BORING OPERATIONS
•
Boring Operation is equivalent to turning , but it is
performed exclusively on internal surfaces.
•
In internal turning, or Boring, the choice of tool is very
much restricted by the workpiece’s hole diameter and
length.
•
Finish boring is particularly noted for its ability to achieve
dimensional and surface.
•
General rule which applies to all machining, is to minimize
the tool overhang to obtain the best possible stability and
accuracy.
•
With boring the depth of the hole determines the overhang.
•
The stability is increased when a larger tool diameter is
used, but even then the possibilities are limited since the
space allowed by the diameter of the hole in the work piece
must be taken Into consideration for chip evacuation
and radial movements.
•
Because of the limitations on tooling design imposed by the
fact that the work piece mostly surrounds the tool, boring is
inherently somewhat more challenging than turning, in
terms of decreased tool holding rigidity, increased
clearance angle requirements, and difficulty of inspection
of the resulting surface (size, form, surface, roughness).
BORING OPERATIONS
•
By understanding how cutting forces
are affected by the tool geometry and
the cutting data chosen, and also
understanding how various types of
boring bars and tool clamping will
affect the stability, deflection and
vibration can be kept to a minimum.
HORIZONTAL BORING MACHINE
• It is a machine tool
which bores
holes in a
horizontal direction.
CUTTING FORCES
•
TANGENTIAL FORCE
•
FEED FORCE
•
RADIAL FORCE
CUTTING FORCES
•
TANGENTIAL FORCE is the largest among
the them. It acts perpendicular to the
cutting insert rake surface.
•
TANGENTIAL FORCE will try to force the
tool downward and away from
the centerline.
•
Because of the curving of the internal
hole diameter the clearance angle will
also be reduced.
CUTTING FORCES
•
Therefore, with small diameter holes, it is
particularly important that the clearance
angle of the insert be sufficient to avoid
contact between the tool and the wall
of the hole.
CUTTING FORCES
•
FEED FORCE is the second largest .
•
It acts parallel to the centerline and
does not deflect the boring bar.
•
Typically, the strength of the feed force
is about 50 to 60 percent of the
tangential force.
CUTTING FORCES
•
RADIAL FORCE is the third among the
three forces.
•
RADIAL FORCE is perpendicular to both of
these forces and pushes the bar away
from side of the bore.
•
This force is about 25 to 30 percent of
the tangential force.
•
RADIAL DEFLECTION will reduce the cutting depth.
•
It can also affect the diametrical accuracy, the chip thickness will change
with the varying size of the cutting forces.
•
If the cutting edge transferred to the tool holder it will lead to vibration.
•
The stability of the tool and clamping will be the factor that determines the
magnitude of the vibration and whether it is amplified or dampened.
INSERT GEOMETRY
•
The geometry of the insert has a decisive influence on thecutting process.
•
A positive insert has a positive rake angle, the inserted edge angle and clearance
angle is equal to less than 90 degrees.
•
Inserted rake angle means a lower tangential cutting force, however positive rake
angle is obtained at the cost of the clearance angle or edge angle.
•
If the clearance angle is small there is a risk of abrasion between the tool and work
piece, and friction can give rise to vibration.
• A sharper cutting edge is obtained if the rake angle is large and edge angle is small,
The sharp cutting edge penetrates the material more easily, but it is also more easily
changed or damaged by edge or other uneven wear.
•
Edge wear means that the geometry of the insert is changed, resulting in reduction in
the clearance angle.
•
In finish machining, it is the required surface finish of the work piece that determines
when the insert must be changed.
•
Edge wear should be between 0.004 and 0.012 in. for finishing and between 0.012 and
0.040 in. for rough machining.
LEAD ANGLE
•
The lead angle effects the axial and radial directions of the cutting
forces.
•
A small lead angle produces a large axial cutting force component
while a large lead angle results in a larger cutting force in the radial
direction.
•
Axial cutting force has a negative effect on the operation since the
force is direct along the boring bar.
•
To avoid vibrations, choose small lead angle but, the lead angle
also affects other factors such as chip thickness and the direction
of the chip flow, so that compromise has to be made.
•
Small lead angle has also a disadvantages, cutting forces are
distributed over a shorter section of the cutting edge than the
large lead angle.
NOSE RADIUS
•
The nose radius , is the key factor in boring
operations.
•
The selection of nose radius depends on depth of
cut the feed, and influences the surface finish,
chip breaking and insert strength.
•
The relationship between nose radius and depth
of cut effects vibration tendencies.
CHIP BREAKING AND EVACUATION
•
Chip breaking is affected by a number of factors such as the
insert geometry, nose radius, lead angle, cutting depth, feed
and cutting speed.
•
Make sure chip breaking and evacuation are satisfactory.
•
Chip jamming affects hole quality, reliability and tool life,
Insert geometry and cutting data is crucial.
•
therefore, it is necessary to choose a grade and insert
geometry that work together with selected machining
parameters, fulfills the requirements for good chip
control.
BORING RIGIDITY
• The key to productivity in boring operations is the tool’s rigidity.
• Boring bars are often required to reach long distances into parts to remove stock.
• Hence, the rigidity of the machining operation is compromised because the diameter of
the tool is restricted by the hole size and the need for added clearance to evacuate chips.
• The practical overhang limits for steel boring bars is four times their shrank diameter.
• When the tool overhang exceeds this limit, the metal removal rate of the boring operation
is compromised significantly because of lack of rigidity and the increased possibility of
vibration.
BORING BAR DEFLECTION
•
The size of the boring bar’s deflection is dependent on the bar material, the diameter,
the overhang and size of the radial and tangential cutting forces.
•
Choosing a boring bar made of a material that has higher coefficient of elasticity
can also counteract deflection.
•
Since steel has a lower coefficient of elasticity than cemented carbide, cemented
carbide boring bars are better for large overhangs.
• Tangential deflection means that the insert tip is moved in a downward direction
away from the centerline.
• In both case the size direction of the cutting forces are affected by changes in the
relationship between the chip thickness and insert geometry.
• If the exact size of the deflection of the insert tip is known in advance, then the
problem can be avoided.
• In the same way, setting the machine at a cutting depth that is greater than the
desired cutting depth compensates for the radial deflection.
• The practical outcome will be somewhat different because the clamping is never
absolutely rigid and because it is impossible to calculate the cutting force exactly.
BORING BAR CLAMPING
• The slightest amount of mobility in the fixed end of the
boring bar will lead to deflection of the tool, the best
stability is obtained with the holder that completely
encase the bar.
BORING BAR CLAMPING
2 styles of holder:
 A rigid or flange mounted bar
 A divided block that clamps when tightened
• A rigid or flange mounted bar, the bar is either preshrunk into the holder or welded in,
flange is usually glued onto the shank of the bar at a distance that gives the required
overhang.
• A divided block that clamps, the bar is fed into holder and clamped by screw connection
or by being held in the turret.
• This method must not be used for the clamping cemented carbide bars, because
cemented carbide is more brittle than the steel and cracks will occur as a result of
vibration , which may result in breakage.
MACHINING
It is a process of cutting,
shaping, or removing
material from a work piece
using a machine tool
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