Bridges & Tabs Explored

You may have heard the term Bridges & Tabs mentioned often enough during our many videos and blog posts. Let’s take a moment to explore exactly what they are and how they can be used effectively. Because CNC machining is a subtractive manufacturing process, stock material needs to be removed by the cutter until the resulting part’s shape is achieved. During this process, the remaining part needs to be fixed and stable on the bed of the CNC machine tool so that accuracy is maintained during the entire machining process.

Why are they Important?

While most CNC routers today have vacuum assisted tables to help with keeping the stock stable during operation, some parts may need additional stability due to their size or shape. Bridges & Tabs are simply areas where stock material by design, is allowed touch the part. These uncut areas form bridges between the part and the remaining stock adding to the rigidity and stability of the uncut part. Once machining is complete, the part is removed from the bed of the machine tool and the bridges are removed manually, allowing the part to separate from the uncut stock material. Once removed, the remaining Tabs left on the part are cut, grinded or sanded away manually or with a hand tool.

Bridges & Tab in 2½ Axis Profiling

In the Mill module, Bridges & Tabs can be defined from the Advanced Cut Parameters tab of the 2½ Axis Profiling operation dialog. When the operation is generated, they are added to the resulting toolpath automatically. From this dialog you can select the bridge type (Triangular or Rectangular) as well as the Bridge Height (H), Bridge Length (L) and the Number of Bridges or Distance between Bridges. The dialog and examples of each type are shown below.

Bridges/Tabs section of the 2 ½ Axis Profiling, Advanced Cut Parameters tab


If you prefer to have exact control over the placement of Bridges & Tabs in 2½ Axis Profiling, you can use the controls on the Regions tab of the Machining Objects Browser. Note that by default, this browser is located below the Machining Job tree. This is achieved by placing Bridge Points on a Curve Region that you have defined from the controls on this tab. The controls allow you to Add, Edit and/or Delete Bridge Points from a curve region. You can also Clone Bridge Points to all matching curve regions in your part! The Regions tab is shown below:

Bridge Point controls on the Regions tab of the Machining Objects Browser

 

2 Axis Examples of the use of Bridges & Tabs. Part courtesy of WorksbySolo.

Bridges & Tabs in 3 Axis Machining

3 Axis toolpaths are driven by your underlying 3D surface geometry, so to create Bridges & Tabs in 3 Axis, just add geometry to your part to represent the tabbed areas. These added surfaces will not be cut! The tab geometry can be simple blocks that pass through your part and extend outward past your Control Geometry. The illustrations below identify tab and control geometry in a 3 Axis part for a jewelry design application. The tabs are 3D rectangular bars. The control geometry is 2D curves.

  1. 3D tabs modeled as simple rectangular bars
  2. 2D outer stock boundary
  3. 2D containment boundary (for Roughing)
  4. 2D inner part offset boundary (for Finishing)
  5. 2D outer part offset boundary (for Finishing)
  6. 3D ring model
  7. 3D jig block

 

3 Axis example of tabs and containment regions.  Note that the tabs are simple rectangular bars and the containment regions are simple 2D curves.

Here we see a 3 Axis Radial toolpath being contained over the 3D tabs.

Here we see another example of 3D tabs.  Notice that the tabs extend past the 2D containment region.

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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