Where to Install the Coordinate Measuring Machine
Where is the best place to install the coordinate measuring machine? This is often the dilemma many manufacturers face: Should it be placed in the quality lab or on the shop floor? The choice really depends on what you are trying to achieve and the type of results you want.
Quality lab. This will be the first choice of the majority of CMM installations, as it certainly is a better working environment than the shop floor. The lab provides a stable measuring environment where temperature, humidity, and vibration are controlled. The coordinate measuring machine works optimally in these conditions. Maintenance is also kept to a minimum when dirt and contaminants are not an issue, and therefore costs are kept to a minimum. In most cases the quality lab, located in the centre of a diversified manufacturing facility, is the best option. There the coordinate measuring machine can be used efficiently to measure accurately a wide variety of components and provide a continuous audit on manufacturing performance.
It should also be noted that most coordinate measuring machine manufacturers provide the environmental conditions under which the machine should function optimally. The following temperature conditions are always stated:
- Ambient temperature. A minimum and maximum should be given; for example, 18 - 22°C, although most accuracies are centred at 20°C.
- Maximum air temperature variation in time. This is usually stated as a per-hour range or over a 24-hour period.
- Maximum gradient. This is generally stated in vertical space; for example, 1°C per meter vertical.
- Maximum relative humidity. This value is usually between 25-75%, non-condensing.
When these guidelines, stated by the manufacturer, are exceeded, it is acceptable to de-rate the coordinate measuring machine's performance. As you can see, the above temperature restrictions can only be met in the quality lab. Therefore, when choosing a high accuracy coordinate measuring machine it is important to understand fully the manufacturers' restrictions for optimum performance. In all cases measuring machine accuracy is closely tied to temperature.
The shop floor presents a completely different set of circumstances. Shop floor CMM machines are becoming extremely popular. They do not replace a lab-based CMM, but they add some tangible benefits to the production process. With fixed gauging being extremely expensive, many companies are turning towards a flexible measurement solution. The coordinate measuring machine can be easily programmed to accommodate engineering changes on the part, or reprogrammed for completely new parts; this, of course, is not possible with fixed gauging. Shop floor coordinate measuring machines provide the benefits of real-time process monitoring with the same temperature and environmental conditions as the manufactured part.
The shop floor CMM can be manually loaded and unloaded, automatically loaded and unloaded or totally integrated into the manufacturing process. Not only can they monitor and report dimensional conditions, but they also can provide corrections to the machine tool.
A CMM for the shop floor differs physically from the lab machine. The machine must be thermally stable. Thermally predictable materials must be used in order that the temperature compensation software may calculate the effects of temperature change at varying degrees of change.
Machine guideways are normally covered or protected from dirt and contaminants in the shop. Hard bearings are generally preferred, as the use of air bearings often causes internal contamination of the machine's ways and guidance system.
The ISO 10360-2 calibration specification calls for a hypothetical permissible error over different temperature bands:
MPEE = 3.0 + 3.0 * L / 1000 (18-22°C)
MPEE = 3.3 + 4.2 * L / 1000 (16-26°C)
MPEE = 3.5 + 5.0 * L / 1000 (15-30°C)
This gives the manufacturer using the CMM a quantifiable number as to how the machine will perform under differing conditions. The latest generation of CMMs for the shop floor use predictive temperature software compensation that utilizes a wide range of sensors, which continually monitor the environment and provide real-time compensation to the machine.