_Among the earliest known examples of screw-thread cutting machines
are the screw-cutting lathe of 1483, known only in pictures and
drawings, and an instrument of the traverse-spindle variety for
threading metal, now in the Smithsonian Institution, dating from the
late 17th or early 18th century. The author shows clearly their
evolution from something quite specialized to the present-day tool.
He has traced the patents for these instruments through the early
1930's and from this research we see the part played by such devices
in the development of the machine-tool industry._
THE AUTHOR: _Edwin A. Battison is associate curator of mechanical
and civil engineering in the Smithsonian Institution's Museum of
History and Technology._
Directness and simplicity characterize pioneer machine tools because
they were intended to accomplish some quite specialized task and the
need for versatility was not apparent. History does not reveal the
earliest forms of any primitive machines nor does it reveal much about
the various early stages in evolution toward more complex types. At best
we have discovered and dated certain developments as existing in
particular areas. Whether these forms were new at the time they were
first found or how widely dispersed such forms may have been is unknown.
Surviving evidence is in the form of pictures or drawings, such as the
little-known screw-cutting lathe of 1483 (fig. 1) shown in _Das
mittelalterliche Hausbuch_.
This lathe shows that its builder had a keen perception of the necessary
elements, reduced to bare essentials, required to accomplish the object.
Present are the coordinate slides often credited to Henry Maudslay. His
slides are not, of course, associated with the spindle; neither is there
any natural law which compels them to guide the tool exactly parallel
with the axis of revolution. In this sense the screw-cutting lathe in
the _Hausbuch_ is superior because it is in harmony with natural law and
can generate a true cylinder, whereas Maudslay's lathe can only transfer
to the work whatever accuracy is built into it.
In principle this machine shown in the _Hausbuch_ is very advanced as we
see when we follow the design through to the present time. The artist,
whose drawings give us our only knowledge of the machine, himself was
obviously not very familiar with the details of its function. Reference
to figure 1 shows that the threads on the lead screw and on the work,
wind in opposite directions. This must be an error in delineation since
the two are closely coupled together without any intervening mechanism
so that the only possible result on the work must be a thread winding in
the same direction as on the original screw. The work also is shown
threaded for its entire length; this cannot be accomplished with any one
location of the cross-slide. We are left with the question of whether
this slide was used in two locations or whether the artist, possibly
working from notes or an earlier rough sketch, failed to show an
unthreaded portion on one end or the other of the work.
[Illustration: Figure 1.--EARLIEST REPRESENTATION FOUND OF A
MASTER-SCREW TYPE of thread-cutting machine. From the inconsistencies,
such as right- and left-hand threads on master and work, it appears that
the artist had scant insight into actual function. From plate 62 of _Das
mittelalterliche Hausbuch, nach dem Originale im Besitze des Fürsten von
Waldburg-Wolfegg-Waldsee, im Auftrage des Deutschen Vereins für
Kunstwissenschaft, herausgegeben von Helmuth Th. Bossert und Willy F.
Storck_ (Leipzig: E. A. Seemann, 1912).]
Of at least equal importance with the lead screw and work and their
relationship to each other is the tool-support with its screw-adjusted
cross-slide (fig. 2). Just how this was attached to the frame of the
machine so that it placed the tool at a suitable radius is again a
questionable point. The very well-developed cutting tool is sharpened to
a thin, keen edge totally unsuited for cutting metal but ideal for use
on a softer, fibrous substance: undoubtedly wood, in this instance.
Unfortunately, the angle at which the artist chose to show us this
cutter is not a view from which it is possible to judge whether or not
the tool has been made to conform to the helix angle of the thread to be
cut. This cross-slide, in conjunction with the traversing work spindle,
gives us a machine having two coordinate slides yielding the same effect
as the slide rest usually attributed to Henry Maudslay at the end of the
18th century. Actually, an illustration of coordinate slides independent
of the spindle had been published as early as 1569 by Besson[1] and
knowledge of them widely disseminated by his popular work on mechanics.
These slides are shown as part of a screw-cutting machine with a
questionably adequate connection, by means of cords, between the master
screw and the work.
