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Chapter 5-Designing for CNC Turret and Laser Fabrication

Laser Cutting

Current trends toward just-in-time (JIT) manufacturing, shorter part runs, and limited product life cycles have increased the use of laser cutting machines in production and prototype fabrication. Laser cutters are constantly evolving, as manufacturers find new and innovative ways to apply this basic technology.

Often the capabilities of lasers and turret presses can be combined. Turret presses are very fast and generate acceptable accuracy when punching many holes of the same or different diameters. Lasers are particularly accurate and economical for profiling irregular exterior contours.

These capabilities can be combined to produce accurate, complex parts at acceptable production rates by using each machine to perform that part of the cutting operation for which it is best qualified.

Examples include turret/laser cells which use the capabilities of both machines either separately or linked together, and combination machines which have both turret and laser cutting capabilities, either of which can be selected under computer numeric control.

There are applications where the laser outperforms any other manufacturing tool. Lasers require virtually no set-up time, no special tooling and, with the advent of CAD-CAM, very little engineering time. This means that a laser can be finished with a job before other machines are even set up.

It is not uncommon to produce a part from digital data (using CAD geometry) to finished blank in less than an hour. This provides a quick, smooth path from concept through pre-production to production with all changes during that evolution driven by software.

Some of the most advanced production lasers incorporate features such as automatic loading and unloading, the ability to move through three axis (which provides for the profiling of parts, holes and features after forming), and direct "down loading" of part programs to the laser CNC console from a CAD/CAM system. These features, when available, improve the versatility and enhance the economics of production laser use.

Figure 9. Typical construction of a laser cutting head.

Equipment Characteristics

The typical metal cutting laser consists of an evacuated container filled with CO₂, a high voltage system which excites the gas to emit single wavelength ("coherent") light and an optics system to focus and direct that light (see Figure 9). The optics system reduces the beam diameter to approximately 0.008 in. (0.2 mm) at the point where the beam meets the workpiece.

Several hundred watts of power, so fiercely concentrated, are sufficient to melt or vaporize most metals. The cutting action is enhanced through introduction of an inert shielding gas to blow away the vaporized metal (usually a nitrogen mixture), or an oxidizing gas (generally an oxygen mixture) to promote combustion of the metal.

Operation

Lasers can be operated in either the continuous wave (CW) or pulsed mode. CW operation is faster and generates a smoother edge. It is inherently less accurate because of thermal workpiece expansion due to the higher power levels reaching the work.

When there is a need for intricate or very close-tolerance cutting, the pulsed mode generates less heat but produces a very finely serrated edge. The finished quality of the workpiece is a carefully balanced compromise between speed, workpiece cooling and edge condition.

Lasers are most productive when applied to mild steel and stainless steel and are more difficult to employ on aluminum. Aluminum and certain other metals like zinc and lead continue to reflect light when molten. This scatters the beam, requiring more power. In addition, aluminum and copper alloys conduct heat away from the cutting area which, again, requires more power.

Table I. Laser cutting speeds in three materials under similar conditions. The noted differences in cutting speed can often influence material selection for a part which is designed to use the unique capabilities of lasers.

Table I gives a comparison of laser cutting speeds on three materials using the same machine, identically focused, at a power level of 1.5 kilowatts.

Other Considerations

In addition to production economics, precision and edge condition, the knowledgeable designer considers other characteristics of laser produced parts when designing for lasers:

• Localized Hardening. Lasers cut by melting or vaporizing metal. This can create problems when cutting heat treatable materials as the area around the part will become case hardened.

Laser cut holes in stainless steel or heat treatable steel alloys which require machining (tapping, countersinking or reaming) can be particularly troublesome. By the same token, designers can employ this characteristic to their benefit when a product must be case hardened for wear resistance.

• Edge Taper. The laser is most accurate where the coherent light beam enters the workpiece. As the beam penetrates the part, the light scatters creating an edge taper condition similar but opposite from "breakout" in a sheared or pierced part. (The hole on the side of the workpiece from which the laser beam exits is generally smaller in diameter than on the entrance side).

Thus the designer must carefully consider the final use of the part and, in some cases, may have to specify from which side the part should be cut.

• Minimum Through-Feature Size. The cutting laser beam is focused down to approximately 0.008 in. (0.2 mm) and is therefore capable of cutting holes and features with radii approximating 0.030 in. (0.76 mm). The limits applicable to piercing or blanking with a punch and die, such as the relationship between minimum hole size and material thickness, or the minimum distance between features to avoid distortion, do not apply when laser cutting.

However, some limitations do exist, and are also related to the material thickness. Table II is a guideline to the minimum through-features which are possible by laser. Laser cutting allows for through-features to be 1/6 to 1/8 the size when compared to die piercing.

Table II. Guide to minimum hole diameter and slot width achievable in various material thicknesses.

Also, since no mechanical force is applied, the width of material remaining between cutout features may be very narrow without distortion occurring during metal removal. A typical application would be tightly spaced venting slots on a visually important surface.

Advantages and Limitations

Laser cutting machines offer the capability of producing prototype and preproduction parts both quickly and inexpensively. No other fabrication machine can match the laser on these jobs.

As more powerful units become widely available, lasers are moving from production runs of less than 100 parts to runs of 1,000 or more. The use of lasers in combination with turret presses can expand this production horizon to several thousand pieces.

Good design often includes techniques such as "common line cutting" where the nested edges of two parts are cut simultaneously. Designers rely on the burr-free edge produced by a laser for certain production applications where burr removal is impractical or very costly.

Three-dimensional lasers, in particular, offer the designer the capability of producing a virtually burr-free hole or feature in a part on which the burr side may not be accessible for deburring.

Utilization of expensive materials such as titanium and monel can often approach 100% through nesting of odd profile parts on a common sheet. In addition, a blank need not be prepared for the laser. A small part can be profiled from a large sheet and the balance of the sheet stored for future use.

Dimensioning Practices

As a general rule, the drafting practices outlined for turret press fabrication can be applied to laser design. The designer will want to consider the economies in nesting, common line cutting, and the burr-free nature of laser parts.

It should also be recognized that the laser, like any other CNC servo driven machine, accumulates mechanical, thermal and electromechanical tolerances during the production cycle. For economy and quality, critical dimensions should be highlighted and functional dimensions should be detailed in accordance with their function.

The use of material cutting lasers offers designers the ability to generate intricate, close tolerance designs in any material which can be burned, melted, or vaporized including a variety of plastics, wood products, ceramics and textiles. Neither designers nor fabricators have fully explored the myriad uses for this state-of-the-art production equipment.

Excerpt taken from Design Guidelines for Metal Stampings and Fabrications -- 2nd Edition copyright © 1995 Precision Metalforming Association

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