and it would seem that the former might be improved by bringing it more nearly to the outline of the latter

and

it

would seem

that the former might be

improved by

bringing it

more

nearly to the outline of the latter. Sup-pose this

be

attempted,

and

that the face of the tooth

in-dicated

by

the line cd, Fig. 60, be

changed

to cl>'.

Such

a

change must

result either in

removing

material

from

the tooth,

and

thereby

weakening

it, or in changing the line

cd

to a position cd'. In other words, if the tooth is not

weak-ened, the space

between

it

and

the next will be reduced.

Again, if to

make

the advancingface still

more

acute, the line

^//' is accepted,

and

the tooth ^is not

made

smaller (that is,

weakened),

there will

be no

space

between

it

and

the next tooth.

Having no

spaces, there can

be no

teeth,

and

conse-quently the attempted

change

is impossible. It will thus be seen that the angle of the advancing face of the ripping-saw tooth cannot, unless it is

weakened,

be

much more

acute than

is

shown by

Fig.

60 and

Fig. 61.

The

form of the tooth

may

be wholly changed, however, to the outline

shown by

Fig. 63,

and some

advantage

may

thus be gained in respect of the cutting angle; but such a tooth, while suitable for machine-saws of considerable size, is too complicatedforsmall saws.

Nothing

remains, then, asa possible

means

of improvingthe cutting

edge

of the

saw

tooth, except a modification of the anglef>cd, Fig.60. Ifit could be

shown

that there isan excess of strength inthe tooth, above

what

is

needed

to perform its

work, the anglemight

be changed

to d'c(f, oreven to i>'^cd,

and

the valueof the tooth as a cuttingtool be increased.

More-over, itdoes notat first

seem

unreasonable to attempt such a change, forit is evident that the cutting

wedge

of the chisel (which

we

have regarded as the typical cutting tool), while

much more

acute than the angle /n-d, is yet strong

enough

to be entirely satisfactory.

A more

carefiil

comparison

of the saw

and

chisel, however,

32

discloses the following facts: first, a

saw

tooth

must be

softer than a chisel in orderthat it

may be

set

and

filed,

and

being softer,is therefore

weaker

in its substance; secondly, the width of the

saw

tooth is less than half the width of the narrowest chisel

made,

and, in this respect also, it is at a disadvan-tage; and, thirdly, in using a chisel the operator's atten-tion is given entirely to its

one

cutting edge,

and

if at

any

time it is likely to receive too

much

strain, it is at

once

re-lieved_; while each

saw

tooth,

on

the contrary, forms but a small part of a toolthat receiveslittle attention

and much

vig-orous handlingwhile it is being driven through straight grain,

crooked

grain, or hard knots, as the case

may

be.

From

a consideration of these points, it

seems

clear that the cutting-angle of a

saw

tooth

must be

less acute than thatof achisel.

But the degree of acuteness can

be determined

only

by

use.

Fig.

60 shows

the

form which

years of experience have

proved

the

most

practicable for generalwork,

and

while

some

bench-workers

do

file theirsaws ^"under," producing a tooth similar todch\ as

many more go

to theotherextreme

and

use atooth similar to dcf.

The

typical

form

given is easily kept in order, and,

when

in that condition, willcut freely

and

well.

55.

The Teeth

of Cross-cutting-Saws.

—

If a ripping-saw

is used directlyacross the grain, the fibersof the materialwill

Kij?.G-i ^^ torn

from

each

otherwithout being properlycut;

hence

the necessity for a

saw

thatwill " cross-cut." Fig.64 shows

by

^{its three} views a representative

form

of toothfor thissaw.

It will be seen

by

thefigure that the tooth terminates ^{in a}

trian-P.ENCH TOOLS. 33

gular point;

and

also, thatwhile the pointa is

formed on one

side of the blade, the next, a', is

formed on

the opposite side_; thus throughoutits length,the points of

any two

adjacentteeth being

on

oppositesidesofthe blade. This

arrangement makes

the

end

view oftheblade

show

twoparallel lines ofpoints,

and between them

a triangular depression,which,

when

exaggerated

by

the "set," will appear as

shown by

section

AB,

Fig. 64.

Inaction, the points a

and

a',Fig. 65, score thework,

and

the friction

between

the teeth

and

the cut fibers breaks

up

the latter,

and

they are carried off

by

the saw.

Assuming

that it is a matter of convenience to have these teeth, aswell as those of the ripping-saw, equal to the space

between

any two of them, there are three questions

which may

be considered concerning their proportions. ^First,

what

shall

be

the inclination of the advancing

edge

or " face^" of the tooth, as represented

by

the line al)

compared

withthe linebd.

Fig.

64?

Holly, in his little

work on "The

Art ofSaw-Filing,"

shows

the similarity of action

between

the advancing

edge ab and

the

edge

of a pocket ^knife

when made

to cut across the grain,

and

asserts that a knife with its cutting

edge

perpen-dicular to the surface

upon which

it acts (a position equiva-lent to

bd)

will

make

a roughercut,

and

require

more

force to carryit forward at a given depth, than

when

it is inclined ina position similar to thatof the line ab.

The

result obtained from such

an

experiment cannot be regarded as conclusive, because ofthe great difference in the character of the cutting edges

compared.

But, if it is found that the knife with its

keen

cutting

edge

behaves

more

satisfactorily at

an

inclination tothe work, it

seems

reasonable to conclude that the rougher

edge

of a

saw

tooth ^will give the bestresults

when much more

inclined.

A

consideration of these points justifies the belief

34 BENCH

thatan angle of

60

degrees with the work, that is, with a

Une

passing through the points a^'

and

a, is

none

too great,

and

all practice goes to

show

that teeth so

formed

not only

do

very

smooth

work, butcutwith ease

and

rapidity.

Secondly,

what

shall be the angle of the advancingface of the tooth, as represented

by

lines e'e

and

ef, Sec.

£F,

Fig.

64?

Sincethis angle formsthe cutting

wedge

of the tooth, itshould

be

as acute as is consistent with strength. Greater strength being required for action in hard

wood

than in soft, itfollows that this angle should

be

varied withthe materialin

which

it is

used.

For

general

work

it

may

correspondto the anglee'e/.

Thirdly,

what

shall be theacuteness ofthe point as indicated

by

the angle iaj, Sec.

AB,

Fig.

64?

This, also, is

determined by

thecharacter of the material to

be

cut. It should be

more

obtuse, as iak, forhard

wood

than for soft

wood,

not only be-cause additional strength is required, but also because, if too acute, the scoring will

be done

so easily that the fibers

be-tween

the scores willnot breakout,

and

the saw, being unable to pass

down

into

new

work, will slide along

on

the old.

Kiir.

66 Fi

In document LIBRARY OF CONGRESS. UNITED STATES OF AMERICA. (Page 47-51)