PUNCHING Classic sheet metal processing in new · PDF fileunching machines used in flexible sheet metal processing have a punching power of approx. 25 tons and, for the most part,

he term punching describes a slit-ting process, in which a punching machine or press separates a sheet

of metal by striking it. Initially, eccentricpresses were employed for punchingholes. Sheet contours were processed bystamping machines. By equipping thesewith nibbling tools, the nibbling ma-chine was born.

Nibbling refers to separating a mate-rial bit by bit along any formed cuttingline by punching several individual, over-lapping holes.

Whereas an operator once movedthe workpiece by hand, partial automa-tion was later made possible thanks tothe development of a machine with acoordinate guide and a template. A sty-lus connected to the coordinate guide ofthe nibble machine traced the contourof the template. This in turn moved theworkpiece under the machine head toproduce the component.

A major step towards combiningpunching and nibbling technology wastaken with the introduction of a combi-nation punching and nibbling punchmachine. Adding a clutch to the nibblingpunch enabled the machine to carry outindividual strokes. Thus, it became pos-sible to punch and nibble on the samemachine.

The end of the sixties brought the in-troduction of NC technology to machinetool manufacturing and a precise conti-nuous control system with NC axes. In-stead of templates, punched tapes con-taining the necessary processing

information were used. The floppy diskfinally superseded the punched tape asa data carrier. Moreover, tool changingand loading and unloading of the work-piece were automated.

The development of the electrohy-draulic ram drive finally allowed a work-piece to be processed while taking therequired punching power, stroke speed,and stroke position into account. Pro-cesses such as forming and tapping rounded off the processing possibilitiesof a complete metal sheet on a punchingmachine.

The rapid development of punchingtechnology forms the foundation for to-day’s high-tech machines with theirwide range of possibilities and high pro-cessing speeds while maintaining theutmost precision and a favorablecost/performance ratio. Today we can-not imagine sheet metal processingwithout them.

Punching in terms of making holes orcutting out pieces is done by a processknown as shearing. Shearing is the resultof placing a sheet between an upper tool(punch) and a lower tool (die). The uppertool plunges into the shearing the sheetproducing a punching slug which is pushed through the lower tool.

Punching encompasses the manufac-ture of an outer profile and apertures.When punching holes, the hole in theworkpiece is identical to the geometricshape of the punch.

Aside from punching openings, apunching machine can also be used for

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Classic sheet metalprocessing in new dimensions – punching,nibbling, formingSince the fifties, as advances were made in sheet metal processing machines, thesignificance of the sheet metal as a material increased accordingly. Whereaspunching machines were first used primarily for creating precision holes, theyare used today for a multitude of manufacturing tasks in flexible processing.Their outstanding feature is that when used for punching, nibbling and forming,they can process quickly while retaining high precision.

T

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other work processes. The most impor-tant of these are nibbling and creatingformed areas.

Nibbling describes the step-by stepprocess of creating any kind of contourin sheet metal.

Forming is a manufacturing processin which a workpiece is shaped. Extru-sions, beads, and louvers are examplesof forming areas which, for instance, arerequired in the manufacture of electricalcabinets.

How a punching machineworks

Before processing, the operator placesthe sheet into the clamps of the coordi-nate guide. A tool – punch, stripper, anddie – are placed into the tool adapter.The sheet is positioned under the pun-ching head by the program control: thisis accomplished by quickly moving thecoordinate guide and the machine table.The punch of the tool punches throughthe sheet, producing a hole with thesame shape as the contour of thepunch. The punch punches in rapidsuccession – up to 1000 strokes perminute – the programmed contours intothe sheet.

All the processing functions and posi-tioning of the sheet metal are CNC con-trolled. The CNC control coordinates thedrives to ensure that the individual moti-ons and the exact assignment of the co-ordinate guide (tool positioning) and ram(punching stroke) are synchronized.

