US3693675A

US3693675A - Shear blade

Info

Publication number: US3693675A
Authority: US
Inventors: Francis E Allen
Original assignee: Francis E Allen
Current assignee: KOCKUMS CANCAR Inc A CORP OF CANADA
Priority date: 1970-07-07
Filing date: 1970-07-07
Publication date: 1972-09-26
Anticipated expiration: 1989-09-26

Abstract

Shear blade for use in band mills used in lumber processing, the shear blade adapted to restrict propellering of lumber. Leading edge of shear blade is tight, being at higher stress than remaining portions of blade, tightness attained by using one or more of three approaches. First approach is to apply asymmetrical load to the blade, load being applied forward of center line of blade. Second approach is to make trailing edge longer than leading edge, and third approach is to roll-tension shear blade.

Description

United States Patent Allen 1 SHEAR BLADE [72] lnventor: Francis E. Allen, 956 Beaumont Drive, North Vancouver, British Columbia, Canada [22] Filed: July 7, 1970 [21] Appl. No.: 52,943

[52] US. Cl. ..143/22, 143/17, 83/201.l5, 143/ 159 S [51] Int. Cl. ..B27b 15/08 [58] Field ofSearch ..l43/l59 8,156,159,159 S, 143/22, 133 C, l9, 17, 157, 17 R, 19 R;

[ 56] References Cited UNITED STATES PATENTS 3,516,458 6/1970 Hedrei ..l43/2 2 3,456,539

7/1969 Amada ..83/201.l5

[ 51 3,693,675 [451 Sept. 26, 1972 1,790,282 l/l93l Phillips ..l43/l33 R 592,936 11/1897 Pryibil 143/156 R 142,036 8/1873 Merritt 143/156 R 1,723,389 3/1929 Thiel ..l43/l 56 R Primary Examiner-Donald R. Schran Attorney-Brian J. Wood [5 7] ABSTRACT 9 Claims, 8 Drawing Figures its PATENTEDsP2s 1972 SHEET 2 or 3 Francis E. Allen,

PATENTEDSEPZB I972 SHEET 3 [IF 3 Francis E. Allen,

Inventor SHEAR BLADE 1 Field of the Invention The invention relates to band mills as used in saw mills and in particular to twin band mills used to produce flitches or to rip cants.

2. Prior Art A fiitch is a slab of lumber having two opposite parallel sawn sides and waney edges. A cant is a length of lumber having two pairs of sawn opposite parallel sides.

Twin band mills have been used for many years, a difficulty with existing twin band mills being in maintaining a flat kerf accurate within required limits, and in maintaining the sawn sides parallel. The kerf can undulate or twist from causes such as, knots, and variation in hardness or friction, in the log. Unequal cutting forces on each blade produce an unbalanced couple tending to produce rotation of thelog as it travels through themill, such rotation being referred to as propellering. Propellering tends to produce helical surfaces, rotation of the log being limited by interference of sawn faces of the log on side walls of the band saws. Such interference consumes power, reducing effectiveness of cutting, and heats the saw. Heating changes tension and change in tension tends to cause the saw to wander which, as is well known in the art, is both disadvantageous and dangerous.

One attempt to reduce the above difficulty is described in Canadian Pat. No. 804,359, Constantin Hedrei inventor and granted to Forano Limitee in 1969. The patent teaches a guide within the kerf mounted behind and co-planer with a cutting portion of a saw blade of a twin band mill, that is to say the log encounters the guide after it has been cut by the saw.

Cutting portion, as used herein, means the one or more teeth which, at a particular instant, are actually cutting. Stiffness of the guide is increased by applying a tensile load, the guide being shaped so that a leading edge nose portion is maintained under a tension lower than that ina remaining portion of the guide. The lower tension results in a slack leading edge adapted to follow slight curves or twists in the kerf, and thus tends to produce sawn sides which are not parallel. The guide is thicker than the kerf and tends to wedge it open, thus relieving friction on the side walls of the saw.

The patent above also teaches inclination of the leading edge of the guide to compensate somewhat for cutting forces produced by the saw as it cuts the log, the cutting forces acting downwardly on the log and forcing the log onto the tracks or conveyor. Inclination of the leading edge reduces load on the conveyor or tracks which may reduce wear on the conveyor and power consumed in feeding logs through the saw.

SUMMARY OF THE INVENTION The present invention contemplates a thin shear blade or guide with a stiff leading edge. The thin blade does not wedge the kerf open, thus frictional forces on sides of the shear blade are low. The stiff leading edge maintains alignment of the log with the saw, maintain- The shear blade has a thickness less than the width of the kerf and has a leading edge and a trailing edge separated by a center portion. The leading edge is maintained at a higher tensile stress than the center portion and trailing edge, a leading edge so stressed being known in the trade as a tight leading edge.

