Bits about Bits
A drill bit is the sharp
component of a drill tool that penetrates materials such as rock or
wood to create holes. Powered by axial force, the bits drill
cylindrical and non-cylindrical holes into materials, dependent on
the type used. Inserted into the core of a rotary drill, bits are
used in a variety of applications from mining to
Prior to the invention of drill bits, materials were put together
using rivets and bolts. Made from carbon steel or high-speed steel,
the bit was developed around 1845. Some bits are made from tungsten
carbide, a material capable of drilling into some of the hardest
materials found on sites. Polycrystalline diamond (PCD) is another
type of hard material capable of cutting through even the most
There are many types of drill bits, each designed for cutting holes
in different types of materials, such as brick, glass, rock,
plastic, and wood. Bits are available in different types:
flat-faced, convex, concave, and kavex. Bit sizes for the cones are
typically between four and seven inches (10.2 and 16.5 cm).
Flat-faced bits are used in heavy-duty drilling applications where
the drilling is deep. As described, the face of the bit is flat as
The convex bit is used in applications where the rock is somewhat
worn away and drills with two buttons as opposed to one.
The concave bit is one of the most common. It is used in medium to
hard rock applications, and drills a straight hole.
The kavex is a combination of both the concave and the convex with
a bit that works best on abrasive materials, drilling straight
Bits have different types of faces, which are suitable for varying
Close Up of a
Polycrystalline Diamond Drill Bit
Dome bits have spherical
faces and are mostly utilized for DTH drills. Well-suited for a
variety of applications from medium to hard rock as well as broken
formations, the dome shaped face is less likely to break than the
The double-domed face is similar to the dome shape, but with an
additional dome positioned on top of the other. The additional bit
adds life to the bit while providing a sharper penetration.
Ballistic bits are capable of drilling in more rapid motions, but
are used less because they tend to break when used in certain
formations they were not designed for.
Learn to enjoy dull reading
Bits, that is. Smith produces an excellent handbook on dull bit
characteristics. Get a copy (a .pdf file is available on their
website http://smith.com). A careful study of a
worn bit can help identify drilling problems and suggest solutions.
Being able to identify exotic characteristics like heat checking
and tracking has in the past enabled me to modify BHAs and reap
huge benefit in ROP and/or extended bit runs. We tend to be fairly
rushed into grading a bit as soon as we get it out of the
Understandable, since we want to be heading the other was as soon
as possible. Make a point of going back and having a more careful
study of the dull when you get the chance. Be methodical; start in
the middle, work to the outside along each rib or cone, then down
each gauge surface. Turn it over, examine the pin (or box) and
finally look inside. A second careful look, especially in daylight,
can reveal useful information and on rare occasions turn you into a
hero if you spot something important.
Occasionally a bit company will send out a specialist to nurse a
particular bit or application. He sees more dulls in a week than
you see in a year. Take advantage. Get with him and pick his
First problem is recognizing that you have a balled up bit. If the
ROP suddenly slows and the torque settles at a low value it could
be a hard stringer. But if there is no expected change in formation
then start thinking balled bit, especially in sticky clay with
simple water based mud.
One frequent but often unrecognized indication of balling problems
is a sudden increase in ROP after a connection. I don’t believe in
coincidences and formation changes rarely happen at connections.
While back-reaming and surveying at the connection you have
probably accidentally un-balled the bit.
PDC bits are especially prone to balling, usually by running too
much WOB. To clean it, pick up till you are just off bottom,
circulate and rotate at max rates. The idea is to wash off the plug
of clay using the backwash of the jets hitting bottom. The heat
from drilling can bake clay surprisingly hard. How hard depends on
how long you have been drilling on it. I have had to use a chisel
to chip clay out of water courses that had been baked almost to a
ceramic. So it can be difficult to wash off. After say five minutes
re-start drilling very gently looking for torque and being careful
not to run too much WOB again.
Hycalog put a lot of good information in their bit boxes, much more
than just makeup torques. Several pages of “how-to” drilling
information and well worth making a copy. They have a section on
un-balling and more importantly on preventing balling.
Rock bits are usually less likely to ball and easier to free up. If
you are not running a motor then pick up off bottom, stop rotating
and spud the bit firmly into bottom; the idea is to push the teeth
through the ball and engage the formation beneath. Start rocking or
slowly rotating the bit and if you have broken through the ball the
weight should drill off quickly and with a single bound you
It gets a little trickier with a motor. Try the PDC method first
but if no success then stop rotating, grit your teeth and pile on a
lot of weight till the motor stalls; you may have to cut back on
the flow rate to do this. Again the idea is to push the teeth
through the ball. With the motor stalled, pick up slowly till it
spins free, increase flow rate at this point and hopefully the
weight will drill off and you can drill ahead.
