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Processes introduction:
Piercing
Piercing is a shearing process where a punch and die are used to
create a hole in sheet metal or a plate. The process and machinery are
usually the same as that used in blanking, except that the piece being
punched out is scrap in the piercing process.There are many specialized
types of piercing: lancing, perforating, notching, nibbling, shaving,
cutoff, and dinking.
The amount of clearance between a punch and die for piercing is governed by
the thickness and strength of the work-piece material being pierced. The
punch-die clearance determines the load or pressure experienced at the
cutting edge of the tool, commonly known as point pressure. Excessive point
pressure can lead to accelerated wear and ultimately failure.
Burr height is typically used as an index to measure tool wear, because it
is easy to measure during production.
Process:
Lancing
It has been suggested that Lancing (shearing) be merged into this article or
section.
Lancing is the specialized piercing process of forming a line cut in the
workpiece without completely removing the tab. Usually a die pad is used to
form the tab at the same time. A common example of this is the louvers in a
locker door.
Perforating
Perforation refers to the puncturing of a material with a harder
(usually sharp) object to create a hole or aperture. Peforation or
microperforation can be created by pins, needles, die and punch or laser.
Rotary pinned perforation rollers are precision tools that can be used to
perforate a wide veriety of materials. The pins / needles can be used cold
or heated. Cold perforating can cause a volcanic structure in the material,
which can be beneficial for a number of applications. Hot needle perforation
melts the material being perforated, and creates a reinforcing ring around
the hole. This can also be beneficial in many applications, as the ring
assists in keeping the integrity of the material. There are a handful of
manufacturers that specialize in hot and cold needle perforation tooling and
equipment. Pinned rollers can be made from a veriety of materials to include
plastic, steel, aluminum, or other materials. Pinned sleeves can be slid
onto a central shaft, so that pin patterns may be interchanged at will.
Die and punch sets can be used for thicker materials, or materials that
require large gauge holes. The material is sheared by the sharp edges of the
punch and die, and the chad is removed. One will see punched perforations in
fruit produce bags.
Laser perforation can place many precise holes in a web. Laser perforations
look similar in many respects to hot needle perforations. However there is a
price to pay for a laser system. They can be expensive to purchase and
maintain. Many use an array of mirrors to direct the beam of light.
One of the most common perforations are found in stamps.
Perforation holes on a pair of coil stampsA perforation is a hole made by
puncturing a surface. However, the term frequently refers to the practice of
creating a long series of holes so that paper can be torn more easily along
a given line. Postage stamps are one common application of this, where small
round holes are cut in lines to create individual pieces (see postage stamp
separation). Since the creation of perforation devices in the 1840s and
1850s, it has seen use in several areas. It is common for notebooks and
legal pads to have perforations making it easier to tear out individual
pages. Perforations on stamps are rather large, on the order of a
millimeter, in comparison other perforated materials often have smaller
holes. Film stock is perforated to allow it to be moved precise distances at
a time continuously. Screwcaps on glass or plastic bottles are sealed with a
ring at the bottom of the cap attached by perforation. Twisting the cap has
the effect of rupturing the perforation and indicating that the original
seal has been broken.
In todays cases, perforation is used in ways to separate loose leaf (or even
a form of graph paper from a ringed binder). A fine perforation next to the
rings allows the page to be separated from the book with no confetti.
Notching
Notching is a metal-cutting process used on sheetmetal or thin barstock,
sometimes on angle sections or tube. A shearing or punching process is used
in a press, so as to cut vertically down and perpendicular to the surface,
working from the edge of a workpiece. Sometimes the goal is merely the notch
itself, but usually this is a precursor to some other process: such as
bending a corner in sheet or joining two tubes at a tee joint, notching one
to fit closely to the other.
Notching is a low-cost process, particularly for its low tooling costs with
a small range of standard punches. The capital cost of the punch press can
be expensive though, so small fabrication shops often out-source their
notching work to a press shop. Notching of large or heavy sections,
particularly for large tube fabrication or HVAC, is increasingly carried out
by plasma cutting rather than punch tools.
The accuracy of punch notching is good, depending on the care with which
it's carried out. For manual folding work, prior notching can often improve
resultant accuracy of the folding itself.
The speed of notching is usually limited by manual handling when loading the
workpieces into the press. Pieces some feet long may be manually loaded into
a single-stroke press. Smaller pieces are still generally hand-fed, limiting
speeds to perhaps 100 strokes / minute.
Almost any workable metal can be notched. It's particularly suitable where
the metal is otherwise awkward to drill, such as stainless steels, titanium
or previously heat-treated aluminium alloys.
Tube notching
Bicycle frame in steel tube, notched at the joints before TIG welding or
brazingTube notching is commonly performed before joining light-gauge tubes
to make a tee or similar joint, as by welding. Either one or both tubes may
be notched before assembly. A familiar example of tube notching is in the
manufacture of bicycle frames.
End notching works the end of the tube, such as a semicircular concavity to
make the base of a tee, or a convex vee to fit into a mitre.
Vee-notched tube, bent for assembly of a mitred jointSide notching (also
called offset notching) works the side of a tube with a vee notch for
bending, semicircular or vee notches for tee joint.
Tube being hollow, it's not practical to use a simple punch operation to
notch it, as it would be squashed. Although punching is possible, it
requires support mandrels and awkward handling. Where tube is worked
with a punch press other than for side notching, this is generally described
as slotting.
Tube notching for fabrication of circular tube is thus usually done with a
rotary hole saw instead, a saw of the diameter of the tube to be attached.
This produces a semi-circular notch. Rather than using large presses, such
saw notching may only require a simple jig, also making it suitable for
on-site working.
Notching in thin-wall tube may also be carried out by abrasive tools,
reducing some of the risk of damage from a hole saw snatching. This also
allows more complex shapes to be performed, such as vee notches. In some
cases, a helical end mill cutter may be used.
Notch and bend
Vee notches in tube, particularly square tube, may be cut so deep as to cut
almost through the tube: three sides of a square tube. This then allows the
tube to be bent into a mitred corner, usually finished by welding.
On a smaller scale for jewellery making, this operation is performed by
hand-filing precious-metal strip before bending and soldering to make box
frames or stone mounts
The notching process removes material from the edge of the workpiece.
Nibbling
The nibbling process cuts a contour by producing a series of overlapping
slits or notches. This allows for complex shapes to be formed in sheet metal
up to 6 mm (0.25 in) thick using simple tools. The process is often used on
parts that do not have quantities that can justify a dedicated blanking die.
The edge smoothness is determined by the shape of the cutting die and the
amount the cuts overlap; naturally the more the cuts overlap the cleaner the
edge.
Shaving
The shaving process is a finish operation where a small amount of metal is
sheared away from an already blanked part. Its main purpose is to obtain
better dimensional accuracy, but secondary purposes include squaring the
edge and smoothing the edge. Blanked parts can be shaved to an accuracy of
up to 0.025 mm (0.001 in).
Cutoff
The cutoff process is used to separate a stamping or other product from a
strip or stock. This operation is very common with progressive die
sequences. The cutoff operation often produces the periphery counter to the
workpiece.
Dinking
It has been suggested that Dinking operations be merged into this article or
section.
The dinking process is a modified form of piercing used on low strength
materials, such as rubber, fibers, and cloth. The die may be pressed into
the material with a hammer or a mechanical press.
Stamping parts