Several identical workpieces are oftenmanufactured from a single metal sheet.In contrast to the classic procedure, bywhich a workpiece was initially cut to sizeand then processed, the sequence todayhas been reversed, particularly for large-scale manufacturing. The punching tool,once it has been loaded into the tool ad-apter, processes all identical elements ofeach part on the single metal sheet. Thenext tool is implemented and the processrepeated. Only after the sheet has beencompletely processed does the machinecut out the individual workpieces with apunching tool. The individual parts arethen removed from the work area via apart chute.

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Work processes: punching, nibbling, forming

unching machines used in flexible sheet metal processing have a

punching power of approx. 25 tonsand, for the most part, employstandardized tools. Punching machinesof this type are particularly suited forsmall and medium sized lots. This is dueto their short setup times, quick toolchanging times, easy to use tool andprogram administration, and finallyautomation components for removingand sorting pieces.

Machine frame

The machine frame of a punching ma-chine absorbs the forces required forpunching. It is either shaped as a C openat the front or as a closed O. The C fra-me allows you to easily access the workarea and also to process oversize sheetsby simply turning them.

Ram drive

The up and down motion of the ram isproduced either electrohydraulicallywith a hydraulic cylinder or electrome-chanically with an excentric drive.

In an excentric drive the excentricconnection of the ram (connecting rod)to the rotating shaft causes the ram stro-ke. A flywheel is located on the shaft,

whose rotational energy is transferred tothe punch by means of a quick-enga-ging clutch.

Oil under pressure moves the hydrau-lic cylinder or punching head. A hydrau-lic pump creates the required pressure(up to approx. 3380 PS) which is then directed into the cylinder. The result ofthis transfer of force is the generation ofa punching stroke.

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Machine conceptsC-Rahmen

Stößel

Maschinentisch

Teilerutsche

Werkstück

Späne und Stanzbutzen

Blechtafel

Querschiene derKoordinatenführung

Hydraulikaggregat

Werkzeug(Stempel und Matrize)

Werkzeugrotation

Stößelsteuerung

A machine frame has the shape of a C or an O.

The schematic diagram displaysthe most important parts of apunching machine.

C frame

ram control

ram

machine table

part chutes

workpiece

chips and slugs

tool rotation

metal sheet

hydraulic unit

transverse rail of thecoordinate guide

tool(punch and die)

P

Hydraulic punching head

A hydraulic punching head works in away that has been optimized for powerand energy. The required punchingpower level is selected automatically de-pending on the required punching po-wer. Thin sheet metal only requires lowoil pressure which results in less pun-ching power. This in turn reduces thepunching machine’s energy require-ments.The hydraulic ram has its own NC-con-trolled axis which regulates the start andreversal points of the stroke movementof the ram according to the metalthickness. Unnecessary stroke lengthsare thus eliminated, increasing the num-ber of strokes and ensuring a perfectpenetrating depth for the punch into thedie. This leads to a significant increase ofthe tool life.

By regulating the ram speed precise-ly, a hydraulic punching head can reducenoise up to 80%. The pressure increasein the punching head is measured andevaluated when the punch strikes theworkpiece. A temporary reduction ofthe ram speed can lead to a more quietpunching of the workpiece.

Tool adapter

In a punching machine we distinguishbetween a tool adapter in which a com-plete tool set is hydraulically tightenedso that it cannot move and a turretwhich serves a a tool storage and guide.

With hydraulically tightened tools theram as the upper tool adapter holds thepunch with the stripper. The lower tooladapter holds the die. The upper andlower tool adapters have the same cen-terpoint. The long, hydraulic ram guideprovides highly precise tool coordinationand allows extreme eccentric loads onthe tools (for example during nibblingand notching).

Tool rotation: The tool adapter can alsobe implemented as a rotational axis allo-wing all the tools to be rotated. The up-per and lower tool adapters rotate syn-chronously and along the shortest routearound the programmed rotationalangle.

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1. Setting the ram positionThe control regulates the distance between the ram and the surface of the sheetmetal. A stepping motor regulates this distance. In this initial position prior to thepunching stroke the lower ram surface is under hydraulic pressure.

