All five of these tools performed powerfully and well in our field-tests, doing just about everything we asked of them, so it was hard to pick a favorite from this group. But it was also hard to ignore how much the ergonomic features and performance meant to us as the test wore on. So we have picked two tools that stand out above the others in this class, one for overall performance and comfort, and the other for power and speed.
Our overall favorite is the Metabo. We liked its heavy-duty housing and thought it did the best job controlling vibration and noise while delivering steady performance. We also liked the removable chuck that could add greatly to the versatility of the tool.
For raw power and performance, we have to hand it to Hitachi. This model provided blazing-fast drilling and chiseling–so this is the tool for just getting the job done.
After these two we felt that the Bosch and DeWalt were about even, with the Makita close behind.
–Erik Elwell owns Thompson Construction, a high-end residential and light commercial remodeling firm in New York City.
–Thanks to DRILTEC for supplying the bits for this test.
Why do different rotary hammer manufacturers deal with the reverse motor function so differently on otherwise remarkably similar tools?
The main reason is that universal motors found in most portable power tools are built with a forward motor pitch or lead, which effectively gives the motor more power in its forward direction by altering the instant of commutation, or firing–similar to advancing the timing in a vehicle's engine. Technically, moving the brushes or stator relative to each other defines motor pitch, while changing the electrical timing of the armature bars defines bar lead. Advancing one of these three components gives the motor more torque because the driving electromagnetic attraction is timed to be applied at the optimal point to provide the highest force to spin the armature in forward mode. Consequently, in this biased configuration, the power in reverse is reduced and the brushes suffer very heavy sparking and diminished life–a situation called "poor commutation," which also causes high levels of radio frequency noise interference.
When tuned for forward performance, the stationary stator's magnetic field distortion caused by the rotating armature's magnetic field is dealt with more efficiently and radio interference transmission is minimized. Motors that only do work in forward, such as in miter saws, are often fully optimized for forward pitch or lead, but tools that are commonly used in reverse, such as drills, are configured in a more neutral position to allow for adequate power in both directions. Some rotary hammer manufacturers, realizing that nearly all the work this type of tool will ever do will be in the forward direction, take advantage of the benefits afforded by wiring a motor with a forward bias. As a result, the reverse power can be compromised. Here's how our tool manufacturers dealt with the variables and how it affects the reverse mode of their tools.
Bosch: Features a patented Rotating Brush Plate that provides equal power and speed in forward and reverse. This feature increases brush life by placing brushes in a position for clean commutation in both directions.
DeWalt: Has a forward-biased motor with reduced power in reverse but full speed at no-load. The reverse power is designed to be adequate to back out masonry anchor screws.
Hitachi: Has a forward-biased motor and a reversing switch that mechanically limits the trigger to half-capacity and disables the trigger lock-on operation. The reduced current flow to the motor in reverse is done to comply with some foreign markets' strict thresholds for radio frequency noise interference transmission.
Makita: Same as Hitachi. The company also notes that reverse limitation also prolongs brush life.
Metabo: Features rotating brushes that provide equal optimized power and speed in forward and reverse. The trigger lock-on can be used in reverse. Interestingly enough, some Metabo rotary hammers in the past actually featured a motor bias that resulted in the tool being 30 percent more powerful in reverse than in forward! It was explained that there was sufficient power in forward for the tasks it was expected to do and that the 130 percent reverse bias was for extra breakaway torque for removing fasteners. Apparently, this feature was originally designed for Europe where multi-use tools are more prevalent and the same tool was intended to be used for drilling, driving, and removing anchors.