theories of interaction of rock cutting tools in contact with the rock, different parameters, specific energy, applications, drag, point attack picks, disc cutters, and their interaction

1. Rock Excavation Systems
Mechanical Excavation
Theories of Interaction of Rock Cutting tools
U.Siva Sankar
Sr. Under Manager
Project Planning
Singareni Collieries Company Ltd
E-Mail :ulimella@gmail.com or
uss_7@yahoo.com
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Rock Breaking Processes
The basic principles of breaking brittle hard rock. The main considerations
in breaking rock are the forces required to induce fractures in the rock and
the energy consumed in breaking rock.
Force is important because it determines the limitation on the type of
machinery that can be used to break the rock and on the materials of
construction that can be used in the machinery.
As the breaking mechanism of the machine changes, so would the energy
required to break the rock since the strength of rock varies depending on
the type of stress induced on the material.
Energy is important because it determines the rate at which rock breaking
can be carried out. All machines are limited in the power that can be
applied to the rock and hardness of the manufactured components of the
machine. Therefore a process that demands substantial energy will result
in a slow rock breaking rate.
The rock breaking process is classified into three major groups: primary,
secondary, and tertiary.
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2. Rock Breaking Processes
Primary
This is the application of a force by means of a hard indenter to a free
rock face much larger than the indenter. This generates chips which are
of a size similar to that of the indenter at the sides of the indenter and a
pulverized zone immediately below the indenter.
Primary breakage processes
1. Impact or hammering. Dynamic forces are applied
2. Percussive drilling. Application of a hard indenter to the bottom of a hole. The
force is applied from one side only and the bottom of the hole is the free face.
The force applied dynamically and after each application the hard indenter is
moved slightly to break out more chips on the next application
3. Button type cutters for raise and tunnel borers. The buttons are loaded
slowly (quasi-statically) and are moved away to be re-applied elsewhere, that is,
indexing occurs by rolling to the next button. Repeated applications over a large
surface area maintain the flat face
4. Disc type cutters for raise and tunnel borers. Hard indenter indexed by
rolling. Forces at a point in the rock rise very slowly.
5. Drag bit. A hard indenter forced onto the rock and indexed by dragging across
the surface.
6. Diamond bits. A very hard surface and very small indenter dragged across the
surface. The real breaking is done by the force thrusting the diamonds against
the rock. Diamonds produce very small fragments because they are small
indenters.
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3. Secondary breakage processes
This is the application of forces inside a hole near to the
rock face. The forces inside the hole generate tension at
the sides of the hole to which produces cracks which
ultimately runs to the free surface.
Secondary breakage processes includes:
1. Wedging. Wedge driven into a hole which produces
crack
2.Blasting. Explosive generates a pulverized zone through
compression but the real breaking process is by driving
tensile cracks.
Tertiary breakage
1. Breaking boulders by impact or mud blasting
2. Crushing
3. Milling
According to theory, the tertiary breakage process is closely related to
breaking the rock in tension. From Figure can be seen that loading of a
sphere by diametrically opposed forces causes a uniform tensile stress
across the diametrical plane. This causes the sphere to split in tension,
that is, at a stress very much lower than the uniaxial compressive stress.
Tertiary
This is the application of forces from more than one side to a free surface.
Tertiary breakage
process
Fig; Tertiary Breakage a Tensile effect
It has been found that the tertiary stress, σt, is also dependent on the
size of the rock, but not as important as the size of the indenter for
primary breakage.
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4. Miscellaneous breakage processes
Several other breakage processes exist, these include:
1. Thermal spalling.
This depends on intense heat (flame) being applied to the rock and
traversed so that a high temperature gradient is produce in the rock
resulting in differential expansion which produces mechanical strains
and ultimately breaking of the rock. It is used in taconite and certain
quarrying operations, usually in cold climates. Thermal spalling is also
used for finishes on rock surfaces and where high forces must be
avoided during breakage. (Commonly used ancient technique)
2. Water jets.
The water jets create high stagnation pressures against the surface it
impinges on. Used for drilling in porous hard rock where water goes
into pores and breaks grains out. Water jets are known to be wasteful
on energy and are used only for special applications.
Mechanical Excavation Systems
Different mechanical excavation systems, like machines with;
Teeth (Dozer, Shovel, Scraper, Bucket wheel excavator, Bucket chain excavator)
Ripping tool (Coal Plough, ripper, rock breaker),
Pick mounted rotary cutting head/drum (Roadheader, Shearer, Continuous
miner, Surface miner)
Disc cutters and button bits (rock drill, Mobile tunnel miner, Tunnel boring
machine)
Auger tool (Continuous Auger Miner, Surface Auger Miner)
Application of Mechanical Systems
Under Ground:
Continuous Miners, Bolter miners, Auger Miners and shearers for coal or soft
nonmetalics
Boom type miners (road headers in soft to medium hard rocks)
Rapid excavation equipment (Mobile tunnel miners,Tunner borers, raise borers,
and shaft sinking rigs) for soft to medium hard and hard rocks)
Surface:
Rippers for very compact soil, coal, and weathered or soft rock
Bucket wheel and cutting head excavators for soil or coal
Augers and highwall miners for coal
Mechanical dredges for placers and soil
4

