COUSTIL - Forge / Coutellerie / Bladesmithing

Blacksmithing
and Cutlery
by Gérard HEUTTE

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Blade Grinding

Here is a page providing fundamental notions concerning the shape (section) of blades. Before any decision-making during a realization, it is necessary to know how the knife will be used...
Each grind will be presented in detail, with its advantages and its inconveniences.

Grind
This term indicates both the part of the blade which gets thinner to make the edge and the way the blade gets thinner. It is the grey area on the drawing:

Here are the different grinds:

A : Chisel or asymmetric grind.
B : Partial Flat grind (The bevel does not rise up to the back of the blade).
C : Full Flat grind (The bevel rise up to the back of the blade).
D : Hollow or Concave grind.
E : Convex grind.

The choice for grinding depends on the use of the knife, but can be summarised to a compromise between: Nuance of steel of the blade, cutting capacity, availability of equipment, mechanical strenght undergone by the blade, mastery of the technique.

Blades geometry
Before detailing grinds, it is necessary to speak about the geometry of blades: Length, height (or width), thickness. Each of these dimensions must be chosen according to aesthetic or functional criteria.

Length
It is the dimension which varies most from a knife to another one.
Very short (some centimeters): Small knife, neck knife, small folder.
Short (7-12 cm): Small utility, small kitchen knife, folding knife.
Average (10-15 cm): Utility, kitchen knife, big folder, small dagger.
Long (15-25 cm): Big utility, camp knife, hunting knife, dagger.
Very long (20-30 cm): Big camp knife, big hunting knife, big dagger.
Beyond (35 cm and more): Two-edged sword, machete. We begin to leave the world of the knife...

Heigth (width)
Low (10-15 mm) : Small kitchen knife, folder small utility knife.
Medium (15-25 mm) : Kitchen knife, large folder, utility kinife, dagger.
Large (30-50 mm) : Big kitchen knife, big dagger, camp knife, hunting knife, machete.
Very large (50 mm et plus) : Big camp knife, machete.

Thickness
Thin (1.5-2.5 mm) : Kitchen knife, small folder.
Moyen (3-4 mm) : Big kitchen knife, folder, utility, small dagger.
Thick (4-6 mm) : Dagger, camp knife, hunting knife, machete.
Very thick (6 mm et plus) : Big camp knife.

A good knife thus begins with a geometry suited to its future use.
Furthermore, the choice of the steel is to be taken into account.
For concrete examples, visit the page achievements where I usually indicate the dimensions of blades and used steel.

General rules
For the below drawings, here is the colors legend :
> Blue : Back of the blade.
> Orange : Side of the blade not impacted by grinding.
> Green : Grind profile.
> Red : Blade edge.

Grinding is usually done in two phases.
(1) In a first phase, after forging and annealing, the chosen shape is given. A certain thickness is kept on the edge. Count 0.5mm for the small blades and until 1.5mm for the long blades. This "reserve" allows to avoid the warping of the blade during hardening and also to eliminate decarburised areas on the edge. This first stage can be made with numerous means: Files, abrasive paper or disk, angle grinder, backstand. The grindstone and/or the backstand contact wheel will be more apprropriate for hollow grind.
Then, normalizations, hardening and tempering are done.
(2) It is then necessary to decrease the thickness of the edge and keep the shape of the grind. As the steel is hardened, we cannot use any more the file... It is necessary to decrease the edge thickness to some tenth of millimeters ("e" in below drawings), even zero.

Points relative to the edge thickness "e" and the angle β (secondary or sharpening angle) will be handled in a specific chapter because they are common to all the kinds of grinds.

Flat grind
Here is a drawing showing the cross-section of a flat grind and the important parameters.

Main parameters are:
> Edge thickness "e".
> α angle, called primary primaire.
> β angle, called secondary angle.

Depending on to the wished α angle, the grind will raise up to the back of the blade (left figure: total flat grind) or not (right figure with an bigger angle: partial flat grind).