It was the author's pleasure recently to obtain for the Smithsonian
Institution and identify a small, nicely made, brass instrument which
had been in two collections in this country and one collection in
Germany as an unidentified locksmith's tool (fig. 3). This proved to be
an instrument of the traverse-spindle variety for threading metal.
Fortunately, all essential details were present including a cutter (A in
figure 4); this instrument was identified by the signature "Manuel
Wetschgi, Augspurg." The Wetschgis were a well-known family of gunsmiths
and mechanics in Augsburg through several generations. Two bore the
given name Emanuel: the earlier was born in 1678 and died in 1728. He
was quite celebrated in his field of rifle making and became chief of
artillery to the Landgrave of Hesse-Kassel shortly before his death in
his 51st year. Little is known of the later Emanuel Wetschgi except that
he was at Augsburg in 1740. Tentative attribution of the instrument has
been made to the earlier Emanuel, chiefly on the basis of his recognized
position as an outstanding craftsman.
[Illustration: Figure 2.--CROSS-SLIDE for the thread-cutting lathe of
_Das mittelalterliche Hausbuch_, shown in figure 1. It is remarkable not
only for its early date, but also for its high state of development with
a crossfeed screw which had not become universally accepted 300 years
later. The cutter, shown out of its socket, is obviously sharpened for
use on wood.]
In several respects this little machine differs from its predecessor of
the _Hausbuch_, as might be expected when allowance is made for the
generations of craftsmen who undoubtedly worked with such tools over the
roughly 200 years of time separating them. Another factor to consider
when comparing these two machines is that one was used on metal, the
other probably only on wood. Therefore, it is not surprising to find on
the later machine an outboard or "tailstock" support for the work. The
spindle of this support has to travel in unison with the work-driving
spindle so that it is not an unexpected discovery to find that it is
spring-loaded. Figure 5 shows how this spring may be adjusted to
accommodate various lengths of work by moving the attachment screw to
various holes in both the spring and in the frame. Also visible in the
same illustration is a rectangular projection at the other end of the
spring which engages a mating hole in the "tailstock" spindle to prevent
its rotation.
[Illustration: Figure 3.--SMALL THREAD-CUTTING LATHE which was made to
be held in a vise during use. It was found as shown here, with only the
operating crank missing. The overall length is approximately 12 inches,
depending on the adjustment of parts. (Smithsonian photo 46525B.)]
Figure 6 shows the traversing spindle and nut removed from the machine.
Provision has been made for doing this so easily that there is every
reason to believe that, originally, there were various different spindle
and nut units which could be interchangeably used in the machine.
Additional evidence tending to support this concept exists in the
cutting tool (fig. 4), which must have been intended for serious work as
it has been carefully fitted in its unsymmetrical socket. The cutting
blade of this tool, which works with a scraping rather than a true
cutting action, is too wide to form a properly proportioned thread when
used with the existing lead screw. This may well indicate that the tool
was made for use with a lead of coarser pitch, now lost.
[Illustration: Figure 4.--THE WORKING AREA of figure 3, showing the tool
and signature. (Smithsonian photo 46525A.)]
Perhaps the most startling feature of this machine when compared with
the machine of the _Hausbuch_, is the absence of a cross-slide for
adjusting the tool. Possibly this can be explained by the blunt scraping
edge on the tool. In actual use, recently, to cut a sample screw, using
a tool similar to the one found in the machine (fig. 7), it was found
advantageous to be free of a cross-slide and thus be able to feed the
tool into the work by feel rather than by rule, as would be done with a
slide rest. In this way, it was possible to thread steel without
tearing, as the cutting pressure could readily be felt and the tool
could release itself from too heavy a cut. Size on several screws could
be repeated by setting the tool to produce the desired diameter when its
supporting arm came to rest against the frame of the machine. The screws
used in the machine itself were apparently made in just such a way. They
were not cut with a die as the thread blends very gradually into the
body of the screw without the characteristic marks left by the cutting
edges of a die. Threads cut with a single-point tool controlled by a
cross-slide usually end even more abruptly than those cut by a die,
while it would be quite simple with a machine of the nature we are
considering to bring the thread to a gentle tapering end as seen in
figure 8 (another view of the screw A in fig. 3) by gradually releasing
the pressure necessary to keep the tool cutting as the end of the
thread was approached.
