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Accurately Measuring Blade Gap to the Nearest 0.01 Millimeter

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.
 
I have found that measuring blade gaps has two main problems:

1) Two people measuring the same gap with the same tool get different results. That is, personal technique introduces a large variability.

2) Blade gap does not tell you much. Blade exposure tells you much more about how close of a shave you will get. Blade gaps are important when the design of the razor family is the same. But, to compare radically different designs... not so much. For example, when comparing Gillette adjustable razor models I think blade gap works pretty well as the design is so similar. But, compare the blade gap on a Fatboy to a Merkur Progress... doesn't work so well.
 
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.
Great effort and methodology.
Blade gap does not tell you much. Blade exposure tells you much more about how close of a shave you will get.
Fair comment but how do you measure exposure accurately? With the tolerances on blade holes and slots it would be a nightmare getting any sensible information on exposure.
 
Fair comment but how do you measure exposure accurately? With the tolerances on blade holes and slots it would be a nightmare getting any sensible information on exposure.
Don't mean to be flip, but just because blade gap is easy to measure does mean we should be measuring it.

RazorDimensions.jpg
 
I can envision various blade exposures above based on the design of the bottom plate where the blade gap could be unaffected.
 
I have found that measuring blade gaps has two main problems:

1) Two people measuring the same gap with the same tool get different results. That is, personal technique introduces a large variability.

2) Blade gap does not tell you much. Blade exposure tells you much more about how close of a shave you will get. Blade gaps are important when the design of the razor family is the same. But, to compare radically different designs... not so much. For example, when comparing Gillette adjustable razor models I think blade gap works pretty well as the design is so similar. But, compare the blade gap on a Fatboy to a Merkur Progress... doesn't work so well.
Thanks for your feedback. Here are my responses:

1. Yes, personal technique matters. Also, blade width---and to a much lesser extent, blade thickness, too---also matters because blade gap varies with the location of the cutting edge. With the same exact tools and equipment, and using the same definition of blade gap as the largest feeler gauge allowed between the safety guard and the razor blade or cutting edge that makes contact with a minority of the blade or cutting edge---that's the definition that I've come up with which seems to fit the bill---I would think that different people would get very similar results.

2. You are right about the importance of blade gap. I've written about that myself. Blade gap does not directly relate to razor performance, but relates indirectly at best to the really important parameters of blade exposure and guard span. I chose not to comment on the importance of blade gap in the OP because that is a separate issue from measuring it.
 
Great effort and methodology.

Fair comment but how do you measure exposure accurately? With the tolerances on blade holes and slots it would be a nightmare getting any sensible information on exposure.
Thanks. Regarding the measuring of blade exposure, I've done so several times through photo analysis.
 
Don't mean to be flip, but just because blade gap is easy to measure does mean we should be measuring it.
I've measured blade gap through photo analysis with different razors, and now, I'm measuring blade gap physically, too. The physical measurement is more trustworthy. Again, you're right about how blade gap does not really matter much, but since it has been measured so much and continues to be measured, I figured that I'd better get with the program and buy some feeler gauges. :001_smile
 

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I was hoping for each and every Gillette's blade gap to be tested, checked and posted ... (though I think it's already in the ShaveWiki, re-verification is important in science)
That would be nice. I'm sure that I'll analyze at least one vintage Gillette razor one of these days. The thing is that if I get my hands on one, I'd be more interested in doing photo analysis and reviewing the razor. Measuring blade gap would be a tiny piece of the work.
 
I never really thought about blade gap until I got started with the Wolfman, Timeless and Blackbird. Above the Tie has a blade gap comparison chart in their FAQ that shows the various heads he manufactures alongside of popular vintage and modern DEs. Very instructive.
Because Wolfman, Timeless, and Blackbird inform the end user of the blade gap I found that I prefer more than less. As to measuring unknown blade gaps, I would if I had an easy way to do it, but for me the shave is measurement enough. I know a Feather AS-D2, or a Pils, as nice as they look, don't do it for me. A .95 Timeless, .74 Wolfman, and oddly enough, a .58 Blackbird are great ...... for me, YMMV. The fact that the Blackbird is so good shows me that there is indeed more to it than blade gap. Exposure, as already noted, and overall design play a large part too.
 
