The main types of drilling bits used in oil and gas wells are roller-cone bits and fixed-cutter bits. Roller-cone bits use rotating cones with milled teeth or tungsten-carbide inserts; fixed-cutter designs include polycrystalline diamond compact (PDC), natural-diamond, and thermally stable polycrystalline (TSP) bits. This illustrated guide compares their cutting mechanisms, typical applications, and design differences. For bit selection, operating practices, and performance evaluation, see our complete drilling bits guide.
- Development of roller-cone designs with improved cutting structures.
- Refinements to cone geometry and bit hydraulics.
- Improved bearing seals and lubrication systems.
- Natural-diamond and diamond-impregnated bit applications.
- Development of PDC and thermally stable diamond cutters.
Main Types of Drilling Bits: Quick Comparison
Bit selection depends on formation strength, abrasiveness, impact loading, hydraulics, directional requirements, and expected drilling economics. This table identifies common designs; it is not a substitute for a formation-specific bit program.
| Bit type | How it cuts rock | Typical consideration |
|---|---|---|
| Roller cone: milled tooth | Crushing, gouging, and chipping with steel teeth | Tooth geometry varies with formation hardness |
| Roller cone: tungsten-carbide insert (TCI) | Crushing and chipping with inserts in rotating cones | Insert shape, bearing design, and gauge protection matter |
| PDC fixed cutter | Primarily shearing with synthetic diamond cutters | Cutter design, cooling, and vibration control matter |
| Natural diamond | Scraping or grinding with exposed natural diamonds | Specialized hard or abrasive applications; low-depth cutting |
| TSP / thermally stable diamond | Scraping or grinding with thermally stable diamond elements | Thermal resistance and cutter configuration vary by design |
Each development addresses particular formation and operating challenges. However, no single bit design is best for every interval; bit performance depends on the match between cutting structure, rock properties, and operating parameters.
How Drilling Bits Types Break Rock
Rock fails beneath a drilling bit through a combination of mechanisms. Two useful ways to describe these are:
- Shearing or scraping: cutters remove rock by imposing shear stresses as they move across the bottomhole.
- Indentation and crushing: teeth or inserts concentrate load until the rock fractures under contact stresses. See the discussion of rock compressive strength.
In practice, crushing, chipping, gouging, shearing, and abrasion can act together. Drilling fluid carries the generated cuttings away and helps clean and cool the bit. For a closer technical explanation, see rock failure mechanisms in drilling.
Shearing occurs when a cutting element moves across rock and removes a chip. PDC bits primarily use this action, although cutter geometry and rock properties influence the result. A cutter must penetrate sufficiently to engage the formation; excessive rock strength, cutter wear, or poor depth of cut can reduce efficiency (see Fig. 1).
An everyday analogy is a blade cutting firm clay: as the blade enters and moves through the material, it removes fragments. Similarly, a drag cutter engages and shears rock as the bit turns. Actual downhole cutting also depends on confinement, drilling-fluid pressure, cutter angle, and formation properties.
Harder or more abrasive formations may limit cutter penetration and accelerate wear. Under some conditions, a roller-cone bit provides effective indentation and crushing; under others, an appropriately designed PDC or diamond bit performs well. Therefore, formation hardness alone does not determine types of drilling bit.
Indentation and crushing occur when concentrated contact loads create fractures beneath a tooth or insert. Hard-formation roller-cone bits often use robust cutting structures and operating parameters selected to deliver adequate contact force. However, there is no universal high-weight-on-bit or low-RPM rule: use the bit manufacturer’s operating window, BHA limits, and downhole response (see Fig. 2).
Roller Cone Drilling Bit Types
How Roller-Cone Bits Work
Roller-cone bits (rock bits) use two or more cones—most commonly three—that rotate on bearings as the bit turns. Steel teeth or tungsten-carbide inserts indent, chip, and gouge the formation. The two main cutting-structure categories are milled-tooth bits and tungsten-carbide insert (TCI) bits. Both remain useful in selected applications, but their worldwide market share varies by region, formation, and drilling program.
The early Hughes roller-cone design, introduced in 1909, helped extend rotary drilling into harder formations. Two-cone designs were followed by three-cone bits with improved tooth arrangements and better bottomhole cleaning. Modern designs have evolved substantially; for construction details, see our roller-cone bit design guide.
Each cone rotates on a bearing or journal supported by the bit leg. Cone offset, tooth or insert geometry, bearing design, and gauge protection affect how the bit engages rock. The three-cone configuration can provide balanced coverage of the bottomhole, but the resulting gauge quality also depends on design and operating conditions.
Important improvements in roller-cone bit design include:
- Improved cuttings removal and bottomhole cleaning through jet nozzles; nozzle sizing affects total flow area (TFA) and bit hydraulics.
- Use of tungsten carbide in cutting structures, hardfacing, and gauge protection.
- Sealed bearing and lubrication designs that limit drilling-fluid ingress and reduce wear.
Milled-Tooth Roller-Cone Bits
Milled-tooth bits have steel teeth machined into the cone surfaces, often with wear-resistant hardfacing. Longer, more widely spaced teeth are commonly used in softer formations, while shorter, stronger teeth suit harder intervals. Selection also considers tooth wear, drilling-fluid cleaning, and bearing life.
