Laser versus Punch

GLENN BINDER

It was not until 1960 that the first laser appeared, even though as early as 1917 Albert Einstein had described the theory of stimulated emission. Since then the application of lasers has become an integral part of many industrial manufacturing operations. In the fabricating industry, before the advent of lasers, punch presses were used for the production of sheet metal parts, where they were first used for the production of holes. Today these are used for a multitude of processes like punching, nibbling, and forming for producing parts.

In recent years—with the development of faster, more powerful and flexible laser systems for sheet metal part processing—this technology has begun to take over punching applications. This article covers the manufacturing factors affecting this change as well as a description of both processes. A comparison is drawn between the benefits of both processes and recommendations put forward.

The punching process

Punching is performed by shearing, the result of placing a metal sheet between an upper tool (punch) and a lower tool (die). The upper tool plunges into the sheet producing a punching slug, which is pushed through the lower tool. Aside from punching openings, this machine can be used to produce other work processes, the most important being nibbling and creating formed areas. The latter is a process in which a workpiece is shaped, such as extrusions, beads, and louvers.

Before processing the operator places the sheet into the clamps of the coordinate guide. A tool-punch, stripper, and die are placed into the tool adapter. The positioning of the metal sheet is CNC controlled and punch strokes and positioning are synchronized. Once all the internals have been input the machine nibbles the parts out of the sheet through a chute if the parts are small enough. Sometimes the parts are tabbed into the sheet, which acts to hold the parts to the skeleton by micro-joints, which allow for easy removal as an extra operation.

Tools can be manually located in a turret or on a rail and they are either mechanically or hydraulically clamped. The ram, as the upper tool adapter, holds the punch with the stripper with the lower tool adapter holding the die. These tool adapters rotate or index to bring up the next tool for production. Some of the tool stations have a rotational axis, which allows the tools to be rotated.

Workpiece clamps hold the sheet metal and guide it as it is processed. The area of the sheet upon which the clamps are located remains unprocessed, because the clamps would otherwise collide with the punching head. If the collision area is also to be processed the sheet has to be repositioned. The sheet is clamped with the stripper and the coordinate guide moves to the new position with the clamps open. Once in the new position, the sheet is clamped and the stripper retracts. Repositioning is also necessary for sheet lengths greater than the working area. Retractable clamps also allow parts to be nested under the clamps, which move automatically out of the collision area when necessary.

When processing, punching tools create holes and openings of any shape but most often are round, square, rectangular, and obround tools. Outer part contours can be nibbled or processed using slit tools, which are narrow rectangular tools with a corner radius which in combination with the tool rotation can quickly cut out pieces.

Tool life is generally between 400,000 and 600,000 strokes. To attain this tool life, care must be taken to ensure a stable tool guide, material-specific tool lubrication, a sharp cutting edge, and the exact central position of the punch and die.

Formed areas with limited height may be produced on the punching machine using specific tools. Louver cuts, extrusions, and beads are prime examples.

The cost-of-operation for the hydraulic turret press includes electricity and tooling. For the servo-electric turret punch, however, energy costs are considerably lower. Maintenance costs, on the other hand, on mechanical drives and wear items are higher.

The laser process

Lasers are used to cut through steel sheet and plate, vaporize hard reactor steel, and slit the hardest metallic materials. Depending on laser power output they can be used on extremely thin materials as well as thick materials. By the middle of the 1980s, kilowatt-level CO₂ lasers became prominent in sheet metal cutting. For this reason the focus will be on the 2D flatbed CO₂ laser cutter.

The energy in the focused laser beam is supplemented by a cutting gas. The metal is vaporized in a confined area (the size of the focused beam) and blown through using the gas stream, producing a kerf. Flatbed machines with moveable laser cutting heads can cut large sheets up to 6000 x 2000 mm and thickness up to 20 mm without the need for machine modifications.

A popular flatbed laser cutting machine uses moveable (“flying”) optics. The laser beam is created in the resonator and passed through the machine components to the cutting head, so good alignment is critical for efficient operation. The beam is normally completely enclosed and guided by metal mirrors coated with silicon or copper. Focus lenses are made from zinc-selenide because glass absorbs the light completely. The beam and the cutting gas are passed through the nozzle onto the workpiece. A constant distance is maintained from the nozzle to the workpiece to maintain the correct focus point. Capacitive non-contact height sensors are used. The workpiece is supported on a pallet, which consists of slats that are pointed to eliminate the possibility of reflecting the laser light. There are normally two pallets arranged so that they shuttle. While one is under the cutting head the other is either being loaded with a new workpiece or parts and the skeleton are being removed from the previous nest.

An extraction system is located under the pallet to remove particles and gases produced by the cutting process. These are then passed through a filter. Because laser light is potentially very dangerous, maintaining safety is extremely important.

The cost of operation of a 2D flatbed laser includes lenses, nozzle tips, and cutting and laser gases such as nitrogen, oxygen, and helium. The cutting table is also a wear item. Long-term expenses include replacement of turbines, resonators, and laser optics.

Process comparisons

Advantages of laser cutting are:

• Metal can be processed without physical contact and the use of force.
• Nearly any contour can be created without a tool change, in contrast to punching and nibbling.
• Slitting is done at high speed, precisely, with a small kerf.
• High processing speed results in a minimal heat affected zone, leading to minimal workpiece distortion, which is, for all intents and purposes, negligible in sheet metal processing.
• Cut surfaces have very little roughness; the thinner the sheet, the smoother the edge.

Consistent laser power, excellent beam quality, polarization, and gas purity are the main factors contributing to high-quality cuts. Due to the flexibility of the laser cutting process it is becoming a viable process that challenges conventional cutting processes.