[Illustration: Figure 5.--SPRING FOR KEEPING THE FOLLOWER SPINDLE
against the work, showing the method and range of adjustment. Note the
rectangular projection to engage a mating socket in the spindle, to
prevent spindle rotation. (Smithsonian photo 46525.)]
[Illustration: Figure 6.--WORK SPINDLE AND ITS NUT removed from the
machine to illustrate how easily another spindle and nut of different
pitch could be substituted. (Smithsonian photo 46525C.)]
That machines of this general type having the lead screw on the axis of
the work were competitive with other methods and other types of machines
over a long period of time may be seen from figures 9 and 10. The
machine, left front in figure 9 and in more intimate detail in figure
10, can be seen to differ little from that shown in _Das
mittelalterliche Hausbuch_ of 1483. The double work-support is, of
course, a great improvement, while the tool-support is regressive since
it lacks a feed screw.
The development of engineering theory, coupled with the rising needs of
industry, particularly with the advent of the Industrial Revolution,
brought about accelerated development of screw-cutting lathes through
the combination of screw-cutting machines with simple lathes as seen in
figure 9 and in detail in figure 11. One important advance shown here
is driving the machine by means of a cord or band so that any means of
rotary power could be applied, not just hand or foot power. Of greater
interest and technical importance to this study is the provision, seen
to better advantage in figure 11, for readily changing from one master
lead screw to another. This had already been achieved in the Manuel
Wetschgi machine, as far as versatility is concerned, although not in
quite such a convenient way.
[Illustration: Figure 7.--THREAD OF MODERN FORM recently cut, using the
old screw and nut but with a new tool. The material threaded is
carbon-steel drill rod. (Smithsonian photo 49276A.)]
Figure 12, the headstock of another and more advanced lathe than shown
in figures 9 and 11 but of the same type, shows "keys" (D), each of
which is a partial nut of different pitch to engage with a thread of
mating pitch. The dotted lines in figure 13 show the engaged and
disengaged positions of one of these keys, and figure 14 shows the
spindle with the various leads, C. At D is a grooved collar to be
engaged by the narrow key shown in operating position at the left in
figure 12 for the purpose of controlling the endwise movement of the
spindle when used for ordinary turning instead of thread-cutting. In
return for greater convenience and freedom from the expense of the many
separate spindles, as typified by the Wetschgi machine, a sacrifice has
been made in the length of the thread which can be cut without
interruption.
[Illustration: Figure 8.--BINDING SCREW seen at A in figure 3, showing
the long smooth fadeout of the thread below the shoulder. (Smithsonian
photo 49276.)]
[Illustration: Figure 9.--MAKING SCREWS IN FRANCE in the third quarter
of the 18th century. From _L'Encyclopédie, ou dictionnaire raisonné des
sciences, des arts et des métiers ... receuil de planches sur les
sciences, les arts libéraux, et les arts méchaniques, avec leur
explication_ (Paris: 1762-1772), vol. 9, plate 1.]
[Illustration: Figure 10.--DETAILS OF THE MACHINE in the left foreground
of figure 9, showing the crude tool-support without screw adjustment.
From _L'Encyclopédie_, vol. 9, plate 2.]
This reduction in the length that could conveniently be threaded was no
great drawback on many classes of work. This can be realized from figure
16 which shows a traverse-spindle lathe headstock typical of the
mid-19th century. During the years intervening between the machines of
figures 12 and 16, the general design was greatly improved by removing
the lead screws from the center of the spindle. This made possible a
shorter, much stiffer spindle and supported both ends of the spindle in
one frame or headstock rather than in separate pieces attached to the
bed. The screws were now mounted outside of the spindle-bearings, one at
a time, while the mating nuts were cut partially into the circumference
of a disk which could be turned to bring any particular nut into working
position as required. With this arrangement, a wide variety of leads
either right or left hand could be provided and additional leads could
be fitted at any future time. Screw-cutting lathes of this design were
popular for a very long time with instrument makers and opticians who
had little need to cut screws of great length.
