PRESENTATION
ABOUT VERTICAL
BORING MACHINES
AND HORIZONTAL
BORING MACHINES
ME 124
MACHINE SHOP THEORY
BSME-2
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 the cutting
process.
•
A positive insert has a positive rake
angle, the inserted edge angle and
clearance angle is equal to less than
90 degrees.
INSERT GEOMETRY
•
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 BREAKING AND EVACUATION
•
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.
MACHINING
• It is a process of cutting,
shaping, or removing
material from a work
piece using a machine
tool
BORING BAR
CLAMPING
This Photo by Unknown author is licensed under CC BY-SA-NC.
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 a holder that
completely encases the bar.
Holder is available in two style:
• A rigid or flange mounted bar
• A divided block that clamps when tightened.
Less efficient are those tool clamping methods in
which the screw clamps onto the bar. This form
generally results in vibration and is not
recommended. This method most not be used for
the clamping of cemented carbide bars. Cemented
carbide is more brittle than steel and cracks will
occurs as a results of vibration, which in turn may
result in breakage.
BORING BAR
CLAMPING
• With a rigidly mounted bar, the bar is
either preshrunk into the holder and/or
welded in.
• With flange mounting, a flange with a
through hole is normally used.
• The flange is usually glued onto the shank
of the bar at distance that gives the
required overhang.
• The bar is then fed into the holder and
clamped by means of screw connection or
by being held in the turret.
BORING BAR
CLAMPING
• Less efficient are those tool clamping
methods in which
the screw clamps onto the bar.
• This form generally results in vibration
and is not recommended.
• This method most not be used for the
clamping of cemented carbide bars.
• Cemented carbide is more brittle than
steel and cracks will occurs as a results
of vibration, which in turn may result in
breakage.
BORING BAR
• Boring bars are made in a wide variety of
styles.
• Single-point boring bars are easily
ground but difficult to adjust when they
are used in turret and automatic lathes
and machining centers, unless they are
held in an adjustable holder.
BORING BAR
• More expensive boring bars are provided with
easily adjustable inserts.
• These bars are made in standard sizes, with a
range of 0.25 to 0.5 in. on the diameter.
• A fine adjustment is included in increments of
0.001 in., or in some cases 0.0001 in. They are
standard up to about 6 in. in diameter.
• Many times, it may be economical to order
special bars with two or more presets
diameters, set at the proper distance apart.
BORING BAR
• These special bars cost more and are
generally only used when large quantities
make their use economical.
• Sometimes this may be the only way to
hold the required tolerances and
concentricity.
• Other special boring bars, sometimes
called boring heads, are designed with
replaceable cartridges.
BORING BAR TYPES
• Boring bars are available in steel, solid
carbide, and carbide-reinforced steel.
• The capacity to resist deflection
increases as the coefficient of elasticity
increases.
• Since the elasticity coefficient of
carbide is three times larger than that
of steel, carbide bars are preferred for
large overhangs.
BORING BAR TYPES
• The disadvantage of carbide is its poor
ability to withstand tensile stress.
• For carbide-reinforced bars, the carbide
sleeves are prestressed to prevent tensile
stresses.
• Boring bars can be equipped with ducts
for internal cooling, which is preferred
for internal turning.
BORING BAR TYPES
• An internal coolant supply provides
efficient cooling of the cutting edge,
plus better chip breaking and chip
evacuation.
• In this way a longer tool life is
obtained and quality problems, which
often arise because of chip jamming,
are avoided.
BORING BAR CHOICE
•
When planning production, it is very
important to minimize cutting forces and to
create conditions where the greatest possible
stability is achieved so that the tool can
withstand the stresses that always arise.
The length and diameter of the boring bar will
be of great significance to the stability of the
tool.
•
Since the appearance of the workpiece is the
decisive factor when selecting the minimum
overhang and maximum tool diameter that
can be used, it is important to choose the
tool, tool clamping and cutting data which
minimize, as much as possible, the cutting
forces which arise during the operation.
BORING BAR CHOICE
• The following recommendations should be
followed to obtain the best possible stability:
• Choose the largest possible bar diameter, but at
the same time ensure that there is enough room
for chip evacuation.
• Choose the smallest possible overhang but, at
the same time, ensure that the length of the bar
allows the recommended clamping lengths to be
achieved.
• A 0-degree lead angle should be used. The lead
angle should, under no circumstances be more
than 15 degrees.
BORING BAR CHOICE
•
The index able inserts should be
positive rake that results in lower
cutting forces.
•
The carbide grade should be
tougher than for external turning
in order to withstand the
stresses to which the insert is
exposed when chip jamming and
vibration occur.
• Choose a nose radius that is
smaller than the cutting depth.
VERTICAL
BORING
MILL
VERTICAL
BORING MILL
•
A vertical boring mill is a type of
large machine tool designed to
machine large, heavy workpieces that
are difficult to handle on conventional
machine tools.
