Tube, profile cutting with lightning speed

Laser cutting tube with a rotary axis

By Pieter Schwarzenbach

October 11, 2005

For more than 30 years, lasers have been used successfully for flat sheet cutting. Complex 3-D laser cutting is well-established in the automotive industry.

Even though laser cutting tubes and profiles is basically a 3-D task, some can be cut with a 2-D machine equipped with an additional rotary axis. With rectangular tubes and other profiles, however, laser cutting is substantially more complex: The X, Y, and rotary axes move simultaneously; the concave-shaped profile might collide with the cutting head when cutting perpendicular to the surface; and corners with small radii need special attention to prevent excessive wall thickening in the corner.

Additional laser cutting applications&mdash:the processing of tubes and profiles with lasers—are gaining increasing interest. Even though this is basically a 3-D job, it does not necessarily require a complex five-axis machine. Many tasks can be fulfilled with a 2-D machine equipped with an additional rotary axis.

For more than 30 years lasers have been used successfully for flat-sheet cutting. Complex, 3-D laser cutting is well-established in the automotive industry, not only for prototyping, but also for production when other processes are less cost-effective. Examples are cutouts in car bodies for nonstandard options (for example, air conditioner, sunroof, and right-hand steering) and trimming of hot-stamped structural parts (for example, A pillar, B pillar).

Other laser cutting applications, such as cutting tubes and profiles, are gaining increasing interest. Even though tube cutting is basically a 3-D job, it does not necessarily require a complex, five-axis machine. Many tasks can be performed with a 2-D machine equipped with an additional rotary axis.

Round Tube Cutting Comparable to Flat Cutting

Round tube cutting is the easiest in terms of actual machining and programming of the job. The cut geometry can be unrolled into the plane and programmed in the X-Y axis. Then the cutting head is positioned on the Y position of the rotary axis, and the cutting is done by converting the Y coordinates into the rotary angle:

              f = y / pD
where D = tube OD.

The laser beam always is perpendicular to the surface, and the cutting process is comparable to flat-sheet cutting.

For large tube diameters, the only limitations are the machine's Z-axis travel and the position of the rotary-axis chuck. The chuck normally allows feed-through loading of tube up to 6 inches in diameter.A lens with a short focal length is recommended, especially for small-diameter tubes (0.5 in. and smaller), and piercing power and time should be minimal to reduce the effects on the rear side of the tube.

Figure 1 To prevent a collision of a concave-shaped profile with the cutting head when cutting perpendicular to the surface, and to prevent excessive wall thickening, it is advisable to start rotating the tube before reaching the problem area.

A spray of antisplatter oil or a shield inside the tube can prevent slag from sticking. In most cases, such a shield needs to be water-cooled, because the integrated power from the laser beam and the molten material is substantial. As small tubes often are thin-walled, the material allows high cutting speeds, which might present a problem when the acceleration of the rotary axis reaches its limit. With the proper technique, however, small-diameter tubes can be cut with lasers.

Obviously, the contour required for a round hole through a round tube is not a simple circle; rather, it is the projection of the circle onto the surface of the base tube. It gets even more complicated if the wall thickness must be taken into account. The appropriate software, however, can determine the correct contour.

Rectangular Tube and Profile Cutting More Complex

With rectangular tubes and other profiles, the procedure gets substantially more complex. Several problems must be taken into account:

• The X, Y, and rotary axes move simultaneously.
• The concave-shaped profile might collide with the cutting head when cutting perpendicular to the surface.
• Clamping the profile may require special clamps.
• Corners with small radii need special attention, as cutting perpendicular to the surface may result in excessive wall thickening in the corner.

Figure 2 Sophisticated software that allows visualization of the profiles is critical for complex cuts and intersections.

Excessive wall thickening in the corner resulting from cutting perpendicular to the surface can be lessened by starting rotation of the tube before cutting reaches the problem area (see Figure 1).

