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What is a T38 Drill Bit and How does it Work?

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If you work in drilling, mining, or construction, you have likely encountered the T38 drill bit. It is one of the most widely used threaded button bits in top hammer drilling, known for its versatility, durability, and efficient rock penetration.

This article explains what a T38 drill bit is, how it works, its key specifications, and where it is commonly used.

Understanding T38 Drill Bits

A T38 drill bit is a threaded button bit used in top hammer drilling systems. The name "T38 " comes from its thread type: the "T" stands for trapezoidal thread, and "38" refers to the nominal thread diameter of approximately 38 mm (1.5 inches). This threaded connection allows the bit to securely attach to a T38 drill rod or shank adapter, forming a reliable drill string.

T38 drill bits are typically manufactured from high-quality alloy steel with tungsten carbide buttons pressed or brazed into the bit face. The tungsten carbide inserts provide exceptional wear resistance, enabling the bit to maintain its cutting efficiency even in hard and abrasive rock formations.

Thread Design and Torque Capacity

The T38 thread system is engineered for toughness. Epiroc describes the T-drill string thread system as “robust enough to prevent product breakage in the blast hole,” which reduces the need for frequent rod and shank changes. The trapezoidal thread design provides a secure, high-strength connection that transmits impact energy efficiently from the drill rig to the rock face.

How Does a T38 Drill Bit Work?

T38 drill bits operate using a combination of rotary and percussive forces—the fundamental principle of top hammer drilling.

The Percussive-Rotary Action

Impact energy transmission: The piston inside the hydraulic rock drill strikes the shank adapter, generating a compressive stress wave that travels down the drill rod to the T38 bit.
Rock fracturing: The tungsten carbide buttons on the bit face concentrate this impact energy onto the rock surface. Each blow fractures the rock through a combination of crushing and chipping.
Rotation: Simultaneously, the drill rig rotates the drill string. This rotation ensures that the bit’s buttons contact fresh rock with each impact, creating a continuous borehole.
Cuttings removal: Flushing air or water flows through flushing holes in the bit, carrying rock cuttings up the annulus to the surface.

Button Configurations and Face Designs

T38 drill bits are available with different button shapes and face designs to match specific rock conditions:

Button Shape	Best Application
Spherical (domed)	Hard, abrasive rock (granite, quartzite); maximum wear resistance
Ballistic	Medium-hard rock; faster penetration
Parabolic	Soft to medium rock; balanced performance
Face Design	Best Application
Drop center	Improved hole straightness; better flushing
Flat face	Extreme hardness; resists body wear
Convex	Concentrates impact energy

Retrac (retractable) button bits feature a reinforced skirt design that prevents jamming in fractured rock, making them particularly useful in challenging ground conditions.

Key Specifications of T38 Drill Bits

Specification	Details
Thread type	T38 (trapezoidal, ~38 mm / 1.5″ nominal diameter)
Bit diameter range	64 mm to 127 mm (2.5″ to 5″)
Common diameters	64 mm, 76 mm, 89 mm, 102 mm, 115 mm, 127 mm
Button materials	Tungsten carbide (grades such as YK05, T6)
Body material	High-quality alloy steel, heat-treated
Flushing holes	Typically M.5 mm diameter
Rod cross-sections	Hexagonal (H32) or round (Rnd38)
Rod lengths	1,830 mm to 6,095 mm (6′ to 20′)

Applications of T38 Drill Bits

T38 drill bits are used across a wide range of drilling applications:

Bench drilling: Surface mining and quarry bench drilling
Production drilling: Underground production drilling in mines
Tunneling: Tunnel face drilling and rock support
Construction: Foundation drilling and rock anchoring
Long-hole drilling: Deep hole drilling with extension rods

Typical hole diameters for T38 drilling range from 64 mm to 127 mm, making T38 a versatile choice for medium-diameter blast holes and production drilling.

T38 vs. T45: Which Should You Choose?

While both T38 and T45 are common thread types in top hammer drilling, they serve different purposes. The key difference between T38 and T45 is the safety margin and fatigue resistance—T45 provides additional thread strength for more demanding conditions.

Factor	T38	T45
Thread diameter	~38 mm	~45 mm
Hole diameter range	64–127 mm	66–152 mm
Torque capacity	Moderate	Higher
Best application	Bench drilling, production drilling	Heavy-duty production, deep holes

For most bench drilling and production drilling applications, T38 offers an excellent balance of performance and cost. For deeper holes or more demanding rock conditions, T45 provides additional thread strength and fatigue resistance.

Final Thoughts

The T38 drill bit is a cornerstone of modern top hammer drilling. Its robust trapezoidal thread design, combined with high-quality tungsten carbide buttons, delivers reliable performance across a wide range of rock conditions and applications.

Whether you are drilling blast holes in a granite quarry, production holes in an underground mine, or anchor holes in a construction project, the T38 drill bit offers the versatility, durability, and efficiency that drilling professionals rely on.

Choosing the right bit—with the correct thread type, diameter, button shape, and face design for your specific rock conditions—is essential for maximizing penetration rates, extending tool life, and controlling drilling costs. For most medium-diameter bench and production drilling applications, T38 is the proven, reliable choice.

Frequently Asked Questions

Q1: What does "T38" stand for in a drill bit?

"T" stands for trapezoidal thread, and "38" refers to the nominal thread diameter of approximately 38 mm (1.5 inches). This standardized thread design ensures secure attachment to T38 drill rods and shank adapters.

Q2: What hole diameters can a T38 drill bit handle?

T38 drill bits are available in diameters ranging from 64 mm to 127 mm (2.5″ to 5″). Common sizes include 64 mm, 76 mm, 89 mm, 102 mm, 115 mm, and 127 mm.

Q3: What is the difference between spherical and ballistic button bits?

Spherical (domed) buttons are designed for hard, abrasive rock and offer maximum wear resistance. Ballistic buttons provide faster penetration in medium-hard rock but have shorter wear life. The choice depends on your specific rock conditions and drilling priorities.

Q4: What is a retrac button bit and when should I use it?

A retrac (retractable) button bit has a reinforced skirt design that helps prevent the bit from jamming when pulled back through fractured or broken rock. It is recommended for drilling in challenging ground conditions where standard bits may become stuck.

Q5: What is the typical application for T38 drill bits?

T38 drill bits are widely used in bench drilling, production drilling, tunneling, and construction. Typical applications include blast hole drilling in quarries, production drilling in underground mines, and rock anchoring in construction projects.