MATERIAL THICKNESS MATTERS
By Brian Welz
To maximize laser cutting productivity but maintain process reliability, certain considerations beyond laser power must be employed to process thicker material effectively.
Thickness, by definition, is the dimension through an object, as opposed to length or width. This sounds simple enough. But in laser cutting, it is a significant factor. The thickness of a material will affect cutting speed, minimum contour size and the overall process consistency.
The goal of today’s laser users is to maximize their productivity, but maintain process reliability. In order to achieve these goals, certain considerations beyond laser power need to be employed to process thicker material effectively.
FIRST CONSIDERATION: THE PIERCING METHOD
The two most common methods are peck or slow pierce, and blast pierce. There are benefits to each. For example, the blast style pierce is a more aggressive style of pierce. It will provide the quickest overall process time, but will also produce a larger hole. The peck pierce is a controlled pierce which, in general, will take more time than a blast pierce, but will produce a much smaller pierce hole.
The peck pierce will, in most cases, be the desired method of producing holes equal to or less than material thickness in steels 5/8 in thick and less. In thicker material the key is to create a small pierce hole quickly, as a peck style pierce can take more time and potentially apply more heat to the area. This additional heat can, at times, affect the consistency of the hole quality.
If smaller holes are required, an effective method would be the use of a pre pierce. In this process, a blast-style pierce would be used along with a smaller nozzle orifice in order to create a pierce hole smaller than the desired hole. The entire sheet would be pierced and then the nozzle changed back to the proper size for cutting. This method would add some additional time, but it will ensure a very consistent process.
While pre piercing has proven to be a reliable method of producing smaller holes in thicker steels, a separate method should be considered when cutting thicker stainless or aluminum with nitrogen. In general, cutting with nitrogen requires significantly more gas pressure than oxygen and, as material thickness increases, the gas pressure requirement increases. These higher pressures can lead to reliability issues when cutting geometry equal to or less than material thickness.
However, a simple solution is to make an adjustment to the laser cutting parameters used for small contours. The first would be to reduce the cutting pressure and the second would be to increase the nozzle gap. The use of this technique will ensure a consistent and quality hole.
SECOND CONSIDERATION: THE START OF THE CUT
Whether the material is steel stainless or aluminum and the cutting gas is oxygen or nitrogen, the method used after the pierce is critical to reliability. The recommended method is the use of a slow lead in. This method employs two techniques: reduced speed and a higher nozzle standoff. The higher nozzle standoff will actually widen the kerf and ensure the cut gas reaches the bottom of the cut and aids in the coupling. This is especially effective in high pressure nitrogen applications. The additional process time is minimal, as in most cases this lead in is only 0.08 in to 0.12 in long.
MATERIAL QUALITY ALSO PLAYS A ROLE, ESPECIALLY IF THE MATERIAL HAS A LAYER OF SCALE ON THE SURFACE
This scale will have a significant impact on the cutting quality and consistency. During the cutting process this scale will affect the flow of assist gas to the cut, which can result in poor cut quality. In this circumstance, the scale could be removed prior to the material being placed on the machine and a light film of oil applied to the surface, or the part geometry can be etched on the surface of the material removing the scale from the material at this point. After the part is etched on the surface, it can then be cut normally. This will generally eliminate the issues of poor or inconsistent quality.
Effectively overcoming obstacles is a key factor in maximizing productivity and process reliability. This is especially important in thicker material. A solid understanding of a laser cutting machine’s capabilities, as well as putting in place processes which employ reliable practices, may initially look as if they will slow down the overall process. However, the reality is that attention to these kinds of details will result in a highly productive process.
About the Author: Brian Welz is the Applications and Showroom Manager for TRUMPF Inc., Farmington Industrial Park Farmington, CT 06032, 860-255-6112, Fax: 860-255-6424, www.us.trumpf.com. For more information, email him at firstname.lastname@example.org.