A contractor in Guangdong Province burned through three full sets of bullet teeth in just 50 meters of granite. Each tooth costs $12. Each set had 24 teeth. The total bill: $864 in wear parts alone, and the rig sat idle for six hours while the crew swapped them out. The problem was not the rock. It was the wrong tooth for the job. He was running B47K19H teeth rated for 70 MPa maximum in an 85 MPa formation. Once he switched to B47K25H heavy-duty teeth and dropped his RPM from 30 to 18, tooth life jumped to 55 meters per set. His cost per meter dropped 37 percent.
Hard rock does not forgive the wrong tool. When you are drilling foundations in formations above 60 MPa, standard bullet teeth wear fast, break often, and drive your costs through the borehole. The good news: choosing the right bullet teeth for hard rock is not guesswork. It is a matter of matching the carbide size and steel grade to the exact compressive strength of your formation.
This guide gives you an MPa-based selection framework, exact model specifications, drilling parameter recommendations, and real project data you can apply to your next job.
Want the complete picture on bullet teeth across all ground conditions? See our complete bullet teeth selection guide for the full model range and application overview.
What Makes Hard Rock Different for Bullet Teeth?
Rock hardness is measured in megapascals (MPa), which tells you how much compressive force the formation can withstand before it fractures. For foundation drilling, the scale breaks down like this:
- 20-60 MPa: Weathered rock, soft limestone, mixed ground. Standard bullet teeth handle this range well.
- 60-100 MPa: Hard limestone, sandstone, conglomerate, weathered granite. This is where bullet teeth for hard rock become essential.
- 100+ MPa: Fresh granite, basalt, quartzite. Bullet teeth reach their limit here.
In hard rock, three forces attack your teeth simultaneously. First, concentrated impact forces from each rotation strike the carbide tip with thousands of micro-impacts per minute. Second, abrasive wear from silica and quartz content slowly grinds the tungsten carbide away. Third, thermal cycling from friction heat followed by cooling mud causes micro-cracks in the steel body.
Standard teeth simply are not built for this. Their smaller carbide tips overload. Their softer steel bodies fatigue. And their self-rotation mechanism seizes when debris packs into the holder.
The 100 MPa threshold is the practical ceiling for most bullet teeth. Above that, roller bit core barrels become the more economical choice. We cover that switch point in detail later.
If you want to understand the applications of Bullet Teeth, please check out our article about How Bullet Teeth Are Used.
Best Bullet Teeth Models for Hard Rock
Not all bullet teeth are created equal. The B47K series is the industry standard for rotary drilling, and the model number directly relates to carbide tip size. Here is how each model performs in hard rock.
B47K19H: The All-Rounder (Up to 70 MPa)
The B47K19H carries a 19 mm tungsten carbide tip on a 30 mm shank. It weighs roughly 0.8 kg. The carbide grade is typically YG11C, and the body uses 42CrMo alloy steel heat-treated to HRC 48-53 on the upper section.
This model works well in weathered rock, medium-hard limestone, and mixed ground where you encounter occasional cobbles. In soft conditions, service life reaches 150+ meters. In hard rock approaching 70 MPa, expect 80-100 meters. It is the tooth many contractors default to, but that default is a mistake when the formation pushes past 70 MPa.
B47K22H: The Hard Rock Standard (60-100 MPa)
The B47K22H is the workhorse for hard rock foundation drilling. Its 22 mm carbide tip provides 35 percent more cutting material than the B47K19H. The shank remains 30 mm, so it fits standard holders on Bauer, Soilmec, and Liebherr rigs. Weight is approximately 1.0 kg.
The carbide is YG11C or YG13C grade, with a hardness of HRC 88-92. The 42CrMo body runs HRC 48-53 on the upper section and HRC 38-42 on the shank. According to manufacturer selection charts from FES, the B47K22H is rated for formations within 70 MPa, but field data shows it performs reliably up to 100 MPa when drilling parameters are optimized.
In 60-80 MPa limestone, service life averages 40-80 meters. In 80-100 MPa granite, life drops to 30-50 meters. This is the model that handles 90 percent of hard rock foundation work.
B47K25H: The Heavy-Duty Option (80-120 MPa)
When the formation pushes past 90 MPa, the B47K25H steps in. Its carbide tip measures roughly 25 mm, built from super coarse particle cemented carbide for maximum wear resistance. The body is reinforced with 42CrMoA alloy steel with additional cladding on high-wear zones.
Manufacturer data places this model in the 35-90 MPa range, but field experience in granite and basalt shows effective performance up to 120 MPa. Service life in 80-100 MPa rock runs 30-60 meters. In very abrasive granite, expect the lower end of that range.
This is the tooth the Guangdong contractor should have been running from day one.
