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Improvements in
Steels for Wood-Cutting Saws and Knives
HENRY B. ALLEN
PHILADELPHIA. PA.
Chief
Metallurgist, Henry Disston Sons
Paper presented
at the
Fifth National
Wood Industries Meeting, New York, N. Y.,
Oct. 16 and 17,
1930, of the American Society Mechanical Engineers
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(NOTE: Statements
and opinions advanced in papers are to be understood as
individual expressions of their authors; and not those of
the Society.
ACKNOWLEDGMENT: The
author expresses his appreciation and thanks to Mr. S.
Horace Disston, Vice-President, Henry Disston & Sons, Inc.,
for suggestions which have been most helpful in the
preparation of this paper.)
The box below
contains the "teaser" for Allen's paper:
The
important factors in a review of saw steels are
the blade which carries the teeth and the teeth
themselves. There is little in sight to promise
further marked improvements in the solid-tooth
type of saw. The next logical step is to use
inserted teeth made from materials known to be
well adapted to the function of cutting wood.
The same forward step was taken years ago in
metal cutting.
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The two important factors in a review of steels used for
sees are the blade which carries the teeth and the teeth
themselves. The steel for the blade, after heat treatment,
should combine high elastic limit, good ductility,
resistance to fatigue cracking, resistance to impact, and
uniformity. There is little in sight to promise further
marked improvements in the solid-tooth type of saw. The next
logical step is to use inserted teeth made front materials
known to be well adapted to the function of cutting wood.
This same forward step was taken years ago in metal cutting.
Future changes in design, so as to put to use metallurgical
discoveries, will require the best. thought of saw user,
machine builder, and saw maker. Research and untiring effort
alone lead the way to accomplishment.
The saw is a venerable tool. It dates bark to prehistoric
time. The first saw consisted of serrations along the thin
edge of a piece of stone or hone. As bronze and, later, iron
were discovered, the same scheme was followed, only with the
greater refinements made possible by the malleability of the
metals.
When steel came into civilization, still further
improvements were possible. Here was a materiel which could
not only be forged into shape, but subsequently could be
made hard and springy.
It is interesting to note that at this stage of development
Western practice very probably branched off from that of the
East. The bronze or iron saw, because of its lack of
springiness, could be used only to full advantage when
pulled through the material being cut. The Oriental still
operates his hand saws in this fashion. The West, however,
finding that a tempered blade enough spring to prevent
buckling, decided to do his cutting on the push stroke.
Civilization has always been slow to change. This
conservatism has applied with full force in the field of the
mechanic arts. Great changes, however, are taking place in
our day; the teachings of science are more widely accepted
and applied than ever before. if a curve were plotted
between time and the application of progressive ideas in the
field of mechanics, we would see
it long, slowly
rising line, with here and there an up or down trend, until
a comparatively few years ago, when the curve started up the
present rapidly increasing slope.
The saw maker and user have, in the past, been no exceptions
to the general law of human conservatism. The manufacture of
saws has been an art acquired by years of first-hand
experience. Specialized knowledge and equipment have been
required. The manufacture has therefore been retained in the
hands of comparatively few. Probably for these reasons there
is a remarkable scarcity of published information about the
steels used.
Simplicity is a consideration of prime importance in any
device. The saw has been a good exponent of this principle.
Even today the greater number are made from a single piece
of steel, the blade and teeth integral and of the same
temper. The advantage of this design is mainly simplicity of
manufacture, hence low cost. The disadvantage is that the
steel forming the blade must also serve for the teeth: The
blade, as we shall see later, requires properties which do
not coincide with those hest suited to a cutting edge.
The obvious way to eliminate these shortcomings of the
solid-tooth saw is to provide separate teeth which can be
mechanically attached to the blade. Thus each part of the
saw can be made out of a material best suited to its
respective function. Inserted-tooth saws have been used to
some extent for many years, although not primarily for the
reason just stated. In the past they have found their place
mainly because of the ease of keeping them in running order
and also that they hold to their original diameter. More
attention has been lately paid, however, to their great
usefulness in providing the means of applying a more
efficient cutting medium for the teeth: The discovery of new
steels and materials which display valuable properties for
cutting wood, but which cannot be used for the blade, must
result in the further development of. the inserted-tooth
saw.
There are therefore
two general types of saws, the solid tooth and the inserted
tooth, to he included -in a discussion of the steels used.
Inasmuch as the problems of today are concerned mostly with
power-operated tools, the paper. will be confined to that
type.
