Short answer
There are two basic systems in rock drilling, and the difference is where the hammer sits. In top hammer drilling the impact is generated above the machine and transmitted down the drill string; it is fast, but energy loss grows with depth. In down-the-hole (DTH) drilling the hammer sits directly behind the bit, inside the hole; there is no transmission loss, which makes it superior for deep, straight holes. Bit selection builds on that system difference by adding rock hardness and abrasivity.
Two systems, two energy paths
In top hammer drilling, impact energy travels from the hammer above the machine to the bit through a string of rods. Every rod joint loses some energy, so drilling speed falls as the hole deepens and the risk of hole deviation grows.
In DTH drilling the hammer sits directly behind the bit and advances inside the hole. Because impact is applied straight to the bit, depth does not cause transmission loss. The hole also stays straighter.
In practice that means: for shallow holes in large numbers, top hammer is faster and more economical; for deep, straight, large-diameter holes, DTH is superior.
What button bit geometry tells you
- Button shape: ballistic buttons penetrate faster in soft and medium-hard rock; spherical buttons last longer in hard, abrasive rock.
- Button layout and count: face buttons do the drilling, gauge buttons maintain hole diameter. If gauge buttons wear out early the hole narrows and the bit binds.
- Face geometry: flat face, concave face and drop-centre options vary with rock type and the drilling speed wanted.
- Flushing holes: designed to carry cuttings out of the hole; blockage lowers drilling speed directly.
Selection logic by rock type
In soft, low-abrasion rock a ballistic button bit advances quickly and button wear is slow. Here the main criterion is drilling speed.
In hard, abrasive rock a spherical-button, durable bit is preferred. Drilling speed falls but bit life rises markedly; counting the time spent changing bits, total productivity comes out higher.
In fractured rock with discontinuities the risk of binding rises; gauge button condition should be checked more often in such formations.
This selection should be reviewed as layers change within the same quarry; fixing one bit type for the whole quarry costs output.
The right way to measure bit life
Measure bit performance in metres drilled, not in holes drilled. If hole depths vary, hole count is misleading. Cost per metre is the only honest way to compare different bit types, and keeping that record takes a few minutes.
Working mistakes that shorten bit life
- Insufficient air or water flushing: if cuttings do not leave the hole, the bit regrinds material it already cut and buttons wear fast.
- Excessive feed pressure: pushing the bit too hard into the rock breaks buttons and does not raise drilling speed.
- Continuing with a worn bit: once gauge buttons are gone the hole narrows, the next bit binds and the rods are strained.
- Wrong rod and bit match: a combination whose connection geometry does not suit both loses energy and tires the joint.
- Missing the regrinding window: a bit reground on schedule drills more total metres than one run until it is finished.