Fascination of SheetMetal Chapter5

8/13/2019 Fascination of SheetMetal Chapter5

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THERE’S NO POINT GETTING BENT OUT OF SHAPE

During processing, sheets take an enormous amount ofpunishment. Machines may strike a part thousands of times

before it’s all over, and the punching tonnage used to punch

large holes can reach 220 kilonewtons (equivalent to a load

of 22 tons). As parts are formed, the sheet skeleton becomes

increasingly more intricate and delicate. Between strokes,

the sheet is whisked off to different positions on the ma-

chine table at very high speeds. It seems almost inevitable

that a sheet treated in this way would have to become

deformed somehow. Or does it?

Keeping it level | Machine, tool, and steel manufacturers

all work together to ensure that the sheet and the parts made

from it remain as flat as possible during processing. This is

the only way to ensure a high degree of accuracy and elimi-

nate costly reworking. Before each production step, distorted

parts have to be straightened out or flattened.

Sheets and parts become deformed due to stresses in thematerial. Indeed, these stresses are generated while the sheet

Ironing out the wrinkles

itself is still in the production stage. For this reason, it is

a good idea to use sheets specially designated as being “stress-relieved” material.

During machining, the punch and die create additional

stresses in the sheet with each and every stroke. The goal

is to reduce this stress. Machine and tool manufacturers do

this by utilizing special tools and machine functions. When

normal, passive punching strokes are performed, there is a

small gap of around 1 to 1.5 millimeters between the stripper

and the sheet. To reduce deformation, the stripper can be

applied actively, so that it holds down the sheet during the

punching stroke. Other strategies can also help:

• Prepunching and postpunching | Pre-punching sheets

using a smaller tool is advisable for the production of

larger holes.

• Tool care | Blunt punch cutting edges can increase

the amount of stress in the material. For this reason,

it is important to regrind punches at regular intervals.

• Optimum die clearance | Additi onal stres s can also b e

created if the clearance between the punch and die is

too large or too small. The optimum clearance is around

10 percent of the sheet thickness.

MAXIMUM ACCURACY, MINIMUM SCRATCHING The mos t impo rtant criter ia for judgin g the qualit y of p unched

parts are accuracy and degree of scratching.

Accurac y | The more accur ately the machin e is able to

position the workpiece under the punching head, the more

precise the dimensions of the punched part will be. At the

same time, however, positioning the workpiece accurately

becomes progressively more difficult as the speed of the

machine components increases. Machine manufacturers are

overcoming this challenge. In recent years, machine dynam-ics have intensified radically, while positioning accuracy has

remained the same or improved. Values of around one tenth

of a millimeter are considered good.

Anoth er as pect i s the accur acy o f inte rnal geome tries such

as diameters. Parts with high-precision fits are an extreme

example. They are designed to enable form-fit insertion of

connecting pins or other components. Hole diameters must

be accurate to a few hundredths of a millimeter and the sides

of the holes have to be very smooth. The only way to produce

high-precision fits such as these used to be by drilling and

rasping. Today, they can be fabricated with a punch press by

prepunching a smaller hole and then using a slightly larger

punch to produce the final diameter. Postpunching removes

only a very small amount of material, making it possible to

attain a high degree of accuracy and a smooth surface of

which nearly 100 percent is actually cut.

Eliminating scratches | Scratches on the bottom of a

part frequently indicate that it was produced on a punch

press. They result when the sheet rubs up against burrs on

1

1 The sh eet rem ains f lat, e ven af ter nu merous holes.

2 Punch presses produce high-precision fits.

2

the die edge or other machine

build up on the die edge, formin

sheet from the burrs, pads are consist of a 0.3-millimeter-thi

raises the sheet, so that it is hi

no longer come into contact wit

A micr oburr f orms on the d ie edge . Adhe sive p ads pre vent the bo ttom o f the sheet from ru bbing u p again st th e burr.

114 | Punching, nibbling, and forming



die perform the same function. During processing, the sheet

on the machine table is moved around to different positions.

