IMPROVEMENTS IN CUTTING SCREWS AND IN APPARATUS EMPLOYED THEREIN
Year: 1884

page_text: I ALFRED GEORGE BROOKES of 55 Chancery Lane in the County of Middlesex
Fellow of the Institute of Patent Agents do hereby declare the nature of my
invention for IMPROVEMENTS IN CUTTING SCREWS AND IN APPARATUS EMPLOYED
THEREIN to be as follows:-
This invention consists mainly in a mechanism for accomplishing the following
objects:
--
First To cut lathe or other screws to an exact standard of length or so that
the distance between the terminal convolutions of the screw thread at the two ends
measured along any line parallel with the axis of the screw shall be exactly equal
to any equal multiple or to any equal subdivision of the Imperial Yard of Great
Britain at the temperature of 02° Fahrenheit or to any other standard of linear
measurement that may be desired. Screws of any required pitch whether. regular
or irregular may be cut to an exact standard of length.
Second-To produce screws which shall have a uniformn pitch not only for
aliquot parts of a revolution or turn of the thread at any point, but also throughout
the entire length of the screw.
Third To test the accuracy of screws cut by our process.
It may be readily demonstrated that existing screws are far from accurate with
respect to uniformity of pitch and the error in the whole length at the standard
temperature is often so great that a screw cut upon one lathe will not fit a nut cut
upon a lathe by a different maker.
The most serious error found in existing lathe screws belong to a class known as
periodic errors occuring in each turn of the thread as follows:-
Starting at any point in the arc of a revolution, the error in inclination or pitch
of the thread increases by successive small increments until at a certain point a
maximum value of the error is reached and beyond this point the error gradually
diminishes but not usually at the same rate that it previously increased. Thus
though the error in the first one hundredth of a revolution may be for example

page_text: only Too of an inch, the error in one tenth of a revolution may by successive
increments amount to as much as 1005 of an inch, and the maximum error in one
turn of the thread may reach of an inch.
Thus though it may be difficult to detect the error for small arcs of revolution,
the accumulation of these small errors for half a revolution of the screw, may
become a very serious matter.
Hitherto the method most commonly employed to obtain uniformity in the pitch
of screws consists in grinding with emery by means of a nut having a greater or
less length; but it may be easily shown that the class of errors described as periodic
cannot be removed by grinding.
The errors in pitch at one part of the screw are simply distributed in a slightly
different manner. This must necessarily be the case since in grinding we deal
with a combination of the threads of the nut and screw and the said combination
- varies at every point in the rotation of one relatively to the other.
The present process deals with each single turn of the thread. Each turn of
the thread is by the process of cutting made to represent the mean of all the
threads in the screw for every part of the arc of revolution.
In order to cut screws in accordance with this invention with success the
following conditions must be fulfilled.
a-The carriage which carries the cutting tool must move in a plane parallel
with the axis of the screw to be cut. If therefore there is any flexure in the ways
upon which it travels, this flexure must be neutralized by proper methods.
b, The cutting tool must move in a line parallel with the axis of the screw to
be cut. The ways must therefore have no curvature in their plane.
C- -The screw to be cut must describe a true cylinder in its revolution and not
the frustums of two cones.
The flexure of the rod upon which the thread is to be cut must therefore be
neutralized.
In practice these preliminary conditions of success are fulfilled by methods of
mechanical construction which need not be fully described in this connection,
it being assumed that the machine is accurate in these particulars, as such accuracy
is within the range of mechanical construction.
In practising this invention as shown in this instance a screw cutting lathe is
employed the leading or tool feeding screw of which may have greater or less
inaccuracies as is inevitable in screws made previous to this invention, the said
feeding screw, and the spindle carrying the shaft to be threaded being connected
by gearing so as to normally have a definite relative rate of rotary movement
as is usual in screw cutting lathes, and the said feeding screw operating the
carriage for the tool by which the shaft or rod is cut or threaded in the usual
manner.
The invention consists'partly in a lathe having a tool carrage and feeding
mechanism therefor, combined with a standard linear scale or graduated bar fixed
upon the lathe bed, and a cooperating measuring device shown in this instance as a
microscope carried by the said tool carriage whereby the distance travelled by the
tool carriage may be accurately and minutely observed from point to point, and
the inaccuracies of its feeding movement thus determined throughout the entire
length of the said movement.
This shows the amount of correction necessary during the different parts of the
movement of the tool carriage, and the invention further consists of mechanism
for so correcting irregularities of longitudinal movement of the tool in relation to
the rotary movement of the spindle carrying the rod or shaft to be cut as to make
the said relation constant.
The invention also consists in various details of construction hereinafter described
whereby the movements of the different parts are rendered more accurate.
