IMPROVEMENTS IN TAPS FOR CUTTING INTERNAL SCREWS
Year: 1879

page_text: LETTERS PATENT to Alexander Melville Clark, of 53, Chancery Lane, in the
County of Middlesex, Patent Agent, for the Invention of "IMPROVEMENTS
IN TAPS FOR CUTTING INTERNAL SCREWS." A. communication from abroad
by Eugène Dejonc, of Paris, France.
led the 4th November 1879, and dated the 17th May 1879.
PROVISIONAL SPECIFICATION left by the said Alexander Melville Clark at
the Office of the Commissioners of Patents on the 17th May 1879.
ALEXANDER MELVILLE CLARK, of 53, Chancery Lane, in the County of Middlesex,
Patent Agent. "IMPROVEMENTS IN TAPS FOR CUTTING INTERNAL SCREWS." [A
communication from abroad by Eugène Dejonc, of Paris, France.]
This Invention relates to improvements in taps for cutting internal screws,
whether used in the ordinary hand stock or in a machine. The improvements
consist,
1st. In the particular form of the tap in cross section, and in the form and direc-
tion of the flutes or grooves forming the cutting angles and clearing the tap of the
cuttings.
2nd. In a simple and mathematically exact means of guiding taps used in tapping
holes of any depth with precision.
In order that the Invention may be more readily understood I have illustrated
an example of it in the annexed Drawing, and will proceed to describe it with
reference thereto. Figure 1 represents a tap embodying the first part of these
improvements; Figure 2 represents a cross section of the blank from which the tap
is made; and Figure 3 shows a similar section of the same after the screw thread
has been cut upon it.
Figures 4 and 5 are cross sections on lines 1-2, 3-4,
Figure 1, respectively of the finished tap, showing the degree of twist in the flutes
or grooves. The same letters of reference indicate the same parts in all these
Figures wherever they recur.
The form of the tap blank in cross section approaches an oval described by four
ares of circles, as follows:- From point a as a centre I describe a circle, and cut off
two arcs b, c, d, e, each equal to about a fourth of the circumference (this proportion
being varied according to circumstances), and symmetrically placed with regard to
the diameter fg. I then join points b, d, c, e, by two eccentric arcs.
Price Ed71
The blank

