A Measuring Microscope

Tooling for my Taig lathe

Part of the Making Stuff collection
by Douglas W. Jones
THE UNIVERSITY OF IOWA Department of Computer Science

Copyright © 2020. This work may be transmitted or stored in electronic form on any computer attached to the Internet or World Wide Web so long as this notice is included in the copy. Individuals may make single copies for their own use. All other rights are reserved.

Several owners of small lathes have written about the benefit of attaching a microscope to the lathe:

When my father cleaned out his basement, one of the finds I claimed was a Bausch and Lomb measuring microscope. This is not a fancy stereo microscope like John Bently and Roger Hess have used, but it has a reticule scale 0.1" long, graduated in 0.001" increments and it focuses on a point about 1.6" from the objective lens. The microscope came with a cast base designed to sit on some kind of linear way, plus a battery powered light, but with those removed, it is just a metal tube with lenses at each end.

The microscope
the microscope in use
Thinking about how to mount this on my lathe led me to the solution shown here. I used some inexpensive camera accessories:

The photo shows the microscope in its place, adjusted to look down on the tops of my taper attachment mounting bars so I could measure the locations of new holes in the mounting bars for a pair of alignment pins.

With the microscope barrel held in the grip of a rubber clamp, focusing is a simple matter of sliding the barrel up or down in the clamp. This would be an almost impossible thing to do with a microscope that had a shallow depth of field, but the measuring microscope has a very deep field, making it fairly easy to get a sharp focus.

Focused on the scribed surface
view through the microscope
Focused on the scale
view through the microscope
Here are two views taken with an iPad camera looking through the microscope at a small pilot hole drilled through a scribed line on the arm of my taper attachment. My usual camera has a lens that is too big to see through the microscope, while the iPad camera lens is closer to the size of a human eye and can (with a bit of difficulty) be held where it captures the view reasonably well. Unfortunately, it is difficult to control the iPad autofocus; it decides what it wants to focus on, which is not necessarily what I want to see.

Looking through the microscope by eyeball, I had no difficulty seeing the workpiece and the scale at the same time, but the iPad has a shallower depth of field when looking through this microscope than my eyeball. As a it turns out, the scale was in focus only when the bottom of my drilled hole was in focus, while the scale was all but invisible when the surface of the part was in focus. You can't really see the scale here until you click on the bottom photo to see it full sized.

In any case, while my dial caliper says that the drill is 0.040" in diameter, measuring the hole with the microscope scale shows a diameter of about 0.045". You can see that I chamfered the edge of the hole, producing a lip averaging about 0.003" wide. I made the chamfer by gently twiddling the tip of a a 1/8" drill in the hole, which is only about 0.05" deep.

From a practical point of view, a measuring microscope like this can serve many of the same functions as a test indicator. Measuring long distances on the scale is more problematic. When an object is exactly in focus, the scale gives exact measurement, but the depth of focus is deep enough that exact focus is hard to determine. The best way to take exact measurements with this microscope is to slide an object of known dimension into the field in the intended focal plane and then adjust the microscope until that object measures correctly. This is tedious and it must be repeated every time the focus is changed.