1. (
Machinery.) A tool for cutting threads in the hand-lathe; sometimes called a comb, from its having a row of projecting teeth (
c c c,
Fig. 1256).
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Chesers. |
It is made of steel, and the teeth filed by hand or by a cutting
hub. It is first forged in blank in the form of
d for an outside chaser, or in the form of
g for an inside chaser.
The teeth are then filed or made by a
hub (which see). The latter is a steel mandrel rotated on the centers of a lathe and having a section of screw-thread cut upon it. The thread is notched in places, so as to make cutting edges.
By holding the edge of the
chaser-blank against the
hub the teeth are cut in the former, and it is ready for tempering.
It may then be used in cutting or finishing screw-threads on a bolt or rod, or in a socket or coupling, as the case may be, or may be used in making a
hub on a mandrel of softened steel turned down to the right size and shape.
The chaser is dispensed with in power-lathes, which are provided with trains of gears for varying pitch and with automatic feed, the work being rotated slowly, the chaser supported in a rest, and sliding therewith at a rate determined by the pitch of the feed-screw and the rate of rotation of the work.
In the chasing-engine (
Fig. 1256), the cutter
k is made as a ring of steel, which is screwed internally to the diameter of the bolt
l which is to be threaded, and turned externally with an under-cut groove, for the small screw
n and nut
o, by which it is held in an iron stock
s formed of a corresponding sweep; for distinctness, the cutter
k and screw
n are also shown detached.
The center of curvature of the tool is placed a little below the center of the lathe, to give the angle of separation or penetration.
After the tool has been ground away, in the act of being sharpened, it is raised up until its points touch a straight edge applied on the line
m m of the stock.
This denotes the proper hight of center, and also the angle to which the tool is intended to be hooked, namely, 10°. Each ring makes four or five cutters, and one stock may be used for several diameters of thread.
In
Shanks's arrangement for
cutting screws in a lathe, a front and a back chaser are employed, so that one may cut while the slide traverses in one direction, and the other during the return-movement,
p represents the front and
t the back tool, which are mounted on one slide
e e, and all three are moved as one piece by the handle
x. In the first adjustment, the wedge
w is thrust to the bottom of the corresponding angular notch in the slide
e, and the two tools are placed in contact with the cylinder to be threaded.
For the first cut, the wedge
w is slightly withdrawn, to allow the tool
p to be advanced toward the work; and for the return-stroke, the wedge is again shifted under the observation of its divisions, and the slide
e is brought forward towards the workman, up to the wedge; this relieves the tool
p and projects
t, which is then in adjustment for the second cut; and so on alternately.
The command of the two tools is accurately given by the wedge, which is moved a small quantity by its screw and micrometer, between every alternation of the pair of tools, by the screw
y operated by the handle
x.
Punches or gravers are used for embossing or engraving the surfaces of metal, the design being in low relief or cut in intaglio.
See also enchasing.
In the embossing by punches, the object is filled with lead or pitch, and laid on a sand-bag
a, or in a
pitch-block, while the chasing-tool
b is held vertically and driven by a hammer.
Some portions of the metal are thus driven inward, while those around rise up from the displacement and reaction of the pitch.
The chasing-tools are of various kinds, with flat, rounded faces and curved edges, so as to follow a pattern.
Other tools have faces ornamented with designs in cameo or intaglio, which are conferred upon the metal by the action of the punch and hammer.
Chasing by the graver may be merely engraving in lines, but is usually in the form of relief; parts of the metal being cut away, leaving protuberant portions of ornate form, and which are farther beautified by graver-lines, frosting, milling, etc. The sand-bag supports the work while being chased by the graver.
The art of chasing was much practiced among the Greeks.
Two celebrated examples of chasing in iron are: —
[
531]
The iron base of the vase made by Glancus of
Chios, and dedicated to the Delphic oracle by
Alyattes, king of
Lydia.
This had small figures of animals, insects, and plants.
The iron helmet of
Alexander, the work of
Theophilus.
The principal chasing of antiquity was upon weapons, armor, shields, chariots, tripods, quoits, candelabra, chairs, thrones, mirrors, goblets, dishes.
The art arrived at great perfection in Etruria.
“But none the golden bowl can chase, Or give to brass such varied grace, As that renowned, hardy race That dwells by Arno's tide.”
Critias, quoted by
Athenaeus (A. D. 220).
2. (
Metallurgy.) One of the edge-wheels which revolves in a trough, to grind substances to powder.
(See Chilian mill ; mortar-mill ; oil-mill.) Also used in grinding ore for puddling-furnaces, etc.