2. Executing the punching strokeThe punching stroke is initiated by an additional signal from the control. The hy-draulic valve of the linear amplifier is opened and conducts a flow of oil onto theupper ram surface upon which the ram is pressed downwards. When the ram makes contact with the sheet, the hydraulic pressure on the upper ram surface automatically increases until the punch is executed. If a high degree of punchingpower is required, the hydraulic pressure on the lower ram surface is also cut off.

3. Return ram strokeAfter the punch, the ram reaches its lowest position. The hydraulic valve is then closed by the mechanical rack and pinion. The ram is returned to its initial positionby increasing the hydraulic pressure on the lower ram surface while decreasing thepressure on the upper ram surface at the same time.

rinciple of the hydraulic punching head

P

CNC

ram control

pump

stepping motor

tank

die

ram

pinion

rack

upper ram

lower ram

linear amplifier

control

hydraulic valve

Schematic illustration of the hydraulic punching head

This reduces the required number oftools, setup time, and the number oftool changes. Tools specifically designedfor rotation such as multicut and curvedtools allow contours to be processedvery quickly with a minimum of rough-ness which otherwise would have to beperformed by time-consuming nibbling.The versatile rotational axis can also beused as a drive for tapping tools andmultitools.

In turret punching machines rotatingthe turret brings the tool necessary forprocessing under the punching station.The ram strikes the punch, drives itthrough the workpiece into the die andthen moves back up. After the punchingstroke, a spring pulls the punch back tothe initial position.

Tool storage

The tool store contains the tools neces-sary for processing. Apart from internaltool magazines there are also externalmagazines which can automatically pro-vide the punching machine with addi-tional tools.

The tools are exchanged internallyusing a tool changer which is themechanism serving to transport andload the desired tool into the tooladapter. The combination of toolchanger and tool storage is realized bymeans of a linear magazine.

The easily-accessible linear magazineis mounted along the transverse rail andtakes full advantage of the available mo-tion axes. This eliminates the need for aseparate drive. The magazine is equip-ped with tool cassettes which alreadycontain tools arranged in sets. A tool re-quired for processing is inserted into thetool adapter by means of a programmedprocedure of the coordinate guide. Aside from the tools, the clamps forworkpiece are integrated into the linearmagazine.

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The rotating tool adapter allows all tools to be rotated.

In turret punching machines thetools are arranged in a circle.

In turret punching machines the toolsare normally arranged in a circle. Thetools are set up manually. Punches, strip-pers and dies are attached individually inseparate mounts. Rotating the turretbrings the required tool into the definedpunching position of the turret.

Complete tool sets are set up in the linear magazine.

Workpiece clamping

Workpiece clamps hold the sheet metaland guide it as it is processed. They arehydraulically or pneumatically activated.The clamps can either be mounted to setpositions or variably placed on the trans-verse rail.

The area of the sheet upon which theclamps are located remains unproces-sed, because the clamps would other-wise collide with the punching head. Ifthis collision area is also to be processed,you must reposition the sheet. The sheetis then clamped with a stripper, and thecoordinate guide moves in a program-controlled manner with open clamps.Afterwards, the clamps clamp the sheetonce again and processing continues.Repositioning is also necessary for pro-

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A punching tool consists of a punch and die.An alignment ring and stripper complete thetool set. A tool cassette holds the tool set.This means that the complete tool set can beinserted into either the tool adapter or thetool storage in a single step.

Punch with alignment ringThe punch is manufactured from high-speedsteel (HSS). For specific applications, pun-ches with coated or oxidized surfaces can beused. The alignment ring is a clamping ring,which fastens the forming punch (e.g. rec-tangular tool) in the zero position. It trans-fers punching power from the ram to thepunch.