• These mills operate somewhat like a
very large lathe, standing tall and
typically
requiring
a
sturdy
foundation to support their weight
and operation.
VERTICAL BORING
MILL FUNCTIONS
• Boring: The primary function is boring holes
into various materials. These machines can
bore precise, large-diameter holes in heavy
workpieces.
• Turning: Vertical boring mills can perform
turning operations, where the workpiece
rotates against a stationary cutting tool,
creating cylindrical shapes.
• Facing: These mills can perform facing
operations to create flat surfaces on the end of
a workpiece.
VERTICAL BORING
MILL FUNCTIONS
• Cutting
Internal
and
External
Threads:
The machine can be used for
threading operations, both internally and
externally.
• Drilling and Reaming: Additional operations
such as drilling and reaming can also be
performed.
TYPES OF VERTICAL BORING MILLS
1. Turret Mill - it has a table that
moves while the cutting part
stays still. It’s pretty handy for
certain jobs.
2. Ram-Type Mill - the cutting
part can move forward and
backward, giving it a bit more
reach.
ADVANTAGES OF VERTICAL
BORING MILLS
Vertical boring mills offer several
advantages that make them a go-to choose
for many machining tasks. Here are the
main advantages of using a vertical boring
mill:
• Handling Large Workpieces
• Precision and Accuracy
• Accessibility and Operator Visibility
• Efficiency and Time Savings
• Customization Opportunities
SELECTING THE RIGHT
VERTICAL BORING MILL
Factors to Consider: Size, Capacity, Features
Selecting your mill is like choosing your battle
gear.
• The physical size of the mill is crucial — it needs
to fit into your workspace and accommodate the
size of your intended workpieces.
• Evaluating the weight capacity of the mill’s table
and the depth, diameter, and height of the cuts
it can make will ensure that the machine meets
your production needs.
• Also, a close look at the features is essential.
Opt for a mill equipped with an advanced, userfriendly control system, a variety of speed
settings to adapt to different materials and
tasks, and a wide range of tooling options for
enhanced versatility.
OPERATING
PRINCIPLES AND
SAFETY
BASIC OPERATION STEPS
AND PROCEDURES
Like a well-executed battle plan, operating a
vertical boring mill is about strategy and precision.
From setup to execution, every move matters:
 Setup:
1. Secure the workpiece on the table, ensuring
it is properly aligned.
2. Select the appropriate cutting tool and
install it.
3. Set the initial tool position and calibrate the
machine settings.
 Programming:
• Input the necessary program or G-code,
specifying cutting speeds, feed rates, and tool
paths.
BASIC OPERATION STEPS
AND PROCEDURES
• Test Run:
• Perform a test run with the spindle off to verify
the tool path and check for any potential
collisions.
• Machining:
• Turn on the spindle and coolant, then start the
programmed operation.
• Monitor the machining process, adjusting as
necessary.
• Inspection and Finishing:
• Once machining is complete, turn off the mill
and inspect the workpiece.
• Perform any necessary finishing operations,
such as deburring.
TURNING
TURNING
What is turning?
Turning is a machining process
that’s characterized by the use of a
stationary, non-rotating cutting tool to
remove material from the external
surface of a workpiece. Although it can
be
performed
by
hand,
turning is typically performed using a
machine
known
as
a
lathe.
As the workpiece rotates against the
blade, the cutting tool removes a uniform amount of material, thereby reducing the
size and changing the shape of the workpiece.
HORIZONTAL
TURNING
Horizontal turning centers
are the more prominent type across
most industries.
They have a
spindle that is horizontally oriented,
with tools mounted out of the side of
the tool holder. This helps them cut
across the object being processed.
HORIZONTAL
TURNING
Much
like
it’s
the
milling
counterpart, this type of turning
center also benefits from gravity as
it pulls the chips away from the
work-piece.
VERTICAL TURNING
• Vertical turning centers are also
called vertical turret lathes or
VTL. The vertical and horizontal
turning centers are very similar,
but their configuration and shape
are upended allowing the
headstock to sit on the floor and
the faceplate to become a
horizontal rotating table.
VERTICAL TURNING
• Inverted vertical turning centers
are like the regular ones but they
have reversed positions for the
spindle and jaw chuck.
HORIZONTAL
BORING
M AC H I N E
HORIZONTAL BORING
MACHINE
• The boring machine is one of the most versatile
machine tools used to bore holes in large and heavy parts
such as engine frames, steam engine cylinders, machine
housing, etc.
• Which are practically impossible to hold and rotate in
an engine lathe or in a drill machine. Therefore, this is
the main purpose for which the boring machine was
developed.
• In addition to its primary purpose of boring, the range of
speeds and feeds provided to the various traversing parts
allow drilling, milling, and facing to perform with equal
facility.
