What PDC Cutter Grade Is Best for Granite Drilling

Granite is about as unfriendly to drill bits as rock gets. It is dense, abrasive, and packed with quartz, so a cutter that sails through sandstone can dull in a single shift. The PDC cutter grade you pick — the size, the diamond layer, the cobalt content, the edge treatment — decides how fast you drill, how long the bit lasts, and what the job costs in the end.

What a PDC cutter grade actually means

A PDC cutter is a thin disc of polycrystalline diamond bonded to a tungsten carbide substrate. When suppliers talk about a "grade," they usually mean the whole package:

• Size. The four-digit code tells you the dimensions. The first two digits are the diameter in millimetres and the last two are the height. A 1308 cutter is 13 mm across and 8 mm tall; a 1613 is 16 mm by 13 mm. Bigger cutters carry more diamond, so they wear longer, but they also need more power to drive.
• Diamond layer thickness. A thicker table lasts longer in abrasive rock but is more prone to impact damage if it is too thick relative to its support.
• Diamond grit size. Fine grit wears slowly and keeps a sharp edge; coarse grit cuts fast but dulls quicker.
• Cobalt content. Cobalt is the catalyst that bonds the diamond grains together. Standard cutters run roughly 10–16% cobalt. In hard, abrasive rock, lower cobalt (6–8%) keeps the diamond table harder and more wear-resistant.
• Chamfer. The bevel ground onto the cutting edge. A bigger chamfer strengthens the edge against chipping at the cost of a little aggressiveness.

Why granite is so hard on cutters

Granite is made up of interlocking quartz, feldspar, and mica crystals. Quartz is one of the most abrasive minerals in the ground, and granite's compressive strength is high enough to punish a cutter in three ways at once:

• Impact chipping. The cutting edge takes repeated blows and can spall or fracture.
• Abrasive wear. Quartz grinds the diamond table flat, so the cutter stops biting and the bit stops advancing.
• Thermal damage. Friction heats the cutter. Above roughly 750 °C, the cobalt catalyst starts to convert diamond back into graphite, and the cutter loses its edge fast.

That is why a grade that works in shale — a flat, aggressive cutter with high cobalt — fails quickly in granite.

What to look for in a grade for granite

If you are drilling granite, the grade needs to be built for impact and wear resistance rather than raw speed:

• Choose a finer diamond grit (roughly 16–25 µm) so the table stays sharp as it wears.
• Ask for lower cobalt content (6–8%) for hard formations. It keeps the diamond table harder and slows abrasive wear.
• Specify a chamfered edge. A larger chamfer absorbs impact and stops the edge from chipping on entry.
• Consider leached cutters. Leaching removes the cobalt catalyst from the surface of the diamond table, raising thermal stability to around 1,200 °C. It costs more, but in deep or hot granite holes it protects the cutters from thermal damage.
• Go bigger where the bit design allows. A 1308 or 1313 cutter carries more diamond volume than an 0808 and will last noticeably longer in abrasive granite.

Which grade is best for granite?

There is no single universal answer, because the best grade also depends on the bit body, the rig, and the drilling parameters. But for most granite jobs, a 1308 or 1313 cutter with a fine-grit diamond table, 6–8% cobalt, a chamfered edge, and (for hot or deep holes) a leached table is a solid starting point. Pair it with a dome or conical cutter shape — these crush and crack the rock instead of shearing it, which is exactly what hard granite demands — and a negative rake angle for extra edge strength.

If you are buying cutters to mount on your own PDC drill bits, look for consistent batch quality: cutters from the same supplier should match in diameter, height, and diamond layer thickness, because a single oversized cutter in the face can cause uneven loading and premature failure.

Practical tips for drilling granite

Even the right grade fails early if it is run badly:

• Break the bit in. Start at about half the normal weight on bit for the first stretch, then build up gradually.
• Keep the RPM moderate — roughly 120–180 RPM is a common working range — and watch for vibration. Excessive vibration chips cutters faster than any rock.
• Keep the hole clean. Adequate flow and good hydraulics clear cuttings so the cutters cut rock instead of re-grinding debris.
• Monitor the bit. If penetration rate drops and the bit is still sharp, the cutters are likely worn or the grade is too soft for the formation.

Questions to ask your supplier

Before you commit to a grade, ask:

• What cobalt content and diamond grit does this grade use, and is it uniform through the table?
• Has the cutter been leached? What is its thermal stability?
• Can you show field results from granite or similar hard, abrasive formations?
• How do you control batch-to-batch consistency?

Frequently asked questions

Q: What does 1308 mean on a PDC cutter?
A: The first two digits are the diameter (13 mm) and the last two are the height (8 mm). 1313 is 13 mm by 13 mm, and 1613 is 16 mm by 13 mm.

Q: Is a leached PDC cutter worth the extra cost for granite?
A: Usually yes for hot or deep holes. Leaching removes the cobalt catalyst and raises thermal stability, protecting the cutter from thermal damage in hard, abrasive formations.

Q: Should I use flat or dome cutters for granite?
A: Dome or conical cutters. Flat cutters are aggressive and fast in soft rock but chip quickly in granite; dome cutters crush the rock and survive impact far better.

Q: Why does my PDC bit wear out fast in granite?
A: Likely a grade mismatch — too much cobalt, too coarse a grit, or no chamfer. Match the grade to the formation and break the bit in properly.

The bottom line

For granite, the best PDC cutter grade is one built for wear and impact resistance: a larger cutter such as 1308 or 1313, fine diamond grit, lower cobalt content, a chamfered edge, and leaching where heat is a concern. Match that with sensible drilling parameters and the bit will hold its edge far longer than a standard grade.

If you need PDC cutters or PDC drill bits for granite and other hard formations, TY Drill Bits (Xi'an Heaven Abundant Mining Equipment) supplies a full range of PDC cutters and PDC drill bits. Their team can help you match the right grade to your formation.