Laser bevel cutting in Japan’s shipyards

Hideo Koike

Fifty-five years of technology development have enabled numerous product and process advances in the shipbuilding industry

Cutting is a major operation in a shipyard, and the use of lasers in this industry has increased through the years. In 1954, flame cutting machines for oxy-fuel plate edge trimming started to be used, and these became the main cutting machine for shipyards. These machines had been introduced in European shipyards before they were used in Japanese shipyards.

In 1955, automatic plate marking devices were developed, and EMP (Electro Photo Marking) equipment started to be used in shipyards around 1960. The ancillary marking process is used to mark the cutting lines, fitting lines, base line, and work order serialization. In Europe, 1/10 scale optical follower tracer oxy-fuel cutting machines became popular and started being exported to Japan.

In the period between1960 and 1965, a Norwegian company developed and introduced software for shipyards and also developed a CNC controller to be retrofitted to existing optical follower tracer machines. The CNC controller was widely used worldwide and cutting machine manufacturers in England, Germany, and France started manufacturing CNC cutting machines. Large shipyards in Japan also started to use software and CNC cutting machines at that time. Koike developed the first Japanese made CNC cutting machine in 1966, because of requests for higher performance for machine accuracy and durability from shipyards that were using imported machines.

Developed in 1970, powder marking, an oxy-fuel process, makes cut lines by spraying melted powder (mainly zinc) on the plate; this spread to replace punch marking, where cut lines were made mechanically. Also, oxy-fuel bevel cutting units, with automatic rotation of the cutting head, were developed and equipped with CNC cutting machines. By 1973, specialized machines for line marking were being used to meet the requirements of shipyards and bridge builders.

Around 1975, an air-plasma system was developed in Eastern Germany and started to be used in several shipyards. The purpose of this technology was to improve heat distortion. However, nitride formation on the cut edge became an issue that affected subsequent sub-arc welding procedures.

The first oxygen-plasma system in the world was developed by Koike in 1981 and started to be used in shipyards together with a new machine design that also had a plasma fume collector on the machine. This was necessary because on a long rail system it was difficult to remove fumes whereas with the fume collector as part of the cutting head motion system, fumes are removed at the cutting point.

The first CNC oxygen-plasma bevel cutting machine to be used in large shipyards was introduced in 1983. Eventually CNC plasma bevel cutting machines became mainstream in shipyards. Ink-jet marking devices became available in 1987, mounted on CNC line-marking machines with automatic rotation of the marking head. These began to be used by shipyards and bridge builders because of the flexibility to mark in any direction.

A 3kW gantry style CO₂ laser cutting machine was developed and started to be used in the bridge building industry in 1990. In shipyards, laser cutting machines started to be used in 1995, and because of the demand for thicker plate cutting, 6kW CO₂ laser systems were adopted by shipyards eventually.

A laser bevel cutting machine was developed in 1997 and this machine was installed in a shipyard in Italy. Y bevel, beveled edge with land (land is a U.S. term for the flat plate) by plasma was developed and started to be used in shipyards in 1997. Y bevel was accomplished by using two plasma torches on one cutting carriage where one torch makes straight cuts and the other cuts the bevel. Shipyards used two to four sets of these on one machine.

A 5kW gantry style CO₂ laser cutting machine was announced at the 2002 Japan International Welding Show and started to be used at actual shipbuilding sites from around 2006. In 2005, shipyards started to use specialized CNC ink-jet marking machines to print information on the whole plate with short operation times. This device was developed together with major shipyards in Japan.

Cutting in shipyards

The typical process flow in shipyards starts with marking the plate, then cutting the hull and the internal structure, and finally the ship-block construction. Plate printing systems, used in the marking process, include specialized machines to produce printing all over one large plate at one time. CNC oxy-fuel/plasma/laser cutting machines, flame planer machines, and portable cutting machines are used in the cutting process. Heating torch and portable welding machines are used in the assembly/welding process, and lift-lug cutters (used to hook and hoist large structures that are later removed) are used in the finishing process. Also, there are marking/cutting machines in use for angle-bar cutting to size or for cutting holes.

Among all the equipment used in the shipbuilding industry, CO₂ laser cutting machines are gaining a good reputation because they reduce labor, reduce welding cost/time, and reduce secondary processes after cutting. In the beginning, it was said laser cutting would find it difficult to be adopted by the shipyard industry because of the various and complicated requirements for bevel cutting. As a result, plasma cutting machines, fabrication machinery, and grinders are used in many cases. This situation has changed recently because of the development of various cutting procedures with a laser bevel torch, and its programming system. It was proven in shipyards that laser bevel cutting could realize high accuracy in high-value-added bevel cutting and could reduce the time and cost of the post-cutting processes, such as assembling and welding.

Benefits of laser cutting

The kerf width of a laser cut is about 1/8 that of a plasma cut, and the part dimension is at maximum +/- 1 mm of tolerance with less heat distortion. Therefore laser cut parts make it easier for the subsequent assembly and welding processes and reduce fitting adjustments and welding failure. Flux copper backing one side sub-arc welding is getting popular in shipyards, and the welding procedure requires high accuracy Y bevel size that is difficult to accomplish with plasma Y bevel cutting.

All Koike laser gantry style cutting machines run on a rail system. Therefore, they are able to cut large plate. The machines are also designed to cut multiple plates in one operation without changing nozzles or focus lenses, even when the plate thicknesses are different. This feature enables the machine to run unattended for long periods of time including night shifts, in a stable and safe condition. When the machine runs unattended it reduces labor cost compared to plasma cutting.

A most important factor in the laser cutting process is material quality, especially in plate where surface condition is important. When a plate is coated with black scale or rust it will cause unstable cut results, so end users are required to manage the plate quality to avoid this. Recently, high tensile plates have been used to reduce the total weight of products. This too can affect the cutting quality. The zinc rich prior coated plate (ZRP) used in shipyards has to have surface coating less than 20 microns. Surface coating greater than that will affect laser cut quality. The 6kW CO₂ laser cutting machine is designed to cut 30 mm thickness for a straight cut, however end users have to be aware that material and plate condition could affect the cut quality.

Laser bevel cutting

The requirements for laser bevel cutting in the shipyard are growing rapidly because of the lack of skilled operators and higher labor cost. There are many types of bevel cutting used in shipbuilding, such as V bevel cutting, Y bevel cutting with variable land width, CVBA (Continuously Varying Bevel Angle) cutting, pipe-hole cutting, and 3mm slit cutting for construction holes and water drain cutting. Koike has worked on developing each bevel cutting method to minimize the waste of time and machine motion. Cutting software development has been done in parallel to program each bevel cutting method, to avoid part dimensions being out of tolerance. For example, bridge cutting was included in the automatic programming process because two cutting passes are required in the Y bevel cutting with land. Koike created the required shape for laser bevel cutting with the capability of automatic programming. Many shipyards have appreciated this development and purchased several machines.

Future for laser cutting in shipyards

As described, complicated bevel shapes are now able to be cut automatically by pushing one command button on a laser cutting machine, compared to the days when skilled operators were needed to create these shapes. As a result, shipyards are able to achieve hourly labor reduction for beveling shapes and for secondary processes such as welding. Laser bevel cutting still requires unnecessary motions when piercing, at corners, and at positioning, because of the multi-axis requirements. The future plan is to make progress on developing a way to reduce these unnecessary motions and to improve productivity. ¿