The lathe bed may be of any suitable or usual construction, the ways upon
which the tail stock, and the tool carriage travel being préferably made as cylinders
for greater accuracy,

page_text: The cylindrical ways upon which the head and tail stocks of the lathe are
mounted, are provided with longitudinal dove-tail shaped ribs rigidly fastened to
then to facilitate the clamping or fastening of the said stocks upon the said
ways.
The spindle of the head stock to which the shaft or rod to be cut or threaded
is fastened by any suitable clamp, dog, or chuck, preferably as hereinafter
described, and is provided with an actuating worm gear meshing with a worm
mounted loosely on the driving shaft provided with a pulley and mounted in
bearings upon a shelf at the head of the lathe bed.
The said worm is provided with a grooved collar, running in a bearing which
prevents any longitudinal movement of the said worm relative to the worm gear,
and headstock spindle and the worm is provided with pins passing through sockets
in a collar fixed upon the driving shaft the said pins and collar operating as a spline
to permit the said shaft to move longitudinally through the said worm but causing
both to have the same rotary movement.
The driving shaft has fixed upon it a worm engaging a worm gear fixed upon
the leading or feeding screw which engages and operates the tool carriage in the
usual manner.
It will be understood that if the loose and fixed worms are of equal pitch and the
corresponding worin gears are exactly equal, then the feeding screw and shaft to be
threaded will be revolved at exactly the same speed as long as the worms remain
in the same relative position, or in other words as long as the driving shaft and
worm fixed thereto have no longitudinal movement, but if the said shaft and worm
be moved longitudinally by mechanism hereinafter described, the said movement
will give a rotary movement to the worm gear and feeding screw independent of
that produced by the rotary movement of the shaft, and worm, and the said rotation
produced by the longitudinal movement of the shaft, and which may be called the
correction, may be such as to increase or diminish the rotation of the feeding screw
relative to that of the shaft to be threaded according to the direction of the longitu-
dinal movement of the said driving shaft. The requisite for making a correct screw
thread on the shaft to be threaded is that for equal angular or rotary movements of
the said shaft, the tool shall be moved by the tool carriage equal distances measured
parallel with the axis of the shaft, and it will be seen that if there is no inaccuracy
in the construction of the actuating and feeding screw worm gears, and worms and if no
error arises in the cornection between the actuating gear and shaft to be threaded"
or between the said screw and the cutting tool, then if the feeding screw itself were
accurate the desired relative movement would be imparted to the shaft to be
threaded and the cutting tool and a correct screw would be cut of the same pitch as
the feeding screw, or it might be of different pitch by varying the gears and
respective worms.
It is however practically impossible to find a correct screw made by previous
methods and consequently the first step towards making an accurate screw is to
find and tabulate the errors in the feeding screw, or in other words in the longitu-
dinal movement of the tool carriage and tool relative to the rotary movement of the
shaft to be threaded.
Suppose for examp'e it is required to cut a screw the thread of which makes five
hundred turns in one metre (say 393, inches) of length, or as it is commonly called
a screw of five hundred threads to one metre, then for each five rotations of the
shaft to be threaded the tool should travel one centimetre (say 3, of an inch), or
for each one half rotation a millemetre (say 1/2 of an inch) and so on. The actual
relative travel is determined as follows:-it being supposed that the feeding screw
has five hundred threads to one metre although they are not uniform.
In order to accurately measure the number of rotations of the shaft to be threaded
so as to compare its rotary movement with the longitudinal movement of the
carriage the spindle is provided with a plate or disc having one or more projections
according to whether it is desired to measure whole rotations or fractions thereof,.
it being supposed say.to have two opposite projections to measure half rotations.

page_text: The said projections coöperate with an indicating device or gauge consisting of a
lever pivoted at its lower end and normally held by a spring in contact with an
insulated stop the said lever and stop forming a key for the circuit of an electrical
signal or sounder which thus announces each time that the projection engages and
moves the said lever.
In order to determine the point for stopping the said shaft to be threaded so
that it may be always stopped at exactly the same point the said lever has a long
index finger which is moved by the action of the said projection over a graduated
are so that the said shaft may always be brought to rest when the said pointer has
arrived at the same graduation on the scale or arc.
In order to further facilitate observation from any part of the lathe without
approaching the index and arc, the stop gauge lever, has connected with it a mirror
by which a reflected beam of light may be made to travel along a scale as the said
lever is turned, it being used in substantially the same way as in the well known
reflecting galvanometer.
The movement of the lathe will necessarily be very slow when frequent obser-
vations are to be made and it will be hardly practicable to drive it by power; but
it will be turned by hand, and the operator will be enabled by the indicating
devices or gauges last described to stop the shaft at exactly the same point in each
rotation.