The metal balls used in ball tables can leave disti nct marks

on sensitive materials. Brush tables can help. Sheets are ableto glide more easily over the plastic brushes. The brushes,

however, are subject to wear and have to be cleaned regu-

larly to remove oil and metal chips.

Sometimes slugs fail to fall through the die. Instead, they

are pulled up with the punch and become stuck inside the

die. If this happens, the slugs protrude from the die and may

scratch the sheet. There are two ways to prevent this from

happening. The first is to use vacuum slug removal to suction

the slugs out through the bottom of the die. The second way

is to use a slug retaining die, which has small grooves built

into its cutting surfaces. When the part is punched, some of

the material enters the grooves. If the slug catches on the

punch, the material in the grooves keeps the slug firmly in

the die, so that it is not pulled up.

1

1 Brush tables keep the bottom of the sheet from getting scratched.

Eliminating burrs | Burr formation is a typical occurrence

in punching and nibbling. A small amount of material remainson the underside of the punched part, where it forms a rough

edge, or burr. The size of the burr can vary. The burr itself,

however, is usually sharp enough to cause injury. Burrs can

also cause problems when edges are joined or finishes are

applied to the part. For this reason, punched parts are fre-

quently deburred before further processing.

One way of preventing burrs is to reduce the clearance

between the punch and die. Doing so, however, results in

increased punching tonnage and shorter tool life.

Machine and tool manufacturers are working on new

methods of producing parts with minimal burr formation. One

solution involves forming the burr using special tools. Another

alternative is to employ deburring devices on the machine.

Devices such as these, however, produce metal chips.

Slug retaining die: special grooves prevent the slug from being pulled

up with the punch on its upstroke.

YOU MIND TURNI NG I T DO WN A LITT LE?

Punch presses are major contributors to the overall noiselevel in production facilities. There are two ways to reduce

the stress on operators’ eardrums and make punch presses

work more quietly. Either a “soft-punch” function can be used

for punching material at lower speeds, or special “Whisper-

tools” can be employed.

Soft punching | When the punch strikes the workpiece

at top speed, there is a loud pop as the slug is forced out.

Depending on the material, the noise level can be reduced

by up to 80 percent by slowing down the punch as it pene-

trates the sheet. The speed is controlled during the punching

stroke. Changes in the oil pressure tell the sensors when the

punch has made contact with the surface of the sheet. The

punch then continues through the sheet at reduced speed

until it passes through the material.

Whispertools | Unlike ot

edges, “Whispertools” feature apunch surfaces can reduce punc

cent using a “progressive” or “p

the slug is punched out continu

other. Punches with flat surface

the sheet straight on, separating

on all sides.

Anoth er benefi t: Whisp erto

force compared to other punch

makes them an attractive alte

high-tensile materials. At the sa

edges of the punch increase the

in a marginal prolongation of th

tion, the punch’s regrind lengt

that of punches with flat surface

cannot be reground as often.

Thanks to th eir bev eled su rfaces , Whis p

material much more quietly.

Normal punching stroke at full speed (left) and “soft” punching stroke

with controlled speed (right)

Decibels versus perceived loudnessspeeds can reduce the noise level of pu

some cases from 90 to 80 decibels (A)but it is. This is because the decibel is a

Richter scale for earthquakes. Thus, a translates into an approximately 50-perlevel as perceived by a person. Now tha






The laser station | The tool of a laser statio n is the laser

beam. Unlike other types of laser flatbed machines, the sheetis moved instead of the laser beam. An opening is located

directly under the laser station. The extraction unit uses the

opening to suck out slag and fumes produced during cutting.

The lower end of the cuttin g optics and the laser beam are

surrounded by a number of brushes, or “laser guard”. The

laser guard performs two tasks. First, it ensures that the sheet

lies flat during processing and does not flap around. Second,

it catches spatter, while functioning like a small safety enclo-

sure to contain reflected or scattered laser radiation.

