Engine 
860 © 
whole length of the divisions on the line divided will 
Graduation. only be 9 22° inches.” 
; F855 
On the for- 
mation of 
the screw of 
dividing 
engines. 
Ramsden’s 
method of 
making .a 
screw. 
PLATE 
CCLXXXL 
Kgs. 7,8, 9. 
Figs. 7, 8. 
Figs.7, 8, 9. 
Fig. 8. 
Fig. 7. 
he screw of a dividing engine, whether for right 
lines or circles, must be considered as a most essential 
, and should be of the most perfect workmanship. 
robably from knowing, that there are several good 
methods of producing a screw now well known to ar- 
tists, Troughton omitted to send an account of that by 
which his was made, at the time he transmitted to us 
the description of his engine. We will, however, an- 
nex the method of making one; and as that employed 
by Ramsden for his straight line engine, may, for aught 
we know, be as good as any, it has been selected. The 
following is Mr Ramsden’s own directions for making 
the screw, and the Plate is a copy from his original. 
‘* The exactness of the above described engine, de- 
pends very much on the correctness of the endless 
screw, which here is required to have some properties, 
that were not absolutely necessary in the endless screw 
for the circular engine. In that, as there were but a 
few threads of the endless screw engaged in the teeth 
of the wheel, it only required that those threads should 
have an equal inclination to the axis of the screw; but 
in this engine, where the whole length of the screw is 
engaged in the teeth of the moveable plate, it is neces- 
sary also that the distance between the threads should 
be the same throughout the whole length of the screw. 
‘This is effected by the screw engine hereafter descri- 
bed. 
Fig. 7. Represents the plan; 
Fig. 8, The elevation ; and 
Fig. 9. A section on the line BO. The same letter re- 
fers to the same part in each of the figures. A represents 
a strong circular plate of brass, having its edge ratched 
bythe method given in the description of the circular en- 
gine. On its centre is firmly fixed the pulley B by four 
screws; a groove is turned on the cylindric part of the 
pulley, perfectly concentric to the plate A. 
C is a steel axis two feet long, terminating in a point 
whereon it rests. The upper part of the axis is firml 
screwed to the plate A, and turns in the collar D, 
’ E-represents an endless screw, which, being turned 
on its axis, moves the plate A round its centre. F, a 
divided circular plate, which may be turned with or 
without turning the endless screw. On the other end 
of the screw arbor is a wheel a, having its outer edge 
cut into teeth. X is a winch whereby-the endless screw 
is turned round. 
G represents a triangular bar of steel, which passes 
over the circular plate A, and is firmly screwed to the 
frame at H and I. 
K, a piece of steel whereon the screw is intended to 
be cut, having its pivots formed in. the manner before 
described. On one end of this steel is a wheel L, ha- 
ving teeth round its circumference, which take into 
those on the wheel a; on the.arbor of the endless screw. 
M and N represent two strong pieces of brass, in 
which the steel whereon the screw is tobe cut turns, 
They are-firmly fixed to the triangular bar G, by tight- 
ening the piece I by the screw n. 
O is a piece of brass, which slides on the triangular 
bar G. Its two extremities are made to. fit the bar, it 
slides regularly thereon, and is prevented from rising 
by the two springing pieces c,c, Near one end of the 
piece O, is an angular groove q, that holds the tool by 
which the threads are cut. As it was necessary to cut 
the screw after the steel was hardened and tempered, 
therefore the tool was pointed with a diamond. The 
cock W serves to fasten the tool, which may be set to 
GRADUATION. 
take proper hold on the steel, by turning the finger Fi 
per 2 ax is fixed there by talon Ne Gr 
- To make a screw perfect, it is only required to give ~~ 
the point which cuts the threads an uniform motion pa- j; 
rallel to itself, and to the axis of the intended screw, ma 
and that this motion be proportioned to the revolution scre 
of the intended screw, as the number of threads may P™ 
require. : ‘ 1 0D, Se 
o effect this, a piece of thin tempered steel, exactly 
of the same shickiteds throughout, is fastened to the 
slide O at r; the other end of the spring is fastened to 
the pulley B, in the groove. Now, while the circle A, y; 
with the pulley, is turning round its centre, by turning 
the endless screw towards the right hand, the spring ¢ 
draws the slide O with the cutter q, along the tri 
lar bar; at the same time the steel K, whereon the 
screw is to be cut, is turned round its axis by the com- 
munication of the wheel a, on the endless screw with 
the wheel L. ; 
Ithath already been mentioned, that the screw of 
the engine before described hath 20 threads in an inch ; 
therefore, if the number of teeth on the wheel a be to 
the number on the wheel L, as the number of teeth on 
the wheel A, is to the number of twentieths of an 
inch round the circumference of the pulley B, allowing 
for part of the thickness the spring ¢, the be- 
tween each of the threads of the screw to be cut will 
be twentieths of an inch. 
The size of the pulley was determined in this man- 
ner: the endless serew being  disen from the 
wheel A, the slide O was drawn back till the end of it 
came nearly to the piece M; the endless screw was 
again engaged in the wheel A ; then having two very 
small dots on the slide O set off parallel to one side, and 
exactly five inches distant from each other ; the slide 
was moved by turning the endless screw, till one of the 
dots was biseeted by a small silver wire fixed across a 
hole in a thin piece of brass attached to the piece N; 
then the O on the divided wheel F was set to its index, 
without moving the endless screw, and the a / was 
reduced, till 600 revolutions of the endless screw 
brought the other dot to be exactly bisected by the 
fixed wire. These bisections were examined by a lens 
of half an inch focus, set in a small brass tube, which 
was fixed perpendicularly over the wire.” 
We believe that the above described right-line en- rR 
gine, has been chiefly employed in dividing such scales Ray 
as surveyors use for mapping their work, barometer * 
plates, &c. But contrary to. what Mr Ramsden has 
stated, it should be remarked, that, for unequal parts, 
(sines, tangents, &c.) its performance must be tedious, 
and of course expensive; for after ev tread, the 
screw would require to be put forward by hand, to 
the amount of at least one significant fi of the 
Tables for every division from which they are gathered. 
Mr Ramsden states the accuracy of this engine to be 
equal to the 4000th part of an inch, which it certainly 
ought to be ; this exceeds in precision; by a small degree, 
what may generally be expected from Bird’s standard 
measures ; yet, as we know that Sir George Shuckburgh 
Evelyn sought in vain when he examined our national 
standards, for some work of Ramsden’s to place in his 
list ; and as in the apparatus for our great trigonometri- 
cal survey, otherwise made entirely by himself, a 42. 
inch scale of Bird’s- was used, it aa be presumed he 
never executed any thing of the kind. 
Soon after the above ‘described engine was finished, 
and. before the description was published, another right- 
line engine appeared, This, with the assistance of an 
1 
