Cutter geometry depends on:
1- Number of Cutters
- Soft rocks can be penetrated easily and hence fewer cutters are used on soft PDC bits as each cutter removes a greater depth of cut.
- More cutters must be added to hard PDC bits for harder formation to compensate for the smaller depth of cut.
2- Cutter Size
- Large cutters are used on softer formation bits and smaller cutters on the harder formation bits.
- Usually a range of sizes is used, from 8 mm to 19 mm is used on any one bit.
3- Back Rake
- Cutter orientation is described by back rake and side rake angles.
- Back rake is the angle presented by the face of the cutter to the formation and is measured from the vertical
- The magnitude of rake angle affects penetration rate and cutter resistance to wear. As the rake angle increase, ROP decreases but the resistance to wear increases as the applied load is now spread over a much larger area.
- PDC cutters with small back rakes take large depths of cut and are therefore more aggressive, generate high torque, and are subjected to accelerated wear and greater risk of impact damage.
- Back rake angles vary between, typically, 15° to 45°. They are not constant across the bit, nor from bit to bit.
4- Side Rake
- Side rake is an equivalent measure of the orientation of the cutter from left to right. Side rake angles are usually small. The side rake angle assists hole cleaning by mechanically directing cuttings toward the annulus.
5- Cutter Shape
- The edge of the cutters may be bevelled or chamfered to reduce the damage caused by impacts.
6- Cutter Exposure
- Cutter exposure is the amount by which the cutters protrude from the bit body.
- It is important to ensure that the exposure is high enough to allow good cleaning of the bit face but not so high as to reduce the mechanical strength of the cutter.
- High exposure of the cutter provides more space between the bit body and the formation face, whilst low exposure provides good backup and therefore support to the cutters.