The divisions of the scale are useful in finding the same point on the shaft with
relation to the point of the cutting tool in case the latter should be reset or ground
before the operation is completed in which case it might be found necessary to stop
the shaft at another division of the indicating scale.
Having determined a number of complete revolutions or an exact fraction of a
revolution of the shaft to be threaded the next step is to measure the distance
travelled by the tool carriage under the influence of the feeding screw, which is
accomplished by means of a linear measuring bar accurately graduated for linear
measurements with the desired number of graduations to correspond with the
distance to be travelled by the carriage in the different measured portions of the
rotary movement of the shaft to be threaded.
For example, the bar may be divided into centimetres and the shaft stopped at
every fifth revolution when the position of the carriage will be determined by a
cooperating measuring device which for extreme accuracy consists in this instance
of a microscope having the usual cross wires to definitely fix a line of vison, it
being fixed upon the said carriage.
The mechanism is set with the shaft to be threaded stopped at a definite point
indicated by the gauge and with the cross wires of the microscope exactly on one
division of the scale or bar and the shaft is turned for five revolutions and an
observation made through the microscope.
If the cross wires are on the proper division of the bar it shows that the feeding
screw is of proper average pitch for the said portion of its length.
If, however, as is usually the case the cross wires are not on the division, the
amount of variation is observed and noted as an inaccuracy to be corrected in
cutting that portion of the shaft to be threaded and the feeding screw will be
turned by the operator independently of that shaft until the cross wires are brought
exactly upon the division after which that shaft will be turned five more revolutions
and another observation made as before and the result tabulated.
In this manner the inaccuracies of the feeding screw are determined throughout
its entire length.
The longitudinal movement of the driving shaft may be produced by various
kinds of mechanism.
This said shaft is provided near one end with collars engaging the ends of a sleeve
in which the said shaft rotates freely, the sleeve being provided with a pin entering
a block mounted in guides of a guide plate pivoted upon a carriage movable in a
longitudinal guide fixed upon or forming part of the bed of the lathe.

page_text: The said carriage is provided with a feed screw, operated by a nut, itself rotated
by a suitable train of gearing from the shaft of the gear by which the feed screw is
actuated, the said gearing being such that the carriage is fed sufficiently to cause
the block to travel the length of its guide during the feeding movement of the tool
carriage along the lathe.
Thus by setting the guide plate at any desired angle to the guide the driving
shaft may be caused to move a certain distance during the entire feeding movement
of the carriage thus varying the relative rate of the shaft being threaded and feed
screw from that made by the simple rotation of the loose & fixed worms.
This enables an error extending through out the entire length of the screw to be
rectified automatically with the lathe running at usual speed, or a screw to be cut
of slightly greater or less average pitch than the feeding screw, so as to bring the
screw that is cut to the desired standard of length although the minor inaccuracies
occuring at different points along the screw will not be corrected.
In order to turn the guide plate on its pivot, it is provided with a worm gear
below the carriage engaged by a worm splined upon an adjusting shaft extending
along the front of the lathe, and provided with any convenient number of graduated
discs or handles by which it may be rotated to turn the guide-plate ou its pivot,
the said worm being free to travel longitudinally on the shaft to accomodate the
longitudinal movement of the carriage.
For correcting the minor inaccuracies which occur in the feeding screw from
point to point the guide plate may be gradually turned by the operator while the
lathe is in operation to produce the proper longitudinal movements in the driving
shaft to make up for the said inaccuracies, and while removing or making the
corrections for the said inaccuracies the gearing may be disconnected from the shaft
of the screw, so that the carriage will have no longitudinal movement while the
corrections are being made, the guide plate merely operating as an arm to move the
shaft endwise.
It may in some cases be desirable however to first cut a screw of correct total
length by means of the inclined guide plate carriage and cooperating parts when
operating automatically as previously described, and then to use the screw thus cut,
to actuate the tool carriage while cutting the screw for which corrections are made
by hand from point to point throughout the entire length of the thread.
The spindle of the head stock of the lathe for carrying the shaft to be cut may
be made hollow and have a slightly tapering socket in its end to receive the corre-
spondingly tapered end of the shaft to be threaded, the said shaft having a small
thread cut on it to be engaged by a threaded socket in a clamping spindle passing
through the head stock spindle and provided with a collar engaging the rear end of
the head. stock spindle and adapted to be rotated by a suitable wrench or key
to draw the tapered end of the shaft to be threaded into its tapered socket' in the
spindle, thus securely fixing it in relation to the said spindle while being operated
upon by the latter.
The clamping spindle has a shoulder engaging a shoulder in the hollow head
stock spindle, so that when the said clamping spindle is turned in the opposite
direction it will throw the shaft to be threaded out from its socket so that it may
be removed from the lathe.
