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Creating an
Efficient Off-Line Band Sawing System
By Doug Harris
President, HE&M Inc.
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When handling piping or tubing material it is important to remember
that there is a constant juxtaposition between the cost of time (or
speed) versus the cost of material and/or equipment (or accuracy).
There is no better example to explain this simple fact than in the
material handling area and the requirements necessary.
In doing off-line cutting, there is certain criterion that must be
considered in designing an efficient operation. Following is a
synopsis of these criteria and guidelines to follow to assist in the
decision-making process.
1. What constitutes an
efficient off-line material handling system and sawing
operation?
Such considerations as space
for the in-feed material handling system, the out-feed material
handling system, and the saw itself are important items to
consider. Most of these items are dependent on the size of the
material to be cut, the size of the final product, and the
availability of cranes and/or forklifts.
2. How much space is
available for the in-feed staging area?
To make a successful
off-line material handling and sawing operation, one of the
necessary components is a staging area to hold the material as it is
being fed into the sawing machine. This allows the plant operator
to load enough material on the transfer trucks to free up the crane
so that it can be used for other tasks. Some systems use feed
transfer trucks on both sides of the in-feed roller tables so that
material is fed alternatively from either side. In this system some
type of squaring pop-up roller system is required. Other systems
load material from one side only. We will be specifically
concentrating on the type of system where material is fed from one
side only, but these items can easily be duplicated for a second
supply line.
3. What size and length
of material will be loaded on the in-feed transfer tables?
This will determine the
quantity, length, and spacing for the in-feed transfer trucks. The
quantity of transfer trucks to adequately support the material to
minimize the amount of deflection in the material when it is being
transported to the powered in-feed roller tables is a critical part
of the planning process.
4. What type of loading
systems is available to load the material onto the material
transfer system?
Cranes and forklifts are
usually used to load the material onto the transfer trucks. The
size, length, and quantity of material that needs to be moved at one
time will determine how to use the crane and forklifts in
conjunction with each other to load the material in an efficient
manner. What type of transfer truck system is purchased will also
determine the crane and forklift requirements. A fork load system
is only capable of end loading. A crane system is capable of
loading anywhere in the staging area including side loading onto
transfer trucks and therefore, is preferable for many applications.
5. What type of
transfer truck system will work most efficiently?
There are two types of
transfer truck systems in common use. The first type is a chain
drag system that feeds material to the in-feed rollers. These side
rollers drag the material down the transfer trucks or they lift the
parts in a uniform manner and carry them on chains, moving them to
the power roller tables. This system is capable of moving material
rapidly but creates the need for back loading the storage table to
hold the material until it can be fed into the saw. Another, and
possibly a preferable, type system is a lift and carry system that
moves one batch of material onto the power roller tables and each
batch of material is delivered to the saw at one time. The crane
would put the material onto the transfer trucks and go on to another
task until that material has been delivered onto the power roller
tables and then return some time later to load another batch of
material onto the side load magazine system.
6. How much space is
available for the out-feed staging area?
The type of transfer system
is just as critical on the out feed or discharge side of the sawing
system. As parts leave the cutting area, a determination must be
made as to the size of the support system needed to handle the parts
coming out of the saw. If the parts are large, say six feet or
more, than three rollers, five feet apart are adequate. However, if
the parts coming out are only two feet or less, obviously the space
between the rollers on the discharge table must be narrow enough to
support greater than one half the weight of the material. The
center of gravity of the material must be supported by at least two
rollers. This problem is further complicated by parts of varying
sizes coming out of the sawing machine. Discharge tabling and
transfer trucks of varying sizes are then required. The
availability of cranes and forklifts will help determine which
system is best suited to each application.
7. Are there parts of
varying sizes being cut?
If there are parts of
varying sizes being cut, the size of these parts will determine the
spacing of the side transfer trucks. Parts need to be supported in
three evenly spaced places and the side transfers will need to have
the availability to move independently to ensure that material can
be off-loaded safely, continuously, and efficiently. Parts of
varying sizes will require transfer trucks on the discharge side of
varying spacing.
8. How will the squaring
of the end of the bundle be accomplished?