Precision and high-speed 2D laser systems have proven themselves in applications requiring a high degree of flexibility for geometrical changes in the workpiece. Overshot and scratching are no consequence when processing with moveable non-contact optics. The machine moves only its cutting head, requiring less space than one that has to position the workpiece.

Punching machines, on the other hand, have been around longer and can be considered the conventional cutting system. Because the set-up-time, which includes both tool setting and NC programming, is high it lends itself more towards larger batch sizes. This is in conflict with modern day lean manufacturing objectives and therefore causes a dilemma in markets demanding manufacturing flexibility. The goal that is often stated is a batch quantity of one. Even though this challenge is growing for the punch machine, the machine does have a niche that lasers are incapable of addressing: the ability to produce forms, extrusions, louvers, lances, and beads in the metal. The turret or rail punch press produces these with high accuracy and speed. It also foregoes the need to produce expensive dies for stamping machines. In this respect there is more flexibility.

Part nesting in punching is not as efficient because provision has to be made for the punching nibble tools between the parts. Even though common line punching may be employed there is still a typical “kerf” loss of around 0.25 in. Nesting programming time is also longer because the appropriate tooling needs to be verified. Some machines are limited by having only two auto index stations. These are tool stations that have a full 360o of freedom. Other tool stations are limited by being fixed, which implies the tools are unable to be rotated. Punch sequencing also needs to be considered because the sheet can lose its strength to be able to be dragged over the table. This takes up even more programming time. Turret or rail set-up is important because the correct tool needs to be fixed in the correct station. This takes up more set-up time. Tool maintenance and grinding is a cost factor, too.

The clamps holding the sheet need to be considered in the programming plan. Many shops simply leave a dead zone where there is no nesting under the clamps, eliminating the need for repositioning. A material utilization improvement of approximately 15 percent may be gained by nesting under the clamps. This necessitates repositioning, which incurs some dimensional error as well as taking up considerable machine time.

Noise produced by the punch machine is another problem in today’s safety-conscious industrial world. Besides the noise from the punch and nibble process, the sheet gets dragged over the machine bed causing further noise. Many brands of punch machine try to address the latter problem by fitting short vertical brushes to the table to dampen the noise.

Automation components used for removing and sorting pieces may be added to the punching machine improving the efficiency of the production process. These would be automatic sheet loaders and part removal systems. Parts are removed either through part chutes or part handlers with suction cups. Parts are stacked on pallets ready for downstream value-added processes. The downside to this is that the machine is idle while the part removal process is in process.

The first observation is that on the laser nest 22 parts are nested while on the punch nest only 16 parts are nested. Therefore there is a substantial increase in material waste using the punching machine. The punch programming took around ten times longer than the laser programming. The cutting process also took twice as long to punch because there is a reposition, whereas the laser had no waiting time. It must be said, however, that this is a random case and the results could be substantially different by using a combination of different parts.

Conclusion

In summing up the advantages and disadvantages of the two processes, it appears that part geometry plays a significant role. For example, parts that require forming will not be able to be produced on a laser cutting machine. Therefore, if forming is required, secondary operations need to be executed. This is not necessarily a downside factor because the secondary operation may be carried out at the same time the laser machine is cutting something else. Second operation forming however may introduce part errors because they have to be repositioned. Parts requiring high accuracy and that have many curves, however, are ideal for cutting on a laser machine.

Part quality also plays a significant role. If tool marks and surface scratches are a quality problem, punching would not be suitable. Because the laser process is non-contact these factors are eliminated. Heat affected zone (HAZ) introduced by laser cutting, however small, needs to be considered. For example parts required in aerospace applications cannot have a HAZ due to metal fatigue considerations. Waterjet cutting, which has no HAZ, then becomes a viable option.

The other important factor considered above is set-up time. It has been shown that significant savings can be made in using a laser machine to cut parts because inventory can be reduced through quick set-ups. Programming is minimal. The cost of ownership can also be significantly less because no tool inventory needs to be stored. It may be concluded that the laser is therefore more aligned to the modern day lean manufacturing philosophies.

A combination of the benefits from each process would be a huge advantage. Fortunately the laser/punch combination machine has been developed over the years to address the needs of the manufacturer that needs both forming and cutting flexibility. Although the initial capital investment is high, the benefits are substantial for the right application. The strengths of each process may be leveraged while the weaknesses of each process are mutually surpassed by the alternate process. The costs of ownership are high because it includes both laser and punch items previously mentioned.

It has been shown that laser cutting as a process will continue to grow and, due to the enormous advances made both in laser and cutting technology, it has become a genuine alternative to punching when economic factors are taken into consideration. In consideration of this, it is safe to say that laser will continue to displace conventional cutting processes for a long time to come.

Bibliography:

• John Powell 1993: CO2 Laser Cutting. Springer-Verlag London Limited
• Dr. Hubert Bitzel, Johanna Borcherdt et al 1996: The Fascinating World of Sheet Metal. Original edition published by Dr. Josef Raabe Verlags GmbH, Stuttgart
• The Techno Book of CAD CAM CNC 2nd Edition 2000: Multitude of references including Machinery’s Handbook, 24 Edition, Tool & Manufacturing Engineers Handbook, SME, 3rd Edition
• Slack, Chambers, Harland, Harrison and Johnson 1995: OPERATIONS MANAGEMENT. PITMAN PUBILISHING
• Charles Caristan 2004: Laser Cutting Guide for Manufacturing. Society of Manufacturing Engineers
• Andrew McCarthy, The Fabricator January 2005: “Punch it; Laser cut it or something else?”

Glenn Binder ([email protected]) is vice president of sales for SigmaTEK Systems, LLC (www.sigmanest.com), Cincinnati, Ohio.