Laser Applications Newsletter
2 November 1997
Laser Kinetics Inc.
Mtn. View, CA 94041
Issue 2 , Vol 1



INTRODUCTION

As promised in our first newsletter, here's the second one. Opting for simplicity, I am calling it the second Laser Kinetics newsletter. Since the initial issue, I had the pleasure of presenting "Introduction to Laser Materials Processing" in Milpitas, CA. This was a rather ambitious new course where I tried to cover the entire subject of laser materials processing in six and a half hours. My options were either to talk very fast or to cut the subject to its bare bones and present its skeleton. Choosing the latter, I was helped greatly by the inputs of those in attendance to expand the presentation in the areas of the greatest interest. One such area was precision laser cutting, as is used for manufacturing medical devices. The requirements for these parts are very tight, requiring the producers to have a very good understanding of the factors affecting cut quality and dimensional variation. These demanding and sophisticated end users are putting laser manufacturers on notice to make more stable and consistent products. I suspect that diode-pumped Nd:YAG lasers will be the tool of choice for the most critical work. While these lasers are more expensive than traditional lamp-pumped units, they are far more stable and repeatable. Their generally better beam quality is also a help.

Another major group of students was from the sheet metal industry. Laser cutters have become standard equipment in most large sheet metal job shops, and their owners want to get the most out of them. The sheet metal industry is highly competitive, so it's important to know exactly what a laser can do and how much it costs to do it. The manufacturers of this equipment do, in general, an excellent job of training, but everyone is trying to get an extra edge.

There was, surprisingly, little interest in welding. I know that there's a lot of laser welding going on; it must be so easy that no one has problems with it.


LASER PROCESSING TIPS

Gas-assisted cutting is one of the most common industrial laser applications. Standard systems, provided by many suppliers, are used to cut materials including paper, alumina, plastic and steel. There are several important steps in setting up for laser cutting, and disappointing results will be obtained unless they are performed correctly.

A laser cutting system typically comprises a laser source (either CO2 or Nd:YAG), a beam delivery system, a focusing lens in a gas jet nozzle and a motion system. We will not, at this time, examine the issues involved in getting the beam from the laser to the focusing lens. These vary greatly with system design and deserve a discussion of their own. We will start, then, with the assumption that the laser beam is going through the center of the focusing lens, and that it is not clipping on any objects or is otherwise distorted. If this is not the case, it has to be corrected before proceeding any further.

Cutting Tip no. 1

Finding Focus
Consistent results in cutting depend on maintaining the relation between the workpiece surface and the focal point of the beam. Since lenses vary, and the wavefront curvature of the laser beam may change, this focal point is not a fixed distance from the lens. It is therefore necessary to determine this distance experimentally. There are many variables that can change the focal point.

1. Laser beam properties change with output power and the age of the internal optics.

2. For any laser beam, the focal point of a lens will vary at different locations along the beam.

3. Lenses shift focus with age, surface condition and incident power.

Because of these variables, focus should be determined at the power level and beam path location of greatest interest. For critical applications, it may be necessary to map the focal position as a function of laser power and path length.

Methods for finding focus

Lasers used in cutting systems may be either Nd:YAG or CO2, and they may be either CW or pulsed. I prefer different methods of finding focus for each of these situations, all intended to locate the waist (the narrowest spot) of the focused beam.

Pulsed CO2: Set the laser up to produce short-duration pulses on a moving sheet of paper. Make a series of runs at various lens-to-paper distances. Examine the diameter of the perforations. The focal distance it that where the diameter is smallest.

CW CO2: If rapid motion can be achieved, make slits in paper. If this cannot be done, make slits in thin (about 0.3 mm) steel. Vary lens-to-object distance and measure to find the narrowest kerf.

Pulsed Nd:YAG: I generally use weld testing for pulsed YAG lasers. Make a series of spot welds on stainless steel over a range of focal distances. Again, for systems with "hard" optics, the laser should be run at the process power level. At focus, the spot will be smallest and have the most ejected material.

CW Nd:YAG: Nd:YAG lasers have large changes in beam divergence as the pump power is varied. If the laser has "hard" optics (not fiber-coupled), it is important to operate it at the power level of interest. At this level, cut thin steel as for CW CO2

Cutting tip no. 2

Nozzle centering
Consistent multi-directional cutting with a coaxial gas jet nozzle requires that the beam be concentric with the nozzle opening. For oxygen-assisted cutting of steel, the concentricity should be within 50 microns. Nozzle centering is done after determining focus and setting the nozzle clearance, and is something to be checked regularly.

First, it is necessary to have some means of adjustment. Most cutting heads attach the nozzle to a plate that can be translated on set screws. Others move the lens. If a cutting head lacks concentricity adjustment, it is necessary to alter the incoming beam alignment to achieve concentricity. This is, in general, possible but awkward.

To examine nozzle/beam concentricity, set a piece of thin (0.3 mm) steel under the nozzle at the focal point of the lens. Send oxygen assist gas at about 50 kPa through the nozzle and pierce a hole. If possible, observe the piercing event (use safety goggles appropriate to the wavelength of the laser being used). An inadequately centered nozzle will eject material preferentially in one direction. For heads which move the nozzle, move it in the direction of the ejected material. For heads which move the lens, move it away from the ejected material. Repeat until material is ejected uniformly in all directions around the nozzle.

If it is not possible to get uniform spatter, the laser mode may be asymmetrical, the nozzle may be damaged, or the beam may be misaligned going into the lens. These conditions must be corrected before high quality cutting is possible.

Call 650 575-4919 or e-mail us for more information.