Tungsten-Carbide Insert (TCI) Roller-Cone Bits
TCI bits use tungsten-carbide inserts fitted into the cones rather than integral milled teeth. Insert shapes and grades vary with formation and expected loading: aggressive chisel shapes may suit softer intervals, while shorter conical or rounded inserts can resist higher contact loads in harder rock. Selection also depends on abrasiveness, impact, and drilling dynamics.
Tungsten carbide combines high hardness and wear resistance with grades engineered for different levels of toughness. It is well suited to inserts and wear-protection features, but brittle fracture can occur under unfavorable impact or bending loads. Bit designers balance carbide grade, insert geometry, and support within the cone to manage these stresses.
Insert requirements differ by location on the cone. Inner rows encounter different contact speeds and rock-loading patterns than outer or gauge rows. Therefore, manufacturers vary insert size, shape, grade, and placement to balance penetration, impact resistance, and gauge wear. For classification terminology, see the IADC roller-cone bit classification guide.
Fixed-Cutter Drilling Bit Types: Natural Diamond, PDC and TSP
Unlike roller-cone bits, fixed-cutter bits have no rotating cones. Their cutters engage the rock as the bit body turns. This family includes PDC bits, natural-diamond bits, and TSP designs. Diamond-based cutters offer excellent wear resistance, but application limits vary with cutter material, temperature, impact, and formation abrasiveness.
Performance depends on cutter placement and exposure, bit profile, hydraulic cleaning, cooling, and compatible operating parameters. An effective design must also manage vibration and cutter damage; diamond hardness alone does not guarantee a faster or more economical run.
Natural-diamond and some thermally stable diamond drilling bit types hold cutting elements in a matrix body. Waterways direct drilling fluid across the bit face to remove cuttings and limit heat buildup. PDC bits use larger engineered cutters mounted on blades and can have either steel or matrix bodies (see Fig. 6).
As a natural-diamond cutting element contacts the formation, it removes small amounts of rock mainly by scraping or grinding. PDC cutters, by contrast, generally remove larger chips through shearing.
Natural Diamond Bits
Natural-diamond bits use small diamonds embedded in a matrix face. They are fixed-cutter (drag) bits with no cones, and their application is specialized rather than universal.
Cutting action: the exposed diamond points scrape or grind rock as the bit rotates. Cutter spacing, exposure, and matrix wear determine how the bit engages the bottomhole.
Diamond is highly wear-resistant but can chip under impact. Minimize damaging drill-string vibration and follow the bit manufacturer’s handling and operating recommendations.
Adequate hydraulics remove cuttings, cool the cutting elements, and reduce the risk of bit balling. Inadequate cleaning may reduce penetration rate and accelerate damage.
Limitations: acquisition cost, impact sensitivity, and application-specific performance can restrict their use. Compare total interval drilling cost and expected bit life rather than assuming a diamond bit will always produce a higher rate of penetration.
PDC Drilling Bit Types (Polycrystalline Diamond Compact)
PDC cutters typically bond a synthetic polycrystalline diamond layer to a tungsten-carbide substrate. Their development in the 1970s marked an important step in fixed-cutter bit design. As the bit rotates, these cutters primarily shear the formation. Cutter geometry, diamond-table quality, substrate support, and hydraulic layout strongly affect bit performance.
PDC drilling bits differ substantially from traditional natural-diamond bit types: they usually engage rock with larger engineered cutters, which can achieve efficient shearing in suitable formations. However, PDC cutters can still suffer impact damage, thermal wear, abrasion, and vibration-related failure.
PDC bit bodies are commonly made from two different materials. See the PDC bit design and components guide for blade profiles, cutters, hydraulics, and body construction:
- Matrix-body bits.
- Steel-body bits.
Matrix bodies use a hard composite, commonly containing tungsten-carbide particles in a metal binder. They provide good erosion and abrasion resistance and support complex hydraulic features. Compared with steel, the matrix material is generally less tolerant of severe impacts. Designers select the body according to expected erosion, loading, repairability, and manufacturing requirements.
Steel bodies offer toughness and manufacturing flexibility. They typically need hardfacing or other wear protection in erosion-prone areas. Neither body style is universally superior; formation characteristics and operating demands govern the choice. For PDC bit coding, see our IADC fixed-cutter bit classification guide.
TSP Bits (Thermally Stable Polycrystalline Diamond)
Thermally stable polycrystalline (TSP) diamond elements are engineered to withstand higher cutting temperatures than conventional cobalt-containing PDC cutter material. They have been used in specialized fixed-cutter and impregnated designs where heat and abrasion are important. Their performance still depends on cutter construction, cooling, loading, and the specific application.
Which Drilling Bit Type Should You Choose?
Use the comparison above to identify candidate bit families, then check formation compressive strength and abrasiveness, expected interbedded or impact-prone intervals, directional requirements, available hydraulic horsepower, bit and BHA operating limits, and offset-well performance. Compare total drilling cost per interval—not only bit purchase price or peak ROP. For the detailed decision process, continue to our drilling bit selection guide.
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