A dose of nut plug pumped as a slug is also a prescription for at
least temporary relief of chronic bit balling. I have not seen it
done; others swear by it.
Balled up bits don’t steer well. If you have a sudden loss of
dogleg capability (I have seen this with the GeoPilot) then a
balled bit may be the problem.
PDC Bits – As mentioned in the section on bit balling Hycalog put a
lot of good information in their bit boxes, much more than just
makeup torques. Several pages of “how-to” drilling information and
well worth making a copy.
Handling PDCs on deck you don't have to treat them with kid gloves
any more. The early cutters used to be brittle and only brazed in
place, but they are a lot tougher now and more securely fixed.
Simply sitting the bit down on a steel deck will not damage it
unless you drop it from a height. Put a wiper rubber under the bit
if you want to lower the BHA down over the pin prior to making it
up by hand, not just to protect it but also to make it easier to
Tripping in watch out for all the usual hang up points, ram
pockets, wear bushings, wellheads, liner laps, casing shoes etc. In
open hole, especially if following a hole drilled by a rock bit, be
aware that a PDC bit is more solid than a rock bit and does not
have the advantage of rolling cones. Hit a ledge with a rock bit
and you will most likely deflect off but with a PDC you will stand
up. A rock bit can squeeze through restrictions but a PDC,
especially if it has a tight junk slot area will get wedged into
any slight restriction. If you anticipate tight spots, consider
precautionary reaming rather than trying to force your way through
(otherwise see Jarring).
PDC bits are not designed for reaming so if you have a long section
of undergauge hole then consider not even using a PDC.
Reaming with a PDC use light WOB and full flow. Light weight
because it is all being supported on the gauge cutters. Rule of
Thumb for weight on PDCs = 500lbs per cutter so if you only have 6
gauge cutters then 3000 lbs max WOB. Full flow is required to keep
the cutters cool.
Heat is death to PDC
cutters. Some people like to ream
with low RPM, which is fine in soft, easily displaced formations,
but if you have to ream through tight limestone ledges then spin
the rotary up - for two reasons.
One - PDC cutters cut faster the more often you present them to the
Two - if you are reaming through restrictions then a higher RPM
will give you more inertia. Should the bit take a bite, you get
more warning, you can react, slow down, even pick up, and therefore
the string is less likely to stall, less likely to screw into the
restriction and get stuck. (See Jarring)
Always wash the last stand down, tag bottom, pick back up a foot
and circulate for a few minutes to blow away anything that might be
lying there. Tag bottom with 50/60 RPM and start drilling a new
profile. The watercourses on any fixed head bit (PDC, diamond, TSD,
impreg) only provide the necessary cooling when the bit is fully
engaged with the formation. Drill at least a foot, the length of
the bit, with light WOB. This can take time in hard formation but
this is exactly when it is most important to take that time.
Remember you are trying to minimize point loading and heat
Once you have a pattern you are free to go. PDC ROP is almost
always in direct proportion with RPM. Some studies suggest a linear
increase up to about 200 RPM, then a plateau to ±250 RPM followed
by a slow decrease at higher revs. Even hard formations often
respond best to higher RPM but it depends on what the formation is
and particularly it's abrasiveness. You hear people worrying about
burning a bit up but as long as you keep the circulation going and
don't pile too much weight on it you won't be far wrong going with
max RPM. However a very hard calcite cemented sandstone is usually
best tackled with a lower RPM and higher WOB. On the other hand
very hard limestone usually responds best to higher RPM.
Speak to geologists. Find a sonic log. Look at the cuttings. Gather
The PDC golden rule is "play with it". You rarely get a PDC bit run
where one set of parameters works best top to bottom.
Formations change and you should vary your parameters to suit. You
can usually push the WOB till you see some torque surging, then
back off till it settles again and this is usually the optimum for
this bit/formation. Avoid the surging if possible since this is
"stick/slip", bad for both bit and drill string and usually not the
best ROP either. "Stick/slip" and "Whirl" can create new patterns
on bottom that can excite the drill string, essentially a classic
feedback problem. Usually all that is needed is to vary either WOB
or RPM by about 15% but sometimes the only escape is to pick up off
bottom and drill a new, clean pattern. Very frustrating in hard
formations but these excitations can destroy an MWD, so it is worth
taking the time.
Most Top Drives are good up to ±160 RPM but watch out for drill
After heat, the next worst thing for PDCs is shock. To minimise
shock it is good drilling practice to land a PDC bit on bottom with
light WOB and 50-60 RPM then bring the rotary up as weight is
added. This habit got lost while we drilled mostly with motors
since flow creates RPM and cooling is more important as noted
above. Now that we are drilling less with motors and more with
rotary steerables like GeoPilot and AGS/AGM it is time to revive
Bit grade 0-0 – Some company men object but why not? – Modern PDC
bits are so durable that it is not unusual to pull a bit that is
almost completely un-marked. It is therefore misleading to call it
0-1 or the like, especially since a PDC bit tooth is seriously worn
and almost useless at grade 4.