Another important
consideration is how to square the material. If you are cutting a
bundle or a single layer of material, squaring the material is
necessary if the cutting tolerances are tight. Since all the cut
pieces going into the sawing process may not be the same length, it
is very important to determine the method used to ensure straight
cuts. Squaring is not necessary when cutting a single piece.
9. What are the cutting
tolerances required?
There are two methods that
can be used. First, the material can be moved against a hard stop
before banding. The second option is to make trim cuts on the saw
to square the material. It depends how much drop is available and
how tight cutting accuracies are required for the finished product.
If the hard stop method has been chosen, cutting accuracies can be
sacrificed. As the material hits the hard stop, the stop can
actually bend, shift, or warp, especially over time and when moving
heavier material. Additionally, the material may rebound against
the stop. Trim cutting the material may be preferable to the hard
stop method of squaring the material because it can be more accurate
and cost efficient. If the material is greater than 12 and the
tolerances do not need to be held closely, then a fixed stop is
adequate.
The material is fed into the
saw and a trim cut is used to achieve a straight end. A shuttle
vise system is used to index the material into the saw. If cutting
tolerances are within plus or minus one inch, trim cutting may be
inefficient.
10. How will trim cut
pieces be removed from the saw?
If the decision is made to
use the trim cut method of squaring the material, how to handle the
trim cut pieces must also be determined. Again, there are basically
two methods that can be used. The first is probably the most
inefficient. A saw operator can stand at the output side of the saw
and manually remove the trim cut pieces. This would mean that the
saw and feed systems would have to be turned off to prevent injury
to the operator thus creating an additional, time consuming step in
the work flow. The second method is to add an automated gate
section that lifts, creating a larger opening for trim pieces to
fall into the automated removal system. The trim parts would be
automatically dumped into a parts container or chip conveyor to
remove them from the sawing area. Obviously, this method increases
the speed of flow through the sawing process.
11. What coolant system
will be used?
Once the area for the
cutting process has been designed and planned, the available coolant
systems to use during the sawing process have to be analyzed as
well. Mist systems can be very effective when cutting tubing or
pipe. They are cleaner with less dripping than conventional flood
systems but are still effective because the sawing operation is
cutting air much of the time thereby creating less heat and
friction. Flood coolant systems can also be used and do provide
better heat transfer, provide better lubricity, and provide better
blade life than a mist system. However, coolant tends to flow
through the tubing and spill on the floor creating OSHA concerns,
EPA concerns, and a general housekeeping nightmare. These issues
can be resolved by using lift and tilt system on the discharge side
of the saw to drain the coolant before using the crane to lift the
material. This system can be used in conjunction with pans on the
in-feed, bar feed, discharge tables, and transfer trucks to return
the coolant to the saw, thus saving the mess and providing the
ability to reuse the coolant.
II. Sawing
Application and How it Applies to Various Types of Saws
12. Will the saw be
expected to perform miter cuts?
The plant manager must
determine according to the customers needs whether miter cutting is
required. Will the material be moved in a bundle or a single
layer? If you are going to cut in a single layer, you have the
availability of miter cutting on a vertical saw since you can cut
all the way across the single layer. If you are going to bundle
cut, than you can only cut 90 degrees and miter cutting is
impossible because the part lengths would vary.
13. If bundle cutting is
required, what are the shapes of the bundles to be cut?
Are they square, hexagonal or some other
shape?
Square material is easily
stacked for bundle cutting since you would create a square or
rectangle bundle for greater stability during clamping. If you are
using round bundles they are typically stacked in a hex bundle that
requires special vise jaws for even clamping. These vise jaws have
to be interchangeable or have the ability to be adjustable.
14. Will the material be
banded before moving through the saw and what type of
banding is being used?
If you are using banded
bundles and cutting tolerances are less than 1/16th of an
inch a shuttle system should be used and a bi-directional vise will
be necessary to allow the banding to pass through the vises of the
saw without getting caught. This banding could affect the trueness
of the cut. Banded material travels easily across roller tables but
where the band is crimped to hold it together is important.
15. Will miter cutting
be done, what percentage of the time, and what tolerances
will be required?
