Knockouts are common in sheet metal fabrication and are most frequently used in electrical enclosures as an access point to bring wires into the enclosure. The tab size, quantity and location are critical to determining the ease or difficulty of removing the knockout. Knockouts can be formed up or down with the direction determined by the finished product requirements and manufacturing method.
Precise, consistent stroke control in the press is very important for producing high-quality, consistent knockouts. Variations in the stroke of the press and changes in the material being punched will result in knockouts that are inconsistent in height and tab strength.
The single knockout punching process uses a single tool to create a slug and the tabs. It is common for the slug to be displaced slightly more than one material thickness to ensure it has been cut free from the sheet. The tabs are stretched and weakened when the slug is displaced. The ductility of the material, tab size, quantity and location all can affect the success of the desired result. For example, small tabs in thicker material may not be able to stretch enough to keep the slug attached to the sheet.
Double-round knockouts are also used in electrical enclosures as access points, but they bring the wires in through two different-sized openings on the same centerline location. The double-round knockout punching process uses a single tool to create a cut ring of material and the tabs. Multiple rings and specifically placed tabs can be formed with customized tooling. For electrical enclosures, double, triple, quad, and even quint knockouts are common, which allow different opening sizes to be selected, depending on the application.
It is common for knockouts to be pressed back into the sheet (planished) to create a closed feature. Although this flattening process will not press a knockout completely flat back into the sheet, it does prevent dust intrusion into the enclosure. Planishing introduces stresses into the sheet that may result in a slight bowing of the knockout and/or the surrounding sheet.