There are at least three approaches to obtaining a tight leading edge.

A first approach to tighten the leading edge is by applying asymmetrical loading to the shear blade, by displacing at least one point of application of the load to anchor plates of the shear blade forward of a center line of the shear blade so that more load is carried by the leading edge of the shear blade than by the trailing edge.

A second approach is to apply biased loading to the shear blade by having the trailing edge longer than and inclined to the leading edge, the leading edge being disposed nonnalto direction of travel of lumber.

A third approach is to roll-tension the shear blade, as in a saw blade of a band mill, so that tire portions at leading and trailing edges of the shear blade are produced and, when the saw blade is under tension, the tire portions are maintained at a higher stress than the center portion.

One, two or all three of the above approaches can be used to attain a tight leading edge for the shear blade.

A detailed description following, related to drawings, gives exemplification of the invention which, however, is capable of expression in structure other than that particularly described and illustrated.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective of a twin band mill equipped with shear blades according to the invention,

FIG. 2 is a fragment of a portion of one shear blade in position behind a cutting portion of a band saw,

FIG. 3 is a plan of a log being cut by saws of a twin band mill equipped with two shear blades,

FIG. 4 is an end elevation of a flitch being cut by a twin re-saw, the re-saw being equipped with one shear blade,

FIGS. 2 4 are simplified and diagrammatic,

FIG. 5 is a fragmented side elevation of the shear blade when unloaded, anchor means mounting the blade being shown partly sectioned,

FIG. 6 is a simplified section of the shear blade on 6-6 of FIG. 5 some parts not being shown,

FIG. 7 is a fragmented section of a leading edge of the shear blade on 7-7 of FIG. 5,

FIG. 8 is a fragmented detail section of an upper end of the shear blade on line 8-8 of FIG. 5, showing in addition means of mounting and loading.

DETAILED DISCLOSURE FIG. 1

Band mills, l0 and 11, have

upper wheels

12 and 13 on which ban saws l and 15 run.

Shear blades

16 and 17 according to the invention are shown aligned with the saw 14 by a distance 25, in the order of 2 or 3 g teeth 21 of the saw 14 travelling in a direction shown by an arrow 26 the teeth actually cutting being a cutting portion, 24, as defined. Upwardly projecting spikes 27' of the conveyor 23 hold the log passing it through the saw. Cutting forces on the log are several orders of magnitude greater than feed forces on the log produced by feeding it through the saw. The log is thus forced onto the spikes, the spikes sinking into the log. A conveyor fitted with knife-type flights can be used in lieu of a spiked conveyor.

With reference to FIG. 3, the log 22 passing through the twin band mills on the spiked conveyor 23 is cut producing a flitch 29.2, and two slabs 29.1 and 29.3. The saw blade 14, having a thickness, or gauge, 32 has teeth 21 swaged at cutting edges as shown in profile and designated 21.1, the teeth producing a kerf 30 having a width 31,. which width is greater than the thickness of the saw 14. The shear blade 16 has a thickness, or gauge, 33 less than the saw gauge 32. Hereinafter planes of the shear blade and the cutting portion of the saw blade refer to planes midway between parallel side faces of the blades. The shear blade is disposed symmetrically behind the saw blade, with the saw and the blade planes co-planar.

Ideally the shear bladedoes not wedge open the kerf,

the blade being co-planar with and positioned closely,

behind the saw, with clearance provided in the kerf on either side of the shear blade. In practice slight rotations and transverse movements, of the log, or closing ofthe kerf, produce binding on one or both surfaces of the shear blade. This restricts further transverse movement or rotation of the log, such restriction being a prime function of the shear blade. Restriction above produces in the logs cut by bandmills equipped with shear blades kerfs within closer limits than if shear blades were eliminated. The shear blade serves as a means within the kerf of the log to maintain the kerf within a plane of the cutting protion of the saw blade, so that movement of the logs through the saw is essentially axial translation with negligible rotation of the log.

FIG. 4

When re-sawing a flitch, as shown in FIG. 4, only one shear blade is used inthe twin band mill.

The flitch 40 carried on a slat bed 42, is cut by twin band saws, planes of the cutting portions of blades being designated 44 and 45, the blades not being shown. A crowder roll 47 having an axis of rotation 48 forces the flitch 40 against a line bar 49, which is parallel to the direction .of feed of flitches through the saw. The crowder roll maintains a datum face 40.1 of the flitch 40 against the line bar,'thus ensuring that a kerf flat within required limits is