How to Repair Drill Bit

February 11, 2018

General

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

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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