1. Jig
Grinders (Tooling)
There is no
previous entryabout jig grinders.
New Answer
Jig
grinders are grinding machines traditionally used to grind holes in hardened steel
parts to high accuracies. The original applications included pressworking dies
and tools. Although these applications are still important, jig grinders are
used today in a broader range of applications in which high accuracy and good
finish are required on hardened components. Numerical control is available on
modern jig grinding machines to achieve automated operation.
(Groover
M.P., Fundamentals of Modern
Manufacturing: Materials, Processes, and Systems 4th Edition,
pp. 620)
A jig
grinder is a precision vertical-spindle machine, similar to a jig borer. It is used
for internal grinding. It has a horizontal table that can be moved and located
very accurately. The spindle head contains a high-speed grinding head that
rotates the grinding wheel about its axis but that axis can also revolve in a
planetary motion. The head can also move vertically with reciprocating motion.
Thus, holes can be ground to very accurate locations and diameters and, as the
head reciprocates vertically, to high levels of cylindricity. The machine is
used for finish grinding of hardened dies, molds, jigs, and fixtures.
(Bralla, J.
G., Handbook Manufacturing Processes, How
Products, Components and Materials Are Made, pp. 103)
2. Disk Grinders
There is no previous entry about disk grinders.
New
Answer
Disk grinders are grinding machines with large
abrasive disks mounted on either end of a horizontal spindle as in Figure
25.14. The work is held (usually manually) against the flat surface of the
wheel to accomplish the grinding operation. Some disk grinding machines have double
opposing spindles. By setting the disks at the desired separation, the work part
can be fed automatically between the two disks and ground simultaneously on
opposite sides. Advantages of the disk grinder are good flatness and
parallelism at high production rates.
(Groover M.P., Fundamentals
of Modern Manufacturing: Materials, Processes, and Systems 4th
Edition, pp. 620-621)
Disc grinding is a means for producing flat surfaces.
The workpiece is held against the flat side (face) of a large rotating abrasive
disc. (See Fig. 3C3.) The operation can be performed manually when dimensional
requirements for the part are not severe. In production situations, double disc
grinding, as illustrated in Fig. 3C7, is sometimes employed. The workpiece is
fed between two abrasive discs which grind the opposite surfaces of the
workpiece at the same time, thus controlling flatness, thickness, and
parallelism in the same operation. Parallelism and flatness of surfaces is
particularly good with these machines because there is no magnetic chuck to
pull down non-flat parts, only to have them spring back to a non-flat condition
after the operation. Double-disc grinders are used in the production of
automotive connecting rods, disc brake rotors, compressor vanes, and cast-iron
rocker arms.
(Bralla, J. G., Handbook
Manufacturing Processes, How Products, Components and Materials Are Made,
pp. 104)
3.Compression Test (Testing)
Previous
Answer
The compressive stress-strain curve is similar to the
tensile stress-strain curve up to the yield strength. Thereafter, the
progressively increasing specimen cross section causes the compressive
stress-strain curve to diverge from the tensile curve. Some ductile metals will
not fail in the compression test. Complex behavior occurs when the direction of
stressing is changed, because of the Bauschinger effect, which can be described
as follows: If a specimen is irst plastically strained in tension, its yield
stress in compression is reduced and vice versa.
(Mark's Standard Hanbook For Mechanical Engineers, P.
5-5)
New
Answer (better)
Compression test is reverse of tensile test. This test
can also be performed on a universal testing machine. In case of compression
test, the specimen is placed bottom crossheads. After that, compressive load is
applied on to the test specimen. This test is generally performed for testing
brittle material such as cast iron and ceramics etc. Fig. 7.8 shows the
schematic compression test set up on a universal testing machine. The following
terms have been deduced using figures pertaining to tensile and compressive
tests of standard test specimen.
(Singh, R., Introduction to Basic
Manufacturing Processes and Workshop Technology, pp. 124-125)
Much higher strains are achievable in compression
tests than in tensile tests. However, two problems limit the usefulness of
compression tests: friction and buckling. Friction on the ends of the specimen
tends to suppress the lateral spreading of material near the ends (Figure 4.1).