[Illustration: Figure 11.--DETAILS OF THE THREADING LATHE seen in the
right foreground of figure 9 showing the method of drive and support for
the work. From _L'Encyclopédie_, vol. 9, plate 1.]
The demands of expanding industry for greater versatility in the
production of engineering elements late in the 18th century set the
stage for the evolution of more complex machines tending to place the
threaded spindle lathes in eclipse. Maudslay's lathe of 1797-1800 (fig.
15) appeared at this time when industry was receptive to rapid
innovation. Unfortunately, the gearing which once existed to connect the
headstock spindle with the lead screw has long been lost. At this time
it is quite difficult to say with certainty whether the original gear
set offered a variety of ratios, as was true of slightly later Maudslay
lathes, or a fixed ratio. The plausibility of the fixed ratio theory is
supported by the very convenient means, seen in figure 15, for removing
the lead screw in preparation for substitution of one of another pitch.
All that is required is to back off its supporting center at the
tailstock end and withdraw the screw from its split nut[2] and from the
driving clutch near the headstock. This split nut also would have to be
changed to one of a pitch corresponding to that of the screw. While more
expensive than a solid nut, it neatly circumvents the need (and saves
the time involved) to reverse the screw in order to get the tool back to
the point of beginning preliminary to taking another cut. David
Wilkinson's lathe of 1798 (fig. 17) which was developed in Rhode Island
at the same time shows the same method of mounting and driving the
master screw. At least in the United States, this method of changing the
lead screw instead of using change gears remained popular for many
years. Examples of this changeable screw feature are to be found in the
lathes constructed for the pump factory of W. & B. Douglas Company,
Middletown, Connecticut,[3] in the 1830's. Middletown, at that time one
of the leading metal-working centers in one of the chief industrial
States, had been for many years the site of the Simeon North arms
factory which rivaled Whitney's. In this atmosphere, it is reasonable to
expect that machinery constructed by local mechanics, as was the custom
in those days, would reflect the most accepted refinements in machine
design.
[Illustration: Figure 12.--WELL-DEVELOPED EXAMPLE of lathe headstock
having several leads on the spindle and provision for mounting the work
or a work-holding chuck on the spindle. Adapted from _L'Encyclopédie_,
vol. 10, plate 13.]
[Illustration: Figure 13.--END VIEW OF THE HEADSTOCK seen in figure 12,
showing the keys or half nuts which engage the threaded spindle, in
engaged and disengaged positions. From _L'Encyclopédie_, vol. 10, plate
13.]
[Illustration: Figure 14.--SPINDLE OF FIGURES 12 AND 13, showing the
several leads and the many-sided seat for the driving pulley. Note the
scale of feet. From _L'Encyclopédie_, vol. 10, plate 16.]
Roughly twenty years later, Joseph Nason of New York patented[4] the
commercially very important "Fox" brassworker's lathe (fig. 18). While
this does have a ratio in the pair of gears connecting the work spindle
and master screw, it is clear from the patent that various pitches are
to be obtained by changing screws, not by changing gears. The patent
sums it up as follows:
A nut upon the end of the stud ... is unscrewed when the guide
screw is to be removed or changed. The two wheels ... should have
in their number of teeth a common multiple. They are seldom or
never removed and their diameters are made dissimilar only for the
purpose of giving to the guide screw a slower rate of motion than
that of the mandrel whereby it may be made of coarser pitch than
that of the screw to be cut and its wear materially lessened.
The introduction of gearing between the spindle and the lead screw, for
whatever purpose, could not help but introduce variable factors caused
by inaccuracies in the gears themselves and in their mounting. These
were of little consequence for common work, particularly when coupled to
a screw which, itself, was of questionable accuracy. The increasing
refinements demanded in scientific instruments and in machine tools
themselves after they had reached a relatively stable form dictated that
attention be dedicated to improved accuracy of the threaded components.
[Illustration: Figure 15.--MAUDSLAY'S WELL-KNOWN screw-cutting lathe of
1797-1800, showing the method of mounting and driving changeable master
screws. (_Photo courtesy of The Science Museum, London._)]
[Illustration: Figure 16.--HEADSTOCK OF A GERMAN INSTRUMENT-MAKER'S
LATHE, typical of the mid-19th century, showing the traverse spindle,
interchangeable lead screws, and semicircumferential nut containing
several leads. The nut may be brought into engagement by the lever at
top rear of the headstock. This releases the end thrust control on the
spindle simultaneously with engagement of the nut. (Smithsonian photo
49839.)]
