5
Changing the grinding cup and chuck
Before fitting the grinding cup, smear the shaft of the cup with a
rust-inhibiting agent such as Shell Ensis Compound 356 or similar.
General rules
Use grinding cups of the right shape and size for the buttons that
are to be ground. The drill bit must be properly secured for grind-
ing, so that it cannot move. Pressing the grinding cup too hard on
to the cemented-carbide button will reduce the service life of the
cup. Both the grinding cup and the cemented-carbide button can
be damaged by excessive heat generation.
• Be aware that the grinding cup is hot after grinding.
CAUTION
A new grinding cup must always be “run in”. Start grinding care-
fully and increase the feed pressure gradually. This will increase
the service life of the grinding cup substantially.
Notice: If anything other than original grinding cups are used,
Epiroc Drilling Tools AB will not accept liability for any faults or
damage, or the consequences thereof.
Grinding hints
The rate of bit wear depends on the rock formation, and is highest
in rocks with a high quartz content. A suitable grinding interval
should be determined according to the rate of bit wear. It is more
economical to regrind too early rather than to suffer poor penetra-
tion rates and risk damaging the drill bit through overdrilling.
Following are a few hints about the care of drill bits.
When to regrind
Button bits should be re ground
when the pene tration rate
drops, or if any of the cement-
ed-carbide buttons are dam-
aged (fractured buttons should
be ground flat). It is both
practical and economical to
redress the buttons when the
wear flat reaches about 1/2 of the diameter of the button.
Look out for “snake skin”
If microscopic fatigue cracks – so-
called “snake skin” – begin to
appear on the cemented carbide
buttons, the cracks must be ground
away. In any event, bits should be
reground after 300 metres of
drilling at the most. This should be
done even if there are no visible signs of wear and the penetra-
tion rate continues to be good. If snake-skin is not removed, the
cracks will deepen and ultimately result in button fracture.
Do not grind away too much cemented carbide
Do not grind too much on the top of the
buttons. Let a few millimetres of the wear
flat remain on top of the button.
Always grind broken buttons flat
A drill bit can remain in service as
long as the gauge buttons maintain
the diameter of the bit. Fractured
buttons must always be ground flat to
prevent chips of cemented carbide
from damaging the other buttons.
Avoid grinding the perimeter
2 mm
Min
0,5
Gauge button anti-taper has to be removed by grinding, although
excessive reduction of the bit diameter should be avoided. Leave
about 2 mm of the wear flat.If necessary, remove some of the
bit-body steel below the gauge buttons, so that a clearance (taper)
of 0,5 mm is maintained. If the flushing holes start to deform,
open them up with the aid of a rotary burr or steel file.
Grinding instructions
To obtain the correct button shape, the grinding machine should
be handled as follows:
• When grinding spherical cemented-carbide buttons, move the
grinding machine with GC cup evenly over and around the centre
of the button.
• When grinding ballistic cemented-carbide buttons, hold the
grinding machine with GC-T cup upright over the button.
• Do not press too hard. The weight of the machine itself is usually
sufficient feed force.
The diamond-impregnated grinding cup can also be used to
remove small amounts of steel from around the button. However,
excessive grinding of steel will soon clog up the grinding cup.
If this happens, the cup can be “opened up” again by rubbing it
against a whetstone (available as an accessory), or by rotating it
against a discarded silicon-carbide grinding wheel. The right cup
to use for removing steel from around the button is a GC-B boron-
nitride cup. Grind down the steel by about 1,5–2 mm by holding
the machine directly over the button.
The Secoroc HG is delivered equipped for air cooling of the
grinding cup. A kit for converting the machine for water cooling is
included in the package.
