Summary Using a digital micrometer with a resolution of 0.001 mm, combinations of one to four metric feeler gauge blades were found and tabulated to create a guide for accurately measuring safety razor blade gap to the nearest 0.01 mm The feeler gauge blades associated with the guide were generally found to be thicker than reported, resulting in feeler gauge blade combinations that are up to 0.04 mm thicker than would be expected from adding labeled blade thicknesses, but this issue was rendered moot due to accurate micrometer measurements Small binder clips are used to tightly hold feeler gauge blades together, which allows the hand to apply a light touch when moving a feeler gauge blade stack and fosters accurate assessment of safety razor blade gap OEM Metric Feeler Gauge On September 10, 2017, I ordered the OEM Metric Feeler Gauge, as shown below, for physically measuring blade gaps. The feeler gauge came with twenty-five (25) blades in total, six (6) blades with marked thicknesses of 0.04 mm to 0.09 mm in steps of 0.01 mm and nineteen (19) blades with marked thicknesses of 0.10 mm to 1.00 mm in steps of 0.05 mm. The blades of the feeler gauge are hardened and tempered from high-carbon steel, reportedly S2 tool steel, and the metric thickness label on one side of each blade is stated to be laser-etched. Cleaned Feeler Gauge Blades The feeler gauge blades came coated in oil, so I took the gauge apart and thoroughly wiped and hand-washed the blade protector, blades, knob, and threaded piece. The composite picture below shows the feeler gauge after reassembly, when all pieces were fully dry. The feeler gauge was disassembled so that the blades would be free for stacking as desired for blade gap measurements. All of the feeler gauge blades are shown in the picture below. Some of the blades have stains, which were apparently caused by not drying those blades quickly enough during the cleaning process. For storage, I stack the blades in a plastic bag and remove the vast majority of air while sealing the bag shut. Feeler Gauge Guide for Blade Gap Measurements With the metric feeler gauge blades, one might expect that all feasible blade gaps larger than 0.03 mm could be easily measured to a precision of 0.01 mm simply by using stacked blade combinations with total thicknesses based on the labeled blade thicknesses. The problem with this idea is that the marked blade thicknesses do not reflect the true blade thicknesses. Based on measurements with my ratcheting digital micrometer, the actual blade thicknesses along the center edges are 0.008 mm larger on average than the marked blade thicknesses. The minimum and maximum errors among the blades are -0.003 mm and 0.020 mm, respectively. Further decimating the idea that labeled blade thicknesses are accurate, blade thicknesses were found to generally vary a bit along the curved edges. Given the inaccuracies with the reported blade thicknesses, it became apparent that in order to accurately measure blade gap, I would have to measure combinations of feeler gauge blades and find combinations with total thicknesses to the nearest 0.01 mm. The search process started with a little VBA programming in Microsoft Excel to generate all combinations of blades up to four blades per combination. Total thicknesses were based on micrometer measurements of the individual blades, and after programmatically generating the list of blade combinations, I sorted the list in order of expected total thickness. Many hours were then taken with my ratcheting digital micrometer to measure thicknesses of blade combinations, searching for thicknesses at 0.01 mm intervals up to 2.00 mm. Preference was given to combinations that used less blades, thicker blades, and naturally stacked better with more flatness. Thinner blades were placed in the middle of stacks, while the thicker blades in a combination were placed on the outside. As much as possible, combinations were chosen that had fairly even thicknesses along the curved ends within the micrometer precision of 0.001 mm. The micrometer was often cleaned and recalibrated, especially when double-checking measurements for the guide. The guide for measuring blade gaps with my feeler gauge blades is pictured above. For easy reference when using the feeler gauge blades, the PDF file of the guide was printed and slipped into a sheet protector. Total thicknesses, as measured with my calibrated digital micrometer, are shown in bold to the left of their corresponding blade combinations. On average, actual combined thicknesses at the curved ends are 0.015 mm more than what one would expect based on marked blade thicknesses. The difference among combinations varies, however, from -0.003 mm to 0.040 mm, which is significant given that the reported resolution of the larger blades is 0.05 mm. Nevertheless, for thicknesses in increments of 0.01 mm above 0.31 mm, feeler gauge blade combinations were found such that total thicknesses were within 0.001 mm of the desired values. This accuracy is there for many thicknesses below 0.32 mm, but due to a lack of available blade combinations in this region, some desired thicknesses below 0.32 mm could not be met within the micrometer precision of 0.001 mm. Example of Feeler Gauge Blade Combination An example of combined feeler gauge blades is shown below. Consistent with the guide, the 0.30 mm, 0.10 mm, and 0.25 mm blades were stacked in that order for an actual total thickness of 0.660 mm, which is 0.01 mm more than the combined value of the marked blade thicknesses. When using a combination of feeler gauge blades, one or two small binder clips are used to tightly hold the blades together near the curved end, as demonstrated in the picture. This ensures that the total thickness basically matches that found with my micrometer. Further, the binder clips allow light, sensitive movement of the feeler gauge blade stack, which is important for accurately assessing blade gap of a safety razor loaded with a razor blade. Final Thoughts Creating a guide for accurately measuring blade gap to the nearest 0.01 mm was laborious, but I created something that apparently cannot be bought. I paid about $12 for my inexpensive OEM feeler gauge, while the comparable Mitutoyo feeler gauge would have cost over $60 or $70. Mitutoyo makes some of the best micrometers and calipers in the world, but even their 0.65 mm and larger feeler gauge blades have reported tolerances of ±0.01 mm. No matter the manufacturer, feeler gauge blades will not be perfect and errors will generally compound when stacking them for measuring blade gap. My guide for measuring blade gap removes this issue due to accurate micrometer measurements. Without micrometer measurements, using more expensive Mitutoyo feeler gauge blades in the normal fashion with at most two blades stacked together could be considered accurate enough for measuring blade gap, but blade gap measurements would not be assured as accurate to the nearest 0.01 mm.