[Illustration: Figure 17.--DAVID WILKINSON'S SCREW-CUTTING LATHE,
patented in the United States in 1798. Note the ready facility with
which the lead screw may be exchanged for another and the same means of
supporting and driving as in figure 15. (U.S. National Archives photo.)]
An attack on this problem, which interestingly reverts to the
fundamental principle of motion derived from a master screw without the
intervention of other mechanism (fig. 19), is covered by a patent[5]
issued to Charles Vander Woerd, one-time superintendent of the Waltham
Watch Company. The problem is well stated in the patent:
This invention relates to the manufacture of leading screws to be
used for purposes requiring the highest attainable degree of
correctness in the cutting of the screw-threads of said screw ...
as, for example, in machines for ruling lines in glass plates to
produce refraction [sic] gratings for the resolution of the lines
of the solar spectrum, such machines being required to rule many
thousands of lines on an inch of space by a marking device which is
reciprocated over the glass plate and is fed by the action of a
leading screw after the formation of each line. Great difficulty
has been experienced in constructing a leading screw for this and
other purposes, in which the thread is so nearly correct as to
produce no perceptible variation in the microscopic spaces between
the ruled lines or gratings.... Various causes prevent the
formation of a thread on the rod or blank, which is absolutely
uniform and accurate from end to end of the rod. Among other causes
are the variations of temperature from time to time, the
imperfections of the operating leading screw, the springing of the
leading screw and of the rod that is being threaded, and other
unavoidable causes, all of which, although apparently trivial and
producing only slight variations in the thread at different parts
of the rod or blank, are of sufficient moment to be seriously
considered when a screw of absolute accuracy is desired.
[Illustration: Figure 18.--NASON'S LATHE, patented in 1854, showing a
master lead screw driven at less than work speed so that the master
could be of a coarser and more durable pitch than the work. U.S. patent
10383.]
It is interesting to note in figure 19 that Vander Woerd's machine, to
avoid the problems outlined in his patent, has returned to a starkly
simple design. We are not told, however, how he originated this master
screw which is used to produce the accurately threaded work pieces.
Later generations, in the search for ever-greater accuracy, also
returned to the fundamental simplicity of a master screw as we shall see
when we consider the refinements in mechanism necessary to the extended
development of the automobile and the airplane.
[Illustration: Figure 19.--VANDER WOERD'S PATENT, seen here, covered the
combination of a master screw, toolslide and work in a rigid frame to be
supported and driven by outside means of no required precision. U.S.
patent 293930 dated February 1884.]
As the power and speed of automobiles and aircraft increased, critical
parts became more highly stressed. Gears and threaded parts were
particularly troublesome details of the mechanism because of the
stresses concentrated in them, and, in the case of gears, because of the
internal and external stresses originating in minute deviations from the
ideal of tooth form and spacing. The problems were not entirely new but
had hitherto been solved by increasing the size of the parts, an avenue
of limited utility to designers in these fields where total weight as
well as the effects of mass and inertia are so important. By making
these parts of heat-treated steel, the strength could be made suitable
while the size and mass of the parts were kept within bounds. The
necessary processes of heat-treating were not always applicable to
finished parts as they sometimes destroyed both finish and accuracy.
Grinding, which was well developed for the simple plane, cylindrical,
and conical surfaces so widely used in mechanisms, had to be extended to
threads and gears so that they could be finished after heat-treating.
Sometimes the gear teeth themselves were ground; for other applications
it was sufficient to improve the accuracy of the gear cutters.
[Illustration: Figure 20.--A HOB-GRINDING MACHINE patented in 1932 and
incorporating the master-screw principle. Carl G. Olson's U.S. patent
1874592.]
