Buying a CMM or coordinate measuring machine is a major investment for any business both large or small. Selecting the right hardware and software is crucial to the correct return on investment. Here is a short CMM buyers guide that will take you through the steps of making the correct selection for your first or next CMM.
Quality in manufacturing is generally defined as the consistent production of parts that conform to design specifications. However, achieving production consistency requires control over the manufacturing process. The most effective way to establish and maintain process control is by accurately measuring work-piece dimensions. When dimensional information is captured, analysed and fed back into a production operation, it becomes a valuable tool for continuous quality improvement reduce scrap and improve competitiveness.
Coordinate measuring machines or CMM’s offer one of the most efficient ways of measuring and capturing dimensional data because they can replace numerous surface plate tools and expensive fixed gages, and reduce complex measurement tasks from hours to minutes. The ability of CMMs to quickly and accurately evaluate dimensional data and provide the operator with meaningful information concerning the condition of the manufacturing process is what differentiates CMMs from hand-measuring instruments of all types.
CMMs measure by contacting a part with a measuring probe. The acquired data can be combined into a dimensional description of the part or part feature, such as a cylinder, plain of diameter. From the dimensional description, it is relatively easy to determine if the part or feature is within tolerance.
Locating a CMM in the production shop floor
If a CMM is appropriate for your operation, how do you choose the right one? The first decision is which type of CMM configuration to purchase. There are three basic styles to choose from: vertical, horizontal and portable.
Vertical CMMs have the measuring probe attached to the vertical axis. They have the potential to be more accurate than a horizontal CMM because their bridge structure can be more rigid and more stable. Vertical CMMs come in all sizes and can handle work pieces ranging from small gears to engine blocks to commercial aircraft bodies.
Horizontal arm CMMs have the probe mounted on the horizontal axis. They are generally used in applications where large parts, such as automotive vehicle bodies, must be measured with a medium level of accuracy.
Portable CMMs simplify the inspection of parts or assemblies that can’t be transported to a CMM. The portable CMM can be mounted on or even inside the part or assembly. This permits the measurement of interior space and allows users to measure entire assemblies in situ, saving time necessary to remove, transport and measure components. A portable CMM accuracy is much poorer than a bridge CMM.
In any stationary CMM design, increasing mass and rigidity by increasing the cross-section of the structural components and lengthening the distance between bearings, increasing the power of the drive motor and optimizing the selection of structural materials based on weight and thermal capabilities improves measurement accuracy and repeatability along with the speed and acceleration.
Four-axis capability can be added to a CMM by incorporating a rotary table, and dual horizontal arm configurations are also available. Both provide measurement access to all sides of a part. The horizontal configuration also provides accessibility for quick loading and unloading of parts. Small, shop-floor versions are available for high-speed production measurement applications.
Many CMM’s are located on the shop floor but consideration must be given to house the machine in a suitable enclosure to keep the machine form all forms of dust and dirt.
Selecting the Correct CMM Machine
CMMs are available as both manual and CNC models. The choice depends on the application and budget. If you’re inspecting the geometry and tolerances of simple parts, or if you’re measuring small batches of dissimilar parts, a manual machine may be the best choice. Software provided with manual CMMs allows inspection programs to be stored and recalled, facilitating repetitive measuring. For large-batch inspection of similar parts, or where higher accuracy is required, a direct computer control (DCC) machine is the best choice. DCC coordinate measuring machines automate the inspection process and eliminate operator influence on measurement results. Programmed movement means higher measuring speeds without error.
Tolerances are important, too. Tolerances of 0.004mm or less are difficult to achieve repeatedly on a manual machine. A DCC machine with its consistent touch is better suited to high accuracy and repeatability for close tolerance parts.
DCC machines used to measure parts that require massive amounts of data to accurately define their geometry such as gears, cylinder blocks, turbine blades – can be equipped with an analogue scanning probe. These probes provide continuous data collection for parts that are either fully defined mathematically via CAD or completely unknown, as in the case of a component that is to be reverse-engineered. Very small contoured parts can also be ideal for scanning probes due to their smaller measurable surfaces and the need for large amounts of data to define them.
The location of the measuring operation is also important. Ideally, measuring machines should be used as close to the manufacturing process and possible by operators who actually make the parts. These shop-floor CMMs are generally equipped with user-friendly operator software.
Different types of CMMs can be used in the measuring solution adopted. A high-accuracy vertical CMM can be used as a master arbiter of part specifications in a quality lab, while a shop-hardened CMM can be used on-line for auditing work-piece measurement, providing real-time SPC.
Key aspects of a CMM to consider
Once you’ve determined how and where you want to use a CMM, there are key specifications to look for in selecting a particular model: accuracy, repeatability and throughput.
Accuracy is indicated by the ISO10360 specification in the CMM statement of accuracy. By comparing specifications of machines, you can determine the level of confidence to expect from the measuring operation. The lower the number, the more accurate the machine.
Repeatability is the consistency with which a CMM can measure an object having known dimensions. This is determined by the ISO10360. The degree to which manufacturing operations can be fine-tuned depends on the repeatability of the CMM and the machine tool.
The number of data points that can be measured at acceptable levels of accuracy and repeatability determines the CMM machine throughput.
The probing selection is also vital in the selection process. With the introduction of the Renishaw PH20 unrivalled throughput can be achieved. This unique 5 axis probe can access all features with ease and along with it’s 5 Axis Scanning Probe REVO can make a high return on investment.
CMMs can be justified in today’s modern manufacturing operations because they can replace surface plate tools, fixed or custom gages, and precision hand measuring tools. Their flexibility in handling different manufacturing-related jobs makes them very economical. In addition to providing dimensional data for process control, CMMs offer the added advantage of being used for incoming material inspection, machine tool runoffs, customer quality-certification requirements, gauge qualification, tool-wear studies and optimizing machine tool setups. Justification of any piece of capital equipment is determined by a number of factors, but when considering productivity improvements, cost reductions and process control, CMMs are a good choice for your measurement and inspection requirements.
Selecting CMM Measuring Software
Software enables CMMs to fulfil their potential. Today’s CMM software is refined to the point that no computer programming knowledge is required to run even the most sophisticated programs. The newest CMM software is icon driven using Touch, i.e., it asks operators what they want to do and even prompts the most likely choice.
Software programs are available for statistical process analysis and control. Programs for sheet-metal applications facilitate the location and measurement of parts containing hemmed edges, weld nuts and studs, and similar features. A myriad of contour programs permit the CMM to rapidly and accurately define complex, non-geometric shapes without straight edges such as turbine blades, screw and scroll compressors, gears, pistons, cams and crankshafts.
Soft gauging packages make it possible to graphically construct gages that are mathematically inserted into part features to verify dimensions. Additional application modules offer modules for the measurement of non-prismatic parts, such as sheet metal and plastic assemblies, windshields and exhaust systems. This eliminates the need for expensive fixed gauges. Other modules perform 2-D and 3-D best-fit routines and contour comparisons.
Selecting the appropriate software package for your operation is a critical decision in getting the most value from a CMM.
For more advice on our next CMM section please get in touch with CMM Solutions and we can discuss the Coord3 CMM range with you and we can guide you through the process so that the correct selection is made for your application and budget. Click here for our Contact Us Page