تعامل بین قطعه کار و CMM در کنترل کیفیت هندسی در شرایط حرارتی غیر استاندارد

Industrial quality demands have resulted in increasing attention towards the thermal behavior of coordinate measuring machines. The influence of the workpiece on the measurement accuracy is hereby often disregarded. This can lead to significant measurement errors. The described research examines the interaction between the measurement device and the measured object. Four distinct measurement error types that result from non-standard temperatures are listed. Temperature variations during the measurement lead to the most challenging situation. The key to measurement accuracy lies in linking the measurement time to the accompanying temperature variation. The possibilities of this methodology are indicated by an experiment on a reference object.

In today’s industry, there is a growing need for flexible production systems. The increasing quality requirements imposed by the customer give rise to the integration of quality control on the shop floor. As a result, coordinate measuring machines (CMM) are more and more placed outside the protective environment of a metrology room, close to the production process. However, there the reigning ambient conditions are usually not within the boundaries set by international standardization. All dimensional measurements are theoretically considered at 20°C, thus shop floor conditions require managing the influence of deviating temperature on the measurement process [4]. The thermal behavior of a CMM can be dealt with by a software compensation of the thermal deformation of the machine, to complete the geometrical error compensation. Good results were achieved using this method [1], [5], [6], [8] and [9]. The influence of the workpiece however is rarely dealt with, though it can cause huge errors in the resulting measurement even when—or sometimes because—thermal compensation for the machine is used. This paper focuses on the interaction between the workpiece and the CMM during quality control in a non-standardized environment. It first describes the types of errors that can occur and then proceeds to an appropriate measurement strategy for dealing with these errors. Finally the presented ideas are illustrated by an example.

The workpiece forms a source of errors in the measurement process. During temperature variations significant errors might occur. The errors are a result of the temperature dependent relative probe position and the differential expansion between part and scales. Nowadays, CMM-manufacturers commercialise CMMs with thermal compensation systems that try to correct the machine dependent errors. However, most compensation systems lack accuracy during large temperature variations as well as a correct manipulation of the influence of the workpiece. A conscious approach can limit the errors and increase their predictability. The following four basic rules have to be kept in mind while measuring under non-standard conditions.The experiment with the ball bar pointed out that it is indeed possible to attain good results with this method. By using an appropriate measurement strategy, large errors in a length measurement can be cancelled out. If the temperatures and the CTE workpiece and scales are known, it is possible to compensate for the remaining error, i.e., the differential expansion between workpiece and scale. The uncertainty interval of the corrected measurement depends on the uncertainty of the CTE values and the temperature measurement.