HORIZONTAL BORING
MACHINE
• By the fitting of simple attachments, the use of the
machine can extend still further to include screw
cutting, turning, planetary grinding, or gear cutting.
• In types of boring machines, the horizontal boring
machine is one of the most useful and important
machines. Let’s first understand the parts of a horizontal
boring machine before moving on to the different types of
boring machines.
PARTS OF
HORIZONTAL
BORING MACHINE
HORIZONTAL BORING
MACHINE PRIMARY PARTS
• Bed
The bed is that part of the machine which is
fitting on the floor of the shop and has a boxlike casting. The bed supports the columns,
tables, and other parts of the machine.
• Headstock Supporting Column
The column provides support to the headstock
and guides it up and down accurately by the
guideways. The column has hollow houses
and is heavily ribbed to add rigidity.
HORIZONTAL BORING
MACHINE PRIMARY PARTS
• End Supporting Column
The end supporting column is situated at the
other end of the bed houses.
The column may be adjusted on the slideways of
the bed towards or away from the spindle to
support the different lengths of boring bars. It
may be moved at right angles to the spindle as in
the case of a floor-type machine.
• Headstock
The headstock mounting on the column
supports, drives, and feeds the tool. A headstock
may move up and down on the column to set the
tool for different heights of the work.
HORIZONTAL BORING
MACHINE PRIMARY PARTS
• Saddle and Table
The tables serve as work surfaces and have
T-slots for holding various devices.
The saddle allows the work to be moved
longitudinally on the bed. These movements may
be slow or rapid and are performed by hand or
power.
• Boring Bars
The boring bar supports the cutter for holding
operations on jobs having large bore diameters.
For short holes, the bar may support the
headstock spindle end only. For long work,
the bar is supported on the spindle end and on
the column-bearing block.
TYPES OF HORIZONTAL
BORING MACHINES
1. Table Type Horizontal Boring Machine
The table types are the most common of all
horizontal boring machines.
The name horizontal boring machine is given
because the work is mounted on the table which
is adjustable and feeds are given by hand or
power, lengthwise or crosswise with respect to the
bed of the machine.
This type of machine is appropriate for generalpurpose tasks where it is necessary to carry out
more operations in addition to boring.
TYPES OF HORIZONTAL
BORING MACHINES
2. Floor Type Horizontal Boring Machine
The floor-type horizontal boring machine has
notable uses a constant floor plate on which Tslots are provided to hold the work.
The headstock supporting column and the end
supporting column are mounted on the runways
which are placed at right angles to the spindle
axis.
This is designed for holding very large and heavy
workpieces that are difficult to mount and adjust
on a table.
TYPES OF HORIZONTAL
BORING MACHINES
3. Planer Type Horizontal Boring Machine
The planer-type horizontal boring machine
resembles the table type, but the table slides
directly on the bed instead of on a saddle
angle to the spindle similar to a planer.
The end of the supporting column and
headstock supporting column may adjust
towards or away from the table to
accommodate different widths of work.
This type of machine is suitable for
supporting a long work.
TYPES OF HORIZONTAL
BORING MACHINES
4. Multiple Head Type Horizontal Boring
Machine
The machine resembles a double housing
planer or a Plano-miller. The table is
supported by a long bed on which it
reciprocates.
There are two vertical columns at two sides
of the bed, nearly in the middle of the bed.
The two columns are bridged by a cross rail.
The machine may have two, three, or four
headstocks. This type of machine may be
used as a horizontal and vertical machine.
BORING TOOL MOUNTINGS
FOR HORIZONTAL BORING
1. Boring bar
Ordinary boring operations are carried out
with tools mounted on a bar held in a spindle
having a morse taper hole.
The maximum diameter of the bar employed is
ordinarily not larger than the spindle
diameter, and the length is such that it can
reach the end column support.
A boring bar should be of the maximum
diameter and minimum length to reduce
bending or vibration and it may be supported
in various ways to suit to different types of
workpieces.
BORING TOOL MOUNTINGS
FOR HORIZONTAL BORING
2. Boring Head or Cutter Head
The boring head is using for mounting cutters
while machining large diameter holes where a
standard boring bar is unsuitable due to the
smaller diameter or excessive overhang of the
cutter. Boring heads have the maximum
permissible diameter.
This device amply supports the tool and
reduces machining time due to the larger
number of cutting edges. The cutters may
adjust by micrometre dials. it illustrates in the
boring head.
BORING TOOL MOUNTINGS
FOR HORIZONTAL BORING
3. Facing head
The facing head is mounting on the end
of the spindle. It comprises a flange
provided with a diameter slide-way on
which the tool carrying a bracket may be
adjusted.
The bracket may be fed radially or located
and clamped at the center to support a
long boring bar. A facing head enables
the enlarging of large diameter holes,
facing an external turning operation. it is
illustrated in the facing head.
