In this article, I will try to explain the characteristics of a razor’s edge geometry. For cutting tools of all kinds, edge geometry is hugely important. The wrong geometry will always result in very poor performance of any and all cutting tools.
The four elements of a cutting edge that must all work together for optimum performance at any given (coldwork) task are:
- Abrasion resistance
- Edge Geometry
Razors are “push cutting” tools. Other types of cutting edge tools may use other methods of cutting such as slicing, scoring and gouging. All these methods applied to different materials are best suited to different edge geometries. Scissors, razors and serrated bread knives all are very, very different in geometry because of the mechanical properties they need to be ideal for the task they’re designed to perform. And, of course, those properties make them rather unsuitable for other applications. Scissors and bread knives would perform poorly for push cutting beard stubble and hair from skin with minimal damage to skin and close, clean reliable cutting on the hair.
So, we’ll focus on razor geometry here.
And that has to start with an understanding of the task at hand. We want to cut hair from the face at or below the level of the skin on the follicle. The goal is to clear the skin of all hair doing it very close, very comfortable and with little or no damage of any sort to the skin, itself, in the process but mechanically cutting/shearing it.
Additionally, what we want out of the tool is very good edge retention so we spend much more time shaving than honing, stropping and maintaining it. This is an important consideration.
So what is a great geometry for this?
First and foremost, we need an edge with as close to zero radius as possible; no rounding or flat spots at all. This enables it to start cutting with very little pressure. The smaller the radius of the tip of the edge, the greater the shear force exerted in the smallest possible area. This enables the edge to penetrate and begin shearing very quickly and very easily. The edge must, however, widen at an angle sufficient to provide support for that cutting edge so that it doesn’t fail and deform during the cutting. The angle of the bevel on a razor blade provides this geometry. If it is too obtuse, it will not cut as easily. If it is too acute, it will fail while cutting or at the very least will not maintain it’s proper geometry for long. Generally, a bevel of around 14 to 18 degrees will be very good for this.
So, how do we know what the bevel angle really is? There’s an easy way to tell. Proper edge geometry for a razor is set by the maker and consists of a blade width that is 4 times the size of the spine width where the spine contacts the blade and the edge contacts a hone. An 8/8 razor should have approximately a 1/4″ width spine. Slightly thicker or slightly thinner would be OK but it shouldn’t vary considerably from the 4:1 ratio if the blade has good geometry; say somewhere between 4.2:1 and 3.7 to 1.
So that’s the edge geometry of a razor. Understanding this helps us understand how to hone to maintain this geometry, particularly at the very fine edge.