1. Field of the Invention
The present invention relates to a hardness tester.
2. Description of Related Art
Conventionally, hardness testing methods of a pressing type are well known, such as the Vickers hardness test and the Knoop hardness test, in which an indenter having a planar polygonal shape is pressed against a surface of a sample, then a degree of hardness of the sample is measured from a length of a diagonal line in a resulting polygonal indentation in the sample surface. Such hardness testing methods are widely used in evaluating mechanical characteristics of metallic materials.
As is commonly known, the Vickers hardness test employs a quadrangular pyramid diamond indenter and indicates the degree of hardness by a relationship between an average value for the length of the two diagonal lines of the quadrangular pyramid indentation formed in the surface of the sample and a pressing load of the indenter on the sample. The Knoop hardness test employs a rhomboid pyramid diamond indenter and indicates the degree of hardness by a relationship between the length of the longer of the diagonal lines of the rhomboid pyramid indentation formed in the surface of the sample and the pressing load of the indenter on the sample.
Typically, when conducting a hardness test with the Vickers hardness test or the Knoop hardness test, the sample is observed with a field lens to set a test position. A turret is then rotated to replace the field lens with the indenter, and an indentation operation is performed. Then, the turret is switched back to the field lens once again to observe the indentation formed in the surface of the sample. At this point, in order to correctly form the indentation in the set test position, a center position of the indenter that will form the indentation and the center position of the field lens that will conduct the observation must be positioned correctly. In a conventional hardness tester, a centering mechanism is used to automatically adjust the field lens to match the center position of the indenter when the indenter is disposed, or positioned, in a predetermined position opposite the sample.
However, because the indenter is disposable (i.e., can be thrown away) the indenter must be changed when worn out. When the indenter is changed, the center position of the indenter and the center position of the field lens may become offset, thus creating a need to center the field lens. In order to center the field lens, indentation is performed with a reference indenter, then the turret is rotated to switch to the field lens to be centered. Next, while observing the indentation, the field lens is centered such that the center position of the indentation matches the center position on a screen. Finally, the turret switches to the indenter and performs another indentation, then switches to the centered field lens and confirms whether the center position of the indentation matches the center position on the screen.
Existing hardness testers are mounted with a large number of field lenses. Therefore, the above-described centering procedure must be performed for the number of field lenses installed. In addition, some of the existing hardness testers are mounted with two indenters, and thus a second indenter must be centered to match the center position of a first indenter serving as the reference. A user is burdened in performing the above-described sequence of centering work.
To address this, a hardness tester is disclosed that is capable of performing an accurate hardness test even when the center position of the indenter is offset from the center position of the field lens (see, e.g., Japanese Patent Laid-open Publication No. 2004-286541). Specifically, the invention described in Japanese Patent Laid-open Publication No. 2004-286541 calculates a coordinate differential between a center coordinate of a display screen (a field lens) and the center coordinate of an indentation displayed on the display screen. Then, the offset between the center position of the indenter and the center position of the field lens is corrected by displacing a sample stage in a horizontal direction such that the coordinate differential is approximately zero.
However, the invention described in Japanese Patent Laid-open Publication No. 2004-286541 is a correction process performed during hardness calculation after the indentation is formed and cannot correct the offset between the center position of the indenter and the center position of the field lens during formation of the indentation. Therefore, the user may be unable to form the indentation in a desired test position.