Safety Razor Parameters: Illustrated and Defined


Introduction


Safety razor parameters around the blade cutting edge are illustrated and defined here by Badger & Blade (B&B) member ShavingByTheNumbers. The comprehensive illustration shows the blade and shave planes and the parameters that follow, applying equally to double-edge (DE) and single-edge (SE) safety razors. Details are included.

The author hopes that this document will become a standard reference for DE and SE safety razor parameters around the blade cutting edge with respect to neutral-, steep-, and shallow-angle shaving.


Illustration of Safety Razor Parameters


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Definitions of Safety Razor Parameters

  • Blade plane is the midplane of the DE or SE razor blade, approximately flat for each free end of the blade from the cutting edge to the point of contact with the safety razor.
  • Shave plane is the approximately flat plane associated with the shave, tangentially contacting either (a) the guard and the cap for neutral-angle shaving, (b) the guard and the blade cutting edge for steep-angle shaving, or (c) the cap and the blade cutting edge for shallow-angle shaving.
  • Blade angle is the smallest angle between the blade plane and the shave plane.
  • Handle angle is the smallest angle between the handle and the shave plane.
  • Blade exposure is the displacement of the blade cutting edge from the shave plane with the positive direction pointing away from the razor.
  • Guard span is the distance along the shave plane from the blade cutting edge to the guard.
  • Cap span is the distance along the shave plane from the blade cutting edge to the cap.
  • Blade gap is the minimum distance between the blade cutting edge and the guard.
  • Blade reveal is the distance along the blade plane from the blade cutting edge to the point where the cap is reached according to a view above the cap and perpendicular to the blade plane.
  • Free-end distance is the distance along the blade plane from the blade cutting edge to the point where the blade contacts the safety razor.
  • Clamp distance is the distance along the blade plane from the blade cutting edge to the point where the blade is clamped by the cap and the baseplate.


Importance of Safety Razor Parameters

  • Blade gap is commonly associated with razor aggressiveness, but, at best, blade gap only loosely relates to razor performance. Blade exposure, guard span, and blade angle, on the other hand, are very important performance parameters with respect to aggressiveness (the ability to cut and irritate skin) and efficiency (the ability to cut hair close to skin). Aggressiveness regards the potential for cuts and irritation, especially with bad technique, while efficiency regards the potential for getting close shaves, especially with good technique. There is an interconnectivity, but in the simplest approximation, aggressiveness goes with guard span and efficiency goes with blade exposure. More guard span increases the potential for skin to bunch up in front of the blade cutting edge, potentially increasing the blade-to-skin angle and the likelihood of cuts and irritation. Greater blade exposure also increases aggressiveness, but blade exposure is more related to efficiency, since more blade exposure brings the blade cutting edge closer to the skin for a closer shave.
  • Blade rigidity (the resistance to blade cutting edge deflection) also relates to razor performance in that more blade rigidity generally causes less aggressiveness and more efficiency due to extra control of the blade cutting edge that deflects less when cutting hair. Several factors influence blade rigidity. The elastic modulus (stiffness) of the blade material is one such factor, but since elastic modulus is fairly consistent across different steels, regardless of tempering or hardness, differences in blade rigidity from one razor to another are basically due to geometry. Geometric factors that affect blade rigidity include, but are not limited to, free-end distance, clamp distance, blade thickness, and blade curvature between support points. Depending on how the razor secures the blade, blade curvature might increase blade rigidity and decrease the chances of chatter. The potential advantages of blade curvature apply to DE safety razors, which generally curve DE razor blades in order to make smaller and more reasonable handle angles. Reasonable handle angles are achieved with SE safety razors by the tilting of SE razor blades, which remain flat. Since thinner blades use less material and are easier to bend, it makes sense that DE blades became thinner than SE blades, which are thicker to help compensate for a lack of blade curvature.


Details

  • It appears that ShavingByTheNumbers is the first to use the term "neutral-angle shaving" and, within the context of shaving, the term "neutral blade angle". Neutral-angle shaving favors neither the guard nor the cap, justifying use of the word "neutral", while steep-angle shaving favors the guard and shallow-angle shaving favors the cap. Accordingly, we have neutral, steep, and shallow blade angles.
  • Blade width, the distance between cutting edges for a flat DE razor blade or the distance between the cutting edge and the spine edge for a flat SE razor blade, varies from brand to brand. Parameters depend on blade width, so a DE/SE blade having an average DE/SE blade width should be used for analysis. Careful physical and photographic analysis is required for accurate measurements.
  • The shave plane is a linear approximation of the skin surface. In real life, skin is flexible and deforms under pressure from the blade cutting edge, guard, and cap during a shave. Thus, the blade angle may differ significantly from the blade-to-skin angle, which varies during a shave and is the angle that truly relates to razor performance.
  • Regardless of whether blade exposure is positive or negative due to the blade cutting edge being on one side or the other of the shave plane, the blade cutting edge should contact the flexible skin surface for effective cutting of hair.
  • When blade exposure is positive with the neutral shave plane, zero blade exposure can be achieved with shave planes associated with steep and shallow blade angles relative to the neutral blade angle. For the steep shave plane, there is no cap span, and for the shallow shave plane, there is no guard span.
  • Blade gap is best measured by the thickest feeler gauge stack that can fit at most places between the guard and the blade cutting edge with room to spare, typically yielding a median value since blade gap often varies across the blade cutting edge. The author developed a set of micrometer-measured feeler gauge blade combinations in order to accurately measure blade gap to the nearest 0.01 mm (B&B URL).
  • For a given razor, an increase in blade width causes increases in blade reveal and blade exposure. However, when comparing different razors, more blade reveal does not mean more blade exposure. Blade reveal should not be confused with blade exposure.
  • Blade rigidity may seem dependent on blade reveal, but blade rigidity is only related to blade reveal in that free-end and clamp distances increase with increasing blade reveal.
  • Clamp distance, which only exists when the blade is clamped by the razor, curves with the blade and is greater than or equal to free-end distance.


History

  • October 17, 2017: 1st version of illustration was posted in a forum (B&B URL). Some safety razor parameters had been shown before at B&B and elsewhere, but never in such a comprehensive manner.
  • October 23, 2017: 2nd version of illustration was posted in forums (B&B URL 1, B&B URL 2, B&B URL 3). Notes were improved, the demonstration of blade angle between the blade and shave planes was made better, and most importantly, steep- and shallow-angle shaving were included with color delineation versus neutral-angle shaving. B&B member Cal helped make this version even better with his thoughtful feedback and suggestions.
  • November 8, 2017: 3rd version of illustration was posted in forums (B&B URL 1, B&B URL 2, B&B URL 3). Blade reveal was added and the bullet point on blade rigidity was improved.
  • November 26, 2017: This ShaveWiki page was created as a reliable location for the most up-to-date illustration and information about safety razor parameters.
  • November 27, 2017: 4th version of illustration. A short URL was added under the title and the last bullet point was modified.

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