3D CAM for ruled surface cutting

In my previous article “Waterjet cutting in the era of intelligent manufacturing - Part I Soft cutting tool controller”, I have shared my insight of a soft cutting tool controller. This soft cutting tool controller solves the problem of geometrical errors caused by the deformation of the cutting tool. In another word, it deals with the issues of a cutting tool that is supposed to have the form of a straight line, but in fact, it is not so straight. In this article, other issues of cutting programming of line-form cutting tools will be addressed.

I will start the topic with an example. Figure 1 shows the model of a portion of an aircraft engine part. The blades are machined from a forged blank of titanium alloy. Because it is extremely costly and time-consuming for traditional CNC machines to do all the machining work, a better solution is to use waterjet cutting to do the coarse machining, and then use either the traditional CNC machining or electrochemical machining to finish the part to the required geometry and precision. However, waterjet is a kind of line-form cutting tool. Theoretically, it can only cut ruled surfaces. By definition, a ruled surface is a surface generated by a moving straight line with the result that through every point on the surface a line can be drawn lying wholly in the surface. If you look close enough, you should discover the blade surfaces shown in Figure 1 are free-form surfaces, instead of ruled surfaces. In order to use a 3D CAM software product to do the programming for waterjet cutting, the blade has to be modified to be one with ruled surfaces, like the one blade in the middle of Figure 1. It is a great challenge for an engineer to come up with a model of the modified blade that should envelopes the original blade and should have minimal errors in order to reduce the amount of work for the subsequent machining process. Would it be wonderful that the 3D CAM software automatically comes up with this enveloping model with ruled surfaces? Yes, it is. So here I will introduce a 3D CAM software product that can do this.

Figure 1 The 3D model of a portion of an aircraft engine part

Waterjet is not the only line-form cutting tool. Laser, plasma arc, and wire EDM also belongs to the family of line-form cutting tools. They share the following common characteristics:

The cutting tool has the form of a fish-line or the similar. An obvious example is Wire EDM because it uses an electrical wire as the cutting tool. For waterjet, laser, and plasma arc cutting, the cutting tool is actually a high energy beam that takes the line form.

The cutting process takes the form of profile contouring. The cutting tool generates a kerf in the material as it moves along a cutting path. The amount of material removal is minimal.

The cutting tool has to go all the way through the workpiece. No half way cutting. No countersink machining.

The cutting is usually one pass only. It is not like milling process that normally requires multiple passes. Therefore, the cutting quality and accuracy from a line-form cutting tool cannot be modified with the same cutting tool in multiple passes.

Lead-in and lead-out cutting at the entry and exit of the contour is necessary. The cutting tool needs to penetrate the workpiece prior to cutting the contour. The penetration is done either with the cutting tool itself (e.g. all the high energy beam cutting processes), or with a different penetration tool (e.g. wire EDM). The hole generated in the penetration normally has a bigger diameter than the cutting tool itself and therefore it cannot be generated on the cutting path. Instead, the penetration hole is usually placed outside the part and nearby the cutting path and then the cutting tool leads its way into the contour from the penetration hole. Lead-out is just the opposite of lead-in.

The cutting tool is a soft one and it will deform during the cutting, in contradiction to the rigid cutting tool (e.g. milling cutter). All the high energy beam cutting tools bend backward at the bottom of workpiece and the cut kerf typically has a taper (e.g., the kerf width decreasing with the thickness). The wire in wire EDM cutting bows itself backward in the middle of the workpiece thickness. The deformation of the soft cutting tools causes all sorts of geometrical errors in the workpiece.

The main stream 3D CAM software products are focused on traditional CNC machining, especially milling and turning. Little to no attention has been given to the above line-form cutting tools. Therefore, if you use a main stream product of 3D CAM software to do programming for line-form cutting tools, you will find that these 3D CAM products cannot do the job properly. They are lack of solution for ruled surface cutting when the 3D model of the part includes surfaces that are not ruled surfaces. They are definitely lack of solution for the issues caused by the deformation of the soft cutting tools. They may have many advanced features for traditional CNC machining, but these features may be useless for line-form cutting tools and even become the burden of extra cost.

Here comes a 3D CAM software product that is designed for line-form cutting tools as well as soft cutting tools. It solves the above problems and has more advantages to offer.