page_text: for the tap is turned of this form throughout its whole length, atter which it is
screwthreaded, as indicated in Figure 3. Two flutes or grooves are then made in
it to form the cutting angles of the tap. These grooves start at the upper end of
the tap from a position symmetrical with regard to the major diameter f, g, of the
oval, and proceed in the path of a helix of very slow pitch to the point or lower end
of the tap, where they deviate to one side of said diameter, as seen in Figure 5.
With this oval form of tap the heating of the tap and the work being tapped s
avoided, and friction is considerably reduced. Moreover, the tap commences its work
without jerks, as the three or four grooved taps of other kinds do, which jerks are
liable to cause the breakage of the tap, damage the screwthread, and enlarge the
hole. This improved two grooved tap, on the contrary, is perfectly guided and held
to its work from the very beginning of the operation; it advances freely by cutting
a very smooth and mathematically correct thread. The grooves are made semi-
circular in cross section, in order to admit of lubricating the tap freely during its
work, to permit of cleaning the grooves easily with an emery wheel after hardening
and before tempering the tap, thus restoring the cutting edges, which are deteriorated
by the hardening; this is difficult to do with other taps. The sharpening is equally
facile.
This improved tap requires much less power to work it than others; it admits of
tapping holes with a full thread, and is applicable for holes of large diameter. It
is easily unscrewed by hand from a hole closed at bottom.
Figure 6 represents a guided tap embodying the second part of these improve-
ments. This tap consists of a long rod forming a mandrel. The part a is screw-
threaded, and may either be made of the special form herein before described, or
otherwise. The portions b and c are turned to correspond exactly to the internal
diameter of the hole to be tapped, which diameter should by preference be also equal
to the diameter at the bottom of the screwthread.
With a tap of this kind a perfectly regular thread is produced, whatever may be
the depth of the hole to be tapped, at same time that friction is greatly reduced.
Flexure of the tool is rendered impossible, and the tool always remains mathemati-
cally concentric with the axis of the hole.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said Alexander Melville Clark in the Great Seal Patent Office on the 15th
November 1879.
ALEXANDER MELVILLE CLARK, of 53, Chancery Lane, in the "County of Middlesex,
Patent Agent.
"IMPROVEMENTS IN TAPS FOR CUTTING INTERNAL SCREWS." A
communication from abroad by Eugène Dejonc, of Paris, France.
This Invention relates to improvements in taps for cutting internal screws,
whether used in the ordinary hand stock or in a machine. The improvements
consist,―
1st. In the oval form of the tap in cross section, and in the position and direc-
tion relatively to such oval form of the flutes or grooves forming the cutting angles,
and serving to clear the tap of the cuttings.
2nd. In a simple and mathematically exact means of guiding taps used in tapping
holes of any depth with precision.
In order that the Invention may be more readily understood I have illustrated
an example of it in the annexed Drawing, and will proceed to describe it with
reference thereto. Figure 1 represents a tap embodying the first part of these
improvements; Figure 2 represents a cross section of the turned blank from
which the tap is made; and Figure 3 shows a similar section of the same
after the screwthread has been cut upon it. Figures 4 and 5 are cross sections
on lines 1-2, 3-4, Figure 1, respectively of the finished tap, showing the
degree of twist in the flutes or grooves. The same letters of reference indicate
the same parts in all these Figures, wherever they recur. The form of the
tap blank in cross section approaches an oval described by four arcs, as follows:-
From point a as a centre I describe a circle, and cut off two arcs b, c, d, e,
each equal to about a fourth of the circumference (this proportion being varied
according to circumstances), and symmetrically placed upon the diameter f, g. I
then join points b, d, c, e, by two arcs of greater radius described from centres at
opposite sides of the diameter f, g, as shown by the dotted radii. The blank for
the tap is turned of this form throughout its whole length, after which it is screw-
threaded, as indicated in Figure 3. Two flutes or grooves are then made in it to
form the cutting angles of the tap and clear it of the cuttings. These grooves start
at the upper end of the tap from a position symmetrical with regard to the major
diameter f, g, of the oval, and proceed in the direction of a helix of exceedingly slow
pitch to the point or lower end of the tap, where they deviate towards one side of
said diameter, as seen in Figure 5. The object of this slightly helical direction of
the grooves is to give clearance. With this oval form of tap the seizing and heating
of the tap and heating of the work being tapped is avoided, and friction is con-
siderably reduced. Moreover, the tap commences its work without jerks, to which
the three grooved or four grooved taps of other kinds are subject, which jerks are
liable to cause the breakage of the tap, damage the screwthread, and enlarge the
hole. This improved two grooved tap, on the contrary, is perfectly guided and held
to its work from the very beginning of the operation, and it advances freely, cutting
a very smooth and mathematically correct thread.
The grooves are made semicircular in cross section in order to admit of lubricating
the tap freely during its work, and to permit of cleaning the grooves easily with an
emery wheel after hardening and before tempering the tap, thus restoring the
cutting edges, which are dulled by the hardening; this is difficult to do with other
taps. The sharpening is equally facile.
This improved tap requires much less power than others; it admits of tapping
holes with a full thread, and is applicable to holes of large diameter. It is easily
unscrewed by hand from a blind hole. Figure 6 represents a guided tap embodying
the second part of these improvements. This tap consists of a long rod forming a

page_text: mandrel. The part as screwthreaded and may either be made of the special form
constituting the first part of the Invention hereinbefore described, or of other form.
The portions b and c are turned to correspond exactly to the internal diameter of
the hole to be tapped, which diameter should by preference be also equal to the
diameter at the bottom of the screwthread of the tap.
With a tap of this kind a perfectly regular thread is produced, whatever may be
the depth of the hole to be tapped, at same time friction is greatly reduced. Flexure
of the tool is rendered impossible, and the tool always remains mathematically con-
centric with the axis of the hole.
Having described the nature of the said Invention, and the manner of per-
forming the same, I declare that what I claim as the Invention to be protected by
the hereinbefore in part recited Letters Patent is,-
1. Making a screw tap of oval form in cross section with two grooves or flutes
having the position and direction herein shown and described.
2. A screw tap constructed or provided with the means of guidance, substan-
tially as herein described, and shown in Figure 6 of the Drawings.
In witness whereof, I, the said Alexander Melville Clark, have hereunto set
my hand and seal, this Fifteenth day of November, in the year of our Lord
One thousand eight hundred and seventy nine.
AT DE