Restoring Saws, Saw Tools, and Other Equipment
Straightening Saws -
Scientific American, 1877
In the manufacture of saws, the
straightening forms a large proportion of the manipulative
The cutting of the teeth, the grinding, the
polishing, the tempering, and the finishing: each of these
processes is accompanied by a straightening operation; for
in insuring an equal amount of tension at an parts of the
blade lies one of the principal elements necessary to the
production of a good saw, and a blade can hardly have any
mechanical operation performed upon it without affecting its
tension and straightness.
In the use of saws, it is found that band
and frame saws are, under ordinary conditions, comparatively
easily kept true and straight; whereas hand and circular
saws arc readily affected by several causes, among which the
most prominent is the setting of the teeth. The blades of circular saws, moreover,
frequently become hot, and the heating of a blade is almost
certain to impair its straightness, and hence the
equilibrium of its tension.
The set of a saw tooth should all be given
to the tooth itself, and in no case should it extend below
the bottom of the tooth into the solid blade; because in
that case it affects the straightness of the same and
renders it liable to break. The harder any cutting tool is,
the more cutting duty it will perform without becoming dull.
On the other hand, the strength depends upon the degree of
hardness or temper.
In a saw, the temper is made to conform to
the requirements of strength and elasticity, the latter
element including its resistance to becoming bent or taking
a permanent set, if bent much out of the straight line; and
this degree of temper (which is shown by a blue color) is
found to be the highest which it is practicable to give to
the saw teeth: which, being formed out of the plate itself,
are necessarily of the same temper as the plate.
Furthermore, the blue shows the highest
temper which it is practicable to give to the teeth, and
still allow them the capability of being bent to obtain the
set. Indeed, it is only from the fact of their being
weakened by the spaces between them that they will permit of
being set without being broken; for were we to attempt to
set the solid edge of a plate or blade, it would break, if
If then, in setting saw teeth, we allow the
setting to extend below the tooth, the strength of the
latter is destroyed, and the straightness of the plate or
blade is impaired.
What is commonly called a buckle or a bend
in a saw plate is known to the trade as a tight or a loose
place, meaning that the want of straightness is produced by
parts of the blade being unduly contracted or expanded; and
all the efforts of the straightener are directed to the end
of removing the contraction or of accommodating the
expansion, so that, the unequal tension or strain being
removed, the plate will he true and straight.
If we take a saw plate that is quite true,
and lay it upon a truly planed iron plate and allow it to
become first heated and then cooled thereon, we shall find
that it has become warped by the process, and it is apparent
that the warping has he en produced by the expansion and
contraction of the plate, and possibly mainly from irregular
heating and cooling; for it is impossible to insure that the
heat can be imparted to and extracted from the plate equally
in all parts. The varying widths, the extra exposure of the
teeth due to their partial isolation (and hence their
increased susceptibility to heat and cold), and other
elements, would all cause inequalities in heating, against
which it would be impossible to provide.
The circular saw affords the best example of
the vicissitudes caused by unequal tension, as well as the
most striking instance of the minuteness and skill in
mechanical detail required in the saw straightener's art.
Suppose, for example, that we have a
circular saw of three feet diameter, and that it is made
straight and true, and with an equal degree of tension
existing all over it.
Let its circumference travel at a speed of
2,500 feet per minute: it is obvious that the centrifugal
force generated by the motion will tend (and actually does,
to a slight extent) to expand the saw plate, and it is
equally obvious that this expansion decreases in amount as
the center of the saw is approached.
The equality of the tension on the plate is
destroyed; on the outside (or, in other words, center-bound)
when rotated the looseness of the plate decreasing from the
circumference towards the center as the radius shorten s. As
a consequence the extreme edge will, when in motion, flop
over from one side to the other, according to the side on
which the duty offers the most resistance; and this
resistance will vary, from the curves in the grain in the
wood, from knots, and from a variety of more minute causes.
It follows, then, that the sawing cannot be
smooth, and that, as the saw bends or flops over on one
side, the opposite side of the blade will come into close
contact with the work, entailing friction and, as a result,
heating; the latter will cause the saw to dish, and to
remain permanently dished.
The method employed by the saw straightener
to compensate for the expansion due to the centrifugal
motion is to place upon the saw a tension insufficient to
dish the saw when at rest, and yet sufficient to accommodate
the expansion due to the centrifugal force.
This he does by the delivery of blows upon
the plate, the effect of which will be to create a tension
sufficient to tend to enlarge the plate without overcoming
the resistance to enlargement offered by the plate itself
until such time as the centrifugal force diminishes this
resistance: when the tension follows up the advantage
afforded by the centrifugal force, and holds the plate from
becoming loose on its outer circumference.
If from an error of judgment the tension is
insufficient to accommodate the centrifugal force, the saw
becomes loose in the middle, or, in other words, it becomes
rim-bound when in motion; and the result is that it dishes,
as shown in Fig. 1.