Patent Publication Number: US-2010122620-A1

Title: Circular saw blade with thermal barrier coating

Description:
RELATED APPLICATION 
     This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 61/115,885, filed Nov. 18, 2008, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to cutting systems and more particularly to circular saw blades such as circular saw blades for lumber mills and methods of making circular saw blades. 
     BACKGROUND INFORMATION 
     Circular saw systems for lumber mills are described in U.S. Pat. Nos. 3,285,302 and 3,623,520, incorporated herein by reference for background only. These patents describe gang saws having guide arms with wear pads formed of babbitt metal. The guide arms and wear pads improve the accuracy of the cut and reduce the size of the kerf by preventing the saw blade cutting edge from wandering laterally. The wear pads abut opposing sides of the blade radially inward from the outer margin of the blade where the cutting teeth are distributed. In conventional circular saw systems, sawdust spillage along the sides of the saw blade causes friction that tends to increase the temperature of its outer margin, which increases internal stresses that may cause the cutting edge to deviate laterally. Moreover, the gullets between the cutting teeth tend to wear over time, which increases sawdust spillage and exacerbates blade deviation. In some circumstances, an unacceptable amount of lateral deviation of the cutting edge occurs after only a few hours of operation. 
     SUMMARY OF THE DISCLOSURE 
     According to one embodiment, a thermal barrier coating is applied to an outer margin of a circular saw blade to inhibit abrupt increases in temperature of the outer margin during cutting operation. The thermal barrier coating may be applied on opposing major surfaces of the circular saw blade in the outer margin. For example, the thermal barrier coating may cover opposing major surfaces of the cutting teeth. The thermal barrier coating may also cover the cutting edges of the cutting teeth and the blade tooth gullets that separate the cutting teeth. The thermal barrier coating may include a carbide and/or an oxide, and may be applied by using a plasma spray process or another coating process. 
     Additional aspects and advantages will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a saw blade and a guide arm of a saw blade system according to one or more embodiments. 
         FIG. 2  is a sectional view of the saw blade system taken along lines  2 - 2  of  FIG. 1 . 
         FIG. 3  is a perspective view of a portion of the outer margin of the saw blade of  FIG. 1 . 
         FIG. 4  is a sectional view of the saw blade taken along lines  4 - 4  of  FIG. 3 . 
         FIG. 5  is a flow chart of a coating process for depositing a thermal barrier coating on the saw blade of  FIG. 1 . 
         FIG. 6  is a side view of a portion of the outer margin of the saw blade of  FIG. 1  showing tip inserts for the cutting edges of the blade. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to the above-listed drawings, this section describes particular embodiments and their detailed construction and operation. The embodiments described herein are set forth by way of illustration only and not limitation. Those skilled in the art will recognize in light of the teachings herein that there is a range of equivalents to the example embodiments described herein. Most notably, other embodiments are possible, variations can be made to the embodiments described herein, and there may be equivalents to the components, parts, or steps that make up the described embodiments. 
     For the sake of clarity and conciseness, certain aspects of components or steps of certain embodiments are presented without undue detail where such detail would be apparent to those skilled in the art in light of the teachings herein and/or where such detail would obfuscate an understanding of more pertinent aspects of the embodiments. 
     According to one embodiment, a thermal barrier coating is applied to selected areas of a circular saw blade to inhibit the areas from heating up during cutting operation. For example, the thermal barrier coating is applied on opposing major surfaces of the circular saw blade at its outer margin to inhibit abrupt increases in temperature of the outer margin during cutting operation, which, in turn, inhibits the outer margin from deviating laterally. The circular saw blade may be used in a number of different applications including gang saws for lumber mills. 
       FIGS. 1 and 2  depict a circular saw blade  100  for cutting wood and the like according to an embodiment of the present disclosure. Blade  100  includes a core of a standard metal saw blade, for example of the kind used in gang saws of lumber mills. Blade  100  may be made of carbon or stainless steel having a standard plate hardness such as a hardness in the range of about RC 46 to RC 48 on the Rockwell hardness scale. Blade  100  includes a center opening  102  or “eye” sized and configured to mate with an arbor or shaft (not shown), which rotates blade  100  during cutting operation. In one example, center opening  102  may include splines  103  to mate with a complimentary splined arbor. A wide range of plate thicknesses and diameters may be selected for blade  100 . For example, a plate thickness w may be in a range from about 0.1 to 0.76 centimeter (cm), preferably about 0.2 to 0.31 cm. A kerf size k produced by blade  100  will depend on many factors, including plate thickness w, tooth shape and set, and blade flatness, and may be in a range from about 0.6 to 3.6 millimeters (mm) greater than plate thickness w, typically about 0.76 to 1.27 mm greater than plate thickness w. Moreover, a diameter d may be in a range from about 10 to 200 cm. In one example, blade  100  may be a saw mill blade and its diameter d may be in a range from about 40 to 100 cm. 
