How to Maintain a Thread Button Bit

Cleaning is where maintenance really starts, and it should happen at the end of every shift, not just when the bit is about to be changed. Undercut rock flour, wet mud and cuttings pack into the flushing holes and the thread flanks. Left in place, this abrasive paste grinds away at the connection the next time you screw the bit on and off, and it traps moisture against the steel. Rinse the bit with a stream of clean water to loosen the cake, then use a wire brush to work the remaining cuttings out of the gauge, the button faces and the thread channels. Pay attention to the flushing holes; every blocked passage means less air or water reaching the buttons and faster heat buildup on the next run.

The thread is the weakest link on a threaded button bit, and it is the part people check the least. Before every make-up, run the bit onto the rod by hand and feel for any roughness, binding or rocking. Examine the thread flanks closely for deformed or worn teeth, pulled streaks, galling or shiny fatigue lines. A thread that feels loose on a new rod is a warning, not a minor inconvenience; it cannot transfer the percussion energy delivered by your rock drilling tools, so it starts knocking, overheating and wasting energy that should go into the rock. If you see damaged threading, stop using the bit immediately. A small nick can be dressed by hand, but a stretched or galled connection is unsafe to run.

The single most reliable way to judge a button bit is by the width of the flat worn onto the tungsten carbide buttons. As the buttons work against hard rock, a wear flat grows across the top of each button. When that flat reaches roughly a third of the button diameter, penetration has already started to ｄｒｏｐ and it is time to regrind. Let it grow to about half the button diameter and you will be losing anywhere from twenty to thirty percent of your drilling rate, and the risk of chipping or fracturing the carbide becomes much higher. Catching the bit at the one-third stage keeps the buttons running at their sharpest and slows the overall wear of the head.

Alongside the flat, watch the button projection. When the buttons sit noticeably lower in the head, usually below the point where they project about three or four millimeters, the steel has started to contact the rock and the bit is nearing the end of its life. A button that is missing, cracked or spalled is a separate problem; stop using the bit and ｒｅｐｌａｃｅ that button head rather than grinding around a damaged ｉｎｓｅｒｔ.

Regrinding is the cheapest life extension available for a button bit. All it takes is a suitable grinder and a diamond cup wheel whose inner diameter is about one to two millimeters larger than the button being dressed, with a profile that matches the button shape. Work each button in short, light passes and let the wheel do the cutting rather than forcing it. Start with the center or face buttons and move from the middle of the head toward the edge, then finish the gauge or outer row of buttons. Keep the passes brief, because overheating the carbide creates micro-cracks that shorten button life faster than any amount of wear.

After grinding, check that all the buttons sit at similar height and that each wear flat has been removed evenly. A freshly dressed bit should have a clean, crisp profile on every button with no burned or glazed-looking tips. If you are not comfortable regrinding by hand or you need to dress a large lot of heads between projects, it is worth sending the bits to a supplier who can complete the grinding correctly with the right fixtures.

You do not always need to pull the bit to know it is dull. A reduction in penetration with the same feed pressure, rising torque or air pressure to hold a constant rate, a bore that begins to wander or run off gauge, or extra vibration and hammering are all typical signs that the buttons have worn flat and the bit needs regrinding. An unevenly worn gauge row often shows up first as a wobbling orbit, and it is a strong hint that the bit should come out of the hole before the head geometry itself is damaged. Slowing down a little to regrind on time is always cheaper than redrilling a crooked or reamed hole.

Most corrosion damage happens in storage, not underground. Before putting a bit away, dry it thoroughly, coat the exposed steel surfaces, especially the thread, with a light rust-preventive oil or thread compound, and keep it in a dry, ventilated area where it will not knock against other heads. Never rest bits on their button faces; stand them upright or hang them by the shank so the buttons and the gauge stay protected. For bits running in hot conditions, choose a thread compound with copper or similar anti-seize additives so the connection does not bind on the rod after a long, hot cycle.

Finally, keep a simple record for each head. Note how many meters it drilled, how often it was reground and what the buttons looked like when it came out. A few numbers on a board tell you which suppliers and which sizes give you the best life, and they make it easy to decide when a bit has earned retirement.