Stripper/holding-down clampThe stripper or holding-down clamp canperform three functions:Stripping function: the stripper separatesthe sheet metal from the punch during thereturn stroke.This prevents the metal from being pulled up

with the punch. During processing, the stripper is situated approx. 0.5 to 1 mm above the workpiece surface.Holding-down function: The hydraulic control lowers the stripper onto the sheet before the punching stroke and holds it securelyto prevent warping. This function is particularly important when processing thin sheet metal.Self-separating tools are used as an alternative to strippers and holding-down clamps.Clamping the workpiece: to rework oversized sheets, the stripper lowers itself onto the sheet and clamps it down while the coor-dinate guide moves with opened clamps for reworking.

DiePunch and die function as shearing tools which pass alongside each other in a scissor-like motion and cut the sheet. The resultingpunching slug falls through the die and is usually extracted.

unching toolsP

punch with alignment ring

= +

stripper

die

punch alignment ring

The external tool storage can supply the machine automatically with addi-tional tools.

cessing sheet lengths greater than theworking area.

Similarly, retractable clamps also al-low a sheet to be processed completely,making the clamping strip unnecessary.If a clamp is positioned into the pun-ching area, the clamp opens and movesautomatically out of the collision area.

Coordinate guide

The metal sheet held by the clamps is po-sitioned in the longitudinal and cross di-rection during processing by the co-ordinate guide. Rack and pinion systemsor lead screws execute the motion andprecision drives provide a positioningspeed of over 4300 ipm at a positionalrange as small as 0.03 mm.

Machine table

Machine tables support the sheet duringprocessing. A complete support reducesvibrations when positioning. Further-more, a table prevents the sheet metalfrom sagging, thereby adding to thehigh precision of the punches. Machinetables are rigid or can extend out in onedirection.

Ball rollers or brushes which can beintegrated in the table surface keep theeffects of scratching to a minimum. Theyallow the sheet to glide more easily overthe table.

Removing the parts

After processing with punching tools,the finished passes or scrap pieces areremoved from the working area of thepunching machine:■ Finished or scrap pieces of limited size

are removed by movable chutes tothe containers provided.

■ Very large finished or scrap parts areremoved by hand or by an automaticunloading system.

■ Microjoint manufacturing: The partsare held by small webs and are pressedout by hand from the scrap skeleton.

■ Small scrap pieces are usually pun-ched into little pieces.

■ Chips and punching slugs fallthrough the die – often with the helpof a vacuum system – into a chip con-tainer or onto a conveyor belt.

■ Processed parts and scrap skeletonscan also be removed by supplemen-tary unloading and sorting systems.

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Retractable clamps

In microjoint manufacturing, small webs hold the parts in the grid.

Tools and workpiece

When punching, punching tools createholes and openings of any shape in theworkpiece. The tool shapes used mostoften are: round, square, rectangular,and oblong tools. Depending on thecontour shape, the outer contour of anindividual part can be processed withpunching tools in nibbling mode or withslitting tools. Slitting tools are narrowrectangular tools with a corner radiusthat in combination with the tool rotati-on can quickly cut out pieces.

The minimal dimensions for a punchcan be calculated according to the following general rule: punch size (outer circle diameter) ≈ sheet thickness.Smaller punch sizes pose the risk ofbreakage. To avoid breaking the punch,tools with a guided cutter can also beused.

The maximum size of a punch variesbetween 80 and 130 mm and is depen-dent on the following influencing varia-bles:■ the maximum punching power avail-

able (machine type )■ the maximal geometrical size of the

tool adapter or turret bore

■ the sheet thickness■ the sheet’s tensile strength

Nibbling usually requires smaller pun-ches with a diameter of up to approx.30 mm.

Tools and Processing quality

Die clearance and cut-fracture ratio:The technically required die clearance isthe distance between the cutting edgesof the punch and die. It influencespunching quality.

The punching process is divided intofour phases:■ Phase 1: indenting the sheet■ Phase 2: cutting■ Phase 3: fracturing■ Phase 4: ejecting the punching slug

The choice of die and thus the die clear-ance affects the cut-fracture ratio (phas-es 2 and 3) when punching. A die clear-ance of 0.1 x sheet thickness results in acut-fracture ratio of 1/3 to 2/3. If the dieclearance is smaller, the cut ratio in-crease to 2/3.