Let us see how it solves the programming problem when the 3D model of the part includes surfaces that are not ruled surfaces. The software automatically checks the cutting surfaces against the criteria of a ruled surface. If it does not meet the criteria, the software will automatically generate ruled surfaces and build them into the cutting program. Users can check for errors between the generated ruled surfaces and the original surfaces. For the same engine blade shown in Figure 1, this 3D CAM software automatically generate two ruled surfaces (in yellow) that sandwich the blade (see Figure 2). The errors between the ruled surfaces and the original surfaces are shown in Figure 3.

Figure 2 Ruled surfaces (in yellow) are automatically generated

Figure 3 Errors between the ruled surfaces and the original surfaces

Another example of ruled surface cutting is the round-top-squired-bottom part shown in Figure 4. This 3D CAM software successfully generates the ruled surfaces (in yellow) for this part.

Figure 4 Programing for cutting a round-top-squired-bottom part

Because lead-in/out is necessary for line-form cutting tools, this software will automatically add lead-in/out features, and automatically generates a 3D cutting path. In the cutting path, internal contours will be cut prior to external contour so that the part will not move around before the cutting program is completed. The cutting path also keeps an offset distance (e.g., radius of the cutting tool) from the part surface so that the part size will not be affected by the cutting tool diameter. This 3D cutting path is composed of rapid traverses, lead-ins/outs, and all the cutting surfaces, just like what is shown in Figure 5.

Figure 5 A cutting path is automatically generated

The software allows cutting a pattern on a part with the cutting tool always perpendicular to the top surface of the part, e.g., cutting a pentagram on a round tube with the cutting tool (represented with red arrows) perpendicular to the cylindrical surface (Figure 6). The software also allows cutting a pattern on a part with the cutting tool always parallel to a certain axis, e.g., cutting a circular through hole with the cutting tool (represented with red arrows) always parallel to the intersecting axis (in blue) of a pipe joint (Figure7).

Figure 6 A pentagram cut on a round tube

Figure 7 A circular through hole is cut on a pipe joint

In some situations, the part geometry requires cutting of multiple passes. For example, the part shown in Figure 8 has these irregular holes. On one side of the hole, the cutting surfaces are composed of an upper surface and a lower surface. After the lower surface is cut in the first pass, the upper surface can be cut in the second pass, as illustrated in the diagram to the right of Figure 8. This software makes it feasible and simple for this kind of multiple pass programming.

Figure 8 This irregular hole requires cutting of multiple passes

To further simplify the programming, the cutting program for a single hole shown in Figure 8 can be automatically duplicated for the complete circular array of holes on the whole part. Linear array duplication of a cutting program is also possible, as shown in Figure 9.

Figure 9 Duplication of cutting program into a linear array is possible

This software allows user to assign a certain quality level (e.g. 1 to 5 for waterjet cutting) to each of the cutting surfaces. Then the cutting model that is built into the software suite will be used, based on inputs of workpiece material, quality level, and the jet parameters, etc., to calculate the optimal cutting speed as well as compensation angles (for correcting taper and other geometrical errors). The optimal cutting speed and compensation angles are then programmed into the final CNC program.

Once the programming is done, the program can be played out in the software to check for accuracy and possible collision. If necessary the program can be modified. Prior to cutting, the program should be played out on the machine physically but without turning on the cutting tool (e.g. waterjet). It is not abnormal that the cutting program needs some fine tuning because the actual workpiece is somewhat different from the 3D model. This software provides a tool for making batch adjustment of a series of programming points on the cutting path conveniently. For example, here is one practical application of this tool. In Figure 1, we can see that the blades are below the top surface of the blank of material. If the programming is done on the blades or the ruled surfaces generated, the cutting tool is going to collide with the top surface of workpiece. This problem is solved by using this software tool to align all the points on the cutting path with the top surface (see Figure 10).

Figure 10 All the points on the cutting path are aligned with the top surface

The applications of this ruled surface cutting software are continuously expanding. So is the need for continuous development and improvement of this software product. This software is originally developed for 3D waterjet cutting. But it can be also used for other line-form cutting tools (laser, plasma arc, wire EDM, etc.) with some developments.

This article addresses the topic of ruled surface cutting with line-form cutting tools. Due to the limitation of the author, this article is not going to answer all the questions. Neither will the 3D CAM software introduced here. The author wishes that more people will be interested in this topic and their visions and insights can be heard.

Again the author sincerely welcomes feedback, corrections, and discussions. Feedback can be received at my email address:  zengjiyue@lionstek.com.