There are two equally important factors which must be
considered in a review of steels used for saws; namely, the
blade which carries the teeth, and the teeth themselves.
Both are of equal importance to the success of the saw: The
subject of saw steels will therefore be taken up under the
two headings. Knives also can be treated in the same way,
although the problems offered by the knife back or blade, as
distinct from the cutting edge, do not offer the same
difficulty.
The Blade
Most tools can be made with as heavy a shank or body as is
necessary for rigidly supporting the cutting edge. The saw,
however, is an exception. By its very nature it is a thin
tool. The policy of conservation in the wood industries is
increasing the tendency toward greater thinness. While the
saw blade has strength and rigidity in a direction truly
normal to its thickness, it is weak and flexible under side
strain. The rigidity of a saw has a direct bearing on the
accuracy and smoothness of the cut produced. and on the rate
of cutting, which are considerations of first importance. An
increase in the blade thickness is the direct way to produce
greater stiffness, but this method is prohibited by the
equally important consideration of wood wasted. A material
having a higher modulus of elasticity would give greater
stiffness to bending, but there is no relief in sight. We
have to be content with the 29,000,000 elastic modulus of
steel.
The speed of a revolving circular saw and the static tension
applied to a band saw furnish a stiffening effect which
compensates in part for the flexibility of the disk or band.
A circular saw revolving at wood-cutting speeds involves the
complicated mechanics of centrifugal stresses and also
vibrations. The stresses set up by centrifugal force are low
as compared with the strength of steel. A 16-in. saw
revolving at 3600 r.p.m. has a maximum stress from
centrifugal force of about 6000 lb. per sq. in., and this
occurs at the center hole. But while this stress is not
high, it does produce some stretch in the blade, which is
not uniform from center to rim. These strains in turn, if
uncompensated, result not. only in added lack of stiffness,
but even in an actual fluttering of the rim.
To counteract the stretching from centrifugal force, the
circular saw blade is given a prestraining operation called
tensioning. This may be done by hammering, roiling, or
otherwise, and results in a saw which will be "stiff" when
up to speed. The stretching of the saw rim in work due to
frictional heat is compensated for in the same manner. While
the stresses set up by centrifugal forces are low, those
induced by the pre-straining treatment are high.
The steel for the blade, after heat treatment, should
combine: (1) High elastic limit, (2) good ductility, (3)
resistance to fatigue cracking, (4) resistance to impact,
and (5) uniformity. And because most saws in use today are
of the solid-tooth variety, there is an additional
qualification; namely, (6) satisfactory edge holding.
It is self-evident that the steel and its fabrication into
saws must not be too costly. Another consideration that is
becoming increasingly important is that the steel weld
readily.
(1) High Elastic Limit. The saw blade shout behave as
nearly as possible as a perfect spring.- Operating strains,
due to centrifugal force, frictional heating, or other o al
causes, should produce as little permanent distortion as
possible. If a blade-is strained beyond the elastic limit,
it will be stretched or distorted, and the saw will have
lost its tension or have become uneven and lumpy. In any
event, it will not rim true, and so will produce a poor cut
and not stand up under heavy feed.
Plain carbon-tool steel, formerly used for all saws, could
not be heat treated to the high elastic limits found
desirable for the more modern production rates without an
over-sacrifice of other essential properties. For instance,
a high elastic limit would be accompanied by brittleness.
The usual carbon content of the steels used was 0.85 per
cent, although it might range as low as 0.70 per cent up to
1.10 per cent. The carbon content used depended there on the
edge-holding requirements of the teeth, although high carbon
was 'often selected for a thick plate so as to get better
hardening.
Nickel was the first alloy used to any extent in saw steel.
The object was to produce a stronger blade without
sacrificing toughness. The nickel content ranged between
1.25 to .3.5 per cent, with carbon between 0.60 and 1.00 per
cent, depending on the use of the saw. The plain nickel
steel constituted a real advance in saw steels, and it is
still used to some ex-tent. One objection to this steel is
its tendency toward a. striated, crystalline structure which
may result in spalling teeth. Another difficulty encountered
in the use of this steel is sluggishness in tempering, some
of the hardened steel structure remaining, with consequent
brittleness. These objections are more often with when the
nickel and carbon contents are on the high side. However,
when on the low side the amount of benefit derived is open
to question. The compositions shown in Table. 1, (a), (b),
and. (e), are types still used to some extent.
