Tool Maintenance – Drilling Tools

DTH Button Bit Grinding and Sharpening: When to Grind, When to Replace

By Windrilling Technical Team – Updated September 2026 – 9 min read

A quarry crew in West Africa sent me a video last month. Their 4 inch DTH bit was still turning. Still making hole. But the rig had crept from about 1.6 metres a minute down to 0.7, and the compressor was working harder for less. The buttons on the bit face looked like a bag of gravel – flattened on top, polished, no profile left.

Nobody on that crew had touched the bit with a grinder in three weeks. They had a grinder. They had diamond cups. What they did not have was a rule for when to pick it up. In my experience that missing rule is one of the most expensive blank pages in a drilling contractor’s paperwork.

That is what DTH button bit grinding actually is: a schedule, not a repair. Get the interval right and one bit does the work of two or three. Get it wrong in either direction – never grinding, or grinding every shift for no reason – and you pay either in penetration rate or in button life that you cannot buy back.

Below is how we walk crews through it, from the physics on the bit face to the numbers you write on the log sheet.

1. Why a Blunt Button Costs More Than a Worn One

A button bit cuts by concentrating impact energy into a very small area. A new button has a dome, or in some designs a ballistic shape, and the load from the piston is carried through a contact patch measured in square millimetres. Rock fails under that point load and chips away.

Flatten the top of that button and the contact patch grows. The same impact is now spread across a plate, not a point. To keep breaking rock you have to add weight on bit, and the harder the rig works the more of that energy turns into heat and shock instead of chips. Heat softens the carbide surface, shock starts cracking it, and a cracked button stops absorbing impact properly.

None of this is dramatic. It shows up as a slow slide on the penetration chart. Then one day the bit does not come out of the hole for a regrind – it comes out because a pair of buttons is gone and the steel around them is washing away. That is the failure the crew in the video was two shifts away from.

2. Four Ways a Bit Actually Dies

When a bit comes off the string, put it in one of these four boxes before you decide what to do with it. The box tells you whether it is a grinding decision, a rig-settings decision, or an end-of-life decision.

Failure mode	What you see on the bench	Usual cause	What to do about it
Button wear flat	Shiny, mirror-like flat on the dome of the button; bit still structurally sound	Normal abrasion, extended by too long between regrinds	Regrind. This is the cheap problem – and the one that causes most of the expensive ones
Gauge wear	Outer diameter has shrunk; the bit no longer fits the gauge ring; the hole comes in undersize	Peripheral buttons carry the side load, plus string whip or misalignment	Assess the remaining gauge. If it cannot be restored, the bit is finished – an undersize bit makes the next bit ream
Steel body wash	Flushing holes and grooves eroded, sometimes with a washed groove behind the buttons	Abrasive cuttings at high air velocity, or insufficient flushing	Often scrap. Check air volume and cleaning before the replacement bit runs
Cracked or loose buttons	Spalled tops, hairline cracks, buttons sitting low or rattling in the hole	Over-grinding, excess weight on bit, heat, or an abrasive-and-wet combination the bit was not matched to	Retire. A button with no support underneath will leave the hole on the next run

One more mode is worth a mention even though it is not fixed by grinding: worn or broken shank splines. If the shank is rocking in the driver sub, the bit is hammering off-axis and the buttons will wear unevenly no matter how well you sharpen. Our field troubleshooting guide covers the alignment side of that.

3. The Interval Rule: Watch the Wear Flat, Not the Calendar

Grinding on a fixed day of the week is the wrong discipline. A bit in abrasive granite needs attention at a fraction of the metres that a bit in soft sandstone will run. Published guidance from major rock tool manufacturers and DTH bit suppliers converges on the same threshold, but it is a wear threshold, not a clock.

• The wear-flat threshold. Resharpening guidance published by Boart Longyear, Sandvik and several DTH bit suppliers all point at the same idea: sharpen the bit before the flat on a button exceeds roughly one third of the button diameter. Some aftermarket grinding guides widen the practical window to about 25-50% of the original button diameter.
• The protrusion check. The same body of guidance notes that when the carbide protrusion above the steel falls below about half the button diameter, cutting performance drops off sharply. At that point you are grinding to recover performance, not to protect it.
• The metres that usually fall out of it. A commonly quoted field interval in hard rock is 150-200 metres between regrinds, with five to six regrind cycles before the bit is retired. Treat it as a starting number for your own site, not a universal truth – rock type moves it more than any other variable.

