There is no operation in which the young mechanic is so much at fault as
in that of grinding and setting in order the various tools he has to use.
Nevertheless he will never become either an independent workman or a good
one, if he has to depend upon others for this necessary labour.
No doubt, to sharpen a tool which is in very bad order is a tedious and
tiresome job; but it is not so wearisome an affair to keep tools in
condition for work, after they have been once thoroughly sharpened by one
who understands how to do it. Never, therefore, use a blunt tool, but
at once go to the hone or grindstone with it, and put it in first-rate
order. Time thus employed is never wasted, but rather saved; and the
result will appear invariably in the work which you are engaged upon.
You must, in the first place, understand precisely what it is you have
to do; and although the following details may be by some considered
more adapted for advanced students than for young mechanics, a little
attention to the explanations will render the matter clear to any boy
of age and intelligence to take in hand, with reasonable prospect of
success, the tools of the carpenter, turner, and fitter. I can only say,
that boys of this generation are wonderfully well off in having these
things explained to them. Twenty years ago young mechanics had to grope
along in the dark, ignorant to a great extent of the _principles_ of
work, and almost equally uninstructed in the practical part of it.
In Fig. 45 are represented similar angles to those already explained to
you, and you will quickly understand how useful is a little knowledge of
the elements of mathematics. Suppose A to be a tool, the angle of the
point is a right angle, or 90°. B is another of 60° at the point, and I
have drawn a line across to show you that the three sides of this figure
(called a triangle) are equal. So remember that if you want an angle of
60°, you have only to draw a triangle of three equal sides, and each of
these angles will be 60°. Again, I may as well remind you that three
times 60° equals 180°, which is equal to _two right angles_, so we find
here that the three angles of an equal-sided triangle equal two right
angles, and even if the sides are not equal, the same thing is true. For
instance, look at the first tool, across which I have also drawn a line
to make a triangle. The point we know is 90°, and if the sides, _a b_,
are equal (although the third line is _not_ equal to either), the two
small angles are each 45°, _i.e._, 90° between them, so the three angles
again equal 180°.
[Illustration: Fig. 45.]
The third tool (which we may suppose a turner’s chisel held _edgewise_)
is shown to have an angle of 30°, and I have added one more which has an
angle of 45°. Now all tools, if _well_ ground, are ground to a certain
known angle, according to the material which they are intended to cut.
Tools intended to cut soft woods, like deal, are ground to an angle of
20° to 30°, like the chisel seen edgewise. I shall have a word to say
presently as to the direction in which such tools are to be held, in
order to make them cut as well as possible. A tool for hard wood is given
next at E. The angle is now at least 40°, and it ranges up to 80°, giving
a stronger, thicker edge, but not so keen a one. We have, therefore, more
of a scraping tool than a cutting one,—at least, in the way it is usually
held. Then we come to the tools with which iron is turned and steel also.
Fig. F is one of these, and the usual angle is 60°, and thence it ranges
to 90°. Thus you see, advancing from soft wood tools to those for hard
wood, and thence to a substance still harder, we have increased the angle
of the edge, beginning at 30° and ending with 80° or 90°. But now we come
to a material which is harder than wood and not so hard as iron, yet we
use tools with an angle of 90°, which is still greater, and 70° is the
least angle ever used for this metal.
Experience only has taught the proper angle for tools, and it is found,
that if brass and gun-metal are turned with tools of a less angle
than 70°, they only catch into the material, and do not work at all
satisfactorily. You can, however, _scrape_ brass, as a finish, with the
thin edge of a common chisel; but then the tool is held so as to scrape
very lightly and polish; and its edge will not remain many minutes,
unless the maker (intending it to be so used) has made it much harder
than he would make it for soft wood cutting.
If you buy your tools at any _good_ shop, you will find that they are
already ground to nearly the angles named, and when you re-grind them,
you must endeavour to keep them to the same. The _bevel_, as it is
called, of many tools need not be ground at all, as they may be sharpened
solely by rubbing the upper face on a hone, or grinding it, holding it
so that the stone shall act equally on all parts of it. If, however, the
tool should become notched, you must grind the bevel of it, and then you
must try and keep the intended angle. One tool, however, or rather one
pair of tools, viz., turning-gouges and chisels, are very seldom ground
with a sufficiently long bevel when they first come from the maker. The
usual shape of the edge is like G, whereas the angle should be much less,
as seen at H. This you must correct when you first grind the tools for
use, and keep the same long bevel and small angle of edge continually
afterwards, for you will never make good work on soft wood if your
chisels and gouges are ground with too short a bevel.
