To understand both the advantages and disadvantages of using lasers to cut and drill into very hard materials, it is advantageous to examine the accompanying diagram; it illustrates several important properties of a cutting laser.
Cutting Metal With Laser
First, the laser is a noncontact instrument. This makes it ideal for cutting material that is fragile or easily damaged, and if the material is very hard, the laser will not dull. It can also cut patterns that are too intricate for a mechanical blade to cut.
Next, notice in the illustration that although the light rays emitted by the laser are initially parallel, they are passed through a lens, the purpose of which is to bring those rays to a focus. The location in space at which all the rays converge is called the focal point, and it is here that the best cutting occurs. In an industrial laser, cutting may occur away from the focal point as well because the laser beam already has a very high energy density, meaning that it delivers a great deal of power per unit area. The beam is often powerful enough that the mirrors used in many cutting lasers must be water cooled.
Nevertheless, by bringing all the rays to a focus, one can create an even higher energy density and so cut faster and more precisely than would otherwise be possible. Now take a closer look at the geometry of the laser beam in the diagram. After passing through the lens, the laser beam takes on the shape of a conic, a geometric object that has the shape of two ice-cream cones joined at their apexes.
As described, this conical geometry enables the designer to obtain a higher energy density at the focal point for the same amount of laser energy consumption, but it also has the effect of limiting the depth at which the laser cuts. After passing through the focal point, the light cone becomes ever wider, with the result that the energy density far from the focal point is quite low. As the energy density diminishes so does the laser’s ability to cut.
One can compensate for this effect to some extent by using a lens with a long focal length. (The focal length is the distance from focal point to the lens.) A longer focal length makes the sides of the conic steeper and so keeps the light rays closer together longer. Despite this technique, cutting lasers are usually used on relatively thin materials. Recall that cutting lasers actually vaporize rather than cut the target material. In other words, they cause a phase change: The solid material is turned into a vapor. Some of this vapor will condense on the target material, both on the upper and lower side of the cut, and make for a slightly rough edge. Some of the vaporized material also floats in the air above the cut.
Airborne vapor can absorb energy from the laser, or reflect it away, before the light reaches the target. The result is a degraded laser beam and less efficient cutting. One way of minimizing this effect is to blow the vapor away from the cutting zone, where it can be evacuated by exhaust fans. Engineers have found, however, that if they use oxygen to blow away the vapor produced by the laser they can speed the cutting rates by causing the metal at the focal point to combust in a controlled fashion. Visually, this makes for some spectacular sparks. Lasers are also used as drills. Just as for cutting lasers, laser drills vaporize material rather than drill through it mechanically.
Laser drills are based on essentially the same concepts as laser cutters, and the diagram used to describe the principles by which lasers cut also applies to how they can be used to drill. In particular, by using a lens to focus a laser beam for drilling, one obtains a conical rather than cylindrical hole. As with laser cutting tools, the conical shape of the resulting hole can be minimized by using a long focal length lens, but that simply reduces the effect; it does not eliminate it. And there is another effect to consider: Some of the vapor produced by the process condenses and solidifies around the edge of the region that was vaporized. The result is a crater-shaped hole that, depending on the application, may need some mechanical finishing. Drilling large holes is more problematic because using a laser to create a large hole requires a great deal of energy, and in the process of drilling the hole large amounts of vapor are produced. The production of the vapor can interfere with further drilling by absorbing or reflecting the laser energy. For these reasons, most large holes are still drilled mechanically. While there are significant limitations to the use of lasers as drills, and not every drilling task is suitable for a laser, lasers are widely used in certain applications.
One application in which they excel involves drilling very small holes. This is one application that is unsuitable for mechanical drills. Very small bits become dull quickly and break easily. By contrast, lasers, because they are noncontact tools, never become dull or break. (The mirrors and lenses must, however, be protected and kept clean.) Provided that the material is not too thick, therefore, lasers can be used to drill small holes quickly. One of the classic applications of laser drills involves drilling holes in diamonds. Diamonds, renowned for their hardness, are used in the process of forming wires.
Metal is forced through the hole in the diamond to form a wire. The holes are generally quite small, and drilling a small hole through one of the hardest substances in the world with a traditional drill bit is difficult, expensive, and time consuming. This is an ideal application for the laser drill, which vaporizes, rather than cuts, its way through the diamond in a minute rather than in a day. The hole still needs to be smoothed and polished because of the way that the laser created it, but most of the work in drilling diamonds is done by the laser.