Note: Descriptions are shown in the official language in which they were submitted.
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46371-89
APPARATUS FOR PREDICTING THE CUTTING
~ORMANCE OF SAWS
The present invention relates to testing saw blades,
both circular saw blades and band saw blades, and more
specifically to determine the cutting performance of saw
blades before they are used in sawmills and the like.
Circular saw blades today are made of thinner
material than in the past. By making a thinner blade,
one reduces the thickness of cut, thus reducing wood
loss. Using thin saws is only justified when at the same
time sawing deviation is kept at a minimum.
The cutting performance of a saw blade is the
function of many variables. These include properties,
physical condition and dimension of the wood piece,
dimensions, lateral support and operational speed of the
saw blade and stiffness of the saw blade as related to
levelling, tensioning and the internal strength of the
saw blade material. It is known that saw blades differ
widely in their cutting performance for the same wood
variables and the same blade dimensions. It is known
that the stiffness of saw blades varies from blade to
blade and tensioning and internal strength of the saw
blade are also affected.
At present there are no exact methods of measuring
the tensioning and internal strength within a saw blade.
Saw blades may be examined by placing a straight edge
across the saw which is slightly lifted. A light gap is
formed underneath the straight edge and then the gap
thickness across the surface of the saw blade is
measured. This is an indirect method of measuring the
internal stress pattern in a saw blade. It tends to be
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unscientific and based generally on judgement of the
person conducting the test.
When sawing occurs, it is known that a saw blade
tends to deflect to either one side or the other and this
causes a discrepancy in the thickness of wood pieces
exiting from the saw. When lumber is cut, there are
generally a plurality of saw blades mounted on an arbor
referred to as gang saws, the saw blades being spaced
along the arbor. If one of these saw blades should
deflect, then the result is that the pieces of wood on
each side have different dimensions, one being narrower
and one being wider. Clearly this is undesirable,
particularly when tolerances are becoming more and more
important. Furthermore, it has been found that this
tends to happen more today than when thicker blades were
used because people are using the thinner saw blades than
those used in the past to maximize the use of the wood.
Attempts have been made in the past to develop
methods which reliably predict the cutting performance of
saw blades. One method is based on the natural
frequencies of saw blades and has been somewhat
successful with small diameter saws, but when applied to
the type of saws used in sawmill operations, it has not
been found successful. A second method relates to a one
side loading of the saw blade in stationary or rotating
conditions, but again this has not proved satisfactory.
One example of such a method is disclosed by Dyer et al
in U.S. Patent 4,498,345 which measures deflection of a
blade by fluid flow. The deflection being measured by a
sensor to generate a signal related to blade flexure.
It is an aim of the present invention to develop a
testing arrangement for both circular and band saw blades
wherein the saw blade is moved at a cutting speed, braked
on both sides of the cutting teeth and then allowed to
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deflect due to the loading of the saw blade. This
deflection is measured, and from this can be determined a
predictability of the cutting performance of the saw
blade.
In the case of a circular saw blade, it is an aim of
the invention to provide a testing arrangement that can
be located in the filing room where saw blades are
prepared and sharpened, so that saw blades can be tested
in the filing room before being mounted on the saw arbors
in the sawmill. In the case of band saws, whereas a
testing device may be provided in a filing room, such a
device, however, is more complex and expensive, thus it
is more common to carry out tests on band saw blades on
the band saw machine itself.
The testing machine for circular saws has an arbor
on which the saw can be mounted and a variable speed
drive, thus permitting the saw blade to be rotated at a
normal cutting speed. Two plugs are provided operating
from compressed air from either side of the saw blade
pressing just below the gullet of the blade and these
plugs have a braking affect representing the load on a
saw blade when sawing.
The plugs themselves are mounted on each side of a
wishbone style holder which in turn is keyed to a rod or
bar adjacent the saw blade. Thus, if the saw deflects,
the wishbone holder slides on the rod or bar and there is
a probe to determine the movement of this holder which in
turn provides an indication of deflection. With this
indication of deflection and knowing the load applied to
the saw blade and the rotation or speed of the saw blade,
one is able to predict the deflection during cutting and
thus ensure that saw blades all comply within certain
limits of deflection under load and cutting speed.
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The present invention provides in a sawing device
wherein a saw blade moves past a sawing area at a sawing
speed, the improvement comprising: a wishbone member
mounted for restricted movement perpendicular to a plane
of the saw blade, the wishbone member having a slot
therein in which the saw blade moves; brake means on both
sides of the slot in the wishbone member for applying a
braking force on both sides of the saw blade moving in
the slot, and a sensing means to determine linear
movement of the wishbone member from a center position to
determine deflection of the moving saw blade under
application of the braking force.
In a further embodiment there is provided an
apparatus for predicting cutting performance of circular
saw blades comprising: an arbor for supporting a
circular saw blade for rotation; drive means to rotate
the arbor about an axis of rotation at a rotational
cutting speed; a wishbone member mounted for restricted
movement parallel to the axis of rotation of the arbor,
the wishbone member having a slot therein in which the
saw blade rotates; brake means on both sides of the slot
in the wishbone member for applying a braking force on
both sides of the saw blade rotating in the slot, and a
sensing means to determine linear movement of the
wishbone member from a center position to determine
deflection of the rotating saw blade under application of
the braking force.
