"Rock drill bit" covers a whole family of tools, from the small button bit on a quarry jackhammer to a 26-inch tricone opening the top of an oil well. They differ in how they break rock, what rig drives them and which ground they suit. This guide sorts the main types of rock drill bits by working principle, explains where each one is used in Indian borewell, mining, construction and oilfield work, and gives a simple method for choosing between them.
Two ways to break rock: rotary and percussive
Every rock drill bit works by one of two principles, and knowing which one a bit uses tells you most of what you need about it.
Rotary drilling turns the bit continuously under weight. The bit either crushes and chips the rock (roller cone bits) or shears it like a cutting tool (fixed-cutter PDC and drag bits). Drilling fluid or air cools the bit and lifts the cuttings. Rotary bits cover the widest range of hole sizes and depths, from shallow borewells to deep oil and gas wells.
Percussive drilling hits the rock with rapid blows while the bit rotates slowly to index onto fresh rock. The hammer may sit at the surface (top hammer) or directly behind the bit at the bottom of the hole (down-the-hole, DTH). Percussion is very effective in hard, brittle rock such as granite and basalt, typically in smaller hole diameters.
Tricone (roller cone) bits
A tricone bit has three cones mounted on bearings. As the bit turns, the cones roll over the hole bottom and their teeth crush and gouge the rock. There are two main versions:
- Milled-tooth (steel-tooth) tricone — teeth are machined from the cone steel, often with hardfacing. Long teeth drill fast in soft formations such as clay, shale and soft sandstone.
- TCI (tungsten carbide insert) tricone — tungsten-carbide inserts are pressed into the cones. They resist abrasion and impact, so TCI bits handle medium to very hard rock such as limestone, basalt and granite.
Bearings are open or sealed, and the whole design is described by the IADC code: the first digit gives cutting structure and hardness (1–3 steel tooth, 4–8 insert), the second refines hardness, the third describes bearing and gauge protection. Tricones are the most versatile rock bits, used in borewells, blast holes and oil and gas wells. Volgaburmash produces more than 600 roller cone bit designs, from 95.3 mm to 660.4 mm (up to 26"); see our tricone bits.
PDC and other fixed-cutter bits
A PDC bit has no moving parts. Polycrystalline diamond compact cutters are brazed into blades on a steel or matrix body and shear the rock as the bit rotates. In soft to medium, uniform formations — shale, claystone, soft limestone — PDC bits usually give the highest rate of penetration and the longest runs. In hard, fractured or very abrasive rock the cutters can chip, and a roller cone bit is often the safer choice. Volgaburmash makes over 350 PDC designs from 3" to 17½", in steel and matrix bodies; our guide PDC bit full form and how it works explains the design in detail, and the range is on our PDC bits page.
Fixed-cutter bits for mining blast holes and water wells follow the same principle, with designs adapted to air or water flushing.
Button bits: DTH and top hammer
DTH (down-the-hole) button bits
A DTH button bit is a solid steel head studded with tungsten-carbide buttons, fitted to the front of a pneumatic hammer. Because the hammer is at the bottom of the hole, impact energy does not fade with depth, and holes stay straight. DTH is the everyday method for hard-rock borewells across much of peninsular India and for small and medium blast holes in quarries and mines. Button shape matters: spherical buttons are the most durable in hard, abrasive rock, while ballistic or parabolic buttons penetrate faster in medium rock but wear sooner. Face designs (flat, concave, convex) are chosen for straightness and flushing.
Top-hammer button bits
Top-hammer bits screw onto drill rods driven by a hammer at the surface. They are used for small holes in tunnelling, quarrying, construction and bench drilling. Energy is lost through each rod joint, so they are most efficient in shorter holes.
Cross and chisel bits
Older percussive designs with carbide blades instead of buttons. They are still used in some small-hole and hand-held work, but button bits have largely replaced them.
Drag, core and hole-opener bits
- Drag bits (blade or fishtail bits) have fixed steel or carbide blades and cut soft, unconsolidated ground fast. They are cheap and simple but wear quickly in rock.
- Core bits cut a ring and leave a cylinder of rock (core) for geological study. Small-diameter exploration uses diamond-impregnated or surface-set core bits; oil and gas coring uses larger PDC core heads such as the Volgaburmash CoreBits series.
- Hole openers and reamers enlarge a pilot hole to the final diameter, using roller cone segments or fixed cutters. They are common in large water wells, piling and horizontal directional drilling (HDD).
Rock drill bit types compared
| Bit type | Cutting action | Best rock | Typical use | Main limitation |
|---|---|---|---|---|
| Steel-tooth tricone | Rotary crushing and gouging | Soft: clay, shale, soft sandstone | Borewell top holes, oil and gas upper sections | Wears fast in hard rock |
| TCI tricone | Rotary crushing | Medium to very hard: limestone, basalt, granite | Borewells, blast holes, oil and gas | Bearing life limits run length |
| PDC | Rotary shearing | Soft to medium, uniform | Oil and gas, water wells in sediments | Cutter damage in hard, broken rock |
| DTH button bit | Percussion | Hard, brittle: granite, basalt | Hard-rock borewells, blast holes | Needs high air volume; poor in loose ground |
| Top-hammer button bit | Percussion | Medium to hard | Tunnelling, quarrying, small holes | Efficiency drops with depth |
| Drag bit | Rotary scraping | Very soft, unconsolidated | Shallow water wells, soil | Unsuitable for rock |
How to choose a rock drill bit
- Start with the rock. How hard is it, how abrasive, and is it fractured or massive? Soft and uniform points to PDC or steel tooth; hard and abrasive points to TCI or DTH.
- Fix the hole size and depth. Large diameters and deep holes favour rotary bits; small, shallow holes in hard rock favour percussion.
- Check the rig and flushing. A DTH bit needs a hammer and a large compressor; a tricone needs weight on bit and a mud pump or compressor to clean the hole.
- Match the connection. Rotary bits use API Reg pins; hammers and rods have their own threads.
- Compare on cost per metre. Track metres drilled per bit in each formation, not only the purchase price.
For borewells specifically, our DTH vs tricone vs PDC guide applies this method to Indian geology. For blast-hole drilling, see our mining and blast-hole bits and the mining bits in stock in Dubai.
Frequently asked questions
Which drill bit is best for hard rock?
For hard, brittle rock such as granite and basalt, TCI tricone bits in rotary drilling and DTH button bits in percussive drilling are the usual choices. PDC bits are better suited to soft to medium, uniform formations.
What is a button bit?
A button bit is a percussive rock bit with a steel head studded with tungsten-carbide buttons. It is used on DTH hammers or top-hammer rigs, and the button shape (spherical, ballistic or parabolic) is chosen for rock hardness and abrasiveness.
What is the difference between a tricone and a PDC bit?
A tricone bit has three rotating cones that crush rock and suits a wide range of formations, including hard and fractured rock. A PDC bit has fixed diamond cutters that shear rock and is usually faster in soft to medium, uniform formations.
How are rock drill bit sizes measured?
By the diameter of the hole the bit drills, in inches or millimetres. Rotary bits for oil, gas and water wells follow standard API sizes such as 6 1/8, 8 1/2 or 12 1/4 inches; percussive bits are usually quoted in millimetres.