5. Mechanical Excavation Systems
Classifications of cutting tool based on Cutting Action
•Type of •Mode of •Specific tool •Machines using tool
tool action types
•Drag tool •Applies a force •Diamond drill bit •Rotary drilling
roughly parallel machine
to the rock •Pick •Roadheader
surface (point attack and •Continuous miner
wedge) •Shearer
•Indenter •Applies a force •Brazed drill bit •Rotary percussive
normal to the •Button drill bit drilling machine
rock surface •Tricone Roller •Rotary drilling
drill bit machine
•Raise borer
•Disc cutter •Tunnel boring
machine (TBM)
•Mobile miner
•Raise borer
Mechanical Excavation Systems
The main difference between indenters and drag bits is that an
indenter breaks rock by applying a force that is predominantly in a
direction normal to the rock surface.
Comparatively, a sharp drag bit applies the main force in a direction
predominantly parallel to the rock surface. The breaking mechanism for
both is actually a tensile fracture.
Because the drag tool initiates tensile fractures in a more direct manner,
with less crushing, it is more efficient than an indenter.
However, indenters are by far the most widely used type of tool; why is
this? The reason lies in the strength of the tool itself. The materials used
for the cutting edge must be hard but, because of this property, they are
also brittle.
The mode of action of a drag tool induces bending, or tensile stresses in
the tool cutting edge and makes catastrophic failure of the tool more
likely.
An indenter, on the other hand, is loaded mainly by a compressive force
along its main axis and the material of which it is made is inherently
strong in compression. (Hood and Roxborough 1992.)
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6. Mechanical Excavation Systems
These basic cutting methods,
defined in terms of tool type,
are and include:
1.Drag bit cutting.
Drag type
2. Point-attack bit cutting.
3. Disk cutting.
4. Roller cutting. Indenters
5. Button cutting.
Fig: Rock Cutting Techniques
Mechanical Excavation Systems
Drag bit cutting and Point-attack bit cutting.
The application of both drag bits and point-attack bits is similar.
The tools are inserted in tool holders (or boxes), which are integral parts
of the cutting head, and may be held in place by a circlip or spring.
Point-attack bits are commonly free to rotate in their holders. It has
been claimed that this feature promotes more even tool wear (self
sharpening) and better overall tool life.
During cutting, the bits are pushed into the rock, developing cutting
forces parallel to the direction of head rotation and normal forces
parallel to the direction of head thrust.
As these forces build up to critical values, a macroscopic failure surface
develops ahead of the bit, and a piece of rock spalls away.
Road headers, Continuous Miners (Bolter Miners & Surface Miners)
and Shearers use drag and point-attack bits almost exclusively.
These tools also find application on tunnel boring machine (TBM) cutter
heads, but in this role they are generally limited to machines operating
in weaker formations.
6