The α angle is usually between 3° and 7°. On a blade of 3mm thick and 20mm width, the α angle will be 4.3° with a total flat grind.

Realization
This grind can be made with a file or on a backstand with a support to work with a flat.
Advantages
> Easy to realize whatever is the equipment.
> Good compromise.
Inconveniences
> None.
Use
This grind is very versatile.

Convex grind
Here is a drawing showing the cross-section of a convex grind and the important parameters.

Main parameters are:
> Edge thickness "e".
> α angle, called primary primaire.This angle is measured at edge level.
> β angle, called secondary angle.

This grind can be qualified by being more or less convex, extremely almost flat or on the contrary with a strong α angle! The α angle is usually included between 5° and 15°, with an average value of 10°.
Realization
This grind can be made with a file or on a backstand with a floating belt. With the file, make a partial flat grind, then round off the angle on the side. See chapter In the practice!
Advantages
> Easy to realize whatever is the equipment.
> Very strong.
Inconveniences
> Cutting capacity a bit weak, if the convex shape is very strong.
Use
This grind is rather used for heavy duty blades (camp knives, machetes).

Hollow grind
Here is a drawing showing the cross-section of a hollow grind and the important parameters.

(Knife made by Turbotec)

Main parameters are:
> Edge thiskness "e".
> The radius R.
> β angle, called secondary angle.

For this grind, the α angle is meaningless. At edge level, both sides are almost parallel.

Realization
This grind can only be realized with a grindstone or a backstand contact wheel (usually of a diameter from 250 to 350 mm). Many cutlers do not have several contact wheels or grindstones at their disposal. They adapt their grinding according to their equipment.
Advantages
> Good cutting capacity.
Inconveniences
> Fragile (unless e is big, what does not have interest).
> Technique of realization difficult to master.
Use
This grind is rather used for light duty blades requiring high cutting ability.

Chisel (or asymmetric) grind
Here is a drawing showing the cross-section of a chisel grind and the important parameters.

Main parameters are:
> Edge thickness "e".
> α angle, called primary primaire.
> β angle, called secondary angle.

As this grind is asymmetric, there are left-handed and right-handed knives. The flat side is in contact with the material to be cut, the side with the bevel evacuates shavings or slices.

The α angle is usually included between 10° and 20°, with an average value of 15°.

(Knife made by Turbotec - Pictures by Turbotec)

Realization
This grind can be made with a file or on a backstand with a support to work with a flat.
Advantages
> Easy to realize whatever is the equipment.
Inconveniences
> Asymmetric. So, there are left-handed and right-handed knives.
Use
This grind is typically used on japanese hotchos and sometimes on tactical ou utilitary knives.

Edge thickness and secondary angle
These two points are fundamental. Ideally, if we consider only the cutting ability, the edge should have a very small thickness and a secondary angle almost zero! In the practice, it is impossible. It would give a too fragile edge which would break at the first cut.
It is necessary to find the good compromise between:
> Kind of grind.
> Edge thickness "e".
> Secondary or sharpening angle.

Edge thickness
In the practice, the grind is reworked after the hardening to bring the edge thickness to its final value.
For the flat, convex or chisel grinds, the thickness will be:
> Almost zero for light duty or kitchen knives.
> 1 or 2 tenth of millimeter for utility or folding knives.
> 2 to 4 tenth of millimeter for heavy duty knives, camp knives or machetes.

For hollow grind, the thickness will vary from some tenths of millimeter to 1 millimeter. The thickness cannot be reduced to zero, because the particular shape of this grind would give a very fragile edge.

Secondary or sharpening angle
This angle can vary from 9° to 23° as follow:
> 9° to 11° for razor edges.
> 13° to 17° for a common use for a sliding cut.
> 19° to 23° for a use for chopping cut.
For the practical aspects and the sharpening, refer to the page on the maintenance of knives.

N.B: Many cutlers manage the thickness of the edge and the secondary or sharpening angle through an empirical way based on their experience, their cutting tests and the assessment of the quality of their blades...