04-12-2012, 12:14 PM
Metal lathe
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A metal lathe or metalworking lathe is a large class of
lathes designed for precisely machining relatively hard
materials. They were originally designed to machine metals;
however, with the advent of plastics and other materials,
and with their inherent versatility, they are used in a wide
range of applications, and a broad range of materials. In
machining jargon, where the larger context is already
understood, they are usually simply called lathes, or else
referred to by more-specific subtype names (toolroom
lathe, turret lathe, etc.). These rigid machine tools remove
material from a rotating workpiece via the (typically linear)
movements of various cutting tools, such as tool bits and
drill bits.
Construction
The design of lathes can vary greatly depending on the intended application; however, basic features are common
to most types. These machines consist of (at the least) a headstock, bed, carriage, and tailstock. Better machines
are solidly constructed with broad bearing surfaces (slides or ways) for stability, and manufactured with great
precision. This helps ensure the components manufactured on the machines can meet the required tolerances and
repeatability.
Headstock
The headstock (H1) houses the main spindle (H4), speed
change mechanism (H2,H3), and change gears (H10). The
headstock is required to be made as robust as possible due
to the cutting forces involved, which can distort a lightly built
housing, and induce harmonic vibrations that will transfer
through to the workpiece, reducing the quality of the
finished workpiece.
The main spindle is generally hollow to allow long bars to
extend through to the work area. This reduces preparation
and waste of material. The spindle runs in precision bearings
and is fitted with some means of attaching workholding
devices such as chucks or faceplates. This end of the
spindle usually also has an included taper, frequently a
Morse taper, to allow the insertion of tapers and centers.
On older machines the spindle was directly driven by a flat
belt pulley with lower speeds available by manipulating the
bull gear. Later machines use a gear box driven by a
dedicated electric motor.
Feed and lead screws
The feedscrew (H8) is a long driveshaft that allows a series of gears to drive the carriage mechanisms. These gears
are located in the apron of the carriage. Both the feedscrew and leadscrew (H7) are driven by either the change
gears (on the quadrant) or an intermediate gearbox known as a quick change gearbox (H6) or Norton gearbox.
These intermediate gears allow the correct ratio and direction to be set for cutting threads or worm gears. Tumbler
gears (operated by H5) are provided between the spindle and gear train along with a quadrant plate that enables a
gear train of the correct ratio and direction to be introduced. This provides a constant relationship between the
number of turns the spindle makes, to the number of turns the leadscrew makes. This ratio allows screwthreads to
be cut on the workpiece without the aid of a die.
Some lathes have only one leadscrew that serves all carriage-moving purposes. For screw cutting, a half nut is
engaged to be driven by the leadscrew's thread; and for general power feed, a key engages with a keyway cut into
the leadscrew to drive a pinion along a rack that is mounted along the lathe bed.
The leadscrew will be manufactured to either imperial or metric standards and will require a conversion ratio to be
introduced to create thread forms from a different family. To accurately convert from one thread form to the other
requires a 127-tooth gear, or on lathes not large enough to mount one, an approximation may be used. Multiples of
3 and 7 giving a ratio of 63:1 can be used to cut fairly loose threads. This conversion ratio is often built into the
quick change gearboxes.
Carriage
In its simplest form the carriage holds the tool bit and moves it longitudinally (turning) or perpendicularly (facing)
under the control of the operator. The operator moves the carriage manually via the handwheel (5a) or
automatically by engaging the feed shaft with the carriage feed mechanism (5c). This provides some relief for the
operator as the movement of the carriage becomes power assisted. The handwheels (2a, 3b, 5a) on the carriage
and its related slides are usually calibrated, both for ease of use and to assist in making reproducible cuts.
Calibration marks will measure either the distance from center (radius), or the work piece's diameter, so for
example, on a diameter machine where calibration marks are in thousandths of an inch, the radial handwheel dial
will read .0005 inches of radius per division, or .001 inches of diameter. The carriage typically comprises a top
casting, known as the saddle (4), and a side casting, known as the apron (5).
Steady, follower and other rests
Workpieces often need to be supported more than the chuck and/or
centers can support them, because cutting metal produces tremendous
forces that tend to vibrate or even bend the workpiece. This extra
support can be provided by a steady rest (also called a steady, a fixed
steady, a center rest, or sometimes, confusingly, a center). It stands
stationary from a rigid mounting on the bed, and it supports the
workpiece at the rest's center, typically with three contact points 120°
apart. A follower rest (also called a follower or a travelling steady) is
similar, but it is mounted to the carriage rather than the bed, which means
that as the tool bit moves, the follower rest "follows along" (because they
are both rigidly connected to the same moving carriage). [2], [3] Follower
rests can provide support that directly counteracts the springing force of
the tool bit, right at the region of the workpiece being cut at any moment.
In this respect they are analogous to a box tool.
Mini-lathe and micro-lathe
Mini-lathes and micro-lathes are miniature versions of a general-purpose center lathe (engine lathe). They typically
have swings in the range of 3" to 7" (70 mm to 170 mm) diameter (in other words, 1.5" to 3.5" (30 mm to 80 mm)
radius). They are small and affordable lathes for the home workshop or MRO shop. The same advantages and
disadvantages apply to these machines as explained earlier regarding 3-in-1 machines.
As found elsewhere in English-language orthography, there is variation in the styling of the prefixes in these
machines' names. They are alternately styled as mini lathe, minilathe, and mini-lathe and as micro lathe,
microlathe, and micro-lathe.