Direct measurement of total coating thickness, and thickness of individual coats of paint is a unique capability of the “Tooke Gauge.” Thus, in addition to routine use, it often serves as a “referee” instrument to calibrate indirect or “non-destructive” thickness measuring instruments. Other uses include assessment of substrate conditions and coating adhesion, and observation of microscopic cracking, tendency for brittleness, blistering, cratering, or other microscopic film symptoms.

Open the carrying case and remove the instrument. For field use, secure the safety lanyard to the wrist to prevent accidental dropping. Check the positioning of the cutting tips. As originally supplied, the tip positions will be from top to bottom 1X, 2X, and 10X. The numerals 1, 2, and 10 are engraved in the body alongside each tip respectively. The 1X and 10X tips will be bottomed in their slot, thus protruding about 3/16" from the case. The 2X tip (center position) should be in working position, thus protruding about 5/16" from the case. In general, this is the correct configuration for making an initial measurement on a film.

Make a small mark with the brush-pen at the desired measurement location on a painted surface. Grasp the instrument with the cutting tip down as shown in Fig. 2. Place the cutter tip and guide studs in firm contact with the surface with the tip slightly above the mark and aligned to scribe across the mark. Align the forearm with the intended cutting direction to assure a straight cut. Draw the cutter tip straight across the mark, applying just sufficient pressure at the tip to cleanly penetrate through the film into the substrate. In this operation, the cutter trails midway between the two guide studs, and continuous 3-point surface contact should be maintained to assure precise vertical alignment of the groove. Excessive pressure on the guide studs should be avoided.

Turn on the microscope lamp with the slide switch on top of the case next to the eyepiece. Center the foot of the microscope on the scribed line with the mark about one-quarter inch from the foot directly under the microscope objective. Check the microscope for focus (See Fig. 3). If the image is not clear, it may be brought into sharp focus by turning the focusing screw in the body below the microscope. Check the clarity of the reticle (scale) image in the microscope. If it is fuzzy, unscrew or tighten the chrome eyepiece until it sharpens.

With focusing accomplished, view the intersection of the mark and the cut as shown in Fig. 4. Note how the mark delineates the top edge of the cut. Position the microscope as required to align the left edge of the cut with any convenient long line of the reticle and begin counting the small graduations inwardly (to the right) until the next layer or the substrate is reached. This count divided by the tip designation (2) is the film thickness. If the result should be less than 2 or more than 20 mils, the inspector may wish to utilize 10X or 1X cutting tips respectively.

This may be quickly accomplished by using the knurled-head hex key wrench to loosen the tip set screws. (The wrench is conveniently located atop the switch plate, and serves a dual function as a screw closure for the switch plate.) See Fig. 5.

Allow all three tips to bottom in their slots, then pull the selected tip out to 5/16" (approximately) and retighten all tips with moderate finger pressure. Tip changes: Cutting tip designations and appropriate film thickness ranges for each are shown below.

Fig. 4. Micrograph of paint film incision.

For convenience, always keep the tips in their designated locations:

1X—top, 2X—middle, and 10X—bottom. Note also that the narrow face of the tip which bears an angular grind should face toward the guide studs.

1. On wood or other directional material, make incisions in the grain or “machine” direction to avoid ragged cuts.
   
2. Soft or elastic materials can sometimes be cooled or frozen with ice or dry ice to obtain good cutting characteristics.
   
3. Dyes or indicator solutions such as phenolphthalein are sometimes helpful to develop appearance contrast between metals (iron-galvanizing) or paint coats.
   
4. Eraser “white-out” fluid may be useful as a benchmarker on dark surfaces.
   
5. With some coatings, improved cuts can be achieved by wetting the surface, or by speeding or slowing the cutting rate.
   
6. Coatings with poor adhesion will exhibit a ragged line at the substrate interface. Read
   thickness from the left incision edge in the SUBSTRATE in these cases. (Note Reference on “Coatings Adherence Measurement…” below.)