In a still further embodiment there is provided a
method of predicting the cutting performance of a saw
blade comprising the steps of: moving the saw blade at a
cutting speed; applying a braking force to both sides of
the saw blade, the braking force representing a sawing
load on the saw blade, and measuring displacement of the
moving saw blade under load from either side a center
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position to determine deflection of the saw blade under
the sawing load.
In drawings which illustrate embodiments of the
present invention,
Figure 1 is a side elevational view showing a
rotating saw blade mounted on an arbor powered by a
driving mechanism and having a braking device to apply a
braking force to the saw blade according to one
embodiment of the present invention,
Figure 2 is a sectional view showing the braking
device arrangement as shown in Figure 1,
Figure 3 is a detail sectional view showing the
braking plugs applied to a saw blade,
Figure 4 is a partial elevational view showing the
braking device applied to a band saw blade.
Referring now to Figures 1 to 3, a circular saw
blade 10 is shown mounted on an arbor 12 which in turn is
rotated by a mechanical driving mechanism 14 through belt
16. A saw guide 18 is shown on arm 20. The saw guide 18
guides the saw blade, allowing no lateral movement.
A wishbone holder 22 has a slot 24 therein as shown
in Figure 2 in which the saw blade 10 fits. Brake
chambers 26 are positioned one on each side of the slot
24 in the wishbone holder 22 having brake plugs 28
therein that press against the saw blade 10 below the
gullet. The brake chambers 26 have compressed air inlets
30 connected to a compressed air line 32 which in turn
has a valve 34 attached thereto to control air pressure
to the brake chambers 26. As can be seen in Figure 3,
when the brake chambers 26 are pressurized the brake
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plugs 28 are pushed against both sides of the saw blade
10 to provide a braking force. By varying the air
pressure, the braking force applied to the saw blade 10
may be changed.
As shown in Figure 2, the wishbone holder 22 is
mounted on a keyed shaft or rod 36 so that the wishbone
holder 22 can slide backward and forward on the rod 36
which is parallel to the arbor 12 thus the movement of
the wishbone holder 22 is perpendicular to the plane of
the saw blade 10.
The wishbone holder 22 moves freely on the rod 36,
thus deflection of the blade 10 either side of a center
position when it is rotating causes the wishbone holder
22 to move away from the center position representing a
flat saw blade 10. Stops 38 are positioned on each side
of the wishbone holder 22 as shown in Figure 2 so that
the wishbone holder has restricted movement and cannot
move too far, thus preventing the blade becoming too
distorted. Probes 40 are shown attached to the stops 38
and provide a measurement of movement of the wishbone
holder either side of the center position on the rod 36.
This measurement represents the deflection of the saw
blade from the center position. The maximum allowable
deflection depends upon the sawing tolerances permitted,
particularly when saw blades are mounted in a gang
arrangement on an arbor. If the deflection is greater
than a predetermined amount, then the saw blade is
discarded and not used on the saw machine.
A different type of probe 42 is shown on the saw
blade 10 in Figure 1 above the wishbone holder 22 which
measures deflection of the actual saw blade below the
gullet rather than the deflection of wishbone holder.
The predetermined deflection for the location of the
probe on the blade is determined according to the
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required tolerances of thickness for sawing. The probe
may be a laser base non-contact probe or other known
types of probes.
The compressed air and plugs 28 have a predetermined
braking affect or gripping affect on the saw blade 10 and
this can be changed for different types of saw blades,
depending upon thickness. If a saw blade is rejected,
then it may be returned for pretensioning and then
retested to ensure that the deflection of the saw blade
is within the acceptable limits for sawing. These
acceptable limits are determined by the acceptable width
tolerances for the sawn product.
Figure 4 illustrates a band saw blade 50 with guides
52 above and below the wishbone holder 22. Deflection of
the band saw blade 50 is measured in the same way as
shown in Figures 2 and 3. The speed of the band saw
blade 50 is a normal cutting speed and the braking force
applied to the blade in the gullet portion represents the
load on the saw blade 50 when sawing.
Various changes may be made to the embodiments shown
herein. Whereas the brake plugs 28 are shown being
pressured by compressed air, this force could be by
hydraulic or mechanical means. A variation to the force
however is required for different types of blades and
different speeds. The testing device shown in Figure 1
for a rotary blade is not a saw machine, but represents a
testing device suitable for positioning in a filing room,
thus the saw blades can be tested in the filing room
before they are used in a saw machine for sawing a
product. In the case of a band saw blade, it is likely
that the wishbone holder 22 and rod 36 would be mounted
for testing purposes only on the band saw machine at a
location where sawing occurs.
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Various changes may be made to the embodiments shown
herein without departing from the scope of the present
invention which is limited only by the following claims.
