Anatomy of a Hammer Drill Bit
Hammer drills are known by a number of descriptive terms: chipping guns, rotary hammers, demolition hammers, rotary drills, or combi drills. They are designed to quickly drill through brittle materials like concrete by combining the rotary action of the drill bit with a hammer action that moves the drill bit in and out of the hole along the bit’s axis to provide a pounding action along with the drilling motion of the bit.
They work along a couple of different mechanism designs, but ultimately the effectiveness of the drill is only as strong as its drill bit. With that dual action in a tough building material like concrete, the drill bit itself has to withstand incredible pressure.
Are all hammer drill bits created equal?
In basic anatomy, maybe.
However, in real job site experience, a number of small factors can add up to a better experience when drilling concrete. Different materials have been developed to provide both hardness and strength to improve the drill bit’s performance, and the type or make of drill you are using will steer your bit choice as well. So let’s look at the basic anatomy of a hammer drill bit and what is necessary for complete and accurate relative humidity (RH) concrete testing.
1. The Anatomy of a Hammer Drill Bit
- The shank
The shank is the portion of the drill bit that fits into the hammer drill collar and holds it securely as the drill provides both rotation and impact during operation. For most rotary hammer drills, a smooth shank is insufficient because the drill bit can slip during operation and come loose from the drill itself. Common shank designs for rotary hammer drills include SDS, SDS-MAX or spline shanks that lock into the chuck so that the action of the hammer drill will not dislodge the drill bit.
- The land
The land is essentially the raised portion of the spiral portion of the drill bit. The land holds the cutting edges of the bit on its outside diameter.
- The flute
The flute on a drill bit is the “trough” that runs parallel to the land. Its purpose is to move dust out of the hole as it is being drilled in order to prevent jamming.
- The head
The head is the lead point of the drill bit that holds the tip and provides maximum impact in breaking up the concrete. It will also have two or four flat angles (depending on the number of flutes) for the lead cutting action into the concrete.
- The tip
For concrete drill bits (and others), carbide is bonded to the drill bit head to harden and strengthen it for breaking up the concrete. Extreme heat in the drilling hole can sometimes cause damage to the carbide tip, dulling it or melting it away completely, reducing the bit’s ability to break the concrete.
2. Its Role in the Hole
Regardless of the hammer drill bit’s precise composition, the basic elements are the same and its ultimate role is the same: to provide a precise test hole in a concrete slab that will accommodate the RH sensor for accurate relative humidity, or in situ, testing of the slab’s moisture content (MC). Both the depth and the width of that hole play a role in providing accurate MC data to building inspectors, flooring installers, and other professionals and specialists in place to provide a lasting and serviceable concrete slab.
3. Ensuring the Best RH Test Hole
A hammer drill bit is a key part of preparing a concrete slab for RH testing, and because a precise test hole will help ensure the best fit for the RH Smart Sensor, technique is also important.
- Test hole depth
ASTM F2170 says the sensor location (and therefore test hole depth) must be at 40% of the slab depth, when the slab is drying from one side and 20% when drying from two sides, when testing concrete moisture with RH testing. We recommend marking that depth specifically on the drill bit with a tape “flag” that lets the drill operator see when they have reached the precise depth. Holding the hammer drill vertically straight ensures the best torque for the test hole, and the maximum performance of the drill and drill bit.
- Test hole width
RH test method manufacturers supply precise specifications for their test hole widths as well. For example, Rapid RH® 4.0 EX Smart Sensors should be placed in holes drilled with a 4 fluted, carbide-tipped drill bit that falls within the specifications for an ANSI standard ¾” bit. This range is from 0.775” to 0.787”. If your drill bit is at the lower end of this specification, the sensor might be a tight fit that is difficult to properly install. The head design of a drill bit can be an area where experience suggests that four cutting angles (4-flute drill bits) produce a rounder hole, making it easier to seat the RH test sensor.(i) Wagner Meters, for example, makers of the Rapid RH® 4.0 EX, routinely provides a 4-flute bit for optimal test hole dimensions.
Of course, the other reason for a tight fit could be that your drill bit has worn down with use and has lost some of its diameter. This will most likely become noticeable after about 60-70 holes drilled. A worn drill bit also increases the risk of the bit jamming in the hole because it can no longer clear the dust efficiently. Before each job, the drill bit’s head and flutes should be visually examined to ensure they are not damaged from a previous job.
RH testing provides accurate, ASTM-approved concrete moisture measurement, and the hammer drill bit is a vital component of the process.