Introduction to the Toolmaker's Microscope Measurement Method for External Threads
Shadowgraph Method. First, adjust the focal length of the microscope with a focusing rod. Mount the thread workpiece to be measured, select the aperture according to the aperture chart, and tilt the column by the thread helix angle. Then move the microscope and the workpiece to bring the thread profile into the field of view. Adjust until the tooth profile contour is clearly defined on the axial plane, and you may start measuring each parameter, including pitch diameter, thread pitch, thread angle, etc.
1) Pitch Diameter Measurement Align the central dashed line of the reticle crosshair in the eyepiece with the sharp image of the well-focused thread tooth profile and record the horizontal reading. Move the microscope column along the horizontal travel so that the central dashed crosshair line coincides with the image of the opposite tooth profile on the thread diameter. Keep the worktable stationary in the longitudinal direction and read the second horizontal reading. The difference between the two readings equals the thread pitch diameter. To eliminate errors caused when the axis of the threaded part is not perpendicular to the guide rail during mounting, perform multiple measurements on the same tooth profile and take the average value; repeat and calculate the average on the opposite side.
2) Thread Pitch Measurement Tilt the microscope column by the thread helix angle. Align the central dashed line of the eyepiece reticle crosshair with the image of the thread tooth profile and record the vertical coordinate reading. Move the worktable and read the vertical coordinate value of the adjacent corresponding tooth. The difference between the two values is the thread pitch. To eliminate systematic errors arising from non-parallelism between the thread axis and the measuring method, read both left and right tooth profiles once and take the average value during measurement.
3) Thread Angle Measurement Mount the thread in the same way as above. Use the clearance line method to align the reticle crosshair with the thread tooth profile. The angle can be read directly from the angle eyepiece. Measure both left and right flanks, and both sides about the thread axis. Calculate the algebraic sum of the two half-angles measured on the same opposite helical surface to obtain the measured left and right thread half-angles.