7. Mechanical Excavation Systems
Disk cutters generally consist of solid steel alloy discs with a tapered
cutting edge.
The disk is mounted in a bearing and is free to roll in response to applied
forces acting parallel to the rock surface. These rolling forces are
analogous to the cutting forces applied in drag bit cutting.
Thrust and drag forces are applied to the disk through the bearing and
act normal and parallel respectively to the rock surface. Thrust forces
acting on the cutting head push the cutter into the rock building up stresses
which cause local rock failure.
Disks used in practice may be of the simple type, or may consist of
multi-edge varieties, including types with successively smaller disk
diameters giving a tapered or conical arrangement.
Frequently these multi-row disks employ carbide inserts with chisel
points imbedded nearly flush with the circumference.
Simple disk cutters are used primarily on full face TBMs, and multi-row
disks on raise boring machines (RBMs).
Breaking Process Under a Disc Cutter
Fig: Model for disk cutting
(Roxborough and Phillips, 1975a).
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8. Mechanical Excavation Systems
Roller or mill-tooth cutting is similar to disk cutting except that instead of
a tapered disc edge, the tool is equipped with circumferential teeth.
As the cutter moves in response to rolling forces, each tooth in turn is
pushed into the rock, acting like a wedge, and causing local failure.
Button cutters consist of cylindrical or conical tool bodies inset with
tungsten carbide buttons.
The tool is mounted in a bearing in the same way as disk cutters or
roller cutters and is free to roll in response to applied forces acting parallel
to the rock surface.
Thrust forces cause high stress concentrations beneath each button as
they roll across the rock surface, resulting in local failure and pulverization
of the rock. The area of influence of each button is small and results in a
fine-grained product.
Button cutting is used in applications in which high rock strength and
abrasivity preclude the use of other methods. These cutters also find
application as reaming cutters used for final profiling on RBMs and TBMs.
Mechanical Excavation Systems
Pick
The picks consist of a steel body containing a recess into which a
cemented carbide tip is brazed. The cemented carbide tip is the cutting
portion of the pick, and consists of two materials, tungsten carbide and
cobalt, sintered together to form a matrix of car bide grains within a cement
of fused carbon.
The most important physical properties of the cemented carbide are
hardness and toughness. The value of both these properties can be
varied by the amount of cobalt present, as shown in Fig. I. If the carbide is
too hard, premature fracturing will occur, and, if it is too soft, the material
will wear away too quickly. Thus, for optimum cutting performance, a
balance between the two properties is necessary, dependent upon the
quality of the coal being cut.
Fig: variation of Toughness and hardness
of pick with % of Cobalt
8

9. Mechanical Excavation Systems
Drag Pick Types
Radial picks
Forward attack Picks, and
Point attack Picks
Radial and Forward attack
Picks (Chisel or Wedge Picks) Conical or Point Attack
Picks
Radial picks – Chisel or Wedge pick Mechanical Excavation Systems
These tools are designed such that the axis of pick shank is normally
parallel to the radial line of cutting head/drum.
They are generally suitable for cutting soft and medium-hard rocks and
coal.
Radial picks generate lower forces than those of point attack tools,
when pristine. The normal force is of low magnitude compared to
cutting force.
Forward attack Picks - Chisel or Wedge pick
These picks are also termed tangential picks, together with point attack
picks, due to the orientation of their tool axis.
The design and the geometry of tool tip is similar to that found on radial
picks
Chisel or wedge pick may be having either flat bottom surface or round
bottomed surface.
Point attack picks
Conical tip and cylindrical shank. Shank axis is inclined relative to the
rock surface.
The tool is designed to rotate by the action of cutting to produce even
wear and is therefore favoured in abrasive rocks.
Often, however, dirt clogs the tool, so that it cannot rotate.
9

