What is a Dual-Focus Lens?
A Dual-Focus lens has two focal points. The inner portion of a laser beam is intercepted by a section of the lens, of diameter ‘2η’, which has a longer focal length (FL) than the outer part of the lens. The ‘basic focal length’ is F. The difference in FL is dF (See figure 1.01)
Why use Dual-Focus lenses?
Dual-Focus lenses can have significant processing advantages for a variety of materials. A Dual-Focus lens can be designed and supplied as a direct ‘retro-fit’ replacement for all conventional lens types used in CO2 laser cutting. This users guide will show how to use the lenses to best advantage, and will indicate how some of the advantages arise.
Advantages of Dual-Focus Lenses
- 50-100% faster cutting
- Thicker materials may be cut
- Elimination of sub-surface dross
- Cost savings on assist gas
- Immediate cut initiation with no top surface dross
- Reduced sensitivity to errors in setting the focus
- Direct replacement of existing lenses
- Improved cut quality….
The Focus Settings
Discussion of focus settings in this section deals mainly with metal cutting applications. Other applications may require a different approach.
2.1 | Focus & Basic Principles in conventional processing of thick-section metals the (single)
lens focus is usually set some way into the metal. In laser-cutting with dual-focus lenses, the upper
focus is set near to the top surface of the metal, and the lower focus is set near to the lower surface of
the metal. This results in a high energy density both at the upper and lower workpiece surfaces. See
2.2 | The Nozzle Tip
If the basic FL of the Dual-Focus lens you have chosen is the same as the conventional lens used previously, then you will need to move the tip closer to the basic focus. In some cases this may require a new (longer) nozzle tip.
Note that this will usually mean that you can reduce the aperture
of the nozzle tip. (The larger the distance that you were focusing into the metal, the greater the
potential reduction in tip aperture). The reduced nozzle tip aperture will reduce assist gas usage. See