Laser-equipped machine tools are often used to cut parts from sheet metal and relatively thin plate. In such machine tools a laser beam, concentrated by a focusing lens or mirror to a small diameter spot, is directed to position the focal point above, on or below the surface of the material to be cut. The laser beam is directed from the focusing optic through a nozzle disposed immediately above the material workpiece with a pressurized gas being directed through the nozzle, typically coaxially with the laser beam, to assist making the cut. The pressurized gas interacts with the laser beam and material facilitating the cutting process, and creates a gas stream which carries the removed material away from the cut. The removed material consists of fumes, various size particles and drops of molten material some of which are small enough to remain airborne for some time. The amount of fumes created during metal cutting is usually small. More fumes are generated cutting non-metallic materials. Depending upon concentration of fumes in breathed air and the type of material cut, fumes can be a health hazard. Some fumes are poisonous.
Laser-equipped machine tools are Computer Numerically Controlled and are manufactured in many configurations and sizes and with lasers of various types and power. In one configuration, "flying optics", the cutting head is adapted for movement along one axis, such as the Y-axis which is mounted on a bridge adapted for movement in an orthogonal, X-axis. The work is supported on a stationary pallet or table below the bridge. Movement of the cutting head is coordinated with movement of the bridge to define a precise path on the part. The cutting head and laser are controlled to pierce and cut the metal to form holes and shapes in the material, and then to cut the part from the material. In this configuration the laser is mounted on the stationary machine base or on a separate floor mounted stand.
Many same or different parts of common thickness and material type may be cut from a sheet or plate. Such groups of parts are commonly referred to as a nest. Left over material, after the parts have been removed, is referred to as a remnant or a skeleton. A small remnant which falls from a hole cut in a part is called a slug. Remains of material from the cut is called slag. Resolidified material clinging to the part is called dross. The mixture of slugs and slag residue from cutting sheet material is generally called scrap.
When using laser-equipped cutting machine tools it is advantageous to utilize optics with different focal lengths to cut various thicknesses of material. The focal length of the optic contributes to the diameter of the focal spot and thus the energy density, Watts per unit area, at the focal spot. Shorter focal length optics create smaller focal spots having higher energy densities. The focal length of the optic also contributes to depth of focus of the focal spot with longer focal lengths having greater depth of focus. Shorter focal length optics are advantageous for cutting thinner materials while longer focal length optics are advantageous for cutting thicker material.
Various means for dealing with fumes have been utilized and in some cases the problem is ignored. Laser equipped machine tools are available which have no provisions for removing fumes generated by the cutting process. Usually these machines are utilized to cut low carbon steel. In these cases it is assumed that plant ventilation is adequate to prevent fume concentrations reaching hazardous levels.
Providing for efficient fume collection is not a simple problem. The material to be cut is supported on a worktable or pallet which has been sized for the maximum size part to be cut and designed to provide as much open area through the table as possible while providing adequate support for the workpiece and parts cut from it. Normally, a border remains around the workpiece on the worktable or pallet. This border provides space for work locators, clamps and sheet tolerances. Often the workpiece cut is smaller than the maximum size the table is designed to handle. As a result there is a lot of variation in the space around the workpiece through which fumes can escape. Also cutting the workpiece creates holes and open spaces through which fumes can escape. These conditions make it difficult to provide an efficient and reasonably sized and priced blower and filter system.
In some machines the cutting area has been enclosed from the sides by the design of the machine creating a trough. A large duct and blower is placed at the end of the trough creating an air flow under the part to collect the fumes. Because of factors described earlier, these systems are not very efficient fume collectors.
In other cases multiple ducts have been provided under the cutting area providing multiple fume collection points. Efficiency depends upon the number of collection points and the distance between the collection points and where the cut is made.
In other cases the cutting area, under the work support, is partitioned into several zones by sheetmetal and duct work, the duct work provided with valves which open the zone in which cutting is taking place. The other zones are left closed. This reduces the size of the collection area and allows use of a smaller blower and dust collector.
As lasers, with beam qualities suitable for cutting, are developed and become available in higher powered versions, machines are developed having the ability to cut greater thicknesses of material. Adapting high power lasers to cut thicker materials leads to using focusing lenses with longer focal lengths. Below the focal point, a laser beam expands at approximately the same rate that it was focused. For example, if a 35 mm diameter laser beam is focused by a lens with a 10" focal length, then, 10" below the focal point, unless absorbed by the material cut, the beam would be about 35 mm diameter again. Twenty inches below the focal point the beam would be about 70 mm diameter. This remnant, expanding beam from high power lasers has considerable capability to cause damage. For example, in an experiment a 0.125" thick aluminum plate was scuffed with steel slag, then a 38 mm diameter 5500 Watt beam was directed at the surface. The aluminum was cut through in 40 seconds. Similar tests were done with 0.25" thick stainless steel and carbon steel. Both were cut through in well under a minute. These tests indicated that a dust collection system, underlying the cutting area of a high power laser system, with long focal length optics in use, would be at considerable risk of being damaged by the remnant laser beam.
It would be advantageous to provide a dust collection system for a high power laser-equipped machine tool which can remove fumes produced by the cutting process and which is not at risk of being damaged by the high power laser beam.