The Challenge of Grinding a Miniature Biconvex Lens 

Alvaro Amaro de Azevedo, Brazil

Photographs by the author.

Introduction

The fascination for the microscopic world has been present in my life since I made my very first simple microscope and saw for the first time protozoans, rotifers and red blood cells. I was just sixteen, and this was almost 30 years ago. Since that time I have searched for improvements to my instruments and have made extensive research into the process of lens grinding.

In this article I describe how I made a 0.75 mm diameter biconvex lens using ordinary material and common resources available to amateurs. The technique I will describe is the combination of melting and grinding processes that took me one and a half years to fully develop.

Figure 1. This miniature biconvex lens made by Alvaro Amaro de Azevedo is only 0.75 mm in diameter.

Materials and Setup

The glass I selected for making lenses was crystal glass in which no apparent imperfection was visible. The overall clarity and transparency of the glass was satisfactory, and the shape of the piece was suitable for the melting step.

Figure 2. Commercial crystal glass from which the author obtained samples for making into micro lenses.

The grinding powders were made by myself. This was done by milling ordinary washed sand and then grading the resulting powder by levigation. I made six fractions, so that the coarsest one has grains with an average diameter of 0.1 mm, while the finest has grains with an average diameter of 0.001 mm.

Figure 3. The abrasive powders used to grind the lenses were made by milling washed sand and separating it into six grades by levigation.

The grinding tool was made by punching an aluminum block with a drill marker having a conical tip. A shaft was attached to the tool so it could be installed in the grinding lathe.

Figure 4. Homemade grinding tool made from a small block of aluminum attached to a shaft.

The grinding lathe was a battery-powered drilling machine held in upright position by a bench vice. I added an extra lead-acid battery, because the one that comes with the drilling machine cannot cope with the long lasting grinding steps. For polishing, I solely used kaolin suspension used for silver polishing.

Figure 5. A battery-powered drill was used for the grinding lathe. Note the lead-acid battery that allowed the drill to operate over longer intervals than when powered by the internal battery alone.

The cement was a solvent based glue, which I evaporated until it became a solid white mass. This material can be softened by heat, which causes it to become sticky. Once cooled, it holds the work piece very firmly to the handle.

Figure 6. The lens to be ground is attached to the handle by means of a solvent-based glue that was evaporated until becoming a mass like the one shown here.

The Process

First, a portion of the glass was heated with a jewelry torch until it glowed with a yellowish light. Then a pair of pliers was used to pull the molten glass quickly away from the solid substrate. This created a hair-like thread. The glass fiber was carefully cleaned with alcohol to remove any smoke residue. Then one end of the fiber was brought close to a candle's flame until the heat of the flame melted the end of the fiber and formed a small droplet of glass. For the sake of good homogeneity, the newly formed bead should never be brought too close to the flame, because the glass will not become homogeneous.

The glass droplet that I made was about 1.0 mm in diameter, and no bubbles were observed. If you make several of them, you can choose the better ones to proceed.

Figure 7. Glass fiber with attached droplet of glass formed by bringing the end of the fiber near a candle's flame.

After the droplet was inspected and no apparent defect or flaw was found, it was glued to the handle. The advantage of using a solvent-based cement was that it is readily soluble in an acetone/alcohol mixture. This allowed its total removal from the lens after the grinding was completed.

Figure 8. The tiny glass sphere with attached fiber is cemented to the end of a suitable handle.

When the glass droplet was firmly glued to the handle, the fiber was carefully broken away and discarded. The glass droplet was then sanded with wet sand paper (600 grade works well) until the droplet was about one-third of its original height.

Caution: Be sure to carefully dispose of glass fibers! They can easily pierce the skin. One method is to place them between two layers of relatively thick tape.

Figure 9. After the glass droplet is sanded down to one-third of its original height, it is ready to be ground to a spherical shape.

At this point the grinding step was begun. I began with powder number 3 (average particle size 0.01 mm). Gradually finer and finer powders were used until the surface of the glass became very smooth. For the final stage of polishing, a suspension of commercial silver polish was placed on a sheet of plastic film lying over a fluffy layer of cotton or fabric. The lens was rubbed very gently against the plastic film in a back and forth motion. Every time the handle was moved back and forth, it was slightly rotated so that the direction of polishing was changed with each stroke. Eventually the lens had a polished surface in which no imperfection could be observed.

The pressure was kept very slight, because it's easy to damage the curvature of the lens by excess abrasion. After only a few minutes, the entire surface of the lens clearly reflected an overhead light bulb. At this point the handle was gently heated with a flame so that the lens could be carefully detached from the handle and placed in a pool of solvent to dissolve away the glue.

Miniature lenses are easily lost and should be carefully stored in a suitable container. It was unnecessary to store the lens I made, since it was intended for use in a homemade microscope. I installed the lens in the microscope receptacle and then used the microscope to observe various objects. The magnified image seen through the lens showed that the lens was well made, and, consequently, the level of resolution and sharpness was surprisingly high.

Figure 10. A completed miniature lens is shown next to a needle.

Final Notes

Despite the fact that the technique itself is relatively simple, the making of miniature lenses requires repetition to be fully mastered. Gluing the glass ball to the handle requires good ability and skill, as the sizes of such objects are quite small. The polishing procedure is the only step where the curvature of the lens can be damaged. It takes practice to learn the correct pressure so that the surface of the lens will become glossy while keeping its perfectly curved shape.

Finally, I would like to say that making these miniature lenses has shown me how much can be achieved despite the lack of resources and limited hardware. This is particularly consistent with the fact that van Leeuwenhoek and William Herschel long ago made similar achievements using only crude resources from their remote times.

Comments to author Alvaro Amaro de Azevedo are welcomed.