10. Mechanical Excavation Systems
Cutting parameters
Cutting Geometry of
Drag Pick
Schematic
Drawing of Forces
acting on a Conical
Bit
Cutting parameters Mechanical Excavation Systems
A simple Drag pick with the forces acting on it is illustrated in Figure.
The resultant force Pa may be resolved into three mutually
perpendicular components:
Cutting force (Fc), acting in the direction of cutting;
Normal force (FN) perpendicular to the direction of Fc; and
Sideways force (Fs) normal to the plane on which Fc and FN lie.
Clearance Angle:
Clearance angle, which is between the
lower surface of pick and a plane parallel
to the cutting direction, also has
pronounced affects on the pick forces.
Investigations have shown that tool forces
drop sharply after a value of around 5°and
stay sensibly constant.
To meet the kinematic needs, the
clearance angle is generally designed to
be around 10 degrees.
10

11. Mechanical Excavation Systems
Rake Angle:
Cutting and normal forces decrease
monotonically with increasing rake
angle as seen in Figure.
Most of the benefit to pick forces has
been achieved at a rake angle of 20° ,
beyond which further marginal
improvement is at an increasing
penalty to pick strength and its
potential to survive.
Rake angle can be either +ve or -ve.
Rake angles between +20 and +30
degrees can be chosen for weak rocks
and coal cutting.
High rake angles may not be beneficial
since picks with these angles are more
susceptible to gross failure.
Mechanical Excavation Systems
Attack Angle
The angle of attack which is the angle between the tool axis and the
tangent of the cutting path, is another parameter affecting the
performance of point attack picks.
This angle provides a good contact between the pick and rock and failure
to position the pick at its correct angle of attack will significantly alter the
effective tool geometry.
In order to offset the value of clearance angle, the angle of attack is to be
larger, e.g. at 90 degrees cone angle, the angle of attack should be at
least 55 degrees. It is also reported that at high rotational speed this angle
should not exceed 48° .
Tilt angle:
It is the angle between cutter axis
to the vertical line normal to
direction of cutting.
Tilt angles of 65 to 70 degrees
offered the lower specific energy
and relative freedom from
vibration problems.
11

12. Breakout Angle:
Fig. Effect of breakout.
Fig. Breakout between neighbouring Picks
When a pick cuts its way through a material, some
of it breaks away at each side of the pick; this is
referred to as side splay or breakout
Usually the sides of the groove are irregular, but
over the total cut length the average slope of the
sides, termed the 'breakout angle', can be
considered constant for a particular material
Efficient cutting is achieved through the maximum
use of breakout, and pick lacing patterns should be
designed so as to continually repeat the cutting
sequence that produces it.
where s = spacing between the tools, d= depth of
cut, and θ = breakout angle. If the breakout angle
for a particular material has been determined then
s/d can be calculated.
Mechanical Excavation Systems
Wear Angle:
The wearflat is almost parallel to the
cutting direction; however, it generally
tends to incline in the opposite direction
and forms a wear angle.
This angle is around few degrees and
becomes smaller for the hardest and
strongest materials.
Occurrence of wearflat changes the tool
tip geometry and, consequently, results in
the generation of higher tool forces.
The normal force is the most affected
component by the wear, e.g. a wearflat
around 1mm can drastically increase
Fn/Fc ratio. Wear Development
It is also reported that a large clearance of Drag Pick
angle relieves the wear effect and
provides better overall efficiency even if,
as a consequence, a small or slightly
negative rake angle is introduced.
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13. Mechanical Excavation Systems
Important measures of cutting performance
•Yield (Q) •The volume f rock produced by cutting - depends on
penetration depth (d), breakout angle (θ) and distance
•Specific Energy cut work done by the cutting force (FC) to excavate unit
•The
(SE) volume of yield.
•Dependent •Rock strength and toughness
on: •Degree of fracturing
•Machine type and method of operation
•Tool type and condition
•Available tool forces (machine size and
power)
•Penetration depth
Specific energy is one of the most important factors in determining the
efficiency of cutting systems and defined as the work to excavate a unit
volume of rock. Hughes and Mellor demonstrated that specific energy might
be formulated as in the following:
Where, SE is specific energy, E is secant elasticity modulus from zero to load
to failure and Sc is compressive strength of rock.
Detailed rock cutting tests, however,
showed that specific energy was not
only a function of rock properties but it
was also closely related to operational
parameters such as rotational speed,
cutting power of excavation machines
and tool geometry.
Roxborough reported that specific
energy decreased dramatically to a
certain level with increasing depth of
cut and decreasing tool angle.
The effect of the spacing between cuts
and depth of cut (or penetration) on
cutting efficiency is explained in Figure.
If the line spacing is too close , the
cutting is not efficient because the rock
is over-crushed; in this region, tool
wear is also high due to the high friction
between tool and rock.
Fig: General effect of cutter spacing on specific energy.
13