The properties of nickel steels were so interesting that the
next developments of alloy saw steels were mostly made with
the view of eliminating its weak points by the use of one or
more additional alloys. Following other alloy-steel
developments, chromium was tried and found to give very high
elastic limits, but at some sacrifice in ductility. The
steel showed a tendency to be "brash," as the old sawmaker
calls it when the steel will not stand considerable cold
swaging without checking. Vanadium was added with the object
of overcoming this "brash" condition, but without more than
a partial cure. It does, how-ever, produce a marked increase
in toughness. Nickel-chrome steels are used today for many
saws where the ductility requirements are not severe.
Typical compositions are shown in Table 1, (c), (d), (f),
and (j). Chromium is an inexpensive alloy and in some
compositions can be obtained from the scrap metal charged
into the steel melting furnace. Chromium, too, in small
amounts, enhances the hardening power of nickel steels and
so aids the sew manufacturer in hardening the thicker saws.
Another attempt to obtain high strength, together with high
values for the other essential properties, has resulted in
nickel-molybdenum steels, both elements being present in
small amounts. (Table 1 (g) shows a typical composition used
very largely and with good results. The elastic limit can he
made high without. sacrificing ductility and toughness. The
types shown under (j) and (k) are very good steels where no
cutting edge is involved.
(2) Ductility. The elastic properties of steel,
together with its ductility, make possible the tensioning.
or prestraining treatment essential to a hand or circular
saw. Mechanical tensioning is done by hammering or
cold-rolling selected areas of the saw. The raid working
elastically expands the steel adjacent to the spots where
the force is applied, the latter spots being permanently
stretched. A steel low in ductility can be given but little
deformation by the cold working without dangerously
approaching the breaking point.
A phase of ductility, the ability to he cold-worked without.
rupture, is an essential property of solid-tooth saws whose
teeth are swaged for clearance.
The property of ductility is dependent, to a large degree,
upon, the inherent quality of the steel as determined by the
method, of manufacture and the quality of raw materials
used. No alloy will compensate for inferior steel-melting
practice.
As noted previously, chromium does not assist in developing
ductility in saw steel. Nickel does help, and the
combination with molybdenum, Table 1, (g), gives
particularly good ductility with high elastic limit. Alloy
steels have eliminated most of the body cracks which were
fairly common in carbon-steel saws.
(3) Resistance to Fatigue Cracking. The high speeds
and feeds at which production saws are now run have
introduced new problems. Among these is the matter of gullet
cracking, due to repeated stress below the elastic limit of
the steel. Cracks of . this type can usually be identified
by the lines of progressive growth on the adjacent surfaces
of the fracture. Sharp angles or corners should never be
allowed where there is repeated stress, but the saw user
Alen allows this condition to exist. Gullet cracks sometimes
occur which cannot be traced to any such self-evident cause.
Those who have made extensive studies of the-phenomenon of
fatigue cracking are generally agreed that the strength of
steel to resist this type of failure bears a direct
relationship to the ultimate! strength; also that the
freedom of the steel from dirt and other inclusions halt a
direct bearing on its resistance. If such is the case with
saws, the present-day alloy steels, when clean and well
made, should possess remarkable resistance. This is not
always true, and there is reason to believe that fatigue
cracks in band and circular saws may be caused by
complicated stresses which are aggravated rather than
relieved by high physical properties. Good ductility in the
steel is a valuable asset, however.
Special measures can be used to largely prevent fatigue
cracks from forming. Properly designed and spaced rim slots
is one means often used. Another method frequently
practiced, in band-saw mills, is to cold-work the bottoms of
the gullets, thus stretching the metal at those points.
Keeping the saw sharp and preserving well-rounded gullets
and even tension are self-evident and worth-while
precautions.
(4) Resistance to Impact. A saw in service is
subjected to repeated blows. If the teeth are sharp and the
saw is in good shape, the normal impacts of the teeth in the
wood are not excessive. But other shocks incidental or
accidental to service may be heavy. One form of accident
often met with is striking a piece of steel in the cut. The
old-type carbon-steel saw would be apt to crack under such
treatment, or at least he put badly out of shape. A good
alloy-steel saw will either cut through the steel or will
have teeth sheared off, the blade itself being left in fair
condition. Side shocks which would distort a
low-elastic-limit carbon saw will have little effect on the
alloy blade.
