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Creating a Fishmouth Cut

When using tubing it is often desirable to use a fishmouth cut for correct fit and welding purposes. This can easily be accomplished by using a combination of configurations and part booleans. As with most things in Alibre, there are a variety of different techniques that can be used that essentially give you the same end results.

Step 1: Create the Main Tube

Start by making the Main Tube (Figure 1) – both extrusions used for this configuration are active. The first extrusion will be the external diameter circle, with a diameter of 1.00''. The second extrusion will be the internal diameter circle, with a diameter of 0.875''.

Figure 1: Regular Tube Configuration

Now make a solid tube configuration (Figure 2). For this configuration, the internal diameter (0.875'') is suppressed.

Figure 2: Figure 2: Solid Tube Configuration

Step 2: Create the Angled Tube (figure 3)

This part uses a Sweep instead of a linear Extrusion. This allows you to have the tube’s cylindrical dimensions and also specify the straight and curve sections of the path all in one feature.

Figure 3: Angled Tube Part

We'll use a sweep feature to create the angled tube. Figure 4 shows the two circles that make the tube, while Figure 5 is the path that will be used for the sweep (only a 2D sketch was used, but a 3D sketch could have also been used though this can be tricky especially when just learning).

Figure 4

Figure 5

Step 3: Using a Design Boolean to Create the Fishmouth

The design boolean is a feature that essentially adds a second part (or assembly) to the original part and either adds/subtracts material. In this example the Solid Pipe configuration is subtracted away from the Sweep feature; this creates the fishmouth. You may be asking yourself, why does the Main Tube have 2 configurations? When using the Boolean Subtract feature, only the material that intersects will be removed. Since the Main Tube is hollow, there will be extra material that is not cut away; this is demonstrated in Figures 6 and 7.

Figure 6: Regular Tube Configuration

Figure 7: Regular Tube Configuration

When the Solid Tube configuration is used then all of the material will be removed (figure 8).

Figure 8: Regular Tube Configuration

Step 5: Complete the Assembly

At this point, both of these parts can be inserted into an assembly and constrained together (Figure 9). The advantage of using a Part Boolean in this case is for a parametric design. If the Main Tube file changes from 1” to 1.25” the Angled Tube will have those changes reflected automatically.

Figure 9: Final Assembly