IMPROVEMENTS IN WOOD SCREWS, AND IN THE METHOD OF FORMING THE THREADS ON THE SAME
Year: 1877

page_text: LETTERS PATENT to Edwin Gorham Angell, of Providence, Rhode Island,
United States of America, now of 8, Southampton Buildings, London, for the
Invention of "IMPROVEMENTS IN WOOD SCREWS, AND IN THE METHOD OF
FORMING THE THREADS ON THE SAME." A communication from abroad by
Hayward A. Harvey, of Orange, New Jersey, United States of America.
Sealed the 6th July 1877, and dated the 12th May 1877.
COMPLETE SPECIFICATION filed by the said Edwin Gorham Angell at the
Office of the Commissioners of Patents on the 12th May 1877.
EDWIN GORHAM ANGELL, of Providence, Rhode Island, United States of America,
now of 8, Southampton Buildings, London. "IMPROVEMENTS IN WOOD SCREWS,
AND IN THE METHOD. OF FORMING THE THREADS ON THE SAME." A. communication
from abroad by Hayward A. Harvey, of Orange, New Jersey, United States of
America.
My improvements relate to a peculiar formation of the threads upon wood
screws.
According to my Invention I form such threads with a differential pitch and a
variable depth, the pitch and the depth having a constant relation to each other,
due to the fact that the core is of the same length between any two threads what-
ever may be the pitch of the same. Owing to this peculiarity I am enabled to cut
a thread of differential pitch with a tool having a single fixed lead, in other words
without varying the character of the lead given to the tool, in making the series of
cuts necessary to finish the thread to the proper depth. I form the said screw
thread with a gradually increasing pitch from its point of commencement upon the
shank of the blank, which point for convenience I call the heel of the thread.
may gradually increase the pitch from the heel to the point of the screw, or
increase it for only a portion of the distance, and give the remainder of the thread
a uniform pitch. The latter mode especially I contemplate adopting in the case of
very long screws.
My Invention is illustrated in the accompanying Drawing in longitudinal sections
of screws, exhibiting my Invention in the two above specified forms.
Figure 1 shows the differential thread commencing at the heel on the line x, x,

page_text: and terminating at the line y, y, from which line to the point of the screw the
thread has the ordinary uniform pitch.
..
Figure 2 shows the differential thread having the pitch gradually increasing from
the heel to the point.
Figure 3 shows the contour of the threads and the relation of the depth to the
differential pitch.
It will be seen that all of the threads are finished to a sharp edge by the same
tool. In cutting the thread the lead of the tool is gradually increased in speed, and
at the same time the tool is fed inward toward the axis of the blank.
To find
I determine the character of the motion required to be given to the tool by
dividing the greatest depth of the thread into a number of parts corresponding
with the number of turns which the differential, thread makes. For example, in
Figure 1 the differential thread makes only four turns around the core.
the motion for the tool in this case I lay out a single thread, as in Figure 3, and
then draw at equal distances from lines a, b, c, d, parallel with the bottom line e
which represents the length of the core. I make the lines a, b, c, d, of the same
length as the bottom line e, measuring from their respective points of intersection
with one of the inclined sides A of the thread. Three inclined lines b¹, c¹, and d¹
are then drawn parallel to the opposite inclined side e¹ of the thread. The distance
from the line b to the line a indicates the depth attained by the thread at one
revolution. The distance of the line b¹ from the point a³ indicates the gain of the
lead while the blank is making a single revolution. The distances between the
lines a, b, c, d, and e indicate the gain in the depth of thread at each of its
revolutions, and the distances between the point a¹ on the line a and the points
where the lines b¹, c¹, d¹, and e¹ intersect the line a indicate the increase in pitch at
each revolution, or in other words indicate the amount of increased lead required to
be given to the tool for each of the four successive turns of the blank.
It will be seen that the lines representing the core in Figures 1 and 2 are inclined
to the axis of the screw.
In measuring the depth of the thread preparatory to laying off the pitch of the
differential threads, as above described, I measure on the dotted lines z which
indicate the average depth of the deepest thread. In Figure 2 there are twelve
turns of the differential thread, and I therefore divide the greatest depth into twelve
equal parts.
It will of course be seen that the principle of laying out the differential threads,
as above described, for the purpose of measuring the pitch is capable of application
for any variety of pitch required.
Having thus fully described the said Invention, as communicated to me by my
foreign correspondent, and the manner of performing the same, I wish it understood
that I claim,
First. A screw having a thread, which for any portion of its length has a
differential pitch and a correspondingly variable depth, as above described.
Secondly. A screw having a thread of differential pitch for one portion of its
length, and a thread of uniform pitch for another portion of its length.
A
Thirdly. A screw having a thread gradually increasing in pitch from its point of 4
commencement upon the shank.
Fourthly. The method of arranging the motion for the tool in cutting a
differential pitch thread of variable depth, which method consists in giving to the
tool a feeding motion toward the axis of the blank as the pitch increases in speed.
In witness whereof, I, the said Edwin Gorham Angell, have hereunto set my 5
hand and seal, this Eleventh day of May, in the year of our Lord One
thousand eight hundred and seventy-seven.