(5) Uniformity. A saw which has not uniform physical
properties will never be a satisfactory tool. If the blade
has hard or soft spots, it can only he made fiat with
difficulty, it can-not be tensioned evenly, and it will not
retain its trueness or tension in service. Lack of
uniformity can be due to variations in steel composition or
to faulty heat treatment. The first-mentioned cause is most
apt to occur from failure to remove any decarbonized skin
that may have been on the steel sheet. Faulty heat treatment
is the usual reason for ununiformity and may result from
poor heat-treating practice or by reason that the steel
composition is unsuited to good hardening.
On account of the physical dimensions, saws must be hardened
in a comparatively slow quenching medium such as oil. Carbon
steels will not harden thoroughly in oil unless the piece is
thin or unless it has been overheated, which is of course
bad practice. . Carbon steels, therefore, cannot be
considered as suitable for saws to meet present high
standards unless they are of light gage.
The judicious use of small amounts of alloy will give the
steel suitable hardening properties. For this reason many
saws that are to be used only for work of the lighter so are
made from low-priced alloy steels. Chromium is an alloy
often used in this way, such as the composition shown in
Table I, (h). Where a slightly, tougher steel is wanted
composition (i) is a good choice.
Little need be said about the properties of the steel vacs m
machine knives. Sufficient toughness is of course necessary,
together with stiffness. Some types of knives, such as the
hog and chipper, are subject to heavy impacts and pressures,
and therefore the steel must possess more toughness than is
needed for other kinds. The planer or finishing knife, on
the other hand, will have sufficient strength if made out of
a less tough material such as high-speed steel. Some typical
compositions of solid knives are given in Table 2.
The Cutting Edge
The cutting action of a saw and of most knives is similar to
that of other cutting tools. The wood is parted by a wedging
action, and the power required is therefore a function of
the cohesive strength of the material. When the wood is
gummy, an additional factor of increased friction is
introduced. No variety of wood is very strong as compared
with metals, and therefore wood is said to be easy to cut.
There is, however, another and more vital factor than
strength that enters into the cutting of wood, namely,
abrasion. comparatively little power is required to sever
the fibers of any wood while the tool is sharp. The
condition may he short-lived, how-ever. The abrasive action
starts at once to polish away the keen edge until, sooner or
later, dullness results, enough to. cause poor cutting and
excessive power consumption. The saw maker and user have not
sufficiently appreciated the relation between sharpness and
smoothness of cut. A saw tooth is ordinarily considered
sharp if. it appears so to the naked eye, as it is left by a
file or medium-grit abrasive wheel.
The abrasiveness of different woods varies very widely, even
though their other properties may not be greatly different.
An example of this difference is given by the test results
shown in Table 3. The test was made with a 9-in, circular
saw, electrically driven and with a positive feed, the wood
being rip-cut.
Abrasion is a complicated phenomenon about which little is
really known. The type of abrasion met with in wood cutting
can be pictured as a buffing action, the abrasive being
cellulose and mineral salts, including silica. To make a
particular steel more resistant to abrasion, the first
thought would be to make the tool harder. But here enters
one of the curious features about abrasion, at, least under
conditions met with in wood cutting. When the saw speed is
very low, as in hand operations, greater saw hardness may
result in increasing by several times the life of the edge.
When the saw speed is high, however, as with a power tool,
there is little difference in life between the harder and
the softer tool. To obtain a marked improvement in
edge-holding with fast-running tools it is necessary to
change the steel composition.
The difference in tool life with change in tool speed and
tool hardness, but with the same composition steel, is shown
in Table 4. Although the material out was a wood substitute,
the comparison will hold for wood. The saws were made of
0.901 carbon-tool steel.
It will be noted
that at low speeds the increased life brought about by the
greater hardness was 350 per cent, whereas at high speed the
increase was only 20 per cent.
Another example of the effect of tool hardness on the
edge-holding properties is shown in Table 5. The tool was a
knife used in an automatic handle-turning lathe and made
from 0.60 carbon-tool steel. The wood cut was maple,
finishing 1 1/4 in. in diameter. The speed was 3200 r.p.m.
It is interesting to compare the results shown in Table 5
with those of Table 4. The increase of 12 to 14 points in
Rock-well hardness between the 48 and the 62 hardness tools
resulted in both cases in an increased. tool life of 20 per
cent. It is seen from Table 5 that the extreme difference in
tool hardness between the hardest, which was tempered at 350
deg. fahr. and file hard, and the softest, which was
tempered at 90(1 deg. fahr. and easily filed, resulted in a
difference in tool life of only 22 per cent.