It will be found that certain irregularities occur in the relative rotary movement
of the shaft to be operated on and longitudinal movement of the tool carriage
during each rotation of the said shaft, owing to some imperfection in the gearing or
mechanical connection between the said shaft and tool carriage other than the
irregularities in the feeding screw which have before been referred to and provided
for, and these irregularities in rotary movement may be determined by careful
measurement of the longitudinal movement of the tool carriage during the different
fractions of several successive rotations of the shaft being threaded.
Thus in a series of measurements at various points in each rotation of a series of
rotations it may be found that errors of the same character are repeated during the
same part of each rotation, the angle of inclination of the thread being for instance

page_text: too great in the first one tenth of each turn of the thread measured from a given
line along the side of the screw parallel with its axis, and the said angle being too
small during the next one fourth of each turn of the thread and so on, although
the amount of variation will not usually be equal in the different turns, but may
be averaged.
Such irregularities as are found repeated at the same part of each revolution are
compensated for by the connecting mechanism between the head stock spindle and
its actuating gear by which the movement imparted to the said spindle varies
somewhat from that of the gear.
The said spindle has fixed upon it a wrist-plate having a tapering socket
pressed upon a corresponding taper on the spindle by a nut, the head stock gear,
being loose upon the hub of the said plate which is provided with a pin pivoted on
the said plate and having an arm, provided with a stud bearing upon a cam shaped
in accordance with the previously observed irregularities in rotary movement
of the head stock gear with relation to the longitudinal movement of the tool
carriage, the cam being fixed upon the head stock.
The said pin is provided with an eccentric stud connected by a link with the
head stock gear so that as the said pin is partially rotated by the oscillation of the
arm, there is a slight rotary movement of the plate and head stock spindle within
the loose gear which will compensate for the irregularities in the movement of the
said gear.
By fastening the shaft to be threaded to the head stock spindle by means of the
tapering socket the irregular strain that is derived from the usual face-plate and
dog is avoided.
When necessary the entire apparatus up to the level of the graduations on the
bar may be immersed in a liquid such as oil so as to maintain the graduated bar or
standard and the shaft to be threaded, at the same temperature and if the said bar
and shaft are of materials having different co-efficients of expansion the said liquid
will have to be kept at a uniform temperature while the cutting is taking place.
Dated thin 1
Jon of T..1.. 1001

page_text: I ALFRED GEORGE BROOKES of 55 Chancery Lane in the County of Middlesex
Fellow of the Institute of Patent Agents do hereby declare the nature of my
invention for IMPROVEMENTS IN CUTTING SCREWS AND IN APPARATUS EMPLOYED
THEREIN and in what manner the same is to be performed, to be particularly
described and ascertained in and by the following statement:-
This invention consists mainly in a mechanism for accomplishing the following
objects:-
First To cut lathe or other screws to an exact standard of length or so that
the distance between the terminal convolutions of the screw thread at the two ends
measured along any line parallel with the axis of the screw shall be exactly equal to
any equal multiple or to any equal subdivision of the Imperial Yard of Great Britain
at the temperature of 62° Fahrenheit or to any other standard of linear measurement
that may be desired. Screws of any required pitch whether regular or irregular'
may be cut to an exact standard of length.
Second-To produce screws which shall have a uniform pitch not only for.
aliquot parts of a revolution or turn of the thread at any point, but also through-
out the entire length of the screw.
Third To test the accuracy of screws cut by our process.
--
It may be readily demonstrated that existing screws are far from accurate with
respect to uniformity of pitch and the error in the whole-length at the standard
temperature is often so great that a screw cut upon one lathe will not fit a nut cut
upon a lathe by a different maker.
The most serious error found in existing lathe screws belong to a class known as
periodic errors occuring in each turn of the thread as follows:-
Starting at any point in the arc of a revolution, the error in inclination or pitch
of the thread increases by successive small increments until at a certain point a
maximum value of the error is reached and beyond this point the error gradually'
diminishes, but not usually at the same rate that it previously increased. Thus
though the error in the first one hundredth of a revolution may be for example
only 1 of an inch, the error in one tenth of a revolution may by successive
increments amount.to as much as 1000 of an inch, and the maximum error in one
turn of the thread may reach of an inch.
Thus though it may be difficult to detect the error for small arcs of revolution,
the accumulation of these small errors for half a revolution of the screw may
become a very serious matter.
Hitherto the method most commonly employed to obtain uniformity in the pitch
of screws consists in grinding with emery by means of a nut having a greater or less
length; but it may be easily shown that the class of errors described as periodic
cannot be removed by grinding.