     Blade  100  includes multiple cutting teeth  104  and blade tooth gullets  106  located in an outer margin  108 , which is outboard of a region of contact  110  (area between lines  112  and  114 ) of wear pads  116  of guide arms  118 . Although guide arms  118  and wear pads  116  are depicted in  FIGS. 1 and 2 , blade  100  may be used in applications in which guide arms  118  and wear pads  116  are not used. Cutting teeth  104  include cutting edges  104 ′ that may be an integral part of cutting teeth  104 . Alternatively, cutting edges  104 ′ may be formed as part of conventional tip inserts  105  (see  FIG. 6 ) made of stellite or another hard material, which are bonded to the ends of teeth  104 , and may be replaced after they wear down. Blade  100  further includes a thermal barrier coating  120  (shown by hatching in  FIGS. 1 ,  3 , and  6 ) applied to at least a portion of opposing major surfaces  122  and  124  of blade  100  in outer margin  108 . 
       FIGS. 3 and 4  show thermal barrier coating  120  in more detail. In particular, thermal barrier coating  120  covers opposing major surfaces  122 ′ and  124 ′ of cutting teeth  104 . Thermal barrier coating  110  may also cover cutting edges  104 ′ and outermost edges  104 ″ of cutting teeth  104  and gullet edges  106 ′ of gullets  106 . In one example, tip inserts  105  for cutting edges  104 ′ are installed and thereafter thermal barrier coating  120  is applied to outer margin  108 , including tip inserts  105 . In an alternative example, uncoated or coated tip inserts  105  are installed on blade  100  after thermal barrier coating  120  has been applied to blade  100 . 
     Thermal barrier coating  120  may be deposited in a ring, as shown in  FIG. 1 , on each surface  122  and  124  of blade  100  in outer margin  108 . Thermal barrier coating  120  may be applied on opposing major surfaces  122  and  124  from outermost edges  104 ″ of cutting teeth  104  up to gullet edges  106 ′, up to the outer boundary  112  of the region of contact  110 , or up to any dimension therebetween. In some embodiments, thermal barrier coating  120  may also be applied to a center portion  126  of the blade  100 , located inboard of the region of contact  110 . The region of contact  110  where wear pads  116  ride is left uncoated, to provide accurate and smooth bearing surfaces for the wear pads  116 , and to allow blade  100  to be hammered, rolled, or otherwise mechanically leveled for reflattening after use without damaging thermal barrier coating  120 . In one embodiment, the entire central region of the blade, from center opening  102  to within approximately 0.64 cm of gullet edges  106 ′ may be left uncoated to facilitate leveling the blade  100  by hammering, rolling, or otherwise. 
     Preferably, thermal barrier coating  120  includes a ceramic material. Suitable ceramic materials may include carbides, oxides, cermets, mullites, and combinations thereof. Examples of suitable carbides and/or cermets include WC—Co—Cr (example percentages 86/10/4), WC—Ni (example percentages 88/12), WC—CrC—Ni (example percentages 73/20/7), WC—Co (example percentages 88/12), Cr 3 C 2 —NiCr (example percentages 75/25), Mo 2 C, WC—Co—NiSF, TiC, and Cr 3 C 2 . Examples of suitable oxides include Cr 2 O 3  (e.g., 99.5% pure), Cr 2 O 3 —TiO 2  (example percentages 80/20), Cr 2 O 3 —TiO 2 —SiO 2  (example percentages 92/3/5), Al 2 O 3 , Al 2 O 3 —TiO 2  (example percentages 97/3), and ZrO 2 . In one example, thermal barrier coating  120  includes a spherical, hollow micro-balloon shaped mullite clad with nickel. Other ceramics such as nitrides, borides, and silicides may also be used for thermal barrier coating  120 . In one example, thermal barrier coating  120  is relatively smooth and non-abrasive unlike coatings on tools used to grind and/or cut concrete, brick, metal, and other hard materials. Thermal barrier coating  120  may have a surface roughness in the range of about 0.2 to 10 μm R a . In one example, thermal barrier coating  120  has a surface roughness below about 6 μm R a . In another example, thermal barrier coating  120  has a surface roughness in a range from about 4 to 6 μm R a . Surface roughness measurements are typically taken with a profilometer in accordance with a standard such as ISO 4287. 