In our own job files the field signal that arrives before the threshold does is a rate drop. When penetration falls noticeably at the same weight on bit, the same rotation speed and the same air pressure, walk over and look at the buttons before you look at the compressor. Nine times out of ten the buttons have flattened and the bit is being asked to do work it can no longer concentrate.

One exception worth knowing. If the rate drops and the buttons look fine, the problem is probably not the bit at all. Check the flushing first – recirculated cuttings are regrinding the bit face for you – and then the air package. The DTH hammer air requirements guide walks through that calculation properly.

4. How to Measure Wear on the Bench

You do not need a metrology lab. Three checks, done the same way every time, will tell you more than any opinion about “it still looks all right”.

Check	How to do it	What it tells you
Gauge ring	Drop the ring gauge over the bit head. If it passes over, hold it tight to one side and measure the gap on the opposite side – a common field practice is to divide that gap by three to estimate how much the bit is under gauge.	Whether the bit will still drill the hole size your casing and the next bit expect
Wear flat	Measure across the flat on the crown of each button with a vernier or a flat-wear gauge, and compare it with the button diameter	Whether you are still inside the one-third threshold or already past it
Button protrusion	Measure the diameter across the buttons against the diameter across the gauge. One major rock tool maker’s inspection sheet puts the target at roughly 0.8 mm (0.03 in) of protrusion beyond the gauge diameter.	How much support material is left. Once the buttons sit almost flush with the steel, grinding cannot recover much

Write the three numbers against the bit serial number, not against “the bit”. If you run four rigs with twelve bits between them, “the 4 inch one” stops meaning anything by Thursday.

5. What the Grinder Should Actually Take Off

The purpose of a regrind is not to produce a sharp point. It is to restore the button profile – dome or ballistic, whichever the bit was designed with – and to leave the buttons concentric with their holes.

• Leave a small flat on the crown. Sandvik’s published sharpening guidance is explicit about this: leave a little flat on the top and do not grind more than necessary. A perfect needle point on a carbide button has almost no material behind the tip, and one hard stringer will break it off.
• Take off the minimum. Grind until the flat is gone and the profile is back, then stop. Every extra tenth of a millimetre you remove is a tenth of a millimetre of usable button life, and it brings the steel body closer to the unsupported zone.
• Keep the grind concentric. Grinding off-axis produces a chisel edge. A chisel edge cuts worse than a flat and breaks faster, because the impact load is no longer going down the axis of the button.
• Do not grind the steel away. The steel immediately around each button is what keeps the button in place under impact. Clearing “just a little” more room around a button is how buttons get pulled.
• Keep it cool. Interrupted grinding with light pressure, and never grind a hot button. Carbide and its braze do not enjoy thermal shock any more than they enjoy mechanical shock.

6. Hand Grinder or Grinding Machine?

This is the question that decides what your regrind actually looks like six months from now. Both routes work. They do not produce the same bit.

	Hand-held grinder with a diamond cup	Bench or pneumatic bit grinder with a jig
Setup cost	Low. One grinder, one cup wheel, one dressing stone	Higher, but small against the price of the bits it protects
Angle control	Entirely down to the operator’s hand and eye	Fixed by the fixture. The same angle on every button, every time
Speed	Fast on one bit, slower to do a whole set properly	Fast across a bit set, and repeatable by any trained hand
Best for	One rig, occasional touch-ups, mid-job recovery	Contractors running several rigs to a regrind schedule

My honest position on this: a hand grinder with a diamond cup in the hands of someone who has been shown the one-third rule and given a jig or a steady rest will beat a machine in the hands of someone who has not been shown anything. The tool matters less than the standard. But if you are sharpening more than a handful of bits a month, the machine pays for itself in cycle consistency alone – because inconsistent grinding is what turns a bit you could have reground five times into a bit you retire once.

7. Grinding or Replacing: The Arithmetic

A regrind costs you a few minutes of rig time and a little diamond wheel. A replacement bit costs the bit, the freight, the customs paperwork and the days of production you spend waiting for it. That comparison is not close, and it is the reason the interval rule exists.

The comparison flips the moment the button is no longer recoverable. Then grinding is not maintenance, it is a way of delaying a purchase while paying for it in metres per shift.