The machine in action | Production is where the strengths

of combination machines come into play. Complex inner and

outer contours are cut by the laser. The laser is equipped

with a standoff height sensor, which ensures that the dis-

tance between the cutting head and the workpiece remains

constant. This, in turn, allows the laser to make cuts even

over formed areas. The laser cutting head follows the contourand is raised automatically to compensate for higher areas

of the sheet. The punching station is used when standard

contours like round holes need to be produced quickly or for

producing all kinds of formed sections, threads, bends, and

similar features. The processing sequence and techniques

are specified with the help of the programming software. The

software calculates the optimum machining process based

on the machining strategies, technology values, and rules

selected by the programmer.

Pros and cons | The advant ages of c ombi natio n pu nchin g

and laser cutting machines can also pose certain challenges.

Fully exploiting the benefits of both technologies requires

the right processing sequence and optimum parameters .

Throu ghout this proce ss, t he per

software is crucial.

Some sheet metal fabricato

machines are never really runni

of the workstations is always id

pensive. In addition, the machinmachines are higher than those

laser cutting machines.

Nevertheless, using combina

expensive when they can be u

start to finish that would otherw

in two work cycles. Parts that

ent machines have to be transpor

then repositioned on the second

all create costs, which are only

many profitability studies.

Anot her facto r in the com

the part quality. The workpiece

machining steps, resulting in g

parts are machined on two diffe

Punching, nibbling, and forming | Today’s punch press-

es are the most incredibly versatile tools. They are the onlymachines that make it possible to produce complete formed

sections, threads, and bends all on one piece of equipment.

Punch presses can process sheets of mild steel, stainless

steel, aluminum, copper, or brass with a thickness of up to

around 8 millimeters.

Unfortunately, when it comes to extremely brittle or hard

materials, punch presses do not fare very well. As the con-

tours become more delicate and complex, the number of

required tools and the machining time increases. As auto-

mated production cells, on the other hand, punch presses

have a distinct edge. Their part removal flaps, open-design

machine table, and greater gap between parts make it easier

to remove and sort the finished parts.

What this means for part design | Before the design

engineers begin, it should already be clear which manufac-

turing technology is going to be used: punching or laser cut-ting. The engineers need this information, so that they can

optimize the design.

Take for exampl e a strain er being design ed for produc -

tion. The diameter and flow rate are the specified functional

requirements. If the part is produced on a punch press, the

designer will use numerous identical holes, so that no tool

change is necessary. The number of holes is not important.

Things are differ ent if a laser cuttin g ma chine is going to

be used to produce the strainer. Since a piercing operation is

required for each hole, the design engineer will try to get by

with fewer holes. The holes themselves can be designed in

many different ways. The time required to machine the part

on a laser cutting machine depends on the total length of the

cut contours and the number of piercing operations.

COMBINING LASER CUTTING AND PUNCHING

So what’s it going to be: laser cutting or punching? Eachtechnology offers its own individual benefits.

So why not just combine both of them into one machine?

This was the thou ght that led to the devel opmen t of com-

bination punching and laser cutting machines, which are

also frequently referred to as “combination machines.” Laser

cutting enables the creation of infinitely complex contours,

while punching can be used to produce formed areas,

threads, bends, and other shapes in the very same part.

The machine | Combination punching and laser cutting

machines are built according to the same principle used in

punch presses. The difference, however, is in the C- or O-frame,

which is widened to provide room for both the punching sta-

tion and the laser cutting station.

The pu nching tools are lo aded in to a li near m agazin e. The

laser and beam guideway are integrated in the machine. The

machine table has flaps for part removal just like on a regularpunch press. Combination machines, however, require two

flaps: one per workstation. The two flaps mean that finished

parts can be separated from the sheet by either the laser

beam or the punch and then removed from the machine.

1

1 The tec hnology determ ines t he desi gn: lar ge num ber of holes ( left)produced by punching or free contours (right) created by laser cutting.

2 A comb ination machin e in ac tion: on the left t he las er cut ting h ead,

on the right the punching head

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Fascination of SheetMetal Chapter5