Basic Drilling Guidelines

Approximate Drill Bit Loads and Feeds for Thriftline Drill Bits 

Formation Type Revs per Centimetre Bit Type Max Load (tonnes)
Clay 10 AXT 1.85
Coal 20 BX 2.16
Shale 40 BQ 2.67
Medium Sandstone 60 NX 3.24
Basalt Sandstone 120 NQ 4.09
Granite 150 HQ 6.14
Quartzite 200 PQ 7.84


Please note that the maximum load calculations exclude the rod weights and are based on optimum conditions.

It may seem that the faster the drill rotates the Drill bit and the harder it is pushed the faster it will penetrate.

This is only true to a limit as other factors affect Drill bitrotation and pressures.

Limiting Factors:

  • The Drill bit must run smoothly
  • Rod string and machine must not vibrate (excessive rotational speed and feed rate)
  • Chuck jaws must not be slipping. Ensure centralization of drill rod in chuck where the chuck is non self-centring.
  • Look out for factors like ground condition changes, loss of water, pressure and ground caving in.
  • Don’t overfeed      
  • Don’t underfeed and polish      
  • Limit torque
  • Choose the best combination between Drill bit weight and speed to achieve optimum performance.
  • Maintain a peripheral speed (cutting speed) of approximately 3 metres per second but no higher than 5 metres per second.