Published In September 2000
One of the easiest ways to maximize productivity on the job is to match tool to task. For example, while a standard electric drill generally works fine for drilling holes into wood or metal, a hammer drill or a rotary hammer should be the tool of choice for setting anchors in concrete and masonry or otherwise drilling into those obdurate materials. Hammer drills and rotary hammers not only rotate, but also add percussion action, causing the bit to spin and hammer concurrently. This boosts output and saves the worker some labor and potential injuries. Because a hammer drill uses a standard drill chuck and round shank bits, it can be used in the drill-only mode to penetrate wood and metal, as well as in the hammer-and-drill mode to penetrate concrete and brick. But hammer drills can only generate fairly low-impact energy compared to rotary hammers, so they are less effective than rotary hammers for drilling larger diameters in hardened concrete or other hardened materials. Rotary hammers drill up to 10 times faster in concrete, depending on the bit size, and, in general, are more durable than hammer drills. Nevertheless, among electricians, 1/2 inch hammer drills are far more popular than rotary hammers, manufacturers report. This is largely because they are less expensive and use the aforementioned chucks and bits for drilling into just about anything, while rotary hammers require more expensive specialized chucks and bits. The lowdown on hammer drills The hammering action in a hammer drill occurs when the two opposing clutch faces—one fixed and one rotating—are pressed together as the operator pushes the drill against a surface while pulling the trigger to rotate the bit and the chuck. Two factors affect the speed of a hammer drill: the pulling of the trigger, which affects the rpm; and the amount of pressure applied with the drill against the accepting surface. The greater the pressure, the greater the hammering action, which comes from the chuck faces sliding up and down against each other. The simultaneous hammering and rotating allows users to drill into masonry or concrete at the rate of 20,000 to 50,000 blows per minute, about three to four times faster than with an ordinary electric drill. This rapid pounding could yield terrific short-term productivity, but will surely take its toll on an operator’s arm and whole body over the workday. A round shank bit is subject to bit slippage, which could slow down productivity when the operator stops to tighten the bit. Another caveat: The chuck is held in place by a screw at its base. During drilling into concrete, the bit eventually gets pushed to the base of the chuck and over time could destroy the screw, making chuck replacement more difficult. Another consideration is that hammer drills produce a higher-pitched and louder noise than rotary drills. Feature-rich hammer drills have two speed ranges and a gearbox for switching speed ranges. Typically, an electrician uses the lower speed for concrete drilling and the higher one for steel and wood. A two-speed gearbox offers more control than the one-speed variety, because the user is relying on more than just the trigger finger to control speed. Too fast an rpm on concrete can dull or glaze the bits and ruin them. Other attractive features to look for in a hammer drill include: a variable speed trigger, a lock-on button, a front handle that can rotate 360 degrees, contoured and cushioned grips to minimize operator fatigue, and a depth gauge. While tool choice is always an operator’s decision, a rule of thumb suggests use of a hammer drill when drilling up to 3/8 inch diameter holes in concrete; use of a rotary hammer when drilling holes larger than 3/8 inch for better productivity. How rotary hammers work Rotary hammers (which do not have the speed to drill into wood or metal and so are used only on concrete and masonry) require expensive slotted drive system or slotted drive shank (SDS) chucks and bits, or the even bigger SDS Plus, SDS MAX, or SDS spline chucks and bits. A rotary hammer uses a motor-driven piston to provide an even more intense hammering action than that of the hammer drill, independent of operator pressure. The tool’s own weight, held against the receiving surface, is enough to engage the bit. (This factor is much appreciated by workers drilling above their heads.) Absent, too, are the numbing hand buzz and high-pitched noise. The tool does the work while a cushion of air absorbs some of the rebound vibration. The impact mechanism strikes the surface with significantly more force than a hammer drill would deliver, and, rather than pulling material out, pulverizes it. The more robust rotary hammers, which can operate in any of three modes—hammer, drill, or hammer-and-drill—are not only helpful in setting anchors, but also in chiseling and breaking concrete and masonry. They typically range in size from 7/8 inch up to 2 1/2 inch breaker hammers or higher. Though more expensive to buy than hammer drills, rotary hammers (which can also be used for driving in ground rods) are also more efficient to operate. A rotary hammer operates at a significantly lower rpm than a hammer drill, but offers a much faster drilling rate because of the design of the mechanism used to provide the impact force. Rotary hammers have two designs. Smaller rotary hammers suitable for electricians setting anchors feature a hollow piston design, while larger ones use a crank-type piston. In the hollow piston design, the ram and the beat piece both travel within the cylinder, or barrel, that is inside the front of the tool. The beat piece is what actually contacts the bit, hitting the SDS bit into the material. The beat piece moves in very small increments when hit by the ram, which is traveling back and forth within the hollow piston, where the compression is created. After the bit is hit, the rebound energy coming back from the bit pops the beat piece back and the ram is rebounded backward toward the rear of the piston, never touching the back of the piston. Air compression is built up and the piston is cranked forward, slamming the ram forward again toward the beat piece. The compressing air pressure when the ram moves backward absorbs the backward forces. The larger rotary hammers use crank-type pistons rather than hollow ones, which cannot take the more powerful forces generated in a larger unit. In a crank-type piston, the gearing spins a crank that moves the piston to reciprocate. Another contributing factor in the overall efficiency of a rotary hammer is the SDS’s chuck and bit system incorporating the slotted drive, which by design prevents the bit from slipping as it could with a rotary hammer. Because of this, there should be none of the bit slippage that can slow down a hammer drill operator. The rotary hammer operator will also benefit from the higher-impact energy, allowing facile drilling of larger diameters and into harder materials. As the material gets harder, the relative importance of the impact energy becomes greater in penetrating the material. And here’s another plus: changing bits on a rotary hammer is very quick: no key is needed and the SDS bits click right into the chuck. The Feldmans provide Web content and write for magazines, trade associations, building product manufacturers, and other companies. They can be reached at firstname.lastname@example.org or (914) 238-6272.