Rotary Hammers

- Nov 29, 2017 -

Rotary hammers, also called "combi drills," are similar in that they also pound the drill bit in and out while it is spinning. However, they use a piston mechanism instead of a special clutch. This causes them to deliver a much more powerful hammer blow, making it possible to drill bigger holes much faster. Rotary hammers have such force, in fact, that the usual masonry bits are no longer adequate. Their smooth shanks would be pounded loose from the tool's chuck in a few seconds. Rotary hammers require special bits which lock into the rotary hammer and continue spinning while smashing away.

A number of "special shanks" have been developed by various manufacturers. Over the years a fair number of these proprietary systems evolved, but the remaining shanks in use today are: SDS+SDS-MAX, and SPLINE SHANK. These shanks were developed in order to allow for the bit to "slide" back and forth while rotating, so that the drill bit can efficiently transfer the force of the electro-pneumatic hammering mechanism to the working surface.

Rotary hammer drills have an oil filled gearbox, which allows them to operate durably despite the large forces and shocks they receive and the grit-filled environments where they are often used.

Apart from their main function of drilling concrete, the rotary action can be switched off and use is made of just the percussive force. Chisel and point accessories are used for small chipping jobs.

The type of work they do means that they require a "slip-clutch" which engages when the drill bit jams and sufficient torque is put onto the "slip-clutch" mechanism. This stops the violent wrenching motion that a drill without a clutch would cause when stopped suddenly from full speed, protecting the drill from damage. The slip-clutch also protects the operator, but does not always prevent injury. Some manufacturers have introduced additional technology to protect the operator. Hilti has a technology called "ATC" or "Active Torque Control"[5] which works by disengaging the drive from the motor when the tool body begins to rotate excessively through the action of a secondary magnetic clutch in addition to the standard slip-clutch. DeWALT has a related system called "CTC" or "Complete Torque Control"[6] which utilizes a two-position slip-clutch so that the operator can select the lower torque setting for greater safety.

Jams are most often caused by hitting reinforcing steel or by a worn bit. In both cases the drill must be disengaged from the bit and the jammed bit backed out of the hole with vise grips or monkey wrench. Some bits utilize a full carbide "four-cutter" head with a geometry that makes jamming less common - even when rebar is present. These full-carbide "four-cutter" bits can even, in some instances, drill through rebar, although this should be done with caution.


Worn masonry drill bit

A worn drill bit will still drill a horizontal hole, although of a slightly smaller diameter than one created when it was new. When a drill like this is used to drill holes down into a concrete slab, the flutes are so worn that they can no longer lift the dust out of the hole; the concrete dust packs up in the hole and jams the bit.


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