14. Mechanical Excavation Systems
Pointed Attack Vs Chisel or Wedge or Radial Picks
In terms of pick shape, when operating at the same rake and clearance
angles and depth of cut, the pointed pick requires the least cutting and
normal force. The chisel pick requires the greatest forces.
Due to the increased penetrating capability of the pointed pick, for a
given available normal force, pointed picks operate more efficiently
than the chisel bit.
Pointed picks can but deeper for a given level of force, whereas chisel
bit cut more material for a given depth of penetration.
Mechanical Excavation Systems
Theoretical and experimental studies on cutting forces
A number of scientists have formulated mathematical models to
improve the design of the excavation machines and find the best
configuration of the cutting tools for more efficient cutting process.
Evans, Evans and Pomeroy extended theoretical works of Evans
were used to establish the basic principles of the cutting process and
these have been widely used in the efficient design of excavation
machines such as shearers, continuous miners and road headers.
Evans demonstrated theoretically that tensile strength and
compressive strength were dominant rock properties in rock cutting
with chisel picks and point attack tools.
He also formulated optimum spacing for chisel picks as three to four
times the pick width.
Where FC is cutting force, d is depth of cut, w is tool width, α is rake angle,
σt is tensile strength, σC is compressive strength and φ is tip angle.
14

15. Mechanical Excavation Systems
Theoretical and experimental studies on cutting forces
Goktan suggested a modification on Evans’ cutting theory for point attack
tools as indicated in Equation below and concluded that the force values
obtained with this equation were close to previously published experimental
values and could be of practical value;
Where ψ is friction coefficient between cutting tool and rock
Goktan used Evans’ theories to compare the cutting efficiency of point
attack tools and wedge–shaped picks and concluded that the ratio of
tensile to compressive strength was the main parameter governing the
relative efficiency.
Performance of Disc Cutters
Fig: Model for disk cutting
(Roxborough and Phillips, 1975a). Fig. Interplay between pick width
and spacing.
Fig: General effect of cutter
spacing on specific energy.
15

16. Performance of Disc, Button and Pineapple cutters
A ranking of cutting efficiency of tool types, in terms of specific energy,
places the steel disk cutter as the most efficient, the disk-button cutter next,
and the pineapple cutter as least efficient.
However, the wear resistance, and therefore the capability of cutting strong
abrasive formations, is the reverse of this efficiency ranking.
Consequently, steel disks tend to be used for cutting weaker, less abrasive
rocks, and pineapple cutters are used for machining the most abrasive and
toughest formations.
First, in contrast with drag bits, the efficiency of the rock breakage process
does not decrease when disk cutters are used in a groove deepening mode.
Second, the value of this optimum spacing depends on the depth of cut
taken and on the rock type and with drag bits an optimum s/d value of 2 to 3
and with disk cutters this value is in the range 5 to 10.
Third, the efficiency of the rock breakage process is independent of whether
the grooves are cut simultaneously, with multiple disks on a single hub,
orequentially, with independent disks.
16