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Cool Tools: Knockout Tools

By Jeff Griffin | Jul 15, 2015
Greenlee knockouts.jpg

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Knockout tools are widely accepted as the best method of punching conduit-size holes in steel electrical panels, boxes and other steel surfaces. While hole saws and step bits on conventional drills and carbide cutters also can cut metal, none can produce a hole as precisely and cleanly as a knockout tool.


Basic knockout tool components are punch and die, draw stud and the mechanism to pull the stud—a hand wrench, a hand-powered hydraulic pump or electrically powered hydraulics. 


Standard hole sizes range from ½-inch to 6 inches in diameter. Hole sizes vary with the tool used and the material being punched. Tools are rated for use in mild steel or stainless steel with some models suitable for both. Knockouts also are used to punch holes in aluminum, fiberglass and plastic.


Basic hand-operated knockout tools have evolved into more sophisticated tools that are easy to use, accurate and productive.


Evolution of knockout tools


Greenlee introduced a line of knockout punches in 1928 for electricians, plumbers and others in construction, said Rob Bentley, product manager.


“Knockout tools quickly became known as the fastest and easiest way to make accurate openings in sheet steel,” he said. “Prior to knockouts, the most common way of making a hole in sheet steel was a bimetal hole saw. Modern advancements in knockout technology have focused on making the job faster, safer and easier.”


Bentley cited past advances in the Greenlee product line: Slug-splitting tools (mid-1980s) split the slug into two pieces during the knockout process, facilitating removal of the slug from the die or around the draw stud. Patented self-centering knockout punch (2004) added a chamfer on the inner diameter of the punch that forces the punch to center itself on the pilot hole and still provides the slug-splitting benefits.


“The traditional method of centering the hole had been to draw crosshairs on the material surface with lines reaching beyond the die radius and to then align the die alignment markings with the crosshairs,” Bentley said. “Using a self-centering punch eliminates the need to refer to the die alignment markings and also eliminates the potential for the punch and die to shift from center before the punch begins to penetrate the steel. For knockouts that do not have the self-centering feature, the user holds the knockout system in place until the cutting edge of the punch makes contact with the material. Once this resistance is encountered on the material surface the tool will not shift.”


The 6-ton battery-powered knockout driver (2011) is a compact, lightweight driver with 360-degree swivel head for easy positioning in tight corners and difficult spaces. A single-trigger control senses when the hole is complete and automatically stops the tool to prevent damage to the punch and die. A light-emitting diode (LED) illuminates the work area.


The threadless speed-punch design (2012) removes traditional threads from the punch and draw stud and eliminates the time and labor required to manually thread the punch on and off the draw stud. Speed-punch sets are compatible with any manual-hydraulic or battery-hydraulic driver.


“The patent-pending no-thread punch design provides up to three times faster setup and disassembly compared to standard threaded knockouts, saving time and money with every punch. Rugged draw stud grooves hold up on the job—no more damaged threads and costly downtime,” Bentley said.


The 11-ton battery-powered knockout driver (2014) provides more power to punch larger holes through thicker materials and can be used with all Greenlee knockouts. Available kits include driver-­only; ½-inch to 2-inch knockouts; and ½- to 4-inch knockouts.


Paige Bovard, senior product manager, ­Milwaukee Tool, said: “Hole-making is an important and time-consuming application for electrical contractors. Knockout tools are the single most accurate tool to punch out holes, which is important when numerous runs of conduit need to fit into an enclosure. The holes need to be precise so that the conduit aligns perfectly, and knockout tools do this better than any other option.


“For a number of years, very few improvements to hole-making products were made, but knockout tools went from hand-hydraulic models to battery operation. Steel punches and dies expanded the capability of knockouts to make holes in stainless steel enclosures.


“Recently, improving the speed of the knockout process has been a focus. Traditionally, the most challenging step in the hole-making process is setting up the punches and dies on the material. Alignment lines can be very hard to see, and the user must support the weight of the tool that is attached to them while making the alignment.


“While all dies have some sort of alignment mechanism, like small hash marks on the side, the tool actually is not the part that is kept in position. The user tightens the punches and dies on the material, and then the pulling mechanism pulls straight up and down to make the punch. This can be challenging because the weight of the tool can work against the user as they try to align for the punch. Milwaukee has developed punches and dies that feature bold, red crosshairs to make alignment fast and easy, and a new quick-connect system does not require the user to juggle the weight of the tool while trying to line up punches and dies. Because punches and dies are set up separately from the tool, the user is able to snap the tool onto the punches and dies once setup is complete and pull the tool trigger to punch the hole,” Bovard said.


When evaluating knockout products, Bovard said there are a variety of variables to consider:


• Capability—Can the tool punch through all of the materials it will encounter? Are knockouts rated for mild steel, stainless steel or both? (Some products require separate punches and dies for each material, which forces the user to purchase more inventory.)


• Capacity—Can the tool punch all of the sizes that will be needed?


• Ease of use—Check alignment, weight, balance, etc.


• System compatibility—Will the battery work on other power tools already in service?


Basic knockout products


Sophisticated battery-powered knockout tools aren’t always needed for knockout work, and basic punch products are available from Greenlee, Milwaukee Tool and other manufacturers in the electrical industry.


Ridgid offers punch and die kits with cases of precision-ground three-point punches and steel dies in sets of ½ to 2 inches; 2½ to 4 inches; manual knockout kits; and hydraulic kits with 11-ton hydraulic ram, pumps, and three-point punch/die sets to 4 inches in diameter. A modern battery-powered hydraulic knockout tool consists of the tool, battery, punch head, draw bolt, spacers (if needed), a knockout punch and a knockout die to make the hole, said Bob Solymos, Ridgid’s global marketing manager.


“Significant steps in the evolution of knockout tools were the manual hydraulic systems, which required pumping by hand to build pressure to pull knockout dies through the material. That evolved into battery-powered hydraulic tools to make it faster and easier to make a hole. Most knockout tools will make holes to accommodate 4-inch conduit. Some tools can punch up to 6-inch holes.


“Typically, a pilot hole is drilled in the location where the hole will be made,” Solymos said. “Alignment occurs automatically as the drawbolt feeds through the knockout die, pilot hole and into the knockout punch. The punch head is held in position by the knockout die and punch after it is set up. The punch usually is attached to the tool that is making the punch so the user must hold the tool in place. Once the hole is made, the user supports the weight of the punch head and tool.


“When comparing knockout products, buyers should consider hole size capacity and the material gauge that the tool is capable of punching. Ease of set-up, as well as punching speed, are other key factors,” he said.


Klein Tools offers three knockout punch sets: a ½-inch kit and ¾-inch kit, each with punch, die and drawback stud, and a knockout punch set with ratcheting wrench and punches, dies and drawback studs for ½- and ¾-inch holes. All are sold with carrying cases.


Maximum capacity is 10-gauge mild steel, and they will cut aluminum, fiberglass and plastic. Slugs are split for easy removal.


Gardner Bender (www.gardnerbender.com) markets single knockout parts and complete kits. Available are punches, dies, draw studs, hand ratchet sets, mechanical driver knockout set, mechanical ratchet set and stainless steel knockout set. Gardner Bender knockouts make holes in diameters from ½ to 4 inches.


Editor’s note: In the June 2015 Cool Tools, the Greenlee 854 conduit bender was misidentified. We regret this error.

About The Author

GRIFFIN, a construction journalist from Oklahoma City, can be reached at [email protected].

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