In a previous article we explained the types of lens that can be used in a laser cutting machine. The most common lens type used in a low power (<100w) laser cutter is the Plano convex lens.
These lenses are available in a variety of focal lengths to provide for a range of varying beam profiles and focal point sizes. When laser cutting sheet materials many people get confused about which focal length of lens to use.
A short focal length lens, like 1.5” for example, provides for a very wide beam profile with a very small focal point to deliver maximum energy density. For laser cutting applications this lens is best used for thin materials such as laser cutting paper, laser cutting card and laser cutting wood veneer because the smaller focal point provides for minimal kerf; kerf is the width of the cut.
When thin materials retain heat they are susceptible to warping, which causes a defocus leading to deteriorated output quality and material waste. The smaller focal point of the 1.5” lens delivers less heat into the edge of thin materials so the 1.5” lens can help to minimise warping.
A long focal length lens, such as one with a 4” focal length, provides for a narrower beam profile but a larger focal point. When laser cutting thick materials the profile of the beam is the most important factor because a narrower profile will be more consistent in width as it passes through a material of greater thickness.
As a very rough guide, you can use a simple formula to choose the appropriate lens for the thickness of material you want to cut as follows:
Focal length of lens in inches x n = max material thickness in mm where n is 2 for the shortest focal length and for each lens thereafter is n+1.
For example:
A lens with focal length 1.5” x2 = Change to a longer focal length after the material is 3mm thickness
A lens with focal length 2” x3 = Change to a longer focal length after the material is 6mm thickness
A lens with focal length 2.5” x4 = Change to a longer focal length after the material is 10mm thickness
A lens with focal length 4” x5 = Change to a longer focal length after the material is 20mm thickness
No matter what the lens used, considerations with regard to available maximum power and the wave guiding properties of the material must be considered too. For example, it’s generally not viable to use a 4” focal length lens with a 10w laser.
At Lotus Laser Systems we manufacture a wide range of laser cutting, laser marking and laser engraving solutions. We pride ourselves on the quality and depth of knowledge of our support staff. Our experts would be happy to advise you on this or any other aspect of your laser system operation.
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