The origins of the three-axis head
Laser marking on non-flat surfaces is the Research and development project with which LASIT was founded in June 1990 and the company is proud to boast that it was the second in the world - after the American company General Scanning - to have built a three-axis head. From 1990 to 2000, more than 90% of our marking machines have been equipped with this technology, adapted to increasingly more applications and meeting the needs of different sectors.
The highest quality laser marking is carried out when the laser spot incises perfectly perpendicular to the plane, concentrating all the energy in a single point, which has a specific diameter and a variable tolerance rate.
However, not all components are flat or have regular shapes. This is why the three-axis technology was developed, guaranteeing high beam focusing on cylindrical, irregular or large surfaces without manually repositioning the laser. While in the past the focal length was fixed and invariable, today we can control and program 3D forms through the software.
High-speed marking and precision
In addition to offering high power, this system lets users select the appropriate laser irradiation method for the application. This contributes significantly to improving the quality of the marking and to shorten marking time, two important factors for production.
Mechanically, the LASIT three-axis head is composed of a system of linear motors, two X and Y rotaries that allow the laser beam to be moved along the axes, and a third axis for focusing: the laser beam passes through a photographic objective equipped with of movable lens, which in turn is mounted on a linear side-shifter. Operation is automatically regulated through the FlyCAD software.
The heart of the three-axis head is its optical design, which varies according to the type of application and the application requirements: the system can adapt to the size of the working field and to the size of the required laser spot.
The scanning mirrors are located after the lens of the photographic objective. The raw laser beam enters the optical system through the dynamic expansion lens. The photographic objective lenses re-draw the Gaussian Beam formed by the dynamic expander lens on the target plane. The movement of the dynamic expander lenses through the linear side-shifter varies the distance of the focal plane and therefore the dynamic focus. The mirrors (MX and MY) - which are located in the XY scanning module - bend the beam and direct it by angular deflection to scan the work plane.
When to use it and when not to?
Considering that a three-axis scanning head has a higher cost than the traditional two-axis system, it is important to understand when its use is actually worthwhile.
As mentioned above, the essential difference between the two systems relates to the different focal tolerance, or rather to the possibility of marking a detail which, due to its geometrical characteristics, is not always at the same distance of focus with respect to the edge of the scanning head.
Considering a 100x100 mm marking area, a three-axis head usually has a focusing tolerance of about 40mm, while the traditional head is limited to a tolerance between two 2mm and 6mm. It goes without saying that larger marking areas have a larger focusing tolerance.
While the first value (marking area) is only dependent on the design of the head (as it is capable of re-focusing based on the drawing), the second (focusing tolerance) is instead variable depending on some external factors, such as, in detail:
- The material being marked: materials such as steel can be marked with a defocus up to 5 or 6mm, while natural aluminum must be positioned at the exact focusing distance;
- The focal length used: a more or less large field is obtained based on this, varying in a range between 100 and 400mm, and which is chosen mainly on the basis of the required marking area, but not only.
The choice of focal length actually also depends on the type of processing to be carried out and on the tolerance considered necessary. In fact, a large focal length is also defined as “long,” or rather with a greater depth of field. In essence, the use of a long focal length often allows the problem of tolerance to be remedied.
Flat field lens system (two axes)
The flat field lens system, commonly referred to with the abbreviation FFL (Flat field lens), makes use of the optical properties of the lens to maintain constant focus on the work surface. It has the advantage of moving only small mirrors and therefore being very fast.
Three-axis hybrid head (dynamic Z)
The image shown in the photo is a three-axis system, in which the third axis is only used to vary the focal point. There is a lens mounted on the linear axis which allows the focus to be varied in combination with the next lens. Just after that is a galvanometric X and Y system to move the laser beam in the marking field, defined by the FFL lens. Without the movement of an external mechanical Z axis, we are able to mark non-flat (different heights or cylindrical) surfaces, speeding up the entire process with respect to the movement of the entire laser head.
Don’t be fooled
We have very often read and heard information on the three-axis head which have been disclosed for commercial reasons, but which lack technical validity. So let us try to shed light on what, on the contrary, is currently the potential of the three-axis head and the advantages we can derive from it, while highlighting the limits that have not yet been exceeded.
What it can do
- Maintain the focus on a non-flat surface
- Mark at different heights without moving the laser head
- Mark on cylindrical surfaces within certain optical limits
What it can’t do
- Mark on shaded surfaces
- Mark when the angle between the laser beam and the surface becomes small, for example on a cylinder beyond a certain limit
- Mark on a surface whose shape is unknown
- Correct errors in piece shape or placement: if the piece moves, the laser does not recognize it
- Correct deformation of the design, which requires additional software
Looking at the image, we realize that the real problem to consider is not the change in focus (which we can actually compensate with the three-axis head), but the angle of incidence.
In the example, we see that at 150° (±75°), the beam “slips” on the component and the energy does not reach the surface, making it impossible to carry out the process.
For example, if we have a 130 diameter cylinder, we will never be able to mark up to 150° because the focal variation is 48mm.
In practice, good results can still be achieved at an angle not exceeding 100° (± 50°).