Complex Contours and Holes in Thick Walls Require Software

Sophisticated software can calculate the complex contours to be cut in a profile (see Figure 2). With basic programming software, three steps are required to load the NC cutting program into the machine:

1. Choose a profile cross section or program a new one. This information will be used later to define the cutting parameters.
   
   
2. Arrange the tube or profile in 3-D space and choose the intersection type. The intersection is calculated automatically and displayed on the screen.
   
   
3. Nest parts for the profile and create the NC code. This step defines the number of different parts to be cut for the profile. The parts are nested automatically according to the profile length available. The cutting parameters, piercing procedures, and so forth, for optimal processing are chosen automatically according to the prior material specification, and the postprocessor creates the NC program.

Figure 3 A fit-through intersection requires cutting out of the nonintersecting tube or profile (A) only on one or both sides. A fit-in intersection may provide the most visually optimal fit, but missing material in the corners may cause welding challenges. A fit-on intersection requires a cut on the intersecting tube only, unless liquid must flow through the intersecting and nonintersecting tubes, in which case both tubes must be cut.

Three intersection types where profiles meet are fit-through, fit-in, and fit-on (seeFigure 3):

• Fit-through (cut-through). This intersection requires that a section of only the main tube or profile (A) be cut, on one or both sides.
  
  
• Fit-in (elbow joint). This intersection requires that sections of both the main and intersecting tubes (A and C) be cut. This type of intersection allows for a visually optimal fit, but there might be a welding disadvantage because of missing material in the corners. The contours are independent of wall thickness, as the touching lines are on the surfaces of the profiles.
  
  
• Fit-on (saddle joint, flow-through) — with or without hole. This intersection requires the intersecting tube (D) be cut to fit onto the nonintersecting tube. If fluid must flow through both tubes, then both must be cut. In both cases the wall thickness must be taken into account to calculate the proper contour to be cut out.

Wall Thickness Affects Accuracy

Tubes and profiles available on the market often have high straightness and dimension tolerances, including wall thickness. Welded tubes may not be free of stress, which after cutting may result in additional deformation. Also, the nonuniform heating from the laser beam may influence the part accuracy because of temporary thermal bending during the cutting process.

Figure 4 Laser machines equipped with 3-D cutting heads can bevel-cut tube and pipe, such as this 6-in.-dia., 0.5-in.-thick mild steel tube. They also may enhance the fit of intersecting parts.

These effects occasionally reveal unpleasant surprises, because laser technology is expected to produce high-accuracy parts. To obtain the expected accuracy, profiles with more consistent wall thickness may be required.

To avoid errors caused by irregular profiles, complex clamping tools and/or an adjustable steady rest and tailstock can force the profiles straight. More advanced machine concepts, however, allow for detection of the tube center and tube orientation before the cutting process. The CNC then adjusts the contour automatically to obtain the optimum accuracy required for the job.

Five-axis Machines Allow Bevel Cut, Better Fit

Laser machines equipped with 3-D cutting heads can provide further advantages in profile cutting, allowing for bevel cuts and better fit of intersecting parts (see Figure 4). Especially in the case of a fit-on intersection, the parts touch not only on the surface but also match throughout the entire wall thickness.

Nonperpendicular cutting, however, requires adjusting the cutting parameters. Not only does the cutting depth grow with 1/cos(j) (j being the angle of incidence), but more important, the deflection of the assist gas stream on the surface makes the cutting process more difficult with an increasing angle of incidence. That's why in CO₂ laser cutting, angles of incidence exceeding 45 degrees are not recommended.

The additional programming of the beam direction represents a minor complication when the programming software accommodates for this option.

The ability to perform specialty jobs, such as laser cutting tubes and profiles, represents a competitive advantage for a job shop to facilitate one-stop shopping for its customers. Last but not least, user-friendly programming software is crucial for the efficient realization of tube and profile cutting projects.

Dr. A. Pieter Schwarzenbach is vice president of laser technology with Prima North America Inc., 711 E. Main St., Chicopee, MA 01020, 413-598-5208, fax 413-598-5308, pschwarzenbach@prima-na.com, www.prima-na.com.