Focusing and Alignment Methods for Toolmaker's Microscope
When using a toolmaker's microscope, accurate focusing and alignment keep the measurement error for the same workpiece generally within 1–2 μm. Therefore, correct focusing and alignment are essential to guarantee measurement accuracy. The proper focusing and line alignment (line pressing) methods are described below:
I. Focusing Methods
① First, adjust the eyepiece diopter to obtain a clear reticle image in the eyepiece field of view. If the reticle lines appear blurred, turn the eyepiece diopter ring to match the diopter to the operator's eyesight until a sharp crosshair pattern is visible.
② Move the main microscope via the focusing handwheel to form a clear image of the workpiece outline in the eyepiece. Then shift the X-Y stage for alignment. If no relative movement occurs between the workpiece image and crosshair when the operator moves their eyes up, down, left and right over the eyepiece, the workpiece image is correctly focused onto the crosshair reticle, and measurement can begin. If relative shift is observed, the microscope is out of focus. Fine focusing is required to bring the workpiece image and crosshair reticle onto the same focal plane.
II. Line Alignment (Line Pressing) Methods
Line alignment (line pressing) means overlapping the crosshair lines with the edge of the workpiece image, also known as aiming. For a given toolmaker's microscope, instrument precision is fixed. High, reliable measurement accuracy largely depends on proper alignment techniques. There are two alignment methods: gap alignment and superimposition alignment.
① Gap Alignment Method: This method applies to angle measurement. For angle measurement, align one of the crosshair dashed lines with one side of the measured angle, leaving a narrow uniform gap between the dashed line and the edge of the angle. The operator judges alignment quality by checking whether this gap remains consistent in width. Direct superimposition of the line onto the edge is not recommended for angle measurement; it is difficult to align and introduces larger error. The workpiece edge in the field of view is not an infinitely thin line but a transition zone of light and shade, and crosshair lines also have physical width. Superimposition causes notable alignment error, especially for short angle sides. The gap alignment method should be adopted for angle measurement.
② Superimposition Alignment Method: Using gap alignment for length measurement will increase length measurement error, as the gap cannot be quantified yet is included in the measured length value. Superimposition alignment is used for length measurement: the crosshair dashed line is aligned so that half of the line lies inside the workpiece outline and half outside. Alignment is referenced to the centre of the crosshair dashed line (its extended segments serve only as guides) to achieve accurate measurement results.
