How To Model Sheet Metal Knockouts in Autodesk Inventor

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Sheet metal design, especially for electrical enclosures, often incorporates cutouts for specific components, connectors, or fittings, but what happens when one of those isn't needed? A two inch hole in the side of your enclosure may normally house a power connector but when it doesn't, you don't want a big hole allowing dust and debris into your box.

Introducing the "knockout"! A knockout is essentially the same cutout but contains little tabs that hold the waste material in place. The tabs are sized so that they can be easily broken out when the opening is needed but large enough to hold the material in place during the manufacturing, plating, shipping, and assembly processes as well as normal use. Contacting your normal sheet metal vendor will help to determine what minimum tab size would be recommended as well as what the kerf thickness of the cutting process (laser, waterjet, etc.) is. Some online vendors, such as Send Cut Send, recommend a kerf of around 0.01" and a minimum tab size that varies for each material. Send Cut Send, for example, calls this the "Min Bridge Size". 

In this example, I'll use a kerf thickness of 0.05in and a tab width of 0.1" so that they're easy to see.

Step 1:

Create two user parameters: Kerf and Tab. As mentioned, these values will vary by vendor, material, and process.

Step 2:

Sketch the shape of the final cutout and offset it by the kerf thickness. Draw lines through the profile where you would like tabs to be located. The lines represent the center of each tab - the length is unimportant and the tab thickness does not need to be sketched.

Step 3:

Start the Grill command from the 3D Model tab.

In the first tab, "Boundary", select the outer edge of the knockout sketch. The boundary thickness can be any value but I typically set it to the kerf thickness. Setting it to a large value could add material that is not intended. Set the boundary height to the sheet metal thickness and the outside height to 0, as shown.

In the "Island" tab, select the inner edge of the knockout sketch as the profile and give it a thickness of 0.

In the "Rib" tab, select each of the lines where you would like tabs to be located. Set the tab thickness to the "Tab" parameter with a height equal to the sheet metal thickness and a top offset of 0. The remaining two tabs, "Spar" and "Draft" are not needed. Click OK to finish the Grill command.

The result:

The knockout on the left is with exaggerated values for kerf and tab whereas the example on the right has more realistic values. Many sheet metal fabricators can work with a step or iges file with features like this directly - no need to create a center line.