24 
THE YOUNG SCIENTIST. 
[Specially written for the Young Scientist.] 
The Art of Sharpening Edge Tools. 
By Joshua Eose, M. E. 
NO. II. 
Our next example shall be the sharpening of 
a pair of scissors, which is a fair representative 
of a class of severing tools which operate by a 
concentration of pressure upon a narrow sec- 
tion of the material to be separated, rather 
than by the acuteness of the edges. Suppose, 
for example, that we open a pair of scissors, 
and, placing a sheet of paper upon a table, or 
upon a piece of board, attempt to cut it with 
one edge of the scissors used as a penknife 
blade. We shall find that, with both consider- 
able pressure and motion, it will be difficult to 
■cut the paper, even though the scissor edge is 
in excellent order. This teaches us that the 
action is not purely one of cutting on the 
wedge principle, and leads us to a considera- 
iion of the difference between cutting and 
shearing. 
Cutting instruments act by reason of the fine- 
ness of the edge which enters the material, and 
which, acting as a wedge, divides the substance 
operated upon; they are therefore dependant 
upon the fineness of the entering end of the 
wedge (as the cutting edge may be properly 
termed) in a far greater degree than is the case 
with shearing tools. Hence, in the former, 
strength is much less important than in the 
latter. The reason for this we may very 
readily perceive by a little investigation. Sup- 
pose, for example, that we take a piece of wood 
^nd cut it slowly with a knife; we shall find 
that in the beginning of the operation the edge 
•enters the wood very easily, but that the re- 
sistance increases as the knife blade enters. 
This is because we have at first to sever the 
^bres of the wood only, while afterwards we 
have to continue that severing process, and 
■sAso to force the already severed portions asun- 
der to allow the thicker parts of the wedge or 
knife blade to pass. We have thus, to a cer- 
tain extent, the strength, as well as the resist- 
ance to severance of the material, to encounter. 
If we rest the back of the knife blade upon a 
support, such as a table, place the piece of 
wood upon the edge of the knife blade, and 
then press another knife edge upon the top of 
the wood, with the two knife edges exactly 
facing each other, each edge will enter easily 
at first, because the separation of the fibres is 
the only resistance offered; but as the knife 
blades enter, the friction between the sides of 
the severed wood and that of the blades will 
increase until it absorbs the greater part of the 
power applied, teaching us that for shearing 
duty the angles of the side facets forming the 
cutting edge should stand at a right angle to 
the surface of the work, so that the latter shall 
not rub against them after severance. This 
then determines that the side facets of each 
leg of the scissors shall stand at least parallel 
with the line of motion of the legs when in 
use. In actual practice we shall find that these 
side facets have a little clearance, being ground 
away as the cutting edge is reeeded from in the 
width of the leg, so that when the edges, at any 
point, are in contact, the side facets form an 
obtuse angle one to the other. In addition to 
this, there is given to these faces a slight twist 
in their planes, and a curve in their lengths, 
whose usefulness will be hereafter explained. 
We may now consider the angle necessary to 
what may be called the top facet of the cutting 
edges — that is to say the narrow surface (at the 
cutting edge) which stands at about a right 
angle to the line of motion of the scissor legs 
when in use, and this is of the utmost impor- 
tance. In a pair of scissors, the material to be 
cut is generally weak and pliable; hence it will 
give way whatever be the angles of the faces, 
so that all we have to do is to consult the best 
angles to produce the required cutting edge. 
The action being that of a wedge, it would 
appear that the more acute they are the more 
easily they would cut; this, however, is not the 
case, for the following reasons. 
It is obvious that the substance is severed at 
the point of contact of the scissor edges, and 
that if those edges become rounded the pliable 
material will be apt to spring the legs apart and 
slip between them; hence we have to consider 
the angle that will best enable the edge to re- 
sist becoming rounded, while at the same time 
give an edge sufficiently acute to cut. Let it 
be remembered that the distance of the ends 
or points of a pair of scissors from the joint 
which holds them together, enables very little 
pressure at the points to spring them apart; 
hence as the edge becomes dull, or the end be- 
comes rounded, the points do spring apart, the 
substance or work passing between their side 
