What Is a Marking Gauge?
The ruler is a length standard for optical measuring instruments. It is widely used in instruments for measuring length, such as tool microscopes, length gauges, and measuring machines. The First Machinery Industry Ministry has established standards for glass scales for reference when selecting. [1]
- A ruler is an integral part of a pointing device composed of an ordered set of ruler marks and related numbers.
- Rulers are generally divided into two types: metal rulers and glass rulers. In the instrument, the former uses reflective lighting and the latter uses transmissive lighting. Generally, short rulers use glass rulers, so glass rulers are widely used in optical instruments. The basic series of glass scales are shown in the table below.
- There are three main factors affecting the accuracy of the scale: 1) scoring errors; 2) errors caused by temperature changes; and 3) errors of the scale bending deformation by gravity. The former one mainly depends on the scribe machine and the scribe process. Generally, the ruler's scoring errors can be divided into short-period errors and long-period errors, which are caused by the scoring machine. The Ministry of Standards issued a clear rule for the accuracy of the scale. According to the maximum inaccuracy between any two dotted lines of the current scale, it is divided into five levels, that is:
- Level 1
- level 2
- Level 3
- level 4
- Level 5
- In the formula, L is the distance (unit: mm) between any two dashes.
- The Ministry of the First Machinery Industry has promulgated the standard for optical instrument scales (JB2215-78). The designer of the instrument should choose a scale with an appropriate accuracy level as much as possible. However, the actual situation is that a high-precision scale cannot meet the needs, and often a lower-precision scale is used instead. The method of error correction table is to use a high-precision scale to verify the ruler, The errors are listed in a table, and then corrected according to this table when used. Example: A ruler of 200 mm is used to measure the error values of 0-1, 0-2, ... 0-200 on the length gauge, a total of 200 score lines. A table is attached to the user for use Can be corrected according to this table. If the two readings of the length of a workpiece are measured to be 78.2194 and 11.66382 mm, use the correction table to compare the data and calculate as follows:
- 78.2194 mm: Set the scoring error found in the correction table to be -0.0005 mm at 78 mm. The actual value after correction is:
- 11.6382 mm: Set the scoring error from the correction table to +0.0008 mm at 11 mm. The actual value after correction is:
- Therefore, the actual measurement length is
- As can be seen from the above example, after using the correction table, the error of the ruler no longer depends on the scoring error, but on the verification error of the ruler, thereby improving the accuracy of the ruler. At present, the photoelectric ratio meter used for the calibration scale can generally achieve a calibration accuracy of ± 0.5 microns. The relevant measuring units in China have developed laser interferometry, and the accuracy of measuring 1 meter can reach ± 0.2 microns. [1]
- The mechanical structure of the ruler base should ensure that the ruler is fixed and reliable without internal stress, and the scored surface is perpendicular to the optical axis, and the score line is perpendicular to the direction of the ruler movement. Therefore, when designing the mechanical structure of the ruler, it is necessary to fully consider the possibility of adjustment of the ruler to meet the above basic requirements. The following description will be given in combination with two specific structures.
- The picture shows the mechanical structure of the longitudinal scale base of the 19JA universal tool microscope. The scale 3 is supported in the scale base 11 with the scored side of the scale facing downwards to prevent dirt and other dirt from falling on the scored surface and the protective glass is glued on the scored surface. The correct position of the ruler in the plane is adjusted by three screws 2, 12, etc. In order to prevent the ruler from being deformed or crushed, the corresponding point on the opposite side of the adjustment screw is supported by an elastic fulcrum. The screw 2 is adjusted left and right, and the two screws 12 are adjusted back and forth. The correct position of the ruler in the vertical plane is by turning the four adjusting nuts 1. Move the entire ruler base up and down to adjust it.
- Mechanical structure of 19JA universal tool microscope ruler base
- Above: 1Adjustment nut 2Adjustment screw 3Scale 4Spring leaf 5Screw 6Cover plate 7Screw 8Inner sleeve 9Spring 10Jacket 11 Ruler base 12Adjustment screws (one on each side)
- The following figure is the mechanical structure diagram of the ruler on the spindle for universal length measuring. The ruler 8 and the protective glass 11 are integrated into the measuring spindle. The scaled surface of the ruler should be measured through the axis of the spindle and perpendicular to the optical axis. The correct position is achieved by adjusting the adjustment screws 7, 9, and 10. Corresponding points on the opposite side of the screw are supported by spring screws 5, 6, 12 to prevent the scale from deforming or crushing. It can be seen that these two structures are designed in accordance with the principles of kinematics, with three constraint points on the bottom surface, two on the side and one on the end.