The errors in pitch at one part of the screw are simply distributed in a slightly
different manner. This must necessarily be the case since in grinding we deal

page_text: with a combination of the threads of. the nut and screw and the said combination
varies at every point in the rotation of one relatively to the other.
The present process deals with each single turn of the thread. Each turn of the
thread is by the process of cutting made to represent the mean of all the threads in
the screw for every part of the arc of revolution.
In order to cut screws in accordance with this invention with success the
following conditions must be fulfilled.
a; The carriage which carries the cutting tool must move in a plane parallel
with the axis of the screw to be cut. If therefore there is any flexure in the ways
upon which it travels, this flexure must be neutralized by proper methods.
b; The cutting tool must move in a line parallel with the axis of the screw to
be cut. The ways must therefore have no curvature in their plane.
c; The screw to be cut must describe a true cylinder in its revolution and not
the frustums of two cones.
The flexure of the rod upon which the thread is to be cut must therefore be
neutralized.
In practice these preliminary conditions of success are fulfilled by methods of
mechanical construction which need not be fully described in this connection, it
being assumed that the machine is accurate in these particulars, as such accuracy
is within the range of mechanical construction.
In practicing this invention as shown in this instance a screw cutting lathe is
employed the leading or tool feeding screw of which may have greater or less
inaccuracies as is inevitable in screws made previous to this invention, the said
feeding screw, and the spindle carrying the shaft to be threaded being connected by
gearing so as to normally have a definite relative rate of rotary movement as is
usual in screw cutting lathes, and the said feeding screw operating the carriage for
the tool by which the shaft or rod is cut or threaded in the usual manner.
The invention consists partly in a lathe having a tool carriage and feeding
mechanism therefur, combined with a standard linear scale or graduated bar fixed
upon the lathe bed, and a cooperating measuring device shown in this instance as
a microscope carried by the said tool carriage whereby the distance travelled by
the tool carriage may be acurately and minutely observed from point to point ;
and the inaccuracies of its feeding movement thus determined throughout the
entire length of the said movement.
This shows the amount of correction necessary during the different parts of the
movement of the tool carriage, and the invention further consists of mechanism for
so correcting irregularities of longitudinal movement of the tool in relation to the
rotary movement of the spindle carrying the rod or shaft to be cut as to make the
said relation constant.
The invention also consists in various details of construction hereinafter described
whereby the movements of the different parts are rendered more accurate.
Figure 1, is a plan view of an apparatus for cutting screw threads embodying this.
invention.
Figure 2, a front elevation, and
Figure 3, an end elevation thereof..
Figure 4, a transverse section on line x Figure 1.
Figure 5, a longitudinal sectional detail of the spindle for revolving the shaft or.
1od being cut, and
Figure 6, details to be referred to.
The lathe bed a, may be of any suitable or usual construction, the ways a¹, a², a³,
upon which the tail stock b, and the tool carriage c travel being preferably made as
cylinders for greater accuracy.
The cylindrical ways a', a², upon which the head and tail stocks of the lathe are
mounted, are provided with longitudinal dove-tail shaped ribs a, see Figure 4,.
rigidly fastened to them to facilitate the clamping or fastening of the said stocks
upon the said ways.

page_text: The spindle d of the head stock to which the shaft or rod S to be cut or threaded
is fastened by any suitable clamp, dog, or chuck, preferably as hereinafter described,
and is provided with an actuating worm gear & meshing with a worin e mounted
loosely on the driving shaft f, provided with a pulley f¹, and mounted in bearings ƒ³,
upon a shelf f³, at the head of the lathe bed.
The worm e, is provided with a grooved collar e' running in a bearing f* which
prevents any longitudinal movement of the said worm relative to the worm gear d³,
and spindle d, and the worm is provided with pins e passing through sockets in a
collar e fixed upon the shaft f, the said pins and collar operating as a spline to
permit the said shaft to move longitudinally through the said worm but causing
both to have the same rotary movement.
The shaft has fixed upon it a worm g engaging a worm gear g¹ fixed upon the
leading or feeding screw g³ which engages and operates the tool carriage c in the
usual manner.
