• In article DTH Drilling Care & Maintenance – Part One and DTH Drilling Care & Maintenance – Part Two , we introduced how to correct balanced system, causes of product failures and the importance of lubrication. Here we will explain the importance of regular repair bit and how to do it
  • Service life for rock drilling tools is very difficult to predict simply because the number of parameters involved
  • Rock drillability, machine type and settings and operator skills are the major factors influencing wear rates and tool life
  • The drill bit service life in particular is very much related to rock properties, its mineral content, grain size and structure
  • Grinding is very important not only for best possible service life on bits, but also for quality and total economy of the drilling operation





When grinding?

  • Well before the wear-flatness develops above recommended limits
  • When penetration rate starts dropping off
  • Well before anti-taper develops above recommended limits
  • When hole deviation tends to increase

Note: Before snake skin appears in the cemented carbide surface Grinding once too often is more economic than once too seldom!

Cemented carbide wear

  • Frontal wear
  • Gauge wear

Grinding intervals and routines

  • Establish grinding routines
  • Button bits should be taken to the grinding station when the wear flat is 1/3 of the button diameter
  • Volume of Cementer Carbide to be grounded away





Penetration rate improvement 

  • Penetration rate is improved significant after grinding bit, please see more
  • Solid line stand for before grinding, dotted line means after grinding,
  • Y axis is Penetration rate, X axis is Drill meters






Over drilling

  • Excessive over drilling of bits will cause damage not only to all drilling tools but also to rock drill and rig
  • The flat  worn area of the button should not exceed ⅓ of its diameter
  • These have all 100% flat buttons and have caused a lot of damage





Snake skin

  • ”Snake skin” wear causes severe damage if not removed
  • Occurs mainly in rock formations that cause little wear on the cemented carbide






  • Anti-taper happens when the gauge buttons are worn down so there is no clearance between the bit body and the hole
  • The bits need proper clearance
  • When necessary, remove some steel below the gauge buttons to maintain a clearance of 0.5 mm
  • Avoid grinding the gauge
    • On the gauge buttons the anti- taper has to be removed without grinding too much of the diameter
    • Leave 2 mm of the flat part of the gauge





Grinding Body

  • Sometimes the steel in the bit body is not worn away at the same rate as the cemented carbide in the buttons
  • Then the steel must be ground away to maintain same button protrusion as on a new bit
  • This to maintain good flushing at the front, good penetration rate and avoid re-crushing of drill cuttings, for importance of flushing please read this





Button Protrusion

  • Sometimes the steel in the bit body is worn away faster then the cemented carbide in the buttons
  • Then the buttons must be ground to maintain same button protrusion as on a new bit
  • If not there is an increased risk of button breakage





Flushing holes

  • Open up flushing holes with a rotating burr or a file for steel





Grinding benefit

  • Optimal penetration rate
  • Good drilling accuracy
  • Less wear on drilling tools, rock drill and rig
  • Improved productivity
  • Lowest possible cost per drill meter

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