     Blade  100 , including the blade edges, may be stressed or tensioned so internal stresses at outer margin  108  are normally different than the internal stresses of the central regions (e.g., central portion  126 , region of contact  110 ) of blade  100  when blade  100  is in a non-rotating state (i.e., when blade  100  is not spinning). For example, outer margin  108  may be put under tension by loosening or stretching the central regions of the blade  100  using hammering and/or rolling techniques (i.e., working the blade  100 ) prior to or after coating blade  100 . Blade  100  may be tensioned to operate at a particular cutting speed. Blade  100  may be susceptible to temperature changes during use. For example, with an un-coated saw blade, sawdust chips that are smaller than the side clearance (e.g., (kerf size k−blade thickness w)/2) of the blade may spill out of the gullets onto opposing major surfaces of the outer margin causing it to heat up and deviate laterally due to expansion of the outer margin. In contrast, thermal barrier coating  120  provides a thermal barrier layer that inhibits abrupt increases in temperature of the metal blade core at outer margin  108  that may otherwise result in an imbalance in internal blade stresses leading to lateral deviation of the outer margin  108 . Reduced deviation of outer margin  108  reduces the kerf size k cut by the blade  100  and/or helps to maintain a relatively straight cut, thereby improving cutting accuracy and reducing waste. Reduced deviation also reduces sawdust spillage from gullets  106 , thereby reducing friction and blade wear, and associated blade changes and equipment downtime. In one example, thermal barrier coating  120  has a thermal conductivity below that of steel. In another example, thermal barrier coating  120  has a thermal conductivity below about 120 watts per meter-kelvin W/(mK). In another example, thermal barrier coating  120  has a thermal conductivity below about 85 W/(mK). In another example, thermal barrier coating  120  has a thermal conductivity below about 50 W/(mK). In another example, thermal barrier coating  120  has a thermal conductivity below about 30 W/(mK). In another example, thermal barrier coating  120  has a thermal conductivity below about 20 W/(mK). In another example, thermal barrier coating  120  has a thermal conductivity below about 10 W/(mK). In another example, thermal barrier coating  120  has a thermal conductivity in a range from about 0.35-1.0 W/(mK). 
     In addition to providing a thermal barrier, thermal barrier coating  120  may also provide a wear coating for cutting teeth  104  and gullet edges  106 ′. For example, thermal barrier coating  120  may inhibit gullet edges  106 ′ from wearing down over time to thereby reduce sawdust spillage and friction. Moreover, thermal barrier coating  120  may be sufficiently strong and well bonded to blade  100  to withstand wearing away from opposing major surfaces  122  and  124  over time so that blade  100  may continue to provide thermal protection for outer margin  108  even after extended use. For example, thermal barrier coating  120  may have a hardness in a range from about 1000 to 1500 in the Vickers hardness scale. In one example, thermal barrier coating  120  has a hardness in a range from about 1250 to 1350 in the Vickers hardness scale. In another example, thermal barrier coating  120  has a hardness in a range from about 1300 to 1400 in the Vickers hardness scale, and in another example thermal barrier coating  120  has a hardness in a range from about 1100 to 1200 in the Vickers hardness scale. 
       FIG. 5  is a flow chart of a method  500  that may be used to coat blade  100  according to one embodiment. Although method  500  corresponds to a plasma spray process, other techniques, such as solution processing, chemical vapor deposition (CVD), physical vapor deposition (PVD), and high-velocity oxy-fuel coating (HVOF), may be used to deposit thermal barrier coating  120  on blade  100 . First, blade  100  is masked to cover the regions that are to remain uncoated (e.g., the region of contact  110 ) and positioned in an arbor (not shown) to allow blade  100  and mask (not shown) to rotate to facilitate application of thermal barrier coating  120  (step  502 ). The exposed surfaces of outer margin  108  are then sandblasted to roughen them so that thermal barrier coating  120  can bond well to blade  100  (step  504 ). An initial bond coating may optionally be applied to serve as an interface between the sandblasted outer margin  108  and the ceramic coating (step  506 ). Preferably, the bond coating is applied when a ceramic oxide is used for thermal barrier coating  120 . The bond coating functions to hold ceramic oxide to the blade  100 . One example of a suitable bond coating is NiCr (example percentages 80/20). The initial bond coating may be applied using a plasma spray gun. When carbide ceramic is used, thermal barrier coating  120  can be applied without applying the initial bond coating. 