It will be understood that if the worms e and g are of equal pitch and the worm
gears d¹ and g¹, are exactly equal, then the screw g² and shaft S, will be revolved
at exactly the same speed as long as the worms remain in the same relative position,
or in other words as long as the shaft f and worm g¹ fixed thereto have no longi-
tudinal movement, but if the said shaft and worm be moved longitudinally by
mechanism hereinafter described, the said movement will give a rotary movement
to the gear gland scrow g² independent of that produced by the rotary movement
of the shaft, and worm, and the said rotation produced by the longitudinal
movement of the shaft f, and which may be called the correction, may be such as
to increase or diminish the rotation of the screw g² relative to that of the shaft S
according to the direction of the longitudinal movement of the said shaft f. The
requisite for making a correct screw thread on the shaft S, is that for equal angular
or rotary movements of the said shaft, the tool shall be moved by the tool carriage
equal distances measured parallel with the axis of the shaft, and it will be seen
that if there is no inaccuracy in the construction of the gears d', g', and worms e, g,
and if no error arises in the connection between the gear d' and shaft S, or between
the said screw and the cutting tool, then if the screw g² itself were accurate the
desired relative movement would be imparted to the shaft S and cutting tool
and a correct screw would be cut of the same pitch as the feeding screw, or it
might be of different pitch by varying the gears d¹ and g' and respective worms.
It is however impossible to find a correct screw made by previous methods and
consequently the first step towards making an accurate screw is to find and
tabulate the errors in the feeding screw 92, or in other words in the longitudinal
movement of the tool carriage and tool relative to the rotary movement of the
shaft S.
Suppose for example it is required to cut a screw the thread of which makes five
hundred turns in one metre of length, or as it is commonly called a screw of five
hundred threads to one metre, then for each five rotations of the shaft S, the too!
should travel one centimetre, or for each one half rotation a millemetre and
so on. The actual relative travel is determined as follows:-it being supposed
that the screw 9' has five hundred threads to one metre although they are not
uniform.
In order to accurately measure the number of rotations of the shaft S so as to
compare its rotary movement with the longitudinal movement of the carriage c,
the spindle d is provided with a plate or disc h having one or more projections /¹,
according to whether it is desired to measure whole rotations or fractions thereof,
it being shown in this instance as having two opposite projections to measure half
rotations.
The said projections cooperate with an indicating device or gauge 2, consisting
of a lever pivoted at h³ and normally held by a spring h' in contact with an
insulated stop h³, the said lever and stop h³ forming a key for the circuit of an
electrical signal or sounder which thus announces each time that the pro-
jection engages and moves the said lever.

page_text: In order to determine the point for stopping the said shaft S, so that it may be
always stopped at exactly the same point the lever h has a long index finger h
which is moved by the action of the projection ', over a graduated are so that
the said shaft may always be brought to rest when the said pointer has arrived at
the same graduation on the scale or arc.
•
In order to further facilitate observation from any part of the lathe without
approaching the index and are, the stop gauge lever h, has connected with it a
mirror by which a reflected beam of light may be made to travel along a scale
as the said lever is turned, it being used in substantially the same way as in the
well known reflecting galvanometer.
The movement of the lathe will necessarily be very slow when frequent
observations are to be made and it will be hardly practicable to drive it by power;
but it will be turned by hand, and the operator will be enabled by the indicating
devices or gauges last described to stop the shaft at exactly the same point in each
rotation.
The divisions of the scale are useful in finding the same point on the shaft with
relation to the point of the cutting tool in case the latter should be reset or ground
before the operation is completed in which case it might be found necessary to stop
the shaft at another division of the indicating scale.
Having determined a number of complete revolutions or an. exact fraction of a
revolution of the shaft S, the next step is to measure the distance travelled by the
tool carriage under the influence of the screw g², which is accomplished by means
of a linear measuring bar i accurately graduated for linear measurements with the
desired number of graduations to correspond with the distance to be travelled
by the carriage in the different measured portions of the rotary movement of the
shaft S.
For example, the bar may be divided into centimetres and the shaft stopped at
every fifth revolution when the position of the carriage c will be determined by a
cooperating measuring device which for extreme accuracy consists in this instance
of a microscope ¹ having the usual cross wires to definitely fix a line of vision,
it being fixed upon the said carriage.
The mechanism is set with the shaft S, stopped at a definite point indicated by
the gauge and with the cross wires of the microscope exactly on one division of
the scale or bar and the shaft is turned for five revolutions and an observation made
through the microscope.
If the cross wires are on the proper division of the bar it shows that the feeding
screw g² is of proper average pitch for the said portion of its length.
If, however, as is usually the case the cross wires are not on the division, the
amount of variation is observed and noted as an inaccuracy to be corrected in
cutting that portion of the, shaft S, and the screw g² will be turned by the operator
independently of the shaft S, until the cross wires are brought exactly upon the
division after which the shaft S, will be turned five more revolutions and another
observation made as before and the result tabulated.
In this manner the inaccuracies of the screw g² are determined throughout its
entire length.
For the sake of illustration it may be supposed that in the first division the
carriage c falls behind one one hundredth of a millemetre and in the second
division it has gained one one hundredth of a millemetre.