This will determine whether
a vertical band saw or a horizontal band saw should be used. When
miter cutting more than 5% or 10% of the time and you need to cut
miters on both ends of the piece (box frame or opposite miters), a
vertical band saw is the logical choice because the vertical saw
pivots around the horizontal axis of the center of the machine,
allowing you to keep the vises closer to the work. The closer the
vises are to the piece being cut the greater stability and accuracy
of the cut. Keeping the vising closer to the cutting zone also
dampens out the vibrations that naturally occur during the cutting
process. When using horizontal mitering band saws, the saw head
moves away from the vises as the head is rotated. Depending on the
style and type of machine the vises can be a considerable distance
from where the blade is cutting, allowing greater vibrations to
occur on the material. Vibration destroys blade life and the
straightness of the cut. A vertical saw will enable you to miter in
one direction, index the material, and miter in the opposite
direction by doing nothing more than indexing the material and
rotating the head. This is extremely important if you are cutting
round, tubular products, since on an automatic index your material
is always in contact with a vise, preventing the material from
rolling or shifting.
16. What shapes are
going to be cut, square or round?
If round material is to be
cut than the saw used can be a standard non-canted vertical or
standard non-canted horizontal. If you are cutting square or
rectangular material than a forward canted blade is very
advantageous. A canted blade cuts a more consistent cutting area
achieving approximately a 30% faster cut rate and better blade
life. If you are going to cut both materials, a forward-canted
blade is the proper choice since you gain all the benefits when
cutting square or rectangular material and will experience none of
the drawbacks when cutting round stock.
Mitering in both directions
with a single miter horizontal machine is also time consuming and
is, therefore, an inefficient method to use. In order to double
miter a piece of material, you would need to cut the material,
remove the material from the machine, rotate it 180 degrees, put the
material back in the machine, adjust the angle, measure the length
between the two angles, and re-cut the material. A vertical machine
allows you to do the same operation in 1/5 the time or less because
it has the capability of double mitering without readjusting the
vises. The only thing you need to do is move the material forward
and tilt the head.
Some horizontal machines are
made to double miter. This means swinging the head in both
directions. However, with this type of design the vises have to be
moved out of the way because the head will be moving into the vise
to miter the second cut. All of this takes time and, once again
your vises are clamping the material at a further distance from the
cut, creating instability.
A vertical band saw also
uses less floor space, since the head retracts out of the cutting
area and the material can be top loaded. So, if you were splitting
a 20 bundle, the amount of space required would only be slightly
larger than the 20 material and the material can be loaded with an
over head crane, a forklift, or even manually. Conversely, if a
mitering horizontal machine were used, the material would have to be
loaded behind the machine then fed through the machine, requiring a
lot more space.
Now, if an automatic bar
feeder is added to a mitering machine, there are several different
scenarios that could occur. First, if it is a horizontal mitering
machine mitering in both directions, the automatic shuttle index
cannot move close to the blade since, as the saw is rotating, the
shuttle vise must be further away since the saw is rotating towards
the bar feeder. This creates a very large remnant length in the
material. Once again, as the saw rotates into the vise it needs to
be moved out of the way either manually or hydraulically. If the
horizontal saw only pivots in one direction, then the saw head
pivots away from the fixed vise and once again the vise is further
away from the cutting zone. In addition, you do not have the
ability to cut box frames since the saw is only capable of cutting
90 degrees and up to 45 or 60 degrees depending on the
manufacturer. This means only one part can be cut at a time unless
the material is stacked up vertically on top of each other in a
single row. Otherwise, the part lengths would vary depending on
their position laying in a flat row.
Using a horizontal saw also
precludes the use of an outboard top clamp. An outboard top clamp
is very difficult to use on a mitering horizontal since the clamp
would have to be so far away from the work piece in order to allow
room for the head to swing. In this situation, full utilization
from the structure of the saw cannot be achieved to provide the
stability needed for accurate cuts.