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Tools

Schneidspalt

Stempel

Blech

Matrize

Die clearance width influencespunching quality

The four phases of the punchingprocess

1

2

3

4

punch

sheet

die

die clearance

Punching with very smooth cuttingsurface: The larger the cut ratio, themore cylindrical the punching resultwhich can be necessary for fittings withan extremely high supporting per-centage. The largest possible cut ratiocan be attained if you pre-punch with anapprox. 1–2 mm small tool and a dieclearance of 0.1 x sheet thickness. After-wards, rework with the desired tool toform a scraping cut with a small dieclearance. This method has also provenitself in reducing punching power forthick sheet processing with large tools.

Processing high-grade steel and al-uminum: When processing high-gradesteel and aluminum, cold welding andbuild-up edges may occur. These effects

can be reduced by in the following man-ner:■ by additionally lubricating the tool ■ by using oxidized or surface-coated

punches

Manufacturing very precise punches:When punching with an active holding-down clamp, the stripper/holding-down clamp rests on the sheet duringthe punch. The sheet is then pressedagainst the underlying die, resulting invery precise punches of flat workpiecesand a further improvement of cuttingquality.

Roughness: If a contour, for example acircle, is nibbled with a round tool, thenibbled edge will have a roughnesswhich correlates to the feed per stroke. Smaller strokes improve the quality ofthe nibbled edge. In other words, thereis less roughness.

Visible edges – slots: Slotting is a two-sided shearing process in which an evencutting edge is produced free of build-ups. This process is particularly suited forvisible edges. During the slotting pro-cess, the chip is rolled up into a spiral coiland finally separated by a deep-plungedstroke.

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Hand-held machines are used anytime a workpiece must be processeddirectly on site. They can be used forthree-dimensional workpieces withformed areas as well as for bulky con-structions that are fragile to transport.Typical examples of hand-held machi-nes are hand-held nibblers for produc-ing cutting edges free of deformationsin sheets up to 10 mm thickness. The-se can then be processed at a workspeed of approx. 1.5 m/min. Hand-held shears which can slit and trimsheets up to 4 mm in thickness with-out producing chips can process atspeeds of approx. 3 m/min. Hand-heldmachines are powered by electric orcompressed air motors.

Hand-held nibbler

and-held machines

H

punching with a 2/3 cut ratio

The slotting tool produces visible edges

Low-scratch processing: In addition tomachine-oriented measures designedfor low-scratch processing, brushes em-bedded in the die are also available toimprove the quality of the finished work-piece.

Multiple tools

If the number of required tools exceedstool storage capacity, or if processingtime is to be shortened, multiple toolscan be used.

The multiple tool Multitool containsup to 12 tool stations of any form butlimited size. It is set up in the tool adap-ter as any other punching tool. A pro-gram command causes the Multitool dieto be activated in approx. 1.5 s and brought into any angular position bymeans of the rotational axis. Multitoolconsiderably increases the number oftools in the tool storage.

Typical fields of application for Multi-tool are workpieces with many differentround hole dimensions.

The Multi-Cut is a 4-sided tool de-veloped for rotating with four convexshaped outer edges of various radii. It

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Left: The multiple tool considerably increases the number of availabletools.

Right: Noise reduction by using punches with an angled cutting surface

At the bottom: The four-sided tool Multi-Cut as an alternative to nibbling

can produce circular openings of variousradii without additional tool changes.Apart from a shorter processing time, anadded advantage is that the cuttingedge is less rough than a nibbled edge.

Tools and operational requirements

Tool life: In general, punching toolshave a tool life of 400 ,000 to 600 ,000strokes.

In order to attain such a tool life, caremust be taken to ensure a stable toolguide, material-specific tool lubrication,a sharp cutting edge as well as the exactcentrical position of the punch and die.The required sharpness of the cuttingedge can be guaranteed only by re-grinding the cutting surface.

