PDC Cutter: Sizes, Grades and How Cutters Wear

A PDC cutter is the small cylindrical cutting element that does the actual work on a polycrystalline diamond compact (PDC) drill bit. The bit body, blades and nozzles exist to hold dozens of these cutters at the right angle and keep them cool and clean. Understanding how a cutter is built, how it is sized and how it wears is the quickest way to read a PDC bit specification and to choose the right bit for a formation.

What Is a PDC Cutter?

A PDC cutter has two layers. On top is the diamond table, a layer of synthetic diamond grains sintered together under high pressure and high temperature (HPHT) with a metal catalyst, usually cobalt. Underneath is a tungsten carbide substrate that gives the cutter toughness and lets it be brazed into a pocket on the bit blade. The diamond table does the cutting; the carbide carries the load and absorbs impact.

Because the diamond grains are randomly oriented and bonded to each other, a PDC cutter has no single cleavage plane like a natural diamond crystal. It keeps a sharp edge as it wears and cuts rock by shearing, the way a lathe tool removes a chip of metal, rather than by crushing like the teeth or inserts of a roller cone bit.

Cutter Structure: Diamond Table, Substrate and Interface

• Diamond table: the wear-resistant working layer. Its thickness, grain size and cobalt content control the balance between abrasion resistance and impact resistance.
• Substrate: tungsten carbide that supports the diamond table and is brazed into the blade pocket.
• Interface: the boundary between diamond and carbide. Non-planar interface geometries are used by cutter makers to reduce residual stress and delamination.
• Chamfer: a small bevel on the cutting edge that reduces chipping when the cutter first engages hard rock.

Cobalt left in the diamond table expands faster than diamond when heated, which can crack the table at high cutting temperatures. Leached cutters have the cobalt chemically removed from the working surface layer, which improves thermal stability and abrasion resistance in hard, hot formations.

PDC Cutter Sizes and Naming

Cutter size usually means the cutter diameter in millimetres. Common diameters on oilfield and water-well bits are 8, 9, 10, 13, 16 and 19 mm. Industry part numbers such as 1308, 1313 or 1613 typically combine the diameter and the cutter length in millimetres. As a rule of thumb:

• Larger cutters (16 to 19 mm) take a deeper cut per revolution and drill faster in soft, homogeneous formations, but carry more load per cutter.
• Smaller cutters (8 to 13 mm) allow more cutters on the bit face, spread the load and survive better in harder or interbedded rock, at a lower rate of penetration.

Cutter shape also varies: flat-faced cylindrical cutters are the standard, while shaped cutters (ridged, conical or multi-chamfer faces) are used by some bit designs to improve durability or rock engagement in specific formations.

How PDC Bits Use Cutters

A PDC bit is a fixed-cutter bit: the cutters do not move relative to the body. How they are arranged decides how the bit drills:

• Blade count: fewer blades leave larger junk slots for cuttings and suit soft rock; more blades add cutters and stability for harder rock.
• Back rake and side rake: the angles at which each cutter meets the rock. A more aggressive (smaller) back rake cuts faster; a larger back rake protects the cutter in hard rock.
• Cutter density and backup rows: a second row of cutters behind the primary row takes over as the primary cutters wear.
• Depth-of-cut limiters: support inserts or studs that stop the cutters from biting too deep, reducing torque spikes and vibration.
• Gauge cutters and pads: maintain the hole diameter and steer behaviour in directional wells.
• Hydraulics: nozzles and flow channels that cool the cutters and clear cuttings from the face.

Cutters on Volgaburmash PDC Bit Series

Volgaburmash PDC model codes describe the cutter layout directly: the number of blades, then the cutter size in millimetres, then the formation category. For example, a FastDrill FD616SM has 6 blades and 16 mm cutters and is designed for soft formations with medium interlayers (SM). Other formation letters are S (soft), M (medium) and MH (medium with hard interlayers); the option letter B marks a second row of cutters and F a reinforced central part.

Two more examples of reading the code: FDM716MHB is a matrix-body bit with 7 blades and 16 mm cutters for medium formations with hard interlayers, fitted with a second row of cutters; WT613SM is a WiperTrip reamer with 6 blades and 13 mm cutters for soft formations with medium interlayers. Reading the code this way lets you compare cutter size and blade count across series before you look at the full datasheet.

FastDrill bits can also be specified with shock studs, up-drill cutters, depth-of-cut support inserts, additional flushing holes and a choice of standard, optimal or premium cutters. Source: Volgaburmash Oil & Gas catalogue.

How PDC Cutters Wear and Fail

Reading the dull cutters after a run tells you whether the bit, the parameters or the formation was the problem. The IADC dull grading system records cutter condition on the inner and outer rows (0 = no wear, 8 = fully worn) plus a code for the main dull characteristic. The most common modes:

• Abrasive wear (worn cutter / wear flat): a flat develops on the diamond table in abrasive sandstone. Normal if gradual; rapid wear flats point to excessive heat or a too-aggressive design.
• Chipped and broken cutters: impact damage from interbedded hard stringers, vibration or tagging bottom too hard.
• Spalling and delamination: the diamond table flakes off or separates from the substrate, usually from impact or thermal stress.
• Heat checking: fine cracks from overheating when flow is insufficient.
• Lost cutters: a cutter leaves its pocket, typically after severe impact or braze failure; it can then damage the remaining cutters.
• Balling: sticky clay packs around the cutters so they stop cutting; a hydraulics and design issue rather than cutter wear.

How to Extend PDC Cutter Life

1. Match cutter size and blade count to the formation: large cutters and fewer blades for soft rock, smaller cutters and more blades for harder or interbedded sections.
2. Break in a new bit gently. The Volgaburmash catalogue recommends 30–40 minutes at no more than 40 rpm and 10–15% of the recommended weight on bit before increasing load smoothly.
3. Keep enough flow through the nozzles to cool the cutters and prevent balling.
4. Control vibration (whirl, stick-slip, bit bounce) with parameters and, where available, depth-of-cut limiters.
5. Use a matrix-body bit such as FDM where erosion of the body, not cutter wear, limits bit life.

For the broader comparison with roller cone bits, see PDC drill bits: advantages and selection. FastDrill sizes held in Dubai are listed under PDC bits in stock, or filter the list by series: FastDrill (FD) in stock. For bit sizes and API pin connections see the oilfield bit size and API connection chart.

PDC Cutter FAQ

What is a PDC cutter made of?

A PDC cutter is a layer of sintered synthetic polycrystalline diamond (the diamond table) bonded under high pressure and high temperature to a tungsten carbide substrate. The substrate is brazed into a pocket on the bit blade.

What sizes are PDC cutters?

Common cutter diameters are 8, 9, 10, 13, 16 and 19 mm. Volgaburmash FastDrill bits use cutters from 8 to 19 mm, and the cutter size is the second number in the model code, for example 16 mm in FD616SM.

Can worn PDC cutters be replaced?

On many bits worn cutters can be removed and new ones brazed in during a repair, provided the bit body and cutter pockets are in good condition. Whether a repair makes sense depends on body wear and how many cutters are damaged.

Why do PDC cutters chip or break?

Mostly from impact: hard stringers in softer rock, vibration such as whirl or bit bounce, or too much weight when tagging bottom. Smaller cutters, more blades, depth-of-cut limiters and a careful break-in reduce the risk.