This will show that during the first five rotations of the shaft S, and the
screw g² the latter will have to gain one two hundredth of a revolution, and in the
second five rotations it will have to lose an equal amount to make the proper
connection in the travel of the tool carriage.
These corrections are made by causing the shaft f to move longitudinally during
its rotary movement by which the shaft S and screw g² are turned..
The longitudinal movement of the shaft f may be produced by various kinds of
mechanism.

page_text: As herein shown the said shaft is provided near one end with collars engaging
the ends of a sleeve k¹ in which the said shaft rotates freely, the sleeve being
provided with a pin entering a block see Figure 1, (where the sleeve and a
portion of the shaft fare broken away) mounted in guides of a guide-plate k*
pivoted as shown in dotted lines at 20 Figure 1, upon a carriage & movable in a
longitudinal guide a fixed upon or forming part of the bed of the lathe.
The said carriage is provided with a feed screw m, operated by a nut m¹, itself
rotated by a suitable train of gearing m³, m³, m³, m³, from the shaft of the gear g
by which the feed screw 9' is actuated, the said gearing being such that the
carriage is fed sufficiently to cause the block to travel the length of its guide
during the feeding movement of the tool carriage c along the lathe.
Thus by setting the guide plate k at any desired angle to the guide he the
shaft may be caused to move a certain distance during the entire feeding
movement of the carriage c, thus varying the relative rate of the shaft S and
screw g from that made by the simple rotation of the worms e and g.
This enables an error extending throughout the entire length of the screw to be
rectified automatically with the lathe running at usual speed, or a screw to be cut
of slightly greater or less average pitch than the feeding screw g², so as to bring
the screw that is cut to the desired standard of length although the minor
inaccuracies occurring at different points along the screw will not be corrected.
In order to turn the guide plate on its pivot, it is provided with a worm
gear n below the carriage k engaged by a worm n¹ splined upon the adjusting
shaft n extending along the front of the lathe, and provided with any convenient
number of graduated discs or handles n³ by which it may be rotated to turn the
guide-plate on its pivot, the said worm being free to travel longitudinally on the
shaft to accomodate the longitudinal movement of the carriage ko.
For correcting the minor inaccuracies which occur in the feeding screw g² from
point to point the plate may be gradually turned by the operator while the
lathe is in operation to produce the proper longitudinal movements in the shaft f
to make up for the said inaccuracies, and while removing or making the corrections
for the said inaccuracies the gearing m³, m³, m, m³, may be disconnected from the
shaft of the screw g², so that the carriage will have no longitudinal movement
while the corrections are being made, the plate * merely operating as an arm to
move the shaft ƒ end wise.
By this mode of operating the thread is correctly cut from point to point of its
length and is consequently correct in its total length.
It may in some cases be desirable however to first cut a screw of correct total
length by means of the inclined guide plate k*, carriage 5, and cooperating parts
when operating automatically as previously described, and then to use the screw
thus cut, to actuate the tool carriage while cutting the screw for which corrections
are made by hand from point to point throughout the entire length of the thread.
As herein shown the spindle d of the head stock of the lathe for carrying the
shaft S to be cut is made hollow and has a slightly tapering socket in its end to
receive the correspondingly tapered end of the shaft S, as shown in Figure 5, the
said shaft having a small thread cut on it to be engaged by a threaded socket in a
clamping spindle o passing through the spindle d, and provided with a collar o³
engaging the rear end of the said hollow spindle d and adapted to be rotated by a
suitable wrench or key to draw the 'tapered end 8 of the shaft S into its tapered
socket in the spindle, thus securely fixing it in relation to the said spindle while
being operated upon by the latter.
The clamping spindle o³, has a shoulder o engaging a shoulder in the hollow
spindle d, so that when the said clamping spindle is turned in the opposite
direction it will throw the spindle S out from its socket so that it may be removed
froin the lathe.
It will be found that certain irregularities occur in the relative rotary movement
of the shaft S, and longitudinal movement of the tool carriage c, during each
rotation of the said shaft, owing to some imperfection in the gearing or mechanical

page_text: connection between the said shaft and tool carriage other than the irregularities in
the feeding screw which have before been referred to and provided for, and these
irregularities in rotary movement may be determined by careful measurement of
the longitudinal movement of the tool carriage during the different fractions of
several successive rotations of the shaft S.
Thus in a series of measurements at various points in each rotation of a series of
rotations it may be found that errors of the same character are repeated during the
same part of each rotation, the angle of inclination of the thread being for instance
too great in the first one tenth of each turn of the thread measured from a given
line along the side of the screw parallel with its axis, and the said angle being too
small during the next one fourth of each turn of the thread and so on, although
the amount of variation will not usually be equal in the different turns, but may
be averaged.
