Bausch & Lomb TD4431

A midcentury masterpiece

Repairing mechanical things is a hobby of mine and I find it to be incredibly relaxing. This microscope is no exception.

I found this microscope at a lab cleanout. When I brought it home, all of the linear bearings had seized. I embarked on a repair, after seeing how it works, I can say that it is one of the most beautiful items I own.

All of the pinions in the microscope are helical cut, and mate with an angled rack. This results in a much more smooth motion than straight cut gears, because more than one tooth is always in contact. This is an absolutely fantastic example of the little things that taking machines apart has taught me. I would have never thought to use helical gears and an angled rack to get a smooth motion, but now I can use this design for things I build in the future.

I turned on some music, got out my tools, and sat down to start fixing the microscope. I started by taking off the optical tube, revealing the beautifully machined rails and gears. However, those rails and gears were covered in a dark brown dried-on grease. I cleaned this paste off using WD-40 and re-lubricated using mineral oil. Every bearing in the microscope is hydrodynamic, and requires some oil to prevent seizing. I repeated this procedure on all other bearings and rails.

What had been a sticky, difficult to use microscope became a silky smooth machine.

Below are the rails for the optical tube after cleaning.

The optical tube is entirely supported by friction in the bearing the dials go through. To adjust this, the small silver screws in the image below can be turned. They actuate on a ramp which squeezes on the bearing, increasing or decreasing friction. This allows the friction to be set perfectly, so that the tube never slides, but the dials run buttery smooth.

Unfortunately, moisture got in to the optical path and fouled the lenses before I got my hands on it, making the microscope useless for looking at tiny things. However, with a set of new lenses, or maybe just a new eyepiece, this microscope will be useful. I am currently waiting to find a good deal for optical components.

An Ingenious Mechanism

This microscope has one of the most unique mechanisms for finely adjusting focus I have ever seen. In most microscopes I've used, a simple gear reduction to the main rack is used. This microscope uses something completely different.

The micrometer dials on either side move a fine-pitched lead screw, moving the base of the brass wedge. This causes the wedge to rotate around the large pivot and hence the top edge of the wedge to move up and down. This motion is transferred to the microscope body using a pin that bears on top of the wedge.

This mechanism is fascinating to me, because it is effectively a reverse bell-crank. In a normal bell crank, a rope pulls a right angled lever, which causes the lever to pull the striker on a bell. This uses the exact same mechanism, but with compression rather than tension.

This mechanism was novel enough to be patented (US 2,093,611), and was proudly stamped into the body of the microscope, as shown above. I found the actual patent, and was able to get the great technical drawing shown on the left. It really is such a smart mechanism, to be able to get an such an extreme reduction into such a small space. The microscope claims 2 micron resolution, less than a ten-thousandth of an inch. Normally, that kind of fine adjustment is limited to watchmaking and extremely precise machinery, but H. F. Kurtz managed to produce a mechanism that did not require extremely tight tolerances to produce extremely precise movements.

Achieving Enlightenment

The microscope comes with a small, very bright light designed to be shined into the mirror. The light is based on a tungsten filament bulb, similar in appearance to the ones used in a Spectronic-20. It is powered by a step down variable transformer. When I obtained the microscope, the light didn't work, and the transformer box was making scary buzzing noises.

I took the transformer apart and found the bakelite insulator for the wiper contacts had broken. I bypassed the wiper and replaced all of the paper-insulated wires with modern plastic-insulated wires. I also added a grounded plug, to prevent me from electrocuting myself.

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