The effect of changing the composition of the steel is shown
in Table 6. The tools were operating on the same job as
that' illustrated in Table ,5, except that the wood cut was
cherry.
Their compositions are shown in Table 7.
It will be seen from
Table 6 that the increase of carbon content, the, hardness
being the same, in the plain carbon steels (a), (b), and (c)
resulted in a consistent increase in tool life, up to 100
per cent. Also that the medium-nickel-low-chromium vanadium
steel (d) gave about the same results as did the plain
carbon steel (b) of like carbon content. The fact that the
low-nickel-low-molybdenum steel (e) with comparatively low
carbon; 0.76 per cent, had the same life as the higher
carbon steel (d), is . interesting. It would indicate that a
steel of greater toughness, resulting from the lower carbon,
can be had with no sacrifice in cutting life. The high-speed
steel tool (f) had an unusually low life for this kind of
steel, most saw and knife applications showing about a 10 to
1 ratio in its favor as compared with carbon-tool steel
A significant point
indicated by the foregoing tests is the far greater effect
produced by a difference in carbon, as shown in Table 6,
than by a difference in hardness, as reported in Table 5.
The difference in hardness between high and low temper gave
only. a 22 per cent variation in tool life, whereas
difference in carbon between high and low gave 100 per cent.
It must be borne in
mind that the conditions under which the saw or knife is
operating may have a mark .l influence on the comparative
performance of different steels. In certain applications a
carbon or-low-alloy tool steel will give no service. Where
the temperature developed in cutting is really high, only a
material (as high-speed steel, for example) which remains
hardat these temperatures will do the work. For some classes
of work a. steel having a tungsten content of from 2 to 8
per cent will give better service than without the addition,
although not so good results as will high-speed steel. It
can be taken as axiomatic that the better a steel resists
softening at elevated temperatures and also the type of
abrasive wear met with in wood cutting, the better service
it will give.
We can make use of the improved properties of the steels in
either of two ways: to obtain longer tool life under the
same conditions of operation, or to maintain the same tool
life but at a substantially higher rate of production. In
either case, to effect a major change in tool performance,
our present-day steels all fell into either one of two
classes, high-speed steels and all other tool steels.
For wood as well as for metal cutting, high-speed steel is
the best type known today. It is enough superior to warrant
wider application to wood sawing than is now the case. The
wood-working industry should profit more from the example
set by the metal-working industry and establish, by
fundamental research, more data about cutting its product.
If the results of such studies were at hand, we would not
only be making more use of the best steels we now possess,
but we also would be better' prepared for new developments
in tool materials.
The recently developed cemented tungsten-carbide tool
material will effect profound changes in wood cutting. It
possesses toughness, together with hardness and resistance
to abrasion, such that cutting speeds and quality of cut
heretofore impossible will be obtained. A typical example of
the results-being obtained with this material operating on a
machine designed for high-speed steel tools is given in
Table 8. The job was planning, tonguing, and grooving
fireproofed oak flooring. The 10-in. cutter head was
operating at a speed of 3600 r.p.m. with a feed of 80 f.p.m.
Other applications of cemented tungsten-carbide knives and
also saws are -giving results similar to those indicated.
The possibilities of such new materials cannot be ignored
and can hardly be overestimated by the wood-cutting
industry.
Conclusions
To one who has been in close touch with the recent
developments of saw steels there is little in sight to
premise further marked improvements in the solid-tooth type
of saw. The two conflicting requirements for the one steel
for the blade and for the cutting edge would seem to take us
for further advances beyond the nature of this metal.
Up to 15 years ago, or thereabouts, the saw maker for a
hundred years had spent an unbelievable amount of effort and
ingenuity in effecting the best compromise possible with
carbon steels. The next step was the development of an
alloy-steel saw, with the object of producing a stronger
blade and also to effect all possible improvement in
edge-holding. A much better blade resulted, but even with
the aid of. modem laboratories and metallurgical knowledge,
there was little improvement in edge-holding properties, far
from keeping pace with the potentialities of the new blade.
The next logical step is therefore to depart from the
old-type saw sad use inserted teeth made from materials
known to be well adapted for the function of cutting wood.
In the field of metal cutting this same step forward was
made years ago.
Future changes in saw design, so as to put to use
metallurgical discoveries, will require the best thought of
saw user, machine builder, and saw -maker. Research and
untiring effort form the only road today to accomplishment,
but the benefits in store for the woodworking industry are
worth the struggle.