     After step  504  or step  506 , thermal barrier coating  120  is applied on outer margin  108  (step  508 ). For example, a plasma spray process may be used in which a plasma gas such as argon, nitrogen, hydrogen or a mixture thereof that is inert to the materials that will form thermal barrier coating  120  is dissociated and accelerated in a nozzle of a plasma gun (also called a plasma arc or plasma torch) through expansion to thereby generate a plasma gas stream (also called a plasma flame). The plasma gas stream is directed toward a deposition site at outer margin  108 . A powder (e.g., carbide powder, oxide powder, mullite powder) is introduced into the plasma gas stream and high temperatures (e.g., up to 20,000 K) of the plasma gas stream melt the powder, and the melted powder is propelled toward the deposition site. The melted powder impacts the deposition site to form thermal barrier coating  120 . The nozzle of the plasma gun may be moveable (e.g., via a robotic control system) to provide an even coat on blade  100 . Moreover, blade  100  may be rotated on the arbor in concert with movement of the plasma gun to evenly coat gullet edges  106 ′, opposing major surfaces  122  and  124 , and cutting edges  104 ′. The plasma spray process provides good bond strength for thermal barrier coating  120  and allows a thin, even coat to be applied. For example, the ceramic coating  102  may have a thickness from about 10 μm to about 500 μm. In one example, the thermal barrier coating  120  has a thickness a range from about 35 to 255 μm. In another example, the thermal barrier coating  120  has a thickness of in a range from about 50 to 70 μm. 
     Thermal barrier coating  120  is capable of significantly reducing lateral deviation of the outer margin  108  during cutting operation. For example, a first gang of 10 un-coated saw blades and a second gang of 10 coated saw blades were used to cut Douglas fir wood. The saw blade specifications and operating conditions for each of the first and second gangs of saw blades are listed in the following table: 
                                                    blade diameter d   62.2   cm           plate thickness w   0.28   cm           original kerf size k   0.38   cm           cutting speed   2400   rpm           depth of cut   20.3   cm                             wood type   Douglas fir                                 feed rate   200   feet-per-minute (fpm)                        
The thermal barrier coating  120  on the blades of the second gang included WC—Co—Cr (86/10/4) and was applied to outer margins  108  of the blades using a plasma spray process. The thermal barrier coating  120  was approximately 64 μm thick per side and covered the opposing major surfaces  122 ′ and  124 ′ of the blade teeth  104  from their outermost edges  104 ″ to approximately 0.64 cm radially inward from the gullet edges  106 ′. The thermal barrier coating  120  also covered the gullet edges  106 ′ and the cutting edges  104 ′ of the cutting teeth  104 .
 
     After approximately two hours of continuous run time, the outer margins of the blades of gang  1  deviated laterally or wobbled side-to-side by unacceptable amounts producing a wider kerf size k, a crooked cut, or both. In some applications, a kerf size k that increases by more than about 0.5 to 1.3 mm is unacceptable. Moreover, in some applications, cuts that deviated by more than about ±0.25 to 0.63 mm from their nominal positions are unacceptable. After two hours the blades of gang  1  needed to be changed, which required approximately 20 minutes of unscheduled equipment downtime, or approximately 80 minutes of downtime on average for a ten-hour shift. The blades also required extra maintenance work to re-level and re-tension them for future runs. 
     On the other hand, the coated blades of gang  2  operated continuously for a ten-hour shift, during which the kerf size k and lateral deviations of the cuts remained within acceptable ranges. Acceptable ranges for lateral deviation of a cut and an increase in kerf size k may depend on the type of application in which blade  100  is used. In some applications, lateral deviation of a cut of less than about ±0.25 mm from its nominal position may be acceptable. In one example, the lateral deviation of a cut from its nominal position may be less than about ±0.13 mm, and in another example, the lateral deviation may be less than about ±0.064 mm. Moreover, in some applications an increase in the kerf size k of less than about 0.5 mm may be acceptable. In one example, an increase in the kerf size k of less than about 0.25 mm may be acceptable, an in another example, an increase in the kerf size k of less than about 0.13 mm may be acceptable. After the shift, the coated blades needed no extra maintenance work aside from standard re-sharpening of the cutting edges  104 ′. Accordingly, the coated blades were able to prevent a substantial amount of unscheduled equipment downtime and blade repair time. 
     It will be obvious to skilled persons the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.