Condition on the bench	Decision	Why
Flat wear inside the one-third threshold, gauge still on size	Regrind and run	The cheapest metres you will drill with that bit
Flat wear past the threshold, buttons still supported, gauge recoverable	Regrind, and shorten the next interval	You lost performance by waiting; the fix is the schedule, not the bit
Gauge worn beyond recovery	Retire from production holes	An undersize bit drills an undersize hole and loads the next bit with reaming work
Cracked, spalled or loose buttons	Retire	A button with no support left will end up in the hole, not on the bit
Body wash between the flushing holes	Retire, then check air and flushing	The replacement will die the same way if the cause is not fixed
Sound body, worn buttons, and a workshop that re-tips	Consider re-tipping	Legitimate on some TCI designs – but ask about the button grade and the bearing or body condition, because a good button set on a tired body is a short-term tool

One habit that pays for itself immediately: keep one reground bit on the truck as the spare, not a new one. It forces the crew to learn what their own regrind quality actually looks like in the hole, at a moment when it is not a crisis.

8. What to Write in the Grinding Log

You do not need software. A page per bit is enough, and after two months it will tell you more about your rock and your crews than any supplier can.

Column	Example entry	What it is for
Bit serial and size	SN 0412 – 4 inch, 5 inch shank	Ties every number to one physical object
Rock drilled	Granite with quartz bands	Explains why two bits in the same fleet behave differently
Metres since last grind	172 m	Builds your own interval to replace the generic 150-200 m
Wear flat and gauge reading	Flat 3.1 mm on a 12 mm button; gauge 0.9 mm under	Shows whether the interval is early, right, or late
Action and operator	Reground, crown profile restored – J.M.	Makes regrind quality traceable, which is how you find the bad grind before it costs a bit

Two months of that log turns into a real number: your metres per bit, per grind, in your rock. That is the number you use when you buy the next batch, and the number that tells you whether a cheaper bit is actually cheaper. We have written about that trap separately in the DTH hammer and drill bit selection guide and in the DTH hammer price guide.

9. Where the Consumables Fit

A regrind schedule only works if the rest of the string is holding up its end. In our experience the three items that decide whether a sharp bit stays sharp are the bit itself, the shank adapter that transmits the impact, and the drill pipe that keeps everything on axis.

• The bit. Our DTH drill bits are supplied with the button profile and gauge matched to the formation you tell us about, in the shank sizes that fit the hammers we stock.
• The shank adapter. A worn shank adapter lets the bit rock in the driver sub, and off-axis impact wears buttons unevenly no matter how careful your grinding is.
• The complete package. Bits, hammers, rods and adapters can leave on one pallet rather than three shipments – see the full drilling tools line, or our kits and consumables for the items that disappear from site fastest.

If you are setting up a regrind bench from scratch, tell us what bit sizes and shank types you run. We will tell you which grinding consumables and spares you actually need, and – just as useful – which ones you can skip.

10. FAQ

How often should I regrind a DTH button bit?

By wear, not by time. The published threshold from the major bit makers is roughly one third of the button diameter of flat wear, and 150-200 metres is a commonly quoted interval in hard rock. Start there, log your own numbers, and adjust for your formation.

Can you grind a button bit too much?

Yes, and it is a common mistake. Grinding past the point where the profile is restored shortens button life, thins the steel support around the button, and risks cracking from heat. Sandvik’s guidance to leave a little flat on top and to grind no more than necessary is the right standard.

How do I know when a DTH bit is finished?

Four signs: gauge wear you cannot recover, buttons ground below the point where the steel still supports them, cracked or loose buttons, and body wash between the flushing holes. Any one of those turns a bit from a maintenance item into a replacement item.

Is a hand-held angle grinder good enough for DTH bits?

For occasional regrinds, yes, if the operator keeps the grind concentric and stops at the profile. A jig or steady rest makes a real difference. For a fleet running to a schedule, a bench or pneumatic bit grinder gives you consistency that hand grinding rarely matches.

Does regrinding change the diameter of the hole?

Grinding buttons does not change the gauge diameter of the bit – gauge is set by the body and the peripheral buttons. What changes the hole is gauge wear. Once the bit drills under size, every following bit has to ream the hole, which is why gauge is a replacement decision and not a grinding decision.

Before You Order

Send us four things and we will come back with a bit and a regrind plan you can defend: the bit size and shank type you run, the hardest rock in the interval, the metres you are getting per bit today, and whether your crew grinds by hand or on a machine. If a cheaper button grade will hold up in your ground, we will say so. If it will not, we will tell you that too – a bit that fails on site costs us more than the margin on the better one.