Discussion
J. P. Potter (American Woodworking Company, New York, N.
Y.): Most of the experience that the Writer has had in the
woodworking field has been with saws and cutters. He has
seen it pass from the old carbon steel to the high-tungsten
steels and come on through to the cemented tungsten-carbide
steel. A great many woodworking tools are made today without
due regard to the safety of the swinging of steel and knives
and saws, especially of the high frequency machine. When you
go up to the neighborhood of 7200 to 9000, the centrifugal
strains become very great. So far as is known, there has not
been anything put on the market that offers absolute safety.
The internal head or the solid-back head which was invented
some 20 years ago comes the nearest to it, but it. will not
support a cutter projecting more than three-quarters or one
inch.
Speaking primarily about saws, the prepared paper seems to
be a very elaborate proposition. It needs very little
addition.
The things noticed in practice regarding saws are the
extreme misunderstanding on the part of those using the
steel in knowing what strengths are required from it.
The matter of Preparing steel for the shop is a subject
which needs a lot of discussion. The stock patterns can be
prepared in manufacturing plants far distant and remote from
the point of manufacture and can be shipped in due time for
ordinary purposes.
In plans for individuals, oftentimes one little house
requires enough steel' to write off the cost of the
manufacture. The use of cemented tungsten-carbide steel is
also costly. The writer h found that on stock runs the steel
costs relatively little in proportion to the amount of wood
run. It will range somewhere from 2 to 4 cents per 1000 ft.
On an ordinary mold of 3 in. width, it will run with about 5
or 6 in. of eel with a 6-knife equipment all around. Those
60 in. of steel will run about 4,000,000 lineal feet of
molding before they are completely worn out. That has been
proved in soft woods along the Pacific Coast.
With the higher priced steel, it becomes a question. By the
per cent of this paper, the steel will run so much longer
than the ordinary carbon-tool steel that perhaps there would
be no limit to the wear of the cuts. That brings up another
problem of using this steel properly supported in very thin
sections.
AUTHOR'S CLOSURE
The points brought out by Mr. Potter are of real practical
interest. The present-day use of saws and knives at higher
speeds is no exception to the modern trend, and, as Mr.
Potter indicates, the necessary safety precautions must be
taken, not only in the design of the tool, but also in its.
use. A point in this connection may well be discussed
briefly. An inserted-tooth saw or cutter head is usually
refitted many times by the user. He must see to it that. the
blade or head is not only properly refitted, but also that
it is maintained-in good mechanical condition and balance.
With higher speeds, the machine also must he kept in better
shape than formerly was necessary.
In calculating the effect of centrifugal forces in rapidly
revolving sews and knives, the subject should he considered
from two angles. A revolving disk will have maximum stresses
from centrifugal forces set up, not at the rim, but at the
center hole. The stresses at the rim will he comparatively
small. For ex-ample, a 16-in. blade with a 1-in. center hole
revolving at 3600 r.p.m. will have stresses set up amounting
to about 6000 lb. per sq. in. at the center hole and 1500
lb. per sq. in. at the rim. A 10-in. disk running at 7200
r.p.m. will result in about 9000 and 2000 lb. per sq. in. at
the center and rim, respectively. These stresses are well
within the factor of safety for a good steel and in
themselves far from dangerous.
Excessive rim-strains set up by poor tooth or cutter-edge
clamping methods vibrations, and accidental strains in
operation are far more dangerous than those from centrifugal
forces. The added danger from centrifugal forces as speeds
increase does not come about as much through the greater
possibility of a well-made and properly operated tool being
broken by these forces as by reason of the much greater
damage done if breakage occurs. The energy of a piece flying
off the rim of a saw or cutter in-creases as the square of
the r.p.m. of the tool. Thus doubling the speed of the rim
increases the energy by, four times, and protective measures
must be bettered correspondingly.
Mr. Potter properly raises the question of economy in using
expensive steels or tool materials for
working-ordinary-woods. The relative tool costs must always
be considered. An extreme case where a cheap steel is
clearly good enough is where short runs of special molding
shapes are made from soft wood with no particular
requirement for surface finish. It would be poor economy to
make an expensive tool only to discard it after a few hours'
run. Special material like cemented tungsten carbide for
woodworking is finding its first applications to production
jobs where the wood is particularly abrasive and where
machine down time or other-factors are important. Users are,
however, beginning to. see its possibilities for more of
their regular production work on common woods whose accuracy
or smooth finish is desired. Its' use will grow as
experience indicates its proper applications.