In precision manufacturing, critical parts and components need to have high accuracy and meet industries’ tight tolerances. After using precise and repeatable techniques such as precision CNC machining and 3D printing, it is compulsory to measure the dimensions of an object and compare it with the original design. One way you can go about this is by inspecting using a coordinate measuring machine (CMM).
CMM verifies prototypes and product designs in various manufacturing industries. There are different types of CMM used to maintain workflow and ensure dimensional accuracy. This article will explain what CMM is, the common types of CMM and how they work, and the applications of CMM in manufacturing.
What is CMM in Manufacturing
A coordinate measuring machine is a device that inspects and measures three-dimensional objects. It checks for angular and geometric features such as a line, circle, or square and compares it with the intended design. Using CMM for inspection and quality control reduces design flaws, shortens lead time, and enhances customer satisfaction.
Furthermore, CMM consists of many major and minor parts that allow it to function. It has a structure made of strong metals, a probing system that interacts with the object, a controller, and a metrology software system. When compared to manual inspections using conventional metrology instruments, CMM is fast, repeatable, and produces accurate results.
How Does CMM Inspection Work?
All types of CMM operate in similar ways and have the same end goal; to validate manufacturing processes and ensure that the final parts meet the desired specifications. Below is a step-wise process for carrying out a CMM inspection
Step 1-Preparation and Set Up
- The first step is placing the object on the stage and allowing it to acclimatize to room temperature.
- Thereafter, mount the probe to the machine’s moving axis and calibrate it.
Note that the calibration object should closely match the inspection object in terms of texture, shape, and size.
Step 2-Measurement and Scanning
- The touch probe then scans and moves around the part along the X, Y, and Z axes. This probe does this either by making contact or using a noncontact process with a vision or a laser.
- The probe then sends a signal to the machine which records the coordinates of points on the object’s profile.
Step 3-Data Processing and Analysis
- Thereafter, the software analyzes and processes the data collected. It calculates geometric dimensions and tolerances.
Step 4-Inspection and Output
- Compare the result to the original CAD model and check for deviations.
- If the results meet the measurement objectives, export them using various formats and archive them for future reference.
Types of CMM and How They Are Used
There are four main types of CMM; Bridge, cantilever, gantry, and horizontal arm. They have different structures, advantages, and limitations.
Bridge CMM
This is the most common and widely used type. It has a mobile bridge structure and a probe that moves in three orthogonal axes, X, Y, and Z. The bridge CMM can either be moving or fixed type. In the moving bridge CMM, the bridge, and probe move in the X and Y axes respectively to collect data points for measurement.
On the other hand, the bridge is stationary in the fixed type while the probe does all the movements. Bridge CMM is fast, accurate, and does not alter the integrity of the object. However, it is better suited for small and medium-sized components.
Cantilever CMM
Cantilever CMM has a long measuring arm with a probe mounted at one end. With this CMM type, you can assess the object on three sides allowing for greater flexibility when measuring complex designs.
It is used for relatively small parts and in tight spaces. Cantilever CMM is less accurate than the bridge CMM and the arm induces a bending effect because of the load.
Gantry CMM
The gantry CMM is similar to that bridge type in terms of structure and accuracy level. However, it is bigger and can measure large parts as you can load them directly on the floor.
Gantry CMM is expensive, robust, and not portable. However, it is perfect for industries such as automotive or aerospace that deal with heavy machine components.
Horizontal Arm CMM
This is one of the oldest styles of CMM that mounts its probe horizontally. It is easy to use and can inspect hard-to-reach geometries and thin-walled parts. However, it is the least accurate of all the types of CMM described because the horizontal arm can bend or deflect.
Horizontal arm CMM can either be plate-mounted or runway-mounted. The plate-mounted CMM has a dedicated plate and is more accurate than the runway model.
Applications of Coordinate Measuring Machine
CMM is used for in-process and final inspections in a wide range of industries including aerospace, automotive, health care, pharmaceuticals, and food processing. It is versatile and suitable for both large and small components. The following are some common specific applications of CMM.
Quality Control
CMM is an efficient way to establish quality control in an organization. It inspects and measures critical components to ensure they are dimensionally accurate and meet tight tolerances.
Reverse Engineering
You can also use CMM for reverse engineering. By recording the full geometry of an object, you can create a 3D virtual model out of it. This is useful when creating and designing CAD, CAM, and CAE software. You can then optimize the software to get new, affordable, and more effective products.
Tooling Inspection
CMM also has uses when measuring and inspecting toolings such as CNC cutting tools, molds, and dies before using them for manufacturing. Accuracy and precision are key in mold making because any mistake will affect the final parts.
Sorting Task
You can also use CMM to group parts into different categories based on their accuracy and tolerance level. You group them into very accurate, less accurate, and the least accurate components.
Prototype and Process Verification
CMM is also used in the prototyping stage of product development. It measures the accuracy of rapid prototypes and validates a manufacturing process before full production runs begin.
Limitations of Using a Coordinate Measuring Machine
CMM offers many benefits hence its use in many applications. However, this machine comes with some shortcomings. The following are some of the limitations of using CMM.
Cost
Most coordinate measuring machines are expensive to purchase and set up. It always needs a skilled operator to carry out the inspection process. All these additional expenses will increase the price of the final part.
Material Limitation
One has to be careful when inspecting parts made with soft and delicate materials. This is because as the probe makes contact, it can make the part carve in. This can cause it to deteriorate with time.
Size
The type of the machine will determine the size of the object that you can inspect. This is a limitation for large and heavy parts when a suitable machine is not readily available.
Conclusion
CMM inspection is the process of assessing and measuring the features of a part using coordinate technology. When compared to conventional metrology operations, CMM is less tedious, error-free and repeatable. Hence, it is used for quality control, reverse engineering, and to inspect molds and dies. However, the setup and maintenance may be expensive for small and medium-scale businesses.
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