Drill Bit Types: Tricone, PDC & DTH Bits — How to Choose
Choosing the right drill bit is the cheapest way to protect your drilling speed and your bottom line. The wrong bit turns competent ground into a week-long struggle. This guide breaks down the main bit types — tricone, PDC, and DTH — and when each one earns its place on the string. It is written for drilling contractors and buyers who need to match tooling to formation, not guess.
Tricone (Roller Cone) Bits
Tricone bits use three rotating cones studded with tungsten carbide inserts or milled teeth. They crush and gouge through a wide range of formations — from soft sand to medium-hard rock — and they are forgiving of mixed ground. For a broader look at the machines that run them, see our drilling rig types guide.
- Best for: soft to medium formations, alternating layers
- Pros: versatile, widely available, handles changes in ground
- Cons: slower in very hard rock than PDC or DTH
PDC (Polycrystalline Diamond Compact) Bits
PDC bits shear the formation with fixed diamond cutters. In soft to medium sedimentary rock they drill fast and last long, which is why they dominate water-well and geotech work in those conditions. They dislike fractured, abrasive, or very hard ground where cutters chip.
- Best for: soft-to-medium homogeneous formations, high-speed runs
- Pros: high ROP, long life in the right ground
- Cons: brittle in fractured or coarse abrasive rock
DTH (Down-the-Hole) Bits & Hammers
A DTH bit sits at the bottom of a down-hole hammer that pounds directly on the bit. Penetration stays high as the hole deepens, and the hole stays straighter than top-hammer methods. This is the workhorse for hard rock and deep water wells — paired with the right DTH drilling method.
- Best for: hard, competent rock and deep holes
- Pros: straight holes, consistent speed with depth, clean cuttings
- Cons: needs adequate air volume from a compressor
Other Bits You Will Meet
Auger bits and drag (fishtail) bits handle loose soil, clay, and soft overburden in shallow holes. They are cheap and fast in the right ground but useless once you hit rock — most crews carry both an auger and a roller-cone or DTH setup.
Bit Type Comparison
| Bit type | Best formation | Speed | Depth suited |
|---|---|---|---|
| Tricone | Soft–medium, mixed | Medium | Shallow–medium |
| PDC | Soft–medium homogeneous | High | Shallow–medium |
| DTH | Hard, competent rock | High (with depth) | Medium–deep |
| Auger/Drag | Soil, clay, overburden | High (shallow) | Shallow only |
How to Choose by Formation
Match the bit to what you will actually drill most of the time, not the hardest layer. Soft ground favours PDC or auger; mixed ground favours tricone; hard rock favours DTH. Carrying a spare bit in each category on the rig avoids the downtime of a mid-job change. Our rig selection guide helps you size the machine to the same ground.
FAQ
What bit is best for hard rock?
DTH bits and hammers are the standard for hard, competent rock because they keep penetration up with depth and hold a straight hole. PDC can work in some hard formations but chips in fractured ground.
Can one bit drill both rock and soil?
A tricone bit is the most forgiving across soil and rock, which is why it is common for mixed formations. Truly soft overburden is faster with an auger, and truly hard rock is faster with DTH.
How do I know if my bit is worn out?
A noticeable drop in penetration rate and visibly rounded or chipped cutting structures are the signs. Running a worn bit wastes air, fuel, and time — change it before it costs you a stuck tool.
Does bit size affect my rig choice?
Yes. Larger diameters need more torque and air or fluid to the bit, which feeds directly into rig and compressor sizing. Size the bit to the well screen and pump you plan to install.