drill rods thread types comparison

Every hole a rock drill cuts is the result of energy carried down a string of steel. From the rotation head to the drill rods and finally to the bit, torque, feed force and impact energy all pass through threaded connections. Choose the wrong thread family and you lose energy at every joint, wear parts faster, and risk a string that unscrews or snaps halfway down the hole. Understanding how the main thread types differ is the first step to buying the right equipment for your rig.

Why the thread type matters

A drill rod thread is far more than a simple way to screw two pieces together. It connects the rod to the thread button bit, the adapter and the coupling, and it is the exact point where mechanical energy is transferred with the least possible loss. A well-matched thread distributes stress evenly across the flank, keeps the connection tight under high vibration and shock, and lets the whole assembly survive thousands of drilling cycles without loosening or cracking at the joint.

In practice, top-hammer threaded extension rods come in a small number of families. Over the years manufacturers used conical pipe threads, old trapezoidal threads and serrated threads in turn, and most of them disappeared because their small pitch and small bearing area concentrated stress, wore out fast, and wasted energy. The designs that survived are the ones that transmit power efficiently, never loosen under shock loading, and are easy to make and to run.

The R thread: the workhorse for medium and small boreholes

The corrugated R thread is the oldest, most widely adopted thread structure in threaded extension drilling. Its dimensions were revised many times and finally written into ISO standards, so R thread rods from different suppliers are highly interchangeable. It is the main thread in the medium and small cross-section range and covers the most common sizes: R25, R28, R32 and R38.

• After surface hardening such as carburizing, R threads offer good wear resistance.
• Energy loss across the connection is low.
• Threading and unthreading is quick and convenient on site.
• The profile can be cut on a special lathe, which keeps manufacturing cost down.

The T thread: built for heavy impact and larger holes

The trapezoidal T thread appeared in the 1970s as hydraulic rock drills took over. Because of its stable helix angle, low tightening torque, large pressure-bearing contact surface and generous allowance for wear, it carries high impact energy for a long service life. For this reason it has become the main connection on medium and large cross-section rods, in the common sizes T38, T45 and T51.

Trapezoidal threads distribute impact stress more evenly along the flank, which gives them noticeably better fatigue life under sustained heavy drilling than the R profile. This is why T thread is standard on production and quarry rigs where the drifter is powerful and the hole is large. The trade-off is that R and T threads are not interchangeable, so rod and bit must always share the same thread designation.

R thread vs T thread at a glance

How to choose the right thread for your job

There is no universally best thread, only the right thread for a given rig and rock. Three things usually decide the choice:

• Drilling depth and hole size. As a general rule, holes up to around 45 mm suit R32, holes in the 45–78 mm range suit R38 or T38, and larger production holes suit T45 and T51.
• Ground conditions. Hard, abrasive and broken rock puts far more stress on the joint, which argues for the stronger T profile on larger bits.
• Drifter power. A high-frequency hydraulic rock drill delivers strong impact energy, so the connection must be rated for it or you will suffer premature wear and fatigue at the thread.

Whichever you settle on, keep the whole string on the same thread designation. A rod, adapter and thread button bit that do not match will vibrate, wear unevenly and can separate under load, so match them by thread, not just by diameter.

Where to find dependable drill rods and thread tooling

Building a reliable drilling rock drilling tool string comes down to choosing a manufacturer that controls quality from the raw steel bar to the finished thread. TY Drill Bits, an ISO9001-certified Chinese manufacturer, has supplied rock drilling tools worldwide since 2010. Its range covers R25, R32, T38, T45 and T51 threaded equipment, hexagon shank adaptors, tapered rods and DTH tooling, so you can build a complete, correctly matched string in one place and have it delivered within about 15 working days.

Before you buy, confirm the thread family of your existing rods and drifter, measure the rod diameter, and share your hole depth and rock type with your supplier. With the right thread selection, your drill rods and bits will hold the connection longer, waste less energy, and cost you less per meter drilled.