B47K30H: The Extreme Rock Specialist (100-150 MPa)
The B47K30H carries a 30 mm carbide tip, the largest in the standard B47K range. It is built for extreme conditions where roller bits are not yet justified, typically due to shallow depth or mixed ground transitions.
Service life in 100+ MPa formations runs 20-40 meters. Cost per meter climbs, but the upfront investment is still far below roller bit assemblies. Use this model as a bridge when you know the formation will vary above and below 100 MPa within the same borehole.
| Model | Carbide Size | MPa Range | Service Life (Hard Rock) | Weight | Best Formation |
|---|---|---|---|---|---|
| B47K19H | 19 mm | Up to 70 MPa | 80-150 m | ~0.8 kg | Medium limestone, weathered rock |
| B47K22H | 22 mm | 60-100 MPa | 40-80 m | ~1.0 kg | Hard limestone, sandstone |
| B47K25H | ~25 mm | 80-120 MPa | 30-60 m | ~1.2 kg | Granite, basalt, high-abrasion |
| B47K30H | 30 mm | 100-150 MPa | 20-40 m | ~1.4 kg | Extreme rock, quartzite |
Want to learn more about the series? Please check out our article on B47K, C31HD, and BK Series Bullet Teeth.
Matching Bullet Teeth to Rock Hardness (MPa Guide)
The most common mistake in hard rock drilling is using the same tooth across different formations. Here is the exact matching framework:
30-50 MPa (weathered rock, soft limestone): B47K19H at standard RPM. This is well within the comfort zone. You get long life and fast penetration.
50-70 MPa (medium limestone, sandstone): B47K19H or B47K22H. If the formation is consistent at the lower end, the B47K19H works. If you hit occasional hard bands, step up to the B47K22H.
70-90 MPa (hard limestone, granite, basalt): B47K22H or B47K25H. This is the critical decision zone. The B47K22H handles the lower end. At 85 MPa and above, the B47K25H becomes the safer choice.
90-100 MPa (very hard granite, quartzite): B47K25H or B47K30H. Expect rapid wear. Inspect teeth every 20 meters.
100+ MPa: Transition to roller bit core barrels. Bullet teeth wear 3-5x faster above 100 MPa than they do at 60-80 MPa. The economics flip.
| Rock Type | MPa Range | Recommended Model | Penetration Rate |
|---|---|---|---|
| Weathered rock | 30-50 MPa | B47K19H | 6-10 m/h |
| Medium limestone | 50-70 MPa | B47K19H / B47K22H | 4-6 m/h |
| Hard limestone | 70-90 MPa | B47K22H / B47K25H | 2-3 m/h |
| Granite / Basalt | 90-100 MPa | B47K25H / B47K30H | 1-2 m/h |
| Very hard rock | 100+ MPa | Roller bits | 0.4-0.7 m/h |
Optimizing Drilling Parameters for Hard Rock
The right tooth is only half the equation. Drilling parameters in hard rock are fundamentally different from those in soft ground.
Rotation Speed (RPM)
In soft soil, faster rotation improves penetration. In hard rock, excessive RPM destroys teeth.
For 60-100 MPa formations, target 15-25 RPM. Slower rotation reduces impact frequency and gives each tooth time to initiate a fracture before the next rotation loads it again. At 30+ RPM, the carbide tip polishes the rock surface instead of cutting it. Heat builds up. Wear accelerates.
The Guangdong contractor dropped from 30 RPM to 18 RPM. Penetration rate did not drop. In fact, it stabilized because the teeth were actually cutting instead of skidding.
Feed Pressure
Insufficient feed pressure is the silent killer of bullet teeth in hard rock. Without enough down force, the carbide tip cannot exceed the rock’s compressive strength. The tooth rubs instead of cuts. The carbide overheats. Micro-cracks form.
For B47K22H in 60-80 MPa rock, target 150-250 kN of feed pressure. Match the pressure to your rig’s weight and the formation response. If the rig bounces, you are too high. If penetration stalls, you are too low.
Torque Requirements
Hard rock demands torque. A rig with insufficient torque will stall under load, causing the teeth to seize in their holders. The self-rotation mechanism stops working. Wear concentrates on one side of the carbide. The tooth fails early.
Minimum 120 kNm torque is recommended for consistent hard rock penetration. If your rig cannot deliver this, consider smaller diameter holes or step up to a larger rig.
Debris Removal
In hard rock, debris is coarse and heavy. If it is not cleared efficiently, it regrinds between the tooth and the formation. This triples abrasive wear.
Use high-volume mud circulation or air flush. Monitor return flow. If cuttings are not coming up, stop drilling and clear the hole before continuing.
Penetration Rates in Hard Rock
Penetration rate (ROP) is the measure of how fast you advance through the formation. In hard rock, realistic expectations prevent frustration and poor decision-making.