A cone-shaped region of dead metal (nondeforming
material) can form at each end, with the result that the specimen becomes
barrel shaped. Friction can be reduced by lubrication and the effect of
friction can be lessened by increasing the height-to-diameter ratio, h/d, of
the specimen. If the coefficient of friction, μ, between the specimen and the
platens is constant, the average pressure to cause deformation is
Pav = Y (1 + (μd/h)/3 + (μd/h)2/12+· · ·), (4.1)
where Y is the true flow stress of the material. If,
on the other hand, there is a constant shear stress at the interface, such as
would be obtained by inserting a thin film of a soft material (e.g., lead,
polyethylene, or Teflon), the average pressure is
Pav = Y + (1/3)k
(d/h), (4.2)
where k is the shear strength of the soft material.
However, these equations usually do not accurately describe the effect of
friction because neither the coefficient of friction nor the interface shear
stress is constant. Friction is usually highest at the edges, where liquid
lubricants are lost and thin films may be cut during the test by sharp edges of
the specimens. Severe barreling caused by friction may cause the sidewalls to
fold up and become part of the ends as shown in Figure 4.2. Periodic unloading
to replace or relubricate the film will help reduce these effects.
Although increasing h/d reduces the effect of
friction, the specimen will buckle if it is too long and slender. Buckling is
likely if the height-to-diameter ratio is greater than about 3. If the test is
so well lubricated that the ends of the specimen can slide relative to the
platens, buckling can occur for h/d ≥ 1.5 (Figure 4.3).
One way to overcome the effects of friction is to test
specimens with different diameter/ height ratios. The strains at several levels
of stress are plotted against d/h. By the extrapolating the stresses to d/h =
0, the stress levels can be found for an infinitely long specimen, in which the
friction effects would be negligible (Figure 4.4).
(Hosford, W. F.,Mechanical Behavior of Materials, pp53-55)
4.Shrinkage
Previous
Answer
Mold shrinkage is a volume phenoemnon , usually refers
to the difference between the linear dimension of the mold at room temperature
and that of the molded part at room temperature within fort-eight hours
following ejection
Shrinkage differentials may be due to any of the
following conditions:
*Differential Orientation: Oriented material with
molecules or fibers aligned or parallel shrinks in a more anisotropic manner
than unoriented material.
*Differential Crystallinity: For semicrystalline
materials, if some part of the mold cools at a slower rate, than area will have
higher crystalline content and hence higher shrinkage
*Differential Cooling: This can ocur when the mold
surfaces are at different temperatures.
*Material Characteristic: Copolymers are better than
homopolymers at resisting warpage. Certain types of homopolymers at resisting
warpage. Certain types of fillers reduce overall shrinkage and increase
stiffness.
(Handbook of molded part shrinkage and warpage ; Jerry
M. Fischer ; pg 9-13 , 2003)
New
Answer (better)
Our discussion of solidification has neglected the
impact of shrinkage that occurs during cooling and freezing. Shrinkage occurs
in three steps: (1) liquid contraction during cooling prior to solidification;
(2) contraction during the phase change from liquid to solid, called solidification
shrinkage; and (3) thermal contraction of the solidified casting during cooling
to room temperature. The three steps can be explained with reference to a cylindrical
casting made in an open mold, as shown in Figure 10.8. The molten metal immediately
after pouring is shown in part (0) of the series. Contraction of the liquid
metal during cooling from pouring temperature to freezing temperature causes
the height of the liquid to be reduced from its starting level as in (1) of the
figure. The amount of this liquid contraction is usually around 0.5%.
Solidification shrinkage, seen in part (2), has two effects. First, contraction
causes a further reduction in the height of the casting. Second, the amount of
liquid metal available to feed the top center portion of the casting becomes restricted.
This is usually the last region to freeze, and the absence of metal creates a
void in the casting at this location. This shrinkage cavity is called a pipe by
foundrymen. Once solidified, the casting experiences further contraction in
height and diameter while cooling, as in (3). This shrinkage is determined by
the solid metal’s coefficient of thermal expansion, which in this case is
applied in reverse to determine contraction.
Solidification shrinkage occurs in nearly all metals
because the solid phase has a higher density than the liquid phase. The phase
transformation that accompanies solidification causes a reduction in the volume
per unit weight of metal. The exception is cast iron containing high carbon
content, whose solidification during the final stages of freezing is complicated
by a period of graphitization, which results in expansion that tends to counteract
the volumetric decrease associated with the phase change [7]. Compensation for
solidification shrinkage is achieved in several ways depending on the casting
operation. In sand casting, liquid metal is supplied to the cavity by means of
risers (Section 10.3.5). In die casting (Section 11.3.3), the molten metal is
applied under pressure.
