In order to measure the surface roughness, heretofore, various methods have been used. For example, according to a stylus method, a stylus is brought into contact with a surface to be measured and moved on the surface, while according to a non-stylus or non-contact method, laser light or the like is focused to illuminate a surface to be measured and then the surface is scanned by the focused laser light to detect its reflected light. In either method, a sectional shape of the surface to be measured is obtained and a surface roughness is determined on the basis of the obtained sectional shape.
By the way, in order to measure the surface roughness of a work piece worked by a machine tool such as a machining center or a lathe, after the working on the machine tool has been temporarily stopped, the measurement is conducted while the work piece is removed from the machine tool and set on a surface roughness tester, or while the work piece is still mounted on the machine tool. Even with measurement of the non-stylus system, the surface roughness could not be measured in a real time manner under the circumstances using a machining liquid or cutting oil. With the measuring methods of the prior art, accordingly, it is difficult or even impossible to detect surface roughness in a real time manner during working of a work piece. Consequently, the machining has to be interrupted for the measuring process of the roughness, which has been an important reason for impeding improvement in production efficiency and product quality.
In order to improve the working accuracy and quality more efficiently, therefore, it has become an important task to establish the technique for clarifying surface roughness of an object to be measured (work piece) in a dynamic condition where the work piece is still being driven or worked, in other words, an in-process measurement has been required. Namely, if the measurement of surface roughness is carried out in-process manner, it becomes possible to monitor the accuracy of the surface being cut so that increase of unmanned machine tools and smooth automatic operation without troubles can be progressed, thereby greatly contributing to improvement in production efficiency and product quality.
Even in the case of post-process measurement of the surface roughness, moreover, there is a following problem. If a narrow portion with a physical limitation such as a fine aperture impossible to insert a stylus in the stylus method, or impossible to detect reflected light in the optical method, the object has to be cut to expose the portion to be measured before the roughness is actually measured. Accordingly, it has been an important theme to establish a technique being capable of efficiently measuring the surface roughness of the narrow portion without cutting or breaking down the object to be measured.
Main methods for measuring the surface roughness of the prior art are shown in the following patent documents 1 to 7. These are classified by measuring principles into methods based on properties of work pieces (patent documents 1 and 3), optical measuring methods (patent documents 2, 5 and 7), method utilizing styluses (patent document 4) and method using ultrasonic waves (patent document 6).    Patent document 1: Japanese Patent Application Opened No. 2003-130,606    Patent document 2: Japanese Patent Application Opened No. H11-148,812 (1999)    Patent document 3: Japanese Patent Application Opened No. 2002-340,518 (2002)    Patent document 4: Japanese Patent Application Opened No. H10-138,095 (1998)    Patent document 5: Japanese Patent Application Opened No. H6-258,060 (1994)    Patent document 6: Japanese Patent Application Opened No. H5-177,512 (1993)    Patent document 7: Japanese Patent Application Opened No. H6-99,336 (1994)