Tools with angled punch (noise reduc-tion): Punches with an angled cuttingsurface require less punching power dueto the lengthened power stroke whenthe punch penetrates the workpiece.

The lower punching power has theeffect of reducing noise by approx. 50 %in comparison to a punch with a flat cut-ting surface of the same size. The maxi-mum processable sheet thickness is,however, smaller.

Forming tools

As a supplement to punching and nibb-ling, a variety of formed areas with limi-ted height can be created in the sheetusing special tools. The most importantformed areas are louver cuts, beads, ex-trusions and threaded forms.

Louver cutting: Louver tools make cutsin a single stroke and form them. Thelouver tool is used mainly for producingventilation slits; for example, in manu-facturing various chemical apparatuses,

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Processing examples:At the top: CaseIn the middle: Footboard for atruck

At the bottom: A louver tool isoften used to produce ventilationslits.

in ventilation and air conditioning tech-nology as well as in electrical cabinets.

Beading: Beading refers to the processof embossing hollow, ring-like formedareas in a nibbling operation. Beads areproduced to reinforce metal sheets. Thequality of their surfaces is determined bythe feed selected. The following generalrule applies: maximum bead height = 2 xsheet thickness.

Extrusions: Extrusions are formed areasproduced in a single stroke with a draw-ing punch and a drawing die. Holes arenormally pre-punched prior to the for-ming process.

Extrusions are used, for example, forcable bushings in electrical cabinets.Threads are also frequently formed inthem as a means for attaching sheets.

Tapping: Tapping refers to the creationof threads in prepared punch holes orextrusions which are used in electricalcabinets. Since the material is displacedand not cut, this process does not pro-duce any chips. Displacement causes astrain hardening of the material whichcontributes to the stability of the thread.Threads from M2.5 to M8 are possible.

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At the top: Beads reinforce thesheet metal.

In the middle: Extrusions

At the bottom: A tapping toolcreates threads without chips.

Machine costs

The following items contribute to ma-chine costs which accrue by the hour:

Calculation basis for fixed costs:■ initial cost■ depreciation period■ interest rate■ required space (cost of floor space)■ scheduled time of use

Calculation basis for variable costs:■ electrical energy consumption of the

entire installation (electricity costs)approximate value: 5–10 kW

■ tool costs■ cost of replacing worn-out and de-

fective parts■ scheduled time of use

Machine hourly rate: The total costs arecalculated by adding the fixed costs tothe variable costs. For the most part, these are determined according to theextent to which the machines are utilized.

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Calculation of fixed costs:calculated depreciation = initial cost / (depreciation period x machine time)calculated interest = (initial cost / 2) x interest rate / machine timespace costs = required space x cost of floor space / machine timeThe infrastructure costs in the Federal Republic of Germany provide the basis for these calculations

ixed costsF

Calculation of variable costs:Energy costs = electrical energy / h x electricity costscost of replacement and wearing partsscheduled time of useThe infrastructure costs in the Federal Republic of Germany provide the basis for these calculations

ariable costsV

space costs (4%)

interest (18%) depreciation (78%)

energy (23%)

variable costs (10%)

fixed costs (90%) fixed costs (80%)

variable costs (20%)

One-shift operation Two-shift operation

tools (37%) maintenance (40%)

From:

The Fascinating World of Sheet Metal

Publisher and Editor:TRUMPF GmbH + Co. KG, Ditzingen

Dr. Josef Raabe Verlags-GmbHStuttgart, Berlin, Bonn, Budapest, Heidelberg, Prag, Wien 1996

© This work is copyrighted. All rights, including thosepertaining to the translation, reprint, and the reproductionof the book or any parts thereof, are reserved. No part ofthe book may be reproduced in any way, nor may it beedited or distributed by electronic means without theexpressed written consent of the publisher.

The authors and the publisher affrim that all statementsmade in this book were checked carefully and in aconscientious manner. Liabilities for any damages, director indirect, arising from the use of this book, in part or total,are excluded except where prohibited by law.