Pattern-makers account for thermal contraction by
making the mold cavities oversized. The amount by which the mold must be made
larger relative to the final casting size is called the pattern shrinkage
allowance. Although the shrinkage is volumetric, the dimensions of the casting
are expressed linearly, so the allowances must be applied accordingly. Special
‘‘shrink rules’’ with slightly elongated scales are used to make the patterns and
molds larger than the desired casting by the appropriate amount. Table 10.1
lists typical values of linear shrinkage for various cast metals; these values
can be used to determine shrink rule scales.
(Groover M.P., Fundamentals
of Modern Manufacturing: Materials, Processes, and Systems 4th
Edition, pp. 217-218)
5. Rotational Molding (Manufacturing method)
Previous
Answer (better)
Rotational molding uses gravity inside a rotating mold
to achieve a hollow form. Also called rotomold'mg, it is an alternative to blow
molding for making large, hollow shapes. It is used principally for
thermoplastic polymers, but applications for thermosets and elastomers are
becoming more common. Rotomolding tends to favor more complex external
geometries, larger parts, and lower production quantities than hlow molding.
The process consists of the following steps: (1) A predetermined amount of
polymer powder is loaded into the cavity of a split mold. (2) The mold is then
heated and simultaneously rotated on two perpendicular axes, so that,the powder
impinges on all internal surfaces of the mold, gradually forming a fused layer
of uniform thickness. (3) While still rotating, the mold is cooled so that the
plastic skin solidifies. (4) The mold is opened, and the part is unloaded. Rotational
speeds used in the process are relatively slow. It is gravity, not centrifugal
force, that causes uniform coating of the mold surfaces.Molds in rotational
molding are simple and inexpensive compared to injection molding or blow
molding, but the production cycle is much longer, lasting perhaps 10 min or
more. To balance these advantages and disadvantages in production, rotational
molding is often performed on a multicavity indexing machine, such as the
three-station machine shown in Figure 13.34. The machine is designed so that
three molds are indexed in sequence through three workstations. Thus, all three
molds are working simultaneously, The first workstation is an unload-load
station where the finished part is unloaded from the mold, and the powder for
the next part is loaded into the cavity. The second station consists of a
heating chamber where hot-air convection heats the mold while it is
simultaneously rotated. Temperatures inside the chamber are around 375°C
(700°F), depending on the polymer and the item being molded. The third station
cools the mold, using forced cold air or water spray, to cool and solidify the
plastic molding inside.
A fascinating variety of articles are made by
rotational moiding. The list includes hollow toys such as hobby horses and
playing balls; boat and canoe hulls, sandboxes, small swimming pools; buoys and
other flotation devices; truck body parts, automotive dashboards, fuel tanks;
luggage pieces, furniture, garbage cans; fashion mannequins; large industrial
barrels, containers, and storage tanks; portable outhouses, and septic tanks.
The most popular molding material is polyethylene, especially HDPE. Other
plastics include polypropylene, ABS, and high-impact polystyrene.
(Mikell P. Groover, Fundamentals of Modern Manufacturing,
Materials, Processes and Systems ,page 292-293)
New
Answer
Rotational molding, sometimes called, rotational casting,
is a means for producing components that are thin-walled, hollow, seamless, and
often large. It utilizes the two-axis rotation of a heated, clamshell-like, thin-walled,
metal mold. A measured amount of liquid or powdered thermoplastic resin is
charged to the mold. The mold is heated as it rotates in two planes. The resin
continuously falls by gravity to the lowest point, and the heated mold walls
become coated with the resin, which fuses together. The mold is then subjected to
cooling by water, cold air, or a sprayed water-air mixture. This cools the
plastic, causing it to solidify. The mold is then opened and the hollow part is
removed. The equipment commonly provides three stations for the mold:
1) a loading-unloading station where the mold does not
rotate,
2) a heating station where the mold has entered a
hot-air oven, and
3) a cooling station.
The mold is mounted on an arm, which carries it
sequentially to these three stations. However, many other machine
configurations are in use, including those with straight line and batch-type arrangements.
Large containers, tanks, and outdoor play equipment, are made from polyethylene
powder by this method. Gaskets, syringe bulbs, beach balls, hollow doll parts
and other toys, are other typical applications and are made from liquid
polyvinyl chloride (PVC)(vinyl plastisol). Fig. 4E illustrates the process.
(Bralla, J. G., Handbook
Manufacturing Processes, How Products, Components and Materials Are Made,
pp. 173)
mehmet, shrinkage terimini bi gruba katabilir misin?
ReplyDelete