It is recommended that glass be used for replacement screens. Glass can be matted and ground to the exact outer diameter of the comparator manufacturer's specifications. When the comparator operator turns the glass screen 180 degrees, the image center should remain in the same place. Overlay charts are placed on top of the comparator's glass screen and are held by chart clips. Standard overlays check radii, angles, X-Y grid coordinates, thread profiles, fillets and much more.
Some magnifications are more available than others.
Custom overlay charts permit the operator to see the total part tolerance envelope and to determine if the part satisfies the specifications. They offer users an opportunity to visually relate surface measurements that are often awkward and difficult to measure with other systems. Construction points and theoretical intersections that do not exist on the work piece can also be determined. Irregular surfaces can be viewed over the entire range, comparing actual part shadow with a maximum and minimum tolerance envelope relative to part datum surfaces. Comparator charts today manage to go right to the fundamentals of geometry.