The Starrett 196 back plunger dial indicator has been manufactured for well over 100 years. There have been changes, but the general construction and operating mechanism are pretty much the same as the first ones out of the factory. There is little documentation for the user about how to open and clean the tool, so a short introduction is given here. Major repair is not covered, and many user may not want to take on a major overhaul, but basic cleaning is straightforward.
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Step 1: How It Works
The operating mechanism is quite simple, and similar to, if more crude than, the Starrett 711 "Last Word" test indicator. The plunger has a pin the engages a helical groove in a drum that rotates in proportion to the plunger displacement. The needle is pressed on a shaft that also is the axle for the drum. The drum is lightly preloaded by a gear and hairspring to insure that only one face of the helical groove contacts the pin, so as to eliminate backlash. A stronger return spring pushes the spindle out with a fairly constant force.
Step 2: Disassembly
This guide applies specifically to the older unit (vintage 1920's) shown, but more modern ones are similar.
The bezel can be removed by carefully working a slot-type screwdriver between the bezel edge and the knurled grip. Only friction holds it.
Once the bezel is removed (the crystal will be freed and may drop out of the bezel or stay with it), the retaining ring for the scale can be removed. The retaining ring is held in by tabs that slip under the retainers in the rotating section. A thin probe or screwdriver blade from the outside will free one end without damaging the face. Then the others can be freed by hand and the ring removed.
The needle is then carefully removed using a watch hand puller or hand removal levers. If you need more information, there are a number of videos showing different techniques, but it must be done with care to avoid bending the shaft.
Once the needle is removed, the face can be worked past the retainers and removed, exposing the mechanism.
There is a washer and a wave spring under the face to support it, and they will lift out.
In this example, grime is visible on the spindle, causing rough, sticky operation. The score in the barrel does not affect the operation in this case, as there are no burrs that interfere with the pin.
The grime is cleaned using a fine tip, lint free swab (available from many hobby shops for painting models, or from industrial suppliers for cleaning precision parts and optics) using a naptha, kerosene, or other light solvent. The spindle, pin, and helical barrel should all be cleaned.
After cleaning, a fine mechanism oil or watch oil should be applied to the working surfaces: where the spindle passes through the body, where the spindle passes through the inner frame, and where the pin contacts the helical groove. VERY little is needed, or grime will re-accumulate. The tip of a needle is used, and any visible excess should be wicked off using a lint free swab. The procedure is the same as lubricating a watch. The point of a needle may be used to apply oil to the pivot points.
Reassembly is the reverse of disassembly.
Care should be used pressing the needle back onto the shaft to avoid damage tot he needle or bending the shaft. Take care to put the wave spring, washer, and face back before putting the needle on. When putting the needle on, be sure that it is the correct side out, and pointing the direction you desire for the unloaded unit.
If the crystal needs replacement, this is a good time to do it. Then carefully work the bezel and crystal back on.
Now you have a nice, clean, smoothly operating indicator.