1. Ruler method
Commonly used straightedge and flat ruler are measured by light gap method and indicator method (see gauge). A straightness meter may also be used. Straightness measuring instrument is a kind of length measuring tool which uses straightedge and indicator method to measure straightness error. It takes the measuring surface of the quartz ruler as the known plane to compare with the measured straight line, and records the profile shape of the measured section or prints out the straightness error through the electrical length sensor, the corresponding electronic part and the recorder.
2. Optical collimation
The straightness error is measured by optical collimating telescope system. The collimating telescope system consists of collimating tube and telescope. The light from the light source is emitted by the cross dividing plate 1 and the objective lens 1, and then emitted by the parallel light, and then converged on the cross dividing plate 2 located on the focal plane of the objective lens 2 in the telescope, and read through the eyepiece. For example, if the telescope and collimator are placed on the measured surface (such as the guide rail), when the collimator is moved, if the image of the cross line emitted by the collimator coincides with the cross line of the reticle 2, it means that the straightness is good; if there is deviation, it means that the optical axis of the collimator tilts an angle with the optical axis of the telescope. The straightness error can be obtained by data processing. The measurement instrument of straightness error by using the principle of optical collimation is called collimating telescope. The collimating tube is also provided with a concentric circle dividing plate. The telescope is also equipped with an internal focusing lens group and a parallel plate glass for the reading system. By using the internal focusing lens group to refocus, the concentric circles on the concentric circle dividing plate can be imaged on the dividing plate 2. In this way, the offset of the two optical axes can be measured. The collimating telescope can also be used to measure the coaxiality error of each supporting hole on a large machine (such as a steam turbine).
Optical autocollimation is measured by autocollimator. The whole length to be measured is divided into several sections, and the inclination angle of each section is measured. Calculate straightness by drawing or calculation.
3. Gravity method
The straightness is measured by the gravity phenomenon that the liquid keeps level automatically or the weight keeps straight automatically. The commonly used measuring instrument is the level instrument. It also uses the liquid level as the measuring surface to measure the straightness error compared with the measured surface.