With B47K22H bullet teeth:
- 20-60 MPa weathered rock: 6-12 m/h
- 60-80 MPa hard limestone: 3-4 m/h
- 80-100 MPa granite/basalt: 2-3 m/h
- 100+ MPa: 0.3-1.0 m/h (bullet teeth not recommended)
Bullet teeth drill 20-30 percent faster than flat teeth in medium-hard rock because the conical point concentrates force. A B47K22H has roughly 300 mm² of contact area versus 1,200 mm² for a flat tooth. With identical down pressure, the bullet tooth generates approximately 4x the pressure at the cutting face.
However, this advantage diminishes as rock hardness increases. Above 100 MPa, even the concentrated point force struggles to exceed compressive strength. That is why roller bits with their multi-tooth cutting action and percussion capability take over.
Cost Analysis: Cost Per Meter in Hard Rock
Upfront tooth price is the wrong metric. Smart contractors calculate cost per meter drilled.
B47K22H scenario:
- Tooth price: ~$15 average
- Service life in 70 MPa limestone: 60 meters
- Teeth per tool (typical rock auger): 24
- Total tooth cost per tool change: $360
- Cost per meter: 360/60=0.30/m
B47K25H scenario:
- Tooth price: ~$22 average
- Service life in 85 MPa granite: 45 meters
- Teeth per tool: 24
- Total tooth cost: $528
- Cost per meter: 528/45=0.59/m
Roller bit core barrel scenario:
- Assembly price: ~$1,800
- Service life: 300 meters
- Cost per meter: 1,800/300=6.00/m
The math is clear. Even with rapid wear, bullet teeth cost one-tenth of roller bits per meter up to 100 MPa. The break-even point where roller bits become cheaper is typically around 110-120 MPa, or when depth exceeds 40 meters and the efficiency gap compounds.
Common Failure Modes in Hard Rock (and How to Prevent Them)
Understanding why teeth fail helps you prevent failures before they happen.
Carbide Spalling
Spalling occurs when the tungsten carbide tip chips or flakes under impact overload. You see it as small missing chunks on the carbide surface.
Cause: Wrong carbide grade for the formation, excessive RPM, or feed pressure that exceeds the carbide’s impact tolerance.
Prevention: Match carbide size to MPa. Use YG13C grade for formations above 80 MPa. Keep RPM under 25 in hard rock. Verify feed pressure is within spec.
Shank Fracture
A clean break across the steel body, usually just below the carbide insert.
Cause: Fatigue from high-frequency impact loading, manufacturing defects, or insufficient body hardness.
Prevention: Verify HRC 48-53 on the upper body. Inspect new teeth for cracks before installation. Replace teeth before 50 percent wear, as worn teeth transfer more shock load to the shank.
Holder Weld Cracking
Cracks appear in the weld between the tooth holder and the auger or bucket body.
Cause: Vibration from worn or broken teeth transfers excessive shock to the holder.
Prevention: Inspect holders daily. Replace worn teeth immediately. Do not run with broken teeth, even for one more meter.
Tooth Seizure in Holder
The tooth stops rotating in its holder. Wear concentrates on one side. The tooth fails asymmetrically.
Cause: Rock dust and mud pack into the holder bore. Insufficient torque from the rig prevents the self-rotation mechanism from working.
Prevention: Clean holders with a wire brush every 50 meters in abrasive ground. Verify each new tooth spins freely before installation.
Real-World Scenario: Three Projects, Three Lessons
Lesson 1: Guangdong Granite (85 MPa)
A foundation crew in Guangdong hit 85 MPa granite with occasional 95 MPa quartz veins. They started with B47K19H teeth at 30 RPM. After 35 meters, half the teeth showed carbide spalling. Two teeth had shank fractures. The crew swapped the full set. The same thing happened again at 38 meters.
The project engineer is called Changsha Mingyi. We recommended B47K25H teeth, 18 RPM, and 200 kN feed pressure. The next set lasted 55 meters. Penetration held steady at 2.1 m/h. Cost per meter dropped 37 percent.
Lesson 2: Turkey Limestone (65 MPa)
A contractor in Izmir drilled 120 boreholes through 65 MPa limestone for a commercial building foundation. He standardized on B47K22H teeth from the start, ran 22 RPM, and kept feed pressure at 180 kN.
Average tooth life: 72 meters. Longest set: 89 meters. Shortest set: 58 meters (through a harder dolomite band). Total project tooth cost: $4,320 for 120 holes, averaging 25 meters deep. The contractor reported zero unplanned downtime from tooth failures.
Lesson 3: Malaysia Mixed Ground (50-80 MPa)
A project in Kuala Lumpur encountered highly variable sandstone, with layers shifting between 50 MPa and 80 MPa within single boreholes. The crew used a hybrid configuration: B47K19H on the outer perimeter, where cutting speed is highest, and B47K22H in the center, where the load concentrates.
