PUNCHING Classic sheet metal processing in new dimensions – punching, nibbling, forming Since the fifties, as advances were made in sheet metal processing machines, the significance of the sheet metal as a material increased accordingly. Whereas punching machines were first used primarily for creating precision holes, they are 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. he term punching describes a slit- information were used. The floppy disk ting process, in which a punching finally superseded the punched tape as Tmachine or press separates a sheet a data carrier. Moreover, tool changing of metal by striking it. Initially, eccentric and loading and unloading of the work- presses were employed for punching piece were automated. holes. Sheet contours were processed by The development of the electrohy- stamping machines. By equipping these draulic ram drive finally allowed a work- with nibbling tools, the nibbling ma- piece to be processed while taking the chine was born. required punching power, stroke speed, Nibbling refers to separating a mate- and stroke position into account. Pro- rial bit by bit along any formed cutting cesses such as forming and tapping line by punching several individual, over- rounded off the processing possibilities lapping holes. of a complete metal sheet on a punching Whereas an operator once moved machine. the workpiece by hand, partial automa- The rapid development of punching tion was later made possible thanks to technology forms the foundation for to- the development of a machine with a day’s high-tech machines with their coordinate guide and a template. A sty- wide range of possibilities and high pro- lus connected to the coordinate guide of cessing speeds while maintaining the the nibble machine traced the contour utmost precision and a favorable of the template. This in turn moved the cost/performance ratio. Today we can- workpiece under the machine head to not imagine sheet metal processing produce the component. without them. A major step towards combining Punching in terms of making holes or punching and nibbling technology was cutting out pieces is done by a process taken with the introduction of a combi- known as shearing. Shearing is the result nation punching and nibbling punch of placing a sheet between an upper tool machine. Adding a clutch to the nibbling (punch) and a lower tool (die). The upper punch enabled the machine to carry out tool plunges into the shearing the sheet individual strokes. Thus, it became pos- producing a punching slug which is sible to punch and nibble on the same pushed through the lower tool. machine. Punching encompasses the manufac- The end of the sixties brought the in- ture of an outer profile and apertures. troduction of NC technology to machine When punching holes, the hole in the tool manufacturing and a precise conti- workpiece is identical to the geometric nuous control system with NC axes. In- shape of the punch. stead of templates, punched tapes con- Aside from punching openings, a taining the necessary processing punching machine can also be used for 42 PUNCHING 43 PUNCHING other work processes. The most impor- tant of these are nibbling and creating formed areas. Nibbling describes the step-by step process of creating any kind of contour in sheet metal. Forming is a manufacturing process in which a workpiece is shaped. Extru- sions, beads, and louvers are examples of forming areas which, for instance, are required in the manufacture of electrical cabinets. How a punching machine works Before processing, the operator places the sheet into the clamps of the coordi- nate guide. A tool – punch, stripper, and die – are placed into the tool adapter. The sheet is positioned under the pun- ching head by the program control: this is accomplished by quickly moving the coordinate guide and the machine table. The punch of the tool punches through the sheet, producing a hole with the same shape as the contour of the punch. The punch punches in rapid succession – up to 1000 strokes per minute – the programmed contours into the sheet. All the processing functions and posi- tioning of the sheet metal are CNC con- trolled. The CNC control coordinates the drives 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 often manufactured from a single metal sheet. In contrast to the classic procedure, by which a workpiece was initially cut to size and then processed, the sequence today has been reversed, particularly for large- scale manufacturing. The punching tool, once it has been loaded into the tool ad- apter, processes all identical elements of each part on the single metal sheet. The next tool is implemented and the process repeated. Only after the sheet has been completely processed does the machine cut out the individual workpieces with a punching tool. The individual parts are then removed from the work area via a part chute. Work processes: punching, nibbling, forming 44 PUNCHING Machine concepts CC-Rahmen frame ramStößelsteuerung control toolWerkzeug (punch(Stempel and und die) Matrize) ramStößel hydraulicHydraulikaggregat unit machineMaschinentisch table transverseQuerschiene rail der of the coordinateKoordinatenführung guide metalBlechtafel sheet partTeilerutsche chutes toolWerkzeugrotation rotation workpieceWerkstück The schematic diagram displays chipsSpäne and und slugs Stanzbutzen the most important parts of a punching machine. unching machines used in flexible Ram drive whose rotational energy is transferred to sheet metal processing have a the punch by means of a quick-enga- Ppunching power of approx. 25 tons The up and down motion of the ram is ging clutch. and, for the most part, employ produced either electrohydraulically Oil under pressure moves the hydrau- standardized tools. Punching machines with a hydraulic cylinder or electrome- lic cylinder or punching head. A hydrau- of this type are particularly suited for chanically with an excentric drive. lic pump creates the required pressure small and medium sized lots. This is due In an excentric drive the excentric (up to approx. 3380 PS) which is then to their short setup times, quick tool connection of the ram (connecting rod) directed into the cylinder. The result of changing times, easy to use tool and to the rotating shaft causes the ram stro- this transfer of force is the generation of program administration, and finally ke. A flywheel is located on the shaft, a punching stroke. automation components for removing and sorting pieces. Machine frame The machine frame of a punching ma- chine absorbs the forces required for punching. It is either shaped as a C open at the front or as a closed O. The C fra- me allows you to easily access the work area and also to process oversize sheets by simply turning them. A machine frame has the shape of a C or an O. 45 PUNCHING Hydraulic punching head Principle A hydraulic punching head works in a of the hydraulic punching head way that has been optimized for power and energy. The required punching control power level is selected automatically de- CNC pending on the required punching po- wer. Thin sheet metal only requires low oil pressure which results in less pun- ram control hydraulic valve linear amplifier ching power. This in turn reduces the stepping punching machine’s energy require- motor ments. tank The hydraulic ram has its own NC-con- pinion trolled axis which regulates the start and rack reversal points of the stroke movement of the ram according to the metal thickness. Unnecessary stroke lengths pump are thus eliminated, increasing the num- ber of strokes and ensuring a perfect penetrating depth for the punch into the die. This leads to a significant increase of upper ram the tool life. By regulating the ram speed precise- lower ram ly, a hydraulic punching head can reduce noise up to 80%. The pressure increase in the punching head is measured and evaluated when the punch strikes the workpiece. A temporary reduction of ram the ram speed can lead to a more quiet punching of the workpiece. Tool adapter In a punching machine we distinguish die between a tool adapter in which a com- plete tool set is hydraulically tightened Schematic illustration of the hydraulic punching head so that it cannot move and a turret which serves a a tool storage and guide. With hydraulically tightened tools the 1. Setting the ram position ram as the upper tool adapter holds the The control regulates the distance between the ram and the surface of the sheet punch with the stripper. The lower tool metal. A stepping motor regulates this distance. In this initial position prior to the punching stroke the lower ram surface is under hydraulic pressure. adapter holds the die. The upper and lower tool adapters have the same cen- 2. Executing the punching stroke terpoint. The long, hydraulic ram guide The punching stroke is initiated by an additional signal from the control. The hy- provides highly precise tool coordination draulic valve of the linear amplifier is opened and conducts a flow of oil onto the and allows extreme eccentric loads on upper 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 the tools (for example during nibbling automatically increases until the punch is executed. If a high degree of punching and notching). power is required, the hydraulic pressure on the lower ram surface is also cut off. Tool rotation: The tool adapter can also 3. Return ram stroke be implemented as a rotational axis allo- After the punch, the ram reaches its lowest position.