On a vertical machine, box
frames can be cut automatically since the head is rotating around
the horizontal axis of the machine, which allows the clamping to be
closer to the cut zone. Mitering in both directions without the
vise interfering with the mitering of the head is achieved. This
also allows you to cut a layer of parts across the table to the same
length substantially increasing the productivity of the saw. Using
a vertical saw to miter cut has the added benefit of leaving a
shorter remnant end compared to that left by a horizontal double
mitering machine, giving you greater material utilization. This is
not true with a single mitering horizontal machine since the arm is
mitering away from the cutting area and the shuttle vise system
always comes to the same forward stop. Thus, you lose the ability
to double miter.
An advantage to using a double
column designed machine is the center of gravity on the head remains
constant. The cutting force, therefore, remains uniform all the way
through the cut giving better controllability. Since the head
travels straight up and down on columns, the sawing head is guided
on both sides of the cut providing greater rigidity and stability in
the arm. This allows the guide arms to be closer to the work since
the saw head is not traveling through an arc.
One disadvantage of the double column design is when cutting
rectangular or square material with a non-canted double column, the
blade wants to enter the material all at one time making it more
difficult to control the squareness of the cut during the entry.
Putting a cant on the blade reduces the square inches on the entry
and the exit allowing the set of the tooth to help guide the head
through the material. The cant also becomes important in cutting
square tubing and bundling applications since it is very important
to have as uniform a cross sectional area as possible in cutting the
material. Since the center of gravity does not shift and the head
weight remains constant, the head descent can be controlled very
precisely through hydraulic cylinders using computerized traversing
systems. As the material size becomes greater, over 16, the saw
head becomes more massive and controlling the head becomes more
difficult. The double column design provides the ability for
greater controllability and stability of the saw head
17. What blade
size machine is needed?
The first thing
to take into consideration is the type of materials to be cut. If
you are cutting harder materials, beam strength is critical. The
harder the material, the more critical the beam strength becomes.
The size and hardness of the material together create the
requirement for a specific blade size. An example is a 1 x 1 piece
of stainless tubing could be cut very successfully with a 1 blade
but a 5 piece of stainless tubing would struggle to make the cut.
The beam strength of the blade decreases by the cube. If the guide
spacing between the fixed guide and the moveable guide moves from 2
to 4, you have 8 times less beam strength available from the blade
to cut the material. This is important since the beam strength and
the set of the blade controls the squareness of the cut and the
amount of deflection as you transition through the material. As a
blade wears it has a greater propensity to cut out. This occurs
because every time the saw head is raised, the blade drags against
the material being held by the fixed vise. The blade dulls to the
backside first so when you try to cut the material the blade wants
to cut out to the front side. The beam strength of the blade
restrains the blade from trying to cut out. Also, as you get into
the tougher materials, the amount of feed force required to cut the
material can increase drastically. It is once again the beam
strength of the blade that is restraining the blade from wandering.
The larger and the harder the material, the more beam strength is
required to provide a straight cut. For example, a 1 blade is
.035 thick by 1 high. A 1-1/4 blade is .042 x 1-1/4 thick. A
1-1/2 blade is .050 thick x 1-1/2. As you can see, the larger
blades have a much larger cross sectional area, providing greater
beam strength.
18. What tooth selection
will be used?
Tooth selection also becomes
critical because the blade is cutting a smaller cross section as it
cuts through a square or rectangular piece of material. As the cut
progresses, the blade addresses a larger cross section of material,
requiring more force. The general rule of thumb is to keep three
teeth engaged in the work at all times. This can be difficult to
achieve when cutting thin walled material. It is critical to have a
very independent feed rate and feed force to ensure that three teeth
are engaged in the work at all times.
In summary, for straight cutting a
horizontal machine is more cost effective and efficient than a
vertical machine. For occasional mitering, a horizontal mitering
saw is appropriate for the application until such time as the
mitering requirements exceed 20% of the time. At that point a
vertical band saw is the best choice to achieve high production
rates and versatility at a reasonable cost. Remember that the
harder and larger the material being cut, the greater the beam
strength required to perform a fast and accurate cut. Greater beam
strength is achieved by using a larger band. Always consider the
proper machine requirements for the application being considered
prior to any purchase of a band saw so that cost effectiveness and
cutting efficiency is achieved. We need to consider the proper
material handling systems as well as the saw. This is critical
because if we cut twice as fast and the material handling system
cannot keep pace, the increase in time-savings is actually
minimal.
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