Reshaping the Plastic Lense of a Cheap Compass

by tonytran2015 (Melbourne, Australia).

I just discovered that the lense of my cheap, imitation compass (bought for making some novel instruments) has neither a good shape nor correct focal length. The focal lenth is too long for that compass: I can see neither scale nor figures on the rotating rose of the compass. So I decided to reshape that lense to a new focal length and learn the tricks of (plastic) lens making.

The required tools are:

A small jewellery flat file,

very fine, wet typed abrasive papers (usually used for polishing car paints),

diatomaceous white tooth paste (having microscopic white diatom skeletons),

and paper serviettes.

The first step is to push the 12mm diameter, plastic lense out of the black tab holding it onto the compass.

The jewellery file was used to shape the plastic lense into the desired shape.

Next, very fine wet sand papers were used to smooth the surfaces of the lense. It was then thoroughly rinsed to remove all sanding particles.

Then tooth paste (0.5cc each time) is placed onto paper serviette and the lense was rubbed by the paper to remove fine marks by abrasive papers by even finer toothpaste particles. It was rubbed until the serviette wore out. After three such rubbing operations with toothpaste, the lense was then rinsed in water.

Figure: The upward face of the lense is unmodified. The two curved white lines are two reflections of one single fluorescent tube by the two faces of the lense. The bigger reflection clearly shows waviness of the unmodified, upper face.

The reflection by the surface of the piece of plastic should show the reflection patterns of a spherical convex mirror. The reflection picture of parallel fluorescent lighting light tubes are used to check the shape of the surfaces. They should be curved lines parallel to one another. The reflection from the piece of plastic shows irregularly wavy lines and that reveals the bad, non spherical shape of that lense. It had to be ground until the reflection gives parallel curves which are smoothly bent and spaced.

Top figure: The ground lense is on the transparent rotatable face of the compass, above position 260 degrees of the compass rose. Bottom figure: The scales and figures on the rotating rose can now be seen through the lense.

The lense now can give readable picture of figures and scale as shown in the photo. The lense is then pushed back into the black tab holding it on the compass. The operation has been ended right in this condition as the lense is found to have developed flaking (probably due to internal stresses) and the four screws holding the plastic base plate are also found to be made of magnetic material (mild steel) making the compass inaccurate when the needle points to any of the screws. If there had been no flaking, the procedure could be repeated to give a much better lense.

The result of this reshaping operation has made the lense just barely acceptable as a magnifiers for the compass but it shows the applicability of the method. The operation for this specific lense can be considered a field repair of an almost unserviceable lense !

The lesson is that we all should pay adequate attention to the lense before considering any compass for purchase. It cannot be fixed by an average user.