Attempts to produce gear hobs free of the imperfections and distortions
introduced by heat treatment led to another return to the use of the
master lead screw. Figure 20 illustrates a machine having this feature
which was patented in 1932 by Carl G. Olson.[6] In speaking of the
spindle-driving mechanism disclosed in earlier patents, the patent goes
on to say:
This driving mechanism includes an integral spindle 20, one
extremity thereof being designed for supporting a hob 22 and the
other extremity thereof being formed so as to present a lead screw
24. The spindle 20 is mounted between a bearing 26 and a bearing
28, the latter bearing providing a nut in which the lead screw 24
rotates.... From the description thus far given it will be apparent
that the rotation of the lead screw 24 within the bearing or nut 28
will cause the hob to be moved axially, the lead of the screw 24
being equal to the lead of the thread in the hob.
Claim 8 which concludes the descriptive portion of the patent states in
part:
In a hob grinding machine of the class described, a rotary work
supporting spindle, means for effecting longitudinal movement of
the spindle, a tool holder for supporting a grinding wheel in
operative position with respect to the work supported by the
spindle during the rotary and longitudinal movement thereof, ...
Even before this patent was applied for, another patent was pending for
the purpose of modifying the pitch of the lead screw without the use of
change gears in spite of the wide acceptance of such gear mechanisms for
over a hundred years.
[Illustration: Figure 21.--A HOB-GRINDING MACHINE OF 1933, showing use
of the master screw with a modifier but without change gears. Carl G.
Olson's U.S. patent 1901926.]
[Illustration: Figure 22.--A SINE-BAR DEVICE to modify the effective
lead of a master lead screw without introducing a complex mechanism
which would be both difficult to make and to operate within the required
close limits. Carl G. Olson's (1933) U.S. patent 1901926.]
Figure 21 shows a plan view[7] of the machine, and figure 22 a detailed
view of the sine-bar mechanism actuated by the master screw, 6, to
modify the effective pitch of the lead screw in accordance with the
realities of practice as stated in the preamble of the patent:
This invention relates to material working machines, and
particularly to machines such as hob grinders and the like, wherein
the work is reciprocated through the agency of a lead screw.
In the manufacture of hobs it is common practice to employ the same
machine for grinding hobs of varied diameters, and in order to
employ such a machine in this manner the pitch of the lead screw,
thereof, which actuates the work carrier, must conform to the axial
pitch of the hob to be ground. This will be readily apparent when
it is understood that the helix angles of hobs vary in accordance
with their diameters and, consequently, the difference between the
normal pitch and the axial pitch correspondingly varies. While the
requirement for the normal pitch may be the same for hobs of
different diameters, it is necessary to change the axial pitch in
accordance with a change in the hob diameter, and this axial pitch
of the hob is equal to the pitch of the lead screw which actuates
the work carrier in grinding machines heretofore used. Hence, in
order to adapt such machines to cover a wide range of leads, it is
necessary to provide a large number of interchangeable lead screws
and obviously this represents a large investment, and the
interchanging of these screws requires the expenditure of
considerable time in setting up the machine for each job.
Thread-grinding machines were being designed concurrent with the
development of hob-grinding machines. Many were entirely concerned with
features peculiar to the problems of wheel-dressing and to automatic
characteristics. An invention to embody the use of a master screw and
concerned with the precision grinding of worm threads, for use in
gearing, was patented by Frederick A. Ward in this era.[8] That part of
the invention pertaining to the use of a master screw, "a rotary work
holder mounted on said carriage and provided with a driving spindle, an
exchangeable master screw and stationary nut detachably secured to said
spindle and head,..." is shown in figure 23.
[Illustration: Figure 23.--DETAILS OF A WORK SPINDLE WITH WORK, showing
the use of a master lead screw to control the pitch of a precision worm
thread being ground. From the 1933 U.S. patent 1899654, of F. A. Ward's
worm-grinding machine.]
Machines embodying the principle of the master lead screw are found in
constant use by industry at the present time for specialized
application. Whenever technological changes again reopen the topic of
thread-cutting to a new degree of accuracy or call for a reevaluation of
popular methods for any other reason, we may expect to see another
resurgence of the master-screw method, for no other design eliminates so
many variables or rests on such firm and fundamental natural principles
as the machine of _Das mittelalterliche Hausbuch_ of 1483, the earliest
such machine now known.
* * * * *