Understanding Cut Levels in 2½ Axis Machining

In 2½ Axis machining, machining is performed in multiple Z levels, where the cutter moves in both the X and Y axes while the Z depth is fixed at each of these Z levels. This fixed Z depth is maintained until all of the stock material is cleared for that level. The cutter then moves down in Z and begins clearing the next XY level. One of the benefits to 2½ Axis machining is that you do not need a 3D model. A 2D drawing works just fine. An obvious limitation is that you can only machine prismatic parts, where all of the side walls are vertical. For many applications however, this limitation is not only acceptable but absolutely required. As you may imagine, an understanding of how cut levels are controlled for 2½ Axis machining in MecSoft’s CAM plugins is essential. In this post we will discuss the toolpath operations that share the majority of the same Cut Levels tab options. These include 2½ Axis Facing, Pocketing, Profiling, V-Carve Roughing and Slotting. If you look at the upper image in the dialog you will see that the Location of Cut Geometry and Cut Depth Control param eters are illustrated. To help you understand these controls we will use the pocket examples shown below.

Location of Cut Geometry

Cut Geometry refers to the Part Regions in the Control Geometry tab that you will select to define the cut pattern. These are typically 2D curve geometry and if you have a 3D model might include edge curves or flat area selections. The three options in the dialog At Top, At Bottom and Pick Top are best explained for any instance by simply asking yourself this basic question: “Where is my cut drive geometry located in relation to the starting Z level?”
Typical 2½ Axis Pocket with Location of Cut Geometry At Top
Typical 2½ Axis Pocket with Location of Cut Geometry At Bottom
In the pockets shown here you can clearly see that the selected cut geometry is located at the bottom of the pocket, so At Bottom is selected from the dialog. This means that the Cut Depth Controls in the dialog are measured from this location upward in Z. In a different part model, it might be convenient to select the curves located at the top edge of the pocket. In such cases the At Top parameter would need to be selected. Let’s continue this example and have a look at the other parameters in the dialog. We will discuss the Pick Top option further down in this post.

Cut Depth Control

The Cut Depth Control parameters allow you precise control of the cutter’s Z depth at each cut level in the pocket. For example, Total Cut Depth will be total amount of Z travel you want when performing the machining. This is an absolute value in the dialog (i.e., no positive or negative). You can enter this depth value directly or use the Pick button to select two points from your 3D part model and the depth is calculated and added to the dialog for you.
Rough and Finish Cut Depths
Rough & Finish Cut Depth Controls
If desired, you can divide this total cut depth into a Rough Depth and a Finish Depth by entering the values or using the sliders provided. You can go even further by entering the depth per cut for each of the rough and finish depths. In our example here, we have the Total Cut Depth set to 1.25. This depth is divided into a Rough Depth of 1 and a Finish Depth of 0.25. The Rough Depth/Cut is set to 0.25 and the Finish Depth/Cut is set to 0.05. You can see in the illustration above that these values divide the rough depth into 4 levels (each 0.25) and the finish depth is divided into 5 levels (each 0.05). Keep reading and we will also discuss the Clear Island Tops option and the Use 3D Model to Detect Depth option.

Using the Pick Top Option

There may be situations when the cut geometry available for selection is neither located at the top nor at the bottom of your desired pocket. For example, you are machining from a 2D drawing and the geometry of the drawing is aligned with the bottom of your stock. You may want the depth of the pocket to be measured from the top of the stock or you may want to start cutting slightly above the stock. Have a look at the illustration and dialog below.
2½ Axis Pocketing from 2D Geometry using Pick Top
This cross-section diagram illustrates how the Pick Top option works. In this example there is no 3D model. The Control Geometry is a 2D drawing located on the XY plane at Z0 (zero). You are looking at the resulting stock after the pocket is cut. Notice that the Stock Height and the Pick Top value are the same (0.5).
2½ Axis Pocketing Cut Depth Control using Pick Top
When we defined the stock, we set the H dimension in the Box Stock dialog (shown on the left) to 0.5. By setting Pick Top to 0.5, the cut starts at the top of the stock. You can also think of this as moving the cut geometry up to this Z location. The Total Cut Depth is set to 0.25 and the Rough Depth/Cut is set to 0.05. This produces the 5 cut levels that you see in the cross-section above. Let’s take this one step further and say for example that the bottom of your stock is nice and flat but the top is not. You could then set the Pick Top value to begin the cut slightly above the stock, say at 0.6. As you can see the Cut Levels tab provides precise control of your cutting.

Clear Island Tops

There may be situations when there are one or more islands located within the pocket. Island refers to material that protrudes up from the floor of the pocket and whose sides do not touch the sides of the pocket. When you check the option to Clear Island Tops, an additional cut level is located at the top of the island. If there are more than one and they are at different heights an additional cut level is added at each island top. It is important to note that if you have selected curves as your cut geometry, the heights of the curves will be honored by the system. This feature allows you to select curves to define island tops in multiple depths as in the model shown here.
Using the Clear Island Tops Option

 Use 3D Model to Detect Depth

As the name of this option implies, checking this box will allow the CAM system to determine the depth of the pocket for you based on your 3D part model. You see that with this option checked, the Total Cut Depth and Rough Depth fields in the dialog become inactive. The Finish Depth is automatically measured from the bottom of the pocket and both Depth/Cut controls are still available. Note that a 3D part model is required to use this option. This option can be very useful when loading a knowledge base for machining automation with various 3D parts. If this option is set in the Knowledge Base operation, there is no need for the user to manually specify the total cut depth depending on the loaded part. It will be automatically computed from the current 3D model.
Cut Controls with Use 3D Model to Detect Depth Enabled

Cut Depth Controls when Machining a Feature

When MecSoft CAM is machining a detected feature, such as the pocket in the above example, the Cut Depth Controls on the Cut Levels tab are adjusted to match the feature. For example, the minimum depth required to machine the feature is displayed in the dialog and cannot be changed. You can however, adjust the Rough Depth/Cut, and specify a Total Finish Depth and a Finish Depth/Cut If desired. The dialog below shows the values that will cut the same pocket shown in the example above.
Cut Depth Control when machining a feature
 The controls discussed in this post apply to the following 2½ Axis Milling toolpath strategies in VisualCAD/CAM (VisualMILL), VisualCAM-MILL for SOLIDWORKS and AlibreCAM-MILL:
  • 2½ Axis Facing
  • 2½ Axis Pocketing
  • 2½ Axis Profiling
  • 2½ Axis Slotting
  • 2½ Axis V-Carve Roughing
Don LaCourse

Don LaCourse

Don LaCourse is an Application Engineer with MecSoft Corporation. Don brings over 20 years of experience in CAD/CAM operations in both automotive and mold design applications. Don also has extensive experience in documenting CAD/CAM products and is actively involved with writing the on-line help as well as creating training tutorials for MecSoft's products.
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