This setup balanced cost and durability. The outer teeth lasted 90 meters. The inner teeth lasted 65 meters. The crew replaced the inner set twice and the outer set once across the project. The total cost was 18 percent lower than running all B47K22H teeth.
For help making this decision based on your specific ground conditions, our guide on how to choose bullet teeth walks through a step-by-step selection process.
When to Switch from Bullet Teeth to Roller Bits
The 100 MPa threshold is the rule of thumb, but the real decision depends on more than rock hardness.
Depth matters. At shallow depths under 20 meters, bullet teeth remain economical even above 100 MPa because setup time for roller bits eats into savings. At 40+ meters, roller bit efficiency compounds. The deeper you go, the sooner the switch pays off.
Downtime matters. If your crew can swap bullet teeth in 45 minutes but roller bit setup takes half a day, the economics shift. Calculate total downtime cost, not just tooth cost.
Formation consistency matters. In mixed ground where 80 MPa bands alternate with 110 MPa layers, a hybrid approach works best. Some contractors run bullet teeth on the outer row for the softer zones and roller bits on the inner cutting face for the hard bands.
For the full comparison, including penetration rates, cost-per-meter, and service life data, see our detailed guide on when to switch to roller bit core barrels.
Maintenance Tips for Hard Rock Conditions
Hard rock accelerates every wear mechanism. Your maintenance routine should match that reality.
Daily inspection checklist:
- Check 10 teeth at random for carbide wear. Replace any tooth below 50 percent carbide height.
- Verify all teeth rotate freely in their holders.
- Inspect holder welds for hairline cracks.
- Check return cuttings. Coarse, angular chips indicate good cutting. Fine powder indicates polishing, which means your RPM is too high or your feed pressure is too low.
- Measure penetration rate per shift. A 20 percent drop is an early warning of accelerated wear.
Replace at 50 percent wear in hard rock. In soft ground, contractors often push to 70 percent. In hard rock, that extra 20 percent costs you holder damage and shank fractures.
Clean holders every 50 meters in abrasive ground. Granite and basalt produce fine silica dust that packs into holder bores. A wire brush and compressed air take five minutes. A seized tooth takes an hour to extract.
Store spare teeth dry. Moisture on the shank causes corrosion that accelerates seizure in the holder. Keep inventory in sealed containers with desiccant.
Frequently Asked Questions
What is the best bullet tooth for hard rock?
The B47K22H is the best all-around bullet tooth for hard rock in the 60-100 MPa range. For formations above 90 MPa or highly abrasive granite and basalt, step up to the B47K25H. For extreme conditions near 100-120 MPa, the B47K30H provides maximum wear resistance.
How long do bullet teeth last in hard rock?
B47K22H teeth last 40-80 meters in 60-100 MPa hard rock. B47K25H teeth last 30-60 meters in 80-120 MPa very hard rock. Life depends on rock abrasiveness, drilling parameters, and rig torque. Consistent inspection and proper RPMs extend life toward the upper end of the range.
What MPa is too hard for bullet teeth?
100 MPa is the practical ceiling for most bullet teeth. Above this threshold, wear accelerates 3-5x and the cost per meter exceeds roller bit economics. The B47K30H can handle up to 120 MPa in shallow holes, but roller bit core barrels are the standard choice for sustained drilling above 100 MPa.
How fast do bullet teeth drill in hard rock?
Bullet teeth achieve 2-3 m/h in 60-100 MPa hard rock with proper parameters. In 20-60 MPa weathered rock, rates climb to 6-12 m/h. Above 100 MPa, penetration drops to 0.3-1.0 m/h, which is why roller bits take over.
Why do my bullet teeth break in hard rock?
The four main causes are: (1) wrong model for the MPa range, (2) excessive RPM causing impact overload, (3) insufficient feed pressure causing polishing and heat buildup, and (4) worn teeth transferring shock loads to the shank. Match the model to the formation, slow your RPM, verify your feed pressure, and replace teeth at 50 percent wear.
Conclusion
Hard rock foundation drilling demands precision in tool selection. The wrong bullet tooth model in the wrong formation burns money and time. The right match, combined with optimized drilling parameters, keeps your project on schedule and under budget.
The framework is straightforward. Match the model to the MPa. Run B47K22H for 60-100 MPa. Step up to B47K25H above 90 MPa. Switch to roller bits at 100+ MPa. Keep RPM at 15-25. Maintain 150-250 kN feed pressure. Inspect daily. Replace at 50 percent wear.
The B47K22H handles 90 percent of hard rock foundation work. Know your formation. Know your model. Drill with confidence.
Need help matching bullet teeth to your geological report? Contact Changsha Mingyi Machinery Equipment Co., Ltd. for custom recommendations based on your project data. We manufacture B47K series bullet teeth with YG11C and YG13C carbide grades, customized for your specific ground conditions.
