TURNING. 
The stronger, the firmer, and the better 
tlie workmanship of a lathe, the easier it 
will be to perform work willi expedition 
and truth ; but a good workman will make 
true and excellent work with a very indif- 
ferent lathe, by taking care to cut so lit^e 
at a time that the parts of the engine may 
never be shaken out of their contact. Me- 
tallic lathes, if ever so strong, have an elas- 
tic tremor, which makes it difiicult to cut 
brass and bell-metal as firmly and smoothly 
as in wooden lathes, but the structure of 
the former admits of greater precision and 
truth. In a well constructed lathe, the back 
centre, the centre of the collar, and the 
fore centre, or centre of the moveable pop- 
pet-head, ought to be in one line, parallel 
to the bed or sheers. To prove this by 
trial, set the moveable poppet-head as far 
to ^lie right hand as possible, and screw a 
stick of wood into the nose of the mandrel : 
into the middle of the right hand end of the 
stick, or nearly so, drive a pin or other 
projecting point, and by gen tie blows against 
the stick, cause the point to remain steady 
in the axis, while tlie mandrel is turned 
round. If the centre point of the moveable 
poppet be truly opposite to this revolving 
point, the three centres are in a line ; and if 
the same continues to be the case when the 
face of the moveable poppet is reversed, 
it is a proof that the hole in the poppet 
is bored parallel to the bed ; and if the 
same adjustment continues when the stick 
. is shortened, it shows that the bed is 
strait and parallel to the axis of work. If 
the collar and back centre, and the chamfer 
and point of the mandrel, in a lathe, be 
truly formed, and set square, the rotation 
slowly made by hand, when the back centre 
is rather firmly set up, will be equally stiflf 
in every part, and the wearing parts, when 
examined, will have the same aspect, slope, 
and grain, in every part of their surfaces. 
The velocity of rotation may be extreme- 
ly swift in wood, slower in brass and bell 
metal, still slower in cast iron, and slowest 
of all in forged iron or steel. The reason 
for these limits appears to be, that a certain 
time is requisite for the act of cutting to 
take place, and that the tool itself, if heated 
by rotation, will instantly become soft, and 
cease to cut. Steel and iron require to be 
kept wetted. For rough work in wood the 
guage is a good tool, and after that the chisel, 
with its edge a little convex rather than 
strait lined. The graver is commonly used 
for metal ; and for strong rough work, the 
hook tool, which is of excellent advantage, 
even in small work, on account of its ex- 
treme steadiness. AFhen steel is to be cut 
extremely clean, a sharp hard tool may be 
useful, but for the most part in metallic 
work, even of steel, (if annealed), the hook 
tool, or graver, need not be harder than 
purple, or even blue. But to cut steel work 
or chill cast iron cylinders at a high tem- 
per, the tool must be very hard, the angle 
of edge obtuse, (say seventy degrees), and 
the motion slow. 
Hitherto we have spoken of plain turn- 
ing, which is indeed the most useful and 
most universally practised. But many other 
nice and very curious operations are per- 
formed by this art. If the poppet-heads, 
supporting the mandrel, be made regularly 
to move from side to side, during the rota- 
tion, or the rest be made to approach to, 
and recede from, the work, any number of 
times in a turn, the cuts will not be circu- 
lar but undulating, indented or waved in 
any curve that^may be required. Work of 
this kind, whi^i is chiefly done in watch 
eases, snuff boxes, and trinkets, is called 
rose-work. Tlie motion is commonly re- 
gulated by certain round plates of brass 
fixed on the mandrel, called roves, which 
have their edges waved, and are called 
roses. 
Another deviation from regular turning 
is effected by causing the chuck, which 
carries the work, to recede crosswise from 
the centre of the mandrel, back and for- 
ward during the rotation. The effect of 
this is, that the diameters of the work are 
not all equal to each other. It is practica- 
ble to produce a variety of curves in this 
way, but in our art the process is confined 
to turning ovals ; and the chuck, by which 
the work is made thus to slide back and 
forward, is called an oval chuck. 
Numerous geometrical figures are pro- 
duced by turning, by an apparatus upon the 
principle of the geometrical pen of Snardi, 
in engines which have been made for curio- 
sity, and at great expense. 
Medallions, and other similar pieces, are 
produced by regulating the action of the 
tool in its advance to, or recess from, the 
face of a piece exposed to its action. 
If the mandrel of a lathe be made to ad- 
vance and recede in the line of the axis, 
once in each turn the cut will not be in a 
plane at right angles to the axis of the 
work, and the line traced upon the work 
will be an ellipsis, produced by the oblique 
section of a cylinder. This kind of work is 
called swash-work, and may be seen in some 