Such irregularities as are found repeated at the same part of each revolution are
compensated for by the connecting mechanism between the spindle d, and its
actuating gear d' by which the movement imparted to the said spindle varies
somewhat from that of the gear.
As herein shown the said spindle has fixed upon it a wrist-plate p, shown as
having a tapering socket pressed upon a corresponding taper on the spindle by a
nut 2, the gear d¹, being loose upon the hub of the said plate which is provided
with a pin p2 pivoted on the said plate, and having an arm p³, provided with a
stud bearing upon a cam p', shaped in accordance with the previously observed
irregularities in rotary movement of the gear d¹ with relation to the longitudinal
movement of the tool carriage, the cam being fixed upon the head stock.
The pin p is provided with an eccentric stud po, connected by a link p6, with the
gear so that as the said pin is partially rotated by the oscillation of the arm p³,
there is a slight rotary movement of the plate p and spindle d, within the gear e,
which will compensate for the irregularities in the movement of the said gear.
By fastening the shaft S to the spindle d, by means of the tapering socket the
avoided.
irregular strain that is derived from the usual face-plate and dog
When necessary the entire apparatus up to the level of the graduations on the
bar i may be immersed in a liquid such as oil so as to maintain the graduated bar
or standard and the shaft S, at the same temperature, and if the said bar and
shaft are of materials having different co-efficients of expansion the said liquid
will have to be kept at a uniform temperature while the cutting is taking place.
It will be seen that the accuracy of a screw thread cut in accordance with this
invention depends upon the accuracy of rectilinear parts of the machine and upon
the accuracy of a standard of linear measurement which points of construction are
attainable in practice.
The essential novelty of the operation .consists in compensating for unavoidable
inaccuracies in existing machines while in the actual process of cutting each
continuous portion of the thread which is measured directly during the process of
cutting.
It follows that a screw can be made accurate to the highest degree that is
capable of measurement.
Having now particularly described and ascertained the nature of my said
Invention, and in what manner the same is to, be performed, I declare that what
I claim is
1. The herein described method of cutting screw threads whereby a screw is
produced having a thread of accurate pitch, substantially as set forth.
2. In a machine for cutting screws, the combination with a carriage for the
cutting tool, and mechanism to move it, of the stationary bar graduated to
standard linear measurement, and the cooperating measuring device connected
with the said carriage, whereby the actual linear movement of the tool carriage
may be accurately and minutely measured, substantially as and for the purpose
described.

page_text: 3. In a machine for cutting screws, the combination with the spindle for the
shaft to be threaded or cut, and the tool carriage, of mechanism for actuating the
said spindle and carriage, and devices for indicating the exact amount of rotary
movement of the said spindle and measuring the corresponding movement of the
said carriage, substantially as described.
4. The bed and head and tail stocks thereon, combined with the graduated bar
fixed on the said bed, the tool carriage having a linear movement on the said
bed and a microscope fixed on the said carriage and cooperating with the said bar,
substantially as and for the purpose described.
5. In a machine for cutting screws the movable tool carriage and means to
measure its movement, combined with the spindle for revolving the shaft to be
threaded, the plate or disc fixed on the sail spindle and provided with projections
and the cooperating iudicating device or gage forming a portion of an electric
circuit, substantially as described.
6. In a machine for cutting screws the movable tool carriage and means to
measure its movement, combined with the spindle for revolving the shaft to be
threaded, the plate or disc fixed in the said spindle and provided with projections
and the cooperating indicating device or gauge provided with an index finger and
cooperating graduations, substantially as and for the purpose described.
7. In a machine for cutting screws the movable tool carriage, and means to
measure its movements, combined with the spindle for revolving the shaft to be
threaded, the plate or disc fixed in the said spindle and provided with projections,
and the cooperating indicating device or gauge provided with a reflector whereby a
reflected beam of light may be moved along a scale, substantially as and for the
purpose described.
8. The spindle for rotating the shaft to be threaded, and the actuating gear loose
thereon, combined with connecting mechanism between the said gear and spindle
whereby their relative position is varied during their rotation, substantially as
described.
9. The longitud nally movable actuating shaft and worms rotated thereby, one
worm being inovable with the shaft and the other having no longitudinal move-
ment, combined with the gears actuated by the said worms, the sleeve on the said
shaft, block, pivotal guide plate, and mechanisın for turning the said plate on its
pivot, substantially as described.
10. The longitudinally movable actuating shaft and worms rotated thereby, one
worm being movable with the shaft and the other having no longitudinal move-
ment, combined with the gears actuate 1 by the said worms the sleeve on the said
shaft, block, guide plate and carriage supporting the said plate and means to impart
to the said carriage a movement proportional to that of the tool carriage,
substantially as described.