I must also guard you against another common error, which, however, is
very difficult to avoid at first, and only long practice will enable
you entirely to overcome it. I, is the chisel (held edgewise as before)
ground as it ought to be; K is the same tool ground as it generally is
by young hands, or, even if it is correctly formed at the grindstone,
one or two applications to the oilstone almost invariably round it off
as shown. The bevel of _all_ tools must be kept quite flat and even, and
when the tool is afterwards rubbed on the oilstone to give a finish to
the edge, another flat, even bevel should be made. In the same figure at
L is an exaggerated view of the chisel, with its first long bevel formed
at the grindstone, and the second very small bright bevel seen at the
extreme edge of all such tools when they have been set upon the oilstone.
This second bevel, slight as it is, you will at once understand makes the
angle of the edge a little larger, therefore you must allow for it, and
grind a little keener edge than you really require.
Now, all this is very simple and easy to understand, and when you have
mastered this much, you will be in a fair way to understand more. The
second part of the subject, nevertheless, requires very close attention,
and very likely may not become quite clear to you when explained. I shall
therefore draw a line here, and make this lesson a special paragraph,
which you can look back to some other day, when you are grown from
a boy-mechanic to a man, and have had more experience in cutting and
turning wood and metal.
The tools above described have their cutting edges formed by the meeting
of two planes at a given angle,—these planes being the flat bevels (or
the flat top and one bevel) formed by the grindstone. But in some tools
three planes meet to form an edge instead of two, and the angle of
the cutting edge is not the same as that of either of these, although
it depends upon them, and can be nicely calculated. This calculation,
however, requires a knowledge of some higher branches of mathematics than
the young mechanic is supposed to be acquainted with, and therefore a
table is added instead, by which, when the angles of two of these planes
are known, the third may be at once seen, which last determines, of
course, the angle of the edge.
As an example, take the graver, of which you will find a drawing among
the other tools, but which I give again in this place. M, Fig. 45, is
the tool, looking at the face or bevel which has been ground upon it,
making a lozenge-shape or diamond. But this face is a _third plane_, and
the cutting edges, _a_ and _b_, depend for their angles upon all three
of these. Now, for iron we want an angle of 60°. How are we to make
the edges, _a b_, of that exact size? The bar is first of all square
in section, like N, which would be its shape before the third face or
bevel is ground, and all the angles are now right angles of 90° each.
But instead of this, we want two of them 60°, the other two being of no
importance. We simply proceed thus:—Determine which angle is to become
the point of the tool (it is no matter in the present case, as all are
alike), then grind away underneath till the new bevel forms an angle of
45° with the back (by which I mean the edge which runs along from the
sharp point towards the handle—the edge _x_ in fig. O). Trigonometry
enables us to find out that an angle of 45° is the one required, but you
will find it in the table annexed to this chapter, and an explanation of
this table is also given to enable you to use it easily. Thus ground, the
edges _a b_ of fig. O will be each formed of two planes meeting at an
angle of 60°. You can make a gauge of card or tin, P, to work by, of the
required angle.
[Illustration: Fig. 46.]
[Illustration: Fig. 47.]
In order to understand the use of this table, it is necessary to give
names to the several angles of a tool. That upon the front or face of
the tool, as A of the point-tool, is called the plan-angle; that made
by the upper surface and the front edge, as B (_a_, being the angle in
question), is called the section angle, because, if you were to saw
right through the central line lengthwise, this is the angle that would
appear at the point, viewing it sideways. Now, if we look at C, Fig. 47,
we shall be able to understand how the front line, _b c_, is obtained,
which constitutes one side of the section angle of a tool. It results
from the meeting of the two diamond-shaped planes at the sides formed
by the grindstone, but is dependent also on the plan-angle. These two
side-planes are to be generally ground at an angle of about 3° from the
vertical, which is to give the clearance of the tool if held in a fixed
position, as in the tool-holder of a slide-rest, the tool being supposed
horizontal. This is in accordance with what I have before told you, viz.,
that the cutting edge should be presented to the work at the smallest
possible angle, 3° being very small indeed. This angle is generally
measured by placing the side ground in contact with a cone of wood or
metal, turned to an angle of 3°, such as D,—_k_ being a tool the front
of which is evidently 3°; or a piece of tin, _l_, cut to the same angle,
and stood on its edge, will answer the same purpose. By 3°, I mean an
angle of 3° measured on the circumference of a circle, as I have already
explained in a former page, such angle being of course at the centre
of the circle where the lines drawn from the several degrees on the
circumference meet.