The fact that you have graded the bit tells the story that it has
been run. If it were new it would be
graded NEW. Next time you pull a tough FM2743 and can’t see a mark
on it then grade it 0:0:NO:A:X:0:NO:TD.
In gauge? Grade it “0”. – I am conducting a one-man campaign to
stop people grading in gauge bits as “I”. (Arthur says it is a
two-man campaign) Everything else is graded 0-8 so why not bit
gauge. With most hand writing and some computer fonts it is
difficult to separate a capital “I” from a “1”. For example
“ Lucida Sans -1I1I”, “Minion Web -1I1I” , “Arial Narrow
Nottalottadifference. But it is not a caligraphical web site...So
back in our plan;-)
– No standardisation between bit companies so check you have the
right one. Should come with the bit but often does not. ( Some
compagnies like to desingne some beautifull plastic, and you look
stupid in a midle of the night when you discover this)
There are a lot of different sizes, even from the same bit company,
so either slip it on the bit beforehand or calliper it.
Check the depth of the slots versus the thickness of the plate.
When you torque up make sure the bit is being pulled into the back
of the breaker. This stops the bit being pulled against the slip
bowl, damaging both bit and bowl, and minimises the forces trying
to warp the breaker plate.
– Screw type jets (Smith and all PDC bits) do not have to be
hammered tight. Hand tight is Smith's recommendation for both the
two-piece rock bit jets and especially the brittle one-piece
tungsten carbide jets for PDCs. Make sure the o-ring is clean and
intact. Quite often you find a little swarf in the jet pockets, so
watch those fingers. With all tungsten jets be careful that you
don't get the threads crossed.
The tungsten is so hard you can cross cut the thread in the bit. If
you feel resistance in the first few turns then back it out till
the thread jumps then try to run it in straight this time. You
should only feel increased resistance as the base of the jet
squeezes into the o-ring and then seats on the base of the jet
Security nail-type fixing. Security used to provide a little claw
hammer for the nails but I haven't seen one for a while. Check you
have the right length of nail for the jet size (four different
lengths). Check the o-rings and nail grooves are clean. Make sure
the o-ring is not in the nail groove (has happened).
Push the jet in and if necessary tap it gently home with the wooden
handle of a hammer. Jets have sharp edges when new so don't try to
push one in with your thumb, you can cut yourself (done that, felt
stupid). Push the nail in and start it round the locking track; you
should be able to feel if there is a clear passage. Tap the nail
the rest of the way in; it should go easily. Leave the head a
little proud so that the nail can be easily removed when required
(there is a recess in the bit to allow for this).
BHI / Reed circlip fitting. Get a good set of internal circlip
pliers, often rare as hen's teeth. Check o-rings and circlip
grooves are clean. These jets are push in same as Security. Take
the same precautions. Compress the circlip making sure it doesn't
free and either smack you in the eye or disappear into a corner, or
drop somewhere where you can’t grap it... Pop the circlip into
place then give it a twist. You can't twist it if it isn't
– In an ideal world each rig should have four gauge rings at each
size – a "GO" and "NO-GO" ring for both roller cone bits and fixed
head bits. Fat chance. So appraise yourself of the type and actual
size of gauge ring available and the permitted tolerances of the
type of bit you are using. IADC tolerances as follows –
Roller Cone Fixed Head
Nominal Size – in Tolerance - in Nominal Size – in Tolerance -
3 3/8 – 13 3/4 -0 : +1/32 up to 6
14 – 17 1/2 -0 : +1/16 6 25/32 - 9 -0.020 : +0
17 5/8 and larger -0 :
+3/32 9 1/32 - 13 3/4 -0.030 : +0
13 25/32 and larger -0.045 :
The point is that if you use an 8½" rock bit GO ring to gauge a new
8½" PDC bit you might grade it 1/16" undergauge when it is
A One-Cone Bit
– Sounds like a bad joke but they did exist. Zublin was (is?) a
Californian company and they made these bits with an almost
spherical single cone set on a 45° cranked shaft. They came in big
sizes and were designed for spudding on land rigs.
The theory was they kept the hole vertical, didn't skid and didn't
ball up. My old boss (a Californian) claimed they worked, at least
in Los Angeles. (Hi Gene ;-)
I have now learned that one cone bits are being used for small hole
drilling in Alaska. The point is that with only one cone you can
maximise the size of the bearings, making them as robust as
possible for any given hole size.