When a manufacturing facility needs to inspect and test a part or assembly against the design specifications, it often uses a Coordinate-Measuring Machine (CMM). CMMs use a variety of mechanical, optical and laser probes to measure the geometry of a given object and are controlled either manually or by Direct Computer Control (DCC). Prominent CMM manufacturers include Mitutoyo, ZEISS, TIGO, Renishaw and Hexagon. They come in different sizes and configurations, each with their pros and cons. Some are stationary and ideal for measurement of large or fragile components, while others are portable for flexible shop operation. All are important to ensure your facility produces output that is of consistent quality.
Inspection machines come in different configurations and with different components depending on your needs, budget, and floor space.
There are four basic types of CMM machines, categorized by arm configuration.
Bridge inspection machines are the most popular type of CMM. Their designs are relatively simple and more robust than most other types, and cost less overall than while still being accurate and reliable.
A typical bridge machine allows probe movement across three axes (X, Y, Z) with each axis containing a sensor that monitors the position for the probe along that axis. When the probe contacts or detects a location on an object, the machine samples the three position sensors and measures the relative location of the point on the object's surface. This process is repeated multiple times, with the probe moving to different locations, until enough measurements have been made to accurately map an object's surface area.
Cantilever CMM machines are machines whose beam is only fixed on one end, with the other projecting outward from the base, and the measuring arm hanging down from said beam. Cantilever machines aren't manufactured as widely as bridge CMMs, but they do have situations where they can excel.
A cantilever machine is open on three sides, which allows it to excel in configurations that need automatic loading and unloading. Cantilever inspection machines are generally used for measuring relatively small parts.
Gantry CMM machines are ideal for measuring large or heavy parts. They need a solid foundation, and are usually installed directly into the floor and take up a lot of real estate. Even smaller gantry machines are require multiple columns to support the measuring beams. Because of the enormous investment required to assemble them, don't expect to see many used gantry inspection machines on the market.
Horizontal arm CMM machines have a vertical column for the X-axis, on which is mounted the horizontal arm, the Y-axis. The probe runs along the Y-axis and goes up and down the vertical column to change height.
Horizontal arm inspection machines are the least accurate of the four types, but can measure large components with sizes or orientations that bridge CMM machines cannot handle, such as car doors, sheet metal, and the like.
CMM probes can be categorized into three general types: touch-trigger, displacement, and non-contact.
Touch-trigger probes are the most common type of inspection machine probe. They touch the surface of the object being measured, then send a signal with the point's coordinates to the CMM. This process is repeated until the scan is complete.
Displacement probes pass over an object's surface. As it does so, the probe is constantly transmitting information to the CMM about what it is detecting. Displacement probes use linear variable differential transformer (LVDT) technology or high-quality optics to scan objects.
Non-contact probes are similar to displacement probes, but use different technologies to collect data. Most non-contact probes use laser, capacitive, or video measurement technology to take their measurements.
Cost shouldn't be the first thing you consider when buying a used coordinate-measuring machine. Instead, you should determine what you're measuring and where you're measuring it. Large, unwieldy items will require a portable unit, while small parts can be measured on a stationary CMM. You should also consider what probes you will need, whether contact or non-contact, and whether speed or accuracy is more valuable to you. Lastly, assess the software of the CMM and see if its functionality match your requirements.