ADVANTAGES OF HORIZONTAL
BORING MACHINE
The following are the advantages of a horizontal
boring machine:
1. Some horizontal boring machines have the
advantage of producing very large machined
parts.
2. Depending on the machine, horizontal boring
machines can make several cuts simultaneously
and run at a high cutting speed.
3. This is especially helpful in situations where
businesses need to produce and deliver a lot of
mechanical parts quickly.
ADVANTAGES OF HORIZONTAL
BORING MACHINE
4. The horizontal boring machines are designed
to guarantee that the drilling is always
reliable and precise.
5. Horizontal boring machines with computer
controls minimize the possibility of human
error and guarantee consistently precise cuts.
HAZARDS AND
RISKS
HAZARDS AND
RISKS
• The principal hazards when working at
or near these machines are mechanical
in nature.
• A review of the accident history shows
that the single largest cause of serious
injury is entanglement at revolving
tools.
• Entanglement also accounts for most
fatalities, particularly at larger
machines.
HAZARDS AND
RISKS
• Crushing and trapping hazards have also
been identified as very significant causes
of injury.
• Most injuries occur during activities such
as setting/ adjustment, swarf removal or
observation for the purpose of process
control.
• Injuries are often very severe and include
limb and skull fractures and
amputations. The potential for fatal injury
at these machines should not be
underestimated.
HAZARDS AND
RISKS
• Reports of investigated accidents show
guarding standards to be generally
unsatisfactory with an over-reliance on
systems of work and the 'skill' of operators
as the principal means of risk reduction.
• The precise combination of safety
measures adopted will depend upon the
outcome of the risk assessment and, in
some cases, the practicability of carrying
out modifications.
• Particularly, care needs to be taken when
selecting the control measures.
HAZARDS AND
RISKS
• The final design of the
safeguarding arrangements should
take into account the need for
observation, adjustment etc. while
still providing for adequate levels of
protection.
• A distinction should be made
between 'normal' machining
operations and those occasions
when access to the work zone may
be necessary for the kinds of
higher-risk activities.
SAFEGUARDING
• The variation in size and configuration
of machines and the specific
applications to which they may be put
precludes the use of a standard
safeguarding solution such as that
which may, for example, be applied to
most lathes.
PRIMARY
SAFEGUARDS
• The hierarchy of controls prescribed
under regulation 11 of PUWER should
be applied. During normal machining
operations access to the work zone
should be prevented by fixed and/or
interlocked guards.
• The height, position and construction
of any new guards should meet
appropriate standards. If physical
guarding is not practicable alternative
types of safety device may be used,
example light curtains or pressure
mats.
BRAKES
• It is not possible to specify a minimum
stopping performance for braking systems.
This is because of the wide range of machine
sizes and other design characteristics.
• Nevertheless, the objective should be to stop
the machine as quickly as possible considering
the circumstances.
• Where brakes are fitted to older machines, care
should be taken to ensure that the machine
can withstand the stresses induced by the
effects of braking.
BRAKES
• A braking system may be mechanical or
electrical or a combination of both.
Preference should be given to disc or
caliper brakes in mechanical braking
systems.
• Advice should normally be sought from
the machine manufacturer before
modifications are carried out.
SUPPLEMENTARY
SAFEGUARDS
• Where powered movement of machine
elements is necessary for setting purposes
etc., and access is required by the operator to
the work zone, the risks can be reduced by
using supplementary safeguards.
• The primary safeguard, guard interlocking,
can be suspended via a key-operated selector
switch.
• Any further hazardous movement of the
machine element should be achieved by using
a hold-to-run control arrangement or enabling
device.
SUPPLEMENTARY
SAFEGUARDS
• The selector switch should also enable the
braking arrangements, example dc injection
braking. On release of the hold-to-run control (or
enabling device) the braking system should be
applied.
• This principle may be incorporated into an
existing pendant control. These aspects of the
control system are safety-critical and should be
designed to meet the requirements of regulation
18 of PUWER.
TRAINING
The provision of information, instruction and
training is a legal requirement. Those matters
which require particular attention include:
•
•
•
•
•
•
Dangers at the machine
Location and operation of controls
Precautions to reduce the risk of
entanglement
Correct use of guards and other safety
devices
Any tests, example daily test of trip devices,
and the system for reporting defects
Safe systems of work for cleaning,
maintenance, setting and adjustment, loading
of workpieces etc.
TRAINING
• Activities such as swarf removal
should normally be carried out
with the spindle stopped.
• The selection and use of suitable
work wear and other personal
protective equipment, example for
eyes, is important to help minimize
other residual risks.
MEMBERS:
PERLAS, JOHN PAUL S.
MAGO, JOHN PAOLO
BATOON, DEILVIN V.
VASQUEZ, LIEZEL B.