That would be nice. I'm sure that I'll analyze at least one vintage Gillette razor one of these days. The thing is that if I get my hands on one, I'd be more interested in doing photo analysis and reviewing the razor. Measuring blade gap would be a tiny piece of the work.
One thing I find interesting is that you go to great lengths to compensate for what appears to be a poorly-made, imprecise tool, which is exactly what most of the members here refuse to do when it comes to razors and blades. I guess you aren't satisfied with the lack of precision of your photograph method, but if you took exactly the same photograph from exactly the same distance and angle of various razors, wouldn't that be precise enough for comparison purposes? Anyway, impressively thorough work, congratulations.
 
I never really thought about blade gap until I got started with the Wolfman, Timeless and Blackbird. Above the Tie has a blade gap comparison chart in their FAQ that shows the various heads he manufactures alongside of popular vintage and modern DEs. Very instructive.
Because Wolfman, Timeless, and Blackbird inform the end user of the blade gap I found that I prefer more than less. As to measuring unknown blade gaps, I would if I had an easy way to do it, but for me the shave is measurement enough. I know a Feather AS-D2, or a Pils, as nice as they look, don't do it for me. A .95 Timeless, .74 Wolfman, and oddly enough, a .58 Blackbird are great ...... for me, YMMV. The fact that the Blackbird is so good shows me that there is indeed more to it than blade gap. Exposure, as already noted, and overall design play a large part too.
Many manufacturers are fine providing blade gap because it is easy to measure and it does not give away critical information, such as blade exposure, guard span, and blade angle. Those things are considerably more important than blade gap, which doesn't really matter. You might like a more efficient and aggressive razor, and maybe the .58 Blackbird has more guard span and/or blade exposure than the .95 Timeless and .74 Wolfman. Larger blade gap generally goes with larger guard span. However, a large blade gap doesn't have to go with a large guard span. It depends on the head profile with the blade. The only way I know to get the important information (exposure, span, angle, etc.) is through photo analysis, which take much more time and skill than using a feeler gauge to measure blade gap.
 
One thing I find interesting is that you go to great lengths to compensate for what appears to be a poorly-made, imprecise tool, which is exactly what most of the members here refuse to do when it comes to razors and blades. I guess you aren't satisfied with the lack of precision of your photograph method, but if you took exactly the same photograph from exactly the same distance and angle of various razors, wouldn't that be precise enough for comparison purposes? Anyway, impressively thorough work, congratulations.
Thanks, @mozartman, for your feedback. As I discussed at the end of the OP, even a much more expensive Mitutoyo feeler gauge will not be perfect. All feeler gauges have tolerances on their blades which generally result in more error when stacking blades. The OEM feeler gauge blades have roughly twice the error than Mitutoyo feeler gauge blades. However, if I had bought and used a Mitutoyo feeler gauge, I'd still have to go through the same process that I followed to make a guide for accurate measurements of blade gap to the nearest 0.01 mm. There's no way around it. The big difference is that it probably would have been easier to find the right blade combinations with the Mitutoyo blades. When I bought the OEM feeler gauge, I didn't know what I know now. I didn't know the laborious process that I'd be going through. That developed in time to get the job done. If I knew then what I know now, maybe I would have paid a lot more for the Mitutoyo feeler gauge, but the end result would have been very similar with a much more expensive product.

Regarding the blade gap that I get from photo analysis, it can be very good, but it can also be off because the profile at the sides of the razor head can be different from the profile along most of the blade edge. The physical measurement takes precedence. Even seeing the blade gap in the photo might be impossible for some razors, which would mean that only the physical measurement with feeler gauge blades would be possible. I've done the photo analysis for the EJ DE89 and the blade gap that I get is similar to the blade gap that I get with my feeler gauge blades. In my review coming up, I'll provide my best explanation for the slight difference.
 
@ShavingByTheNumbers, I posted in the PAA DOC thread, but I'll post again here:

Do you have any thoughts about getting more precise and consistent data for the other razor parameters?
I've got seven popular safety razors in hand, as well as a late-model Gillete SA and a PAA DOC. (I'm excited about the opportunity for verification of measurements, though obviously there are tolerances we're fighting with in cast-metal construction.)
 
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Interesting thread! Nice to understand the key variables here and how difficult it is to get good data.
 
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