Basics of Leverage : Cliff Stamp


Introduction

A common use of knives for wood craft is shaping as ilustrated by the picture on the right of work done with a Mora 2000. The Mora 2000 does very well in such work primarily because the edge is very acute. Such typical scandinavian knives are ground at 9-11 degrees per side, which is very acute, and thus enhances the cutting ability. However there is also another factor which has to do with issues of leverage during the cutting. Using such knives it is immediately obvious that there is little hand and wrist strain during the work, this is just a basic issue of physics and easily illustrated with a basic comparison of two knives with opposing characteristics.

Mora 2000 and Safari Skinner

The two knives are shown at the right with the Mora 2000 being on top, a typical Scandinavian ground blade with a "modern" molded plastic grip. The Safari Skinner is a collaboration between Swamp Rat knives and custom knifemaker Mike Thourot. There is little in common between those two blades but the relevant difference for this discussion is that the Safari Skinner has a large index finger choil whereas the Mora 2000 does not and the blade is sharpened right down to the grip.

When both knives are used to carve wood or make similar cuts, the positions in hand are shown in the pictures on the right. The Safari Skinner is shown on the far right and the Mora 2000 on the left. Because of the large index finger choil on the Safari Skinner there is a large gap between where the blade makes contact with the wood and the hand. Due to distance there is a leverage disadvantage when using the Safari Skinner. The exact amount of disadvantage can be calculated with some basic physics, specifically the force and torque equilibrium equations. First the force equation :

FI= Fm+ Fp

where FI is the force applied through the index finger which comes mainly through the wrist, Fm is the force required to cut the material and Fp is the force applied by the pinky finger. Next the torque equation :

Fm lm= Fp lh .

Here lm is the distance between the center of the index finger and the point on the blade which contact the material, and lh is the distance between the center of the index finger and the center of the pinky. Combining these two equations :

FI= Fm(1 + lm/ lh)

Thus in short, the ratio lm/ lh is the extra force required to cut a given material. Specific to those two knives, lm = 5 cm for the Safari Skinner and lm = 2 cm for the Mora 2000, with lh = 6.5 cm for the authors grip. Thus the Safari skinner requires a 5/6.5 or 77% increase in force but the Mora 2000 a much reduced 2/6.5 or 31% which significantly enhances the cutting ability.

Now of course a forward grip can be used around the Safari Skinner where the index finger is in the choil and this reduces lm down to 1.0 cm, and thus gives a much reduced 15% increase in force. However this is usually not practical in heavy cutting as the force will still be enough so as to make the pressure on the index finger so high as it will be uncomfortable very quickly in extended use.

Recap

Leverage issues during cutting is an important issue in consideration of blade design and one application is the use of a large choil has a significant detrimental impact on cutting ability by inducing a linear increase in force required during many types of cutting.


Contact: Email Articles Reviews Evaluations Forum Home

Written: Sept. 2007 Updated: Aug. 2007 Copyright (c) 2007 : Cliff Stamp