The Optical Comparator - 9: Measuring to an Invisible Datum
VMS Forum | Library | Start Page | Previous Page| Next Page
WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor.
There are hundreds of ways to use the optical comparator but perhaps one of the most unique ways is to find a point (datum) that lies inside of the part. For example the part blueprint calls out a measurement from the vertex of the angled face and the horizontal face, to the the vertical face on the right end.
The problem is that the datum cannot be reached with conventional measurement tools. The fillet radius gets in the way.

Below is an animation that shows how you can find the D distance as shown in the print. To do this procedure it must first be recognized that at one point - and one point only - the screen can be rotated around the vertex such that any line on the screen generated outward from that point will align with the two faces mentioned. So the first thing to do is get the horizontal face to rest exactly on the horizontal line of the comparator. This requires that the stage be lifted until there is a match.

Once the horizontal line is matched then we need to go back to being a screen and stage jockey and get some other line on the screen to align with the angled face on the part WITHOUT changing the stage elevation (we have the vertical distance set so it does not need to move again).

After aligning the two faces the vertex is established horizontally and vertically. Now just rotate the screen back to horizontal and measure the horizontal distance from the front face to the vertex.
VMS Forum | Library | Start Page | Previous Page| Next Page

label: optical comarator, sizes, function, screen, overlay, measuring, manual, DRO,