Now, when you have ground these two surfaces, the line _b c_ of B (or
C) will have a certain slope or inclination depending on the plan-angle
of the point. The exact inclination of it may be therefore said to be
accidental; but, whatever it is, it becomes of great importance in the
final result, being one side of the angle which will give any particular
angle of cutting edge. And here the table comes into use:—Suppose I wish
to have an edge of 60°, for cutting iron. Measure the _plan_-angle,—say
it is 90°, which is that of the graver; then, on the table, under the
words “plan angle,” you will see 90°, and opposite, above 60° of “cutting
edges,” you will see 45°. You have only to grind back the upper face of
the tool, until it makes an angle of 45° (section angle) with the front
edge or line, _b c_, and the edges _x x_ will be angles of 60°. Or take
the tool E, of which the plan angle is 120°, and suppose you want cutting
edges of 80°, for brass, opposite 120°, and above 80°, is 78° 5″. Grind
back the top face to an angle of 78° 5″ (or 78½) with the point line, and
it is done.
Until you have practically proved it, you can have no idea of the vast
importance of having correctly-formed cutting edges, and of placing them
within a hair’s-breadth of the proper position. But it is in slide-rest
work especially, and in cutting metal with tools held rigidly in one
position, that this is of such paramount importance. It makes all the
difference between cutting off a clean shaving, and tearing from the
material by main force a quantity of disjointed particles, the latter
process leaving a rough unfinished surface, the former producing one
as smooth and polished as a sheet of glass; and the advantage of this
short table is, that you can at any time shape your own tools for the
particular work in hand.
After you have had some practice in turning, you should certainly learn
to shape your tools from square bars of steel, worn files, and broken
steel tools of various kinds; and before you have arrived at sufficient
dexterity to do this entirely by yourself, you will get them roughly
shaped for you by the blacksmith, and then with grindstone and file you
will further perfect the angles for use. Steel does not require, and
must on no account be subjected to, a white heat, or you will spoil it
hopelessly; and you can always heat it in a common fire, or in the little
stove that I shall describe in a subsequent chapter, to a temperature
that will allow you to bend it into any required form with the hammer and
anvil—a bright red being the utmost heat it must be brought to.
POSITION OF CUTTING TOOLS.
We must now consider the mode of applying the edge of a tool to the work,
so as to produce the best effect. First, we will consider the case of a
gouge and chisel acting upon soft wood.
[Illustration: Fig. 48.]
In Fig. 48, A represents a piece of wood in the lathe, as you would see
it if you stood at one end of it, and a chisel is being held against
it. The arrow shows the direction in which the wood is supposed to be
revolving. Held thus, the chisel would scrape, and its edge would be
carried off at once; it could not possibly cut. But, held as at B,
it would cut off a clean and continuous shaving as the wood revolved
against it, and this shaving would slide off along the upper face, _b_,
of the tool, so that you can see that this face ought to offer the least
possible resistance to it. The tool acts, in fact, like a very thin,
sharp wedge, which divides the material by pressure, which has to be
great or slight according as the edge is sharp and thin or the contrary.
Now, if you again look at A, you will see that this wedge-like action
cannot take place, so that the tool is in its worst possible position.
Between the two positions, however, here shown, are several others at
a greater or less angle to the surface of the wood; but the smallest
possible angle it can make is the best, so long as the thickness of
shaving removed will suffice for your purpose. This rule holds good
with all tools, whether carpenters’ or turners’, which are made with
sharp-cutting edges. Care must be taken, however, that the lower face
of the tool does not rub against the work, which, again, it is evident,
limits to a given degree the angle at which the cutting edge is to be
applied to the work.
We now pass on to C, which represents the ordinary tool for turning iron,
held flat upon the rest, the position it usually occupies. We see at once
that in this case also we have a scraping tool only, and that, although
the angle of the edge is far greater than that of the chisel, it must
soon be ground off by the action of the metal to which it is applied, or
of the hard wood, which is also cut in this way. But with this form of
tool we shall find it impossible to apply it so as to cut in the best
way; because if we lower the handle, as we did that of the chisel, the
part below the edge will rub against the work, while the edge itself
will be moved out of contact with it. Thus we are obliged to hold the
tool in the position first shown; but we may therefore conclude that the
_tool itself is a badly formed one_ for the intended purpose; and so it
is, although you will see it in almost every workshop in the kingdom.
Let us see what can be done to improve it. At D, I have represented the
same tool, but the blackened part shows what has been filed away from the
upper face, and the dotted lines show that, when this has been done, a
tool is made very similar to the chisel for wood, and that it is also now
in a good position for _cutting_ (_not scraping_), although it is still
held horizontally upon the rest. Shavings of iron curl off the upper face
of this, as wood shavings curl off upon a chisel.
If the angle, however, is too small, the edge will soon be broken off,
and the tool will dig into the work; hence the necessity of knowing
at what angle a tool ought to be ground to cut any particular metal
successfully.
Such a tool as the last named, which is intended only to cut with the
front edge, and which is represented in E, is called a single-edged one,
because it only cuts in one direction, but many others are double-edged,
cutting the shaving at once on the flat and edge—that is, paring it off
from the material below and also from the side. For instance, F is a
cylinder of iron, from which a shaving is supposed to be in process
of being cut. It has to be removed from the shoulder to which it is
represented as still adhering, and also from the flat surface, _e b_,
around which it was, as it were, once coiled. But this requires two
cutting edges, both acting at the same time, but in different directions;
and good mechanics therefore so form the tools, and so use them, as to
cut in both directions, which leaves the work beautifully smooth and even.
These tools are mostly used in the slide-rest, where their true position,
once determined, can be accurately maintained; and it is, perhaps, only
with the slide-rest that perfect work can be done. There is, however,
no reason why you should not use tools of all kinds intelligently, and
understand exactly how they should be formed, and how held. Suppose you
have a tool correctly made by the aid of the table of tool angles already
explained, still looking at fig. F, you can see that the smaller part of
the roller is that which is to be left finished, and that it ought to
be quite smooth, but the shoulder at _a_ is not of the same degree of
importance. A tool fit for such work would evidently be shaped on its
_plan-angle_ or face, like H in fig. C or I; and, if held as seen, both
edges would be brought into action at the same time, as will be at once
evident on inspection. In practice, however, the two edges would not be
allowed to touch for their whole length, or the angle on the right would
leave a scratch upon the finished work; therefore it would be eased off
a little, as at K, L. But this is evidently as nearly as possible the
shape and position to be given to such a tool, and the edge which has to
leave the finished surface should, as it were, _follow_ the other; the
right-hand angle being _just_ and _only just_ kept out of cut.
The hand-tools you will generally use are the heel-tool, M, held on
the rest as shown, which, you see, brings the edge into cut at the
least possible angle to the work, and the nail-head, which is in fact
a heel-tool of four faces, or, if round, a heel-tool _all edge_, and
which can be rolled over as it gets blunted. To these add the graver,
of which I have already spoken. I have tried to show its position at O,
with the bevel of the face pointed in the direction of the shoulder, and
downwards; but it can be held face upwards also, and in one or two other
positions. Always remember that the cutting edge is to be presented at a
small angle with the work, and you cannot go wrong if the tool is well
formed. The nail-head and heel-tools are single edged, and easily ground
without the table of angles, but the graver is a double-edged tool,
properly speaking, although only one edge may perhaps be used.
Having explained the principles upon which you have to work as regards
grinding your tools and holding them when in use, I shall merely add a
few remarks as to the action of the grindstone and oilstone, and the
proper way of using them.
Always let the stone revolve towards you, as if you had to turn it smooth
with the tool you have to sharpen, except when you cannot possibly do so
without cutting grooves in it. Chisels, knives, axes, planes, and all
similar tools with flat edges, are to be ground with the stone running in
that direction, by which means you will avoid giving them a wire edge,
as it is called (_i.e._, a ragged-looking edge), and it will instead be
even and sharp; the filament of metal being, as it were, driven back into
the substance of the tool, instead of drawn away from it. Gouges may be
ground in the same way, but must be rolled about to keep up the form of
edge. It is indeed the easiest way with these to hold them _across_ the
stone, in the same direction as its axis, and then, by rolling them over
backwards and forwards, you can give a very good shape to the edge, which
should run slightly to a point, or rather _tend_ to one. They are never
to be ground square across, like that of the carpenter.
It is generally necessary to have some sort of rest upon which to lay
the tools during the operation of grinding, but do not trust to special
contrivances for holding them at the precise angle needed; rather trust
to your own skill, which will increase more and more by being severely
exercised. Always remember to grind your tools to a sharper angle than
will be ultimately required, that the final angle may be given by the
oilstone. Of the latter there are many kinds. Nothing probably can
surpass a Turkey stone, if good, but this varies considerably in hardness
and other qualities. There is a very quick-cutting, slightly coarse stone
from Nova Scotia, which is very serviceable, as it does this tedious work
with great rapidity, not, however, putting on the tools a very fine edge,
but one that admirably suits for such as are to be used on metal. With
the rest, a rub or two on Turkey, or Arkansas, or Chorley Forest stone,
will impart a finish. Arkansas stone, however, may be had coarse as well
as fine; it is much liked by some, but I prefer the Nova Scotia, as it
cuts more keenly, and even with the sharpest stone, setting tools is a
most laborious process.
The young mechanic will find it very difficult at first to hold the tool
steady, and to move it to and fro upon the oilstone so as not to give it
any rolling movement, by which the edge and bevel would be rounded, as I
before explained, which would in effect enlarge the angle of the cutting
edge, besides preventing it from being held at a sufficiently small angle
to the work to cut effectively. Nothing but practice will overcome this
difficulty; I shall not therefore attempt to describe exactly how the
tool should be held and the sharpening effected, such description being
not only difficult, but, as experience has proved to me, impossible.