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Punches : An insight

A punch is a hard metal rod with a shaped tip at one end and a blunt butt end at the other, which is usually

struck by a hammer. Punches are used to drive objects, such as nails, or to form an impression of the tip

on a work piece. Decorative punches may also be used to create a pattern or even form an image.

Punching is a metal forming process that uses a punch press to force a tool, called a punch, through the

work piece to create a hole via shearing. The punch often passes through the work into a die. A scrap slug

from the hole is deposited into the die in the process. Depending on the material being punched this slug

may be recycled and reused or discarded. Punching is often the cheapest method for creating holes in

sheet metal in medium to high production. When a specially shaped punch is used to create multiple

usable parts from a sheet of material the process is known as blanking. In forging applications the work is

often punched while hot, and this is called hot punching.

Punch tooling (punch and die) is often made of hardened steel or tungsten carbide. A die is located on the

opposite side of the work piece and supports the material around the perimeter of the hole and helps to

localize the shearing forces for a cleaner edge. There is a small amount of clearance between the punch

and the die to prevent the punch from sticking in the die and so less force is needed to make the hole. The

amount of clearance needed depends on the thickness, with thicker materials requiring more clearance,

but the clearance is always less than the thickness of the work piece. The clearance is also dependent on

the hardness of the work piece. The punch press forces the punch through a work piece, producing a hole

that has a diameter equivalent to the punch or slightly smaller after the punch is removed. All ductile

materials stretch to some extent during punching which often causes the punch to stick in the work piece.

In this case, the punch must be physically pulled back out of the hole while the work is supported from

the punch side, and this process is known as stripping. The hole walls will show burnished area, rollover,

and die break and must often be further processed. The slug from the hole falls through the die into some

sort of container to either dispose of the slug or recycle it.

Although the larger and more orthodox types of punches for use with hammers may convey the general

impression that such tools have no great adaptability, yet the specialized varieties render quite easy many

 jobs which would be awkward or even impossible without them. Pin punches of appropriate size, for

example, are used for driving out taper pms or tight rivet shanks, and may be employed, too, for tapping

down heads of small nails or tacks where final hammer blows would be visible on surfaces. Stepped

punches or drifts with reduced spigot ends are for driving out bushes or valve guides; and flat-ended

punches in brass, copper or aluminum, avoid damage to threads, ends of bushes, or other parts which

would burr relatively easily. A pin punch which is parallel and perfectly flat at the end may be used for

punching holes in thin metal when this is laid on a block of lead, and may even be used for punching flat

spring steel, like clock spring material, when pieces are being put to other uses and must be held by

screws or bolts. However, for such work, a punch with a slight reverse taper for a distance from the end isadvisable, and is easily machined from silver steel rod, then hardened and tempered to dark straw color. A

flat-ended hollow punch or holdup backed by a piece of heavy metal or hammer head gives support when

a pin or rivet is being driven from a flexible mounting or bracket (a cotter pin from a cycle crank, for

example); and such a punch may be slipped over a rivet shank to enable sheet material to be driven into

close proximity, before a set is used to form the head on the rivet. A sharp hollow punch of suitable size is

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the best tool for cutting bolt or screw holes in making gaskets of paper, cork, fiber, or asbestos-base

materials, with these laid on the end grain of a flat hardwood block.

In riveting, suitable punches promote generally better results, and give a consistent appearance to work 

which would often be marred without their use. This is particularly so in the case of making and fitting

eyelets. In making them, lengths of soft well-annealed tubing are stood one at a time in a ring on a hold-

up, and the shaped punch is driven on the end. If, from a tendency to splitting, each whole head cannot be

formed at once, it can be half-turned, and the tubmg then re-annealed before the second operation.

Heating brass to red and dropping in water anneals it. In fitting eyelets, the head forming punch becomes

the hold-up for mounting in the vice. A simple turning punch is used on the shank, when a washer has

been fitted and pressed down with a hollow punch. Given finished eyelets, the head holdup and the

turning punch can easily be machined with a round-nosed tool in the lathe. Using strong steel vice,

various flanged or shallow formed parts can be made with punches, in some instances employing rubber

to take the reverse shape of a die. Au annealed disc of copper may be flanged, using a punch with rounded

corner, and a die consisting of a steel ring bored out (slightly larger than punch diameter plus twice

material thickness), and flared for entry. Tapping the ruckling and shaping edge of the disc down with a

hammer, re-annealing and final steady force on the vice will form, for example, a small boiler end. Discsof very thin material, like brass shim stock, may be flanged, provided with ribs or depressions on the

surface, or penetrated with holes, using a shaped punch, a piece of steel tubing, a rubber disc, and a flat-

ended punch to apply pressure. Ends of small floats may be formed in this way; but as in all rubber

presswork, the pressure required is considerable. Parallel-shank, taper-ended punches can be used

effectively to bring out of-line holes into coincidence; but for a car spring eye and bracket, an alternative

punch, with an eccentric end to enter at X, and a hole for a tommy-bar, so that by turning the punch the

holes are pulled positively into alignment-when the punch can be tapped through.