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Considerations for designing parts for laser cutting

Edge requirements, pierce points, and material thickness are just some of the factors to examine

Figure 1
Piercing during laser cutting is a much more controlled process than it used to be, often leaving a clean surface. Older laser cutting technology delivered an explosion of molten metal that often left the surface area covered in splatter.

You might have noticed some co-workers with gray hair. It’s not surprising because baby boomers, the generation born between 1946 and 1964, have been a large part of the manufacturing landscape for most of the last 20 years.

That’s changing, however. As the oldest of the boomers are heading for retirement, a new generation of employees are joining the metal fabricating field, and they probably are having to learn as much as they can from the shop veterans before they make their final exit. Unfortunately, the industrial technology courses that helped to shape a generation of tinkerers and gearheads simply don’t exist today.

That’s why it’s important to revisit some basics of part design. In this case, the focus is on laser cutting. Even though laser cutting has been an integral part of metal fabricating for the last 20 years, it still might be new to novice sheet metal and plate fabricators. These design tips can help get them up to speed.

No. 1: The Laser Changes the Material

Anyone who uses a laser to process parts must realize that a high-intensity light source used to generate a laser beam is so hot that it melts metal in a millisecond. Therefore, any part processed with a laser is exposed to extreme heat and will have a heat-affected zone (HAZ) along the edge of the cut.

For almost all fabricators, this HAZ is a nonissue, but in certain fields, such as aeronautics, it can be a serious issue. Critical parts for these industries often are not laser-cut because the design engineers cannot take the risk that the metal will form microfissures that could pose a problem in the future.

No. 2: Watch for the Taper

All laser-processed parts have a taper of some degree because the focused laser beam is not perfectly straight, but rather is shaped like an hourglass. In thin material the taper is so minimal that it is not an issue. The taper starts to show in material 0.50 in. and thicker.

Fabricators concerned about taper should be aware that machine tool builders have developed new technology that improves the cut quality and speed, while reducing the amount of taper in thicker material. For example, a new nozzle technology is being used to funnel the cutting gas to the cut itself rather than allowing it to spread over the material. Also, beam modulation is being used to improve the cut edge quality.

No. 3: Be Aware of Bend Reliefs

One of the best advantages of using a laser to process sheet metal is that it can create just about any shape required. The only limitation is part size, although it is amazing how small laser-cut parts can be.

One consistent part design error is improper bend reliefs drawn into parts.

Often in thin-gauge material, these reliefs are simply too thin to achieve a good cut straight off the laser. The problem occurs because the assist gas travels the path of least resistance, which is the first segment of the path the laser takes.

As the laser travels back up the other side of the relief, the melted material is not ejected properly, which results in dross forming on the relief edge.

That dross gets in the way of the bend. The part must be redrawn so that the laser can process it correctly.

Figure 2
The part that was cut with oxygen as the laser cutting assist gas is marked by a black edge. The clean part was cut with nitrogen.

No. 4: Put the Pierce in Its Place

Piercing through material with a laser has greatly improved compared to how it worked in the early years (see Figure 1). Fabricators now can use various piercing methods to process their parts. These include fast piercing; slow, gentle piercing; and multi-stage piercing. The gentle and multistage pierces are designed to minimize the amount of splatter that can land on what will be a finished part.

Optical sensors are also being used to determine the moment that the laser has penetrated the material so that the cutting can begin. This feature helps to reduce the production time of parts.

Because a finished part’s appearance is often important, a laser programmer has to place the pierce point in an ideal location. One of the best places to put the pierce is in the middle of a small slug of material. Placing it there greatly reduces the risk of pierce splatter and the heat effect in thicker material.

What happens when the pierce is in the wrong place? Consider a pierce that is placed 1⁄4 in. away from what will be the hole edge in 1⁄2-in. mild steel. If the pierce point is placed close to the edge that will be cut, heat or splatter from the pierce could affect the quality of the cut or even cause the cut to be lost. By moving the pierce point away from the edge, the design engineer circumvents those issues.

No. 5: Will the Parts Be Powder-Coated?

If a part is to be powder-coated, a fabricator has to keep a few things in mind. The most important issue is what assist gas should be used to cut. If oxygen is used, a secondary process such as tumbling or an acid bath will be required to remove the oxide layer that forms on the edge of the part during laser cutting. If that oxide is not removed prior to powder coating, the cured powder finish eventually will flake off because it is attached to the oxide layer and not the metal itself.

To eliminate the need to remove the oxide layer, fabricators can cut with nitrogen as the assist gas because it yields a clean cut (see Figure 2). With nitrogen gas cutting, the more power the laser has, the thicker the material that it can process. Increased speed comes along with increased power. The operator should be aware of the power level of the machine to know the maximum thickness that can be cut cleanly.

Depending on the laser power, a fabricator also needs to consider the machine’s ability to process a material with the desired end results. If, for example, a company wants to purchase a laser to clean-cut 0.1875-in. steel that will be powder-coated, the company would want to analyze the differences in the finished part when using a 3-kW laser cutting machine as opposed to 4-kW equipment. The greater power level of the 4-kW machine may give them the results they want to achieve without nitrogen. The 3-kW machine may require the use of oxygen as the laser assist gas to obtain a dross-free part.

No. 6: Material Thickness May Not Matter Anymore

Flat sheet laser cutting machines are really making strides in their ability to process materials. Today’s solid-state laser cutting technology processes thin material very quickly. For instance, an 8-kW solid-state laser cutting machine can cut 0.039-in.-thick steel at a speed in excess of 2,000 inches per minute (IPM).

Even for thick plate, fabricating technology developers continue to increase the speed at which fabricators can cut. Using the latest solid-state cutting technology, fabricators can cut 1-in. mild steel at speeds greater than 35 IPM. For that material especially, the speed has improved compared to what was possible even just a few years ago. The cut edge quality also is improved greatly.

The latest laser cutting technology also can process up to 1-in. aluminum and 2-in. stainless steel (see Figure 3). For reflective material such as brass and copper, these lasers can cut up to 0.38 in. thick without beam reflection issues.

Figure 3
This assembly comprising varying material thicknesses represents the cutting capabilities of modern solid-state laser cutting technology.

Of course, these are some of the maximum thicknesses possible. In general, as the power level of the laser increases or decreases, so does the maximum thicknesses of the materials that can be cut on the machines.

No. 7: The Part Shape Affects Cutting Efficiency

Solid-state laser cutting technology has increased the machines’ cutting speeds and reliability, but it has not changed the way people think about designing parts. Whether cutting with a traditional CO2 laser or a solid-state laser, a fabricator should take the same considerations into account to increase the reliability of the cutting process and to get the best end result.

For example, if a part is engineered with 90-degree corners, the cut time can increase and the part quality decrease. Because the laser cutting head has to decelerate as it takes the sharp corner, it can overburn the corners, causing dross to form. It might even burn away the corners entirely. Generally, the larger the radius a design engineer is able to allow, the better. This enables a fabricator to increase the cutting speed and part quality.

About the Author

Kevin Fradette

Laser Applications Engineer

Farmington Industrial Park

Farmington, CT 06032

860-255-6000