Probes
Non-contact probes, or optical probes, use white light or laser beams to measure the surfaces of objects without touching them. A non-contact probe is the best choice when the surface of the object to be scanned is too flexible for the probe, or too delicate, risking damage to the part.  

Articulating Probes
These have joints or flexible components that allow them to access difficult-to-reach areas in order to measure complex geometries. They can be used to measure features with varying angles and orientations. Most modern CMMs have articulating probes.

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III. CMM Software

The CMM software creates measurement routines that tell the machine what features to measure and how to navigate around the part. This is also where the “measuring” takes place. CMM software constructs the geometric entities (planes, lines, points, circles, etc.) and generates the dimensional info. Many CMM software allows users to see a part as it is being measured in 3D, as well as the measurement points and paths. A good CMM software integrates with CAD, allowing users to compare measurement data with CAD models directly. They can also utilize other tools for data collection and statistical process analyses.

Design engineer working on CAD drawings.

IV. Principles of CMM

Coordinate measuring machines use a three-dimensional Cartesian coordinate system to measure an object’s geometry in 3D (X, Y, Z coordinates for each point are measured and recorded). CMM software analyzes and processes data collected by the probe to generate detailed reports related to the dimensions and properties of the object.  The calibration of CMMs should be checked regularly, as an incorrectly calibrated CMM can produce inaccurate results. Also, inspections should be carried out in controlled environments, as temperature and humidity fluctuations can affect results. Advanced coordinate measuring machines can be integrated into manufacturing systems to perform inspections automatically.

V. Benefits of CMM

There are several benefits of CMMs. They can take accurate measurements and help reduce production downtime and manufacturing costs. CMM inspection services use CMMs to perform quality and dimensional inspections and can be used in reverse engineering to help create drawings of existing parts.

Here are some advantages of CMM.

Accuracy: A CMM can accurately measure complex geometries that might be difficult to measure using manual measuring tools.  

Efficiency: A dimensional inspection performed using a CMM can be completed efficiently and swiftly.  

Versatility: A CMM can be used to measure a wide variety of geometries.  

Reduces Human Error: CMMs automate inspections to minimize human interference. By reducing human error, coordinate measuring machines improve the reliability of measurement data.  

Data Collection and Analysis: A CMM uses data to create in-depth reports that can help the manufacturing team better understand manufacturing variances and identify process improvement opportunities.  

Cost Savings: A CMM can help reduce wastage to improve manufacturing efficiency.

Quality Control: Companies can conduct CMM inspections at regular intervals to ensure their products are compliant.

VI. The Four Most Common Problems Solved With CMM

1. First Article Inspection:
A first article inspection ensures that the first product sample from a production run meets the specified design requirements, specifications, and standards before mass-producing the product.  Coordinate measuring machines (CMMs) can greatly benefit inspection services. They generate accurate and precise measurements, helping manufacturers identify even minor design problems. It's no surprise that CMMs find extensive application across precision-focused industries such as automotive, aerospace, and medical devices. Given the stringent standards these sectors uphold, products from manufacturers in these fields must adhere meticulously to specifications. Even the slightest deviations can have significant repercussions on their performance and safety. A CMM is capable of measuring complex geometries. CMMs can automate the inspection process to reduce the time required to perform first article inspections. They automatically collect and analyze data and generate detailed reports.

2. Part Validation:
Part validation involves testing and analyzing product parts to ensure they meet specific standards and requirements and are reliable and safe.   CMMs can support the part validation process in several ways. A CMM can take precise measurements of complex shapes and surfaces and help compare the dimensions of the parts of a CAD model with that of the product. Reports and data generated by CMMs can be used in statistical process control (SPC) to identify ways to implement continuous improvement initiatives. A CMM can help check if the dimensions of a part fall within the specified tolerances, allowing manufacturers to identify manufacturing errors early.  

3. Gage R&R:  
Gage Repeatability and Reproducibility (Gage R&R) allows manufacturers to evaluate measurement systems used in production. A Gage R&R study can help compare the measurement system variation with process variation and determine if the measurement system is working properly. The Gage R&R ensures that measurement variability comes from the production process and not the measurement methods. CMMs typically provide excellent Gage R&R results. A CMM can precisely and repeatably measure the geometry of a physical object, giving manufacturers quantitative peace of mind to allow them to hone their production process.  

4. Capability Studies:
A process capability study is carried out to assess the ability of a manufacturing process to produce parts that conform to specifications. It can help measure the consistency and reliability of the process and identify areas of improvement.  When used properly, a CMM can help conduct an in-depth capability study. This is also where a good Gage R&R result is crucial. One cannot provide an accurate capability study if it cannot be determined from where the variability came.  CMMs can measure complex parts with high precision and help determine Cp and Cpk values. CMM offers repeatability and can assist manufacturers in accurately measuring the variability of their manufacturing processes.  

VII. Get In Touch

CMM touch probe measuring a plastic cap

Nel PreTech offers a full range of CMM inspection services. We are committed to helping manufacturers improve the quality of their products and decrease manufacturing costs. Our team uses cutting-edge technology and a quality management system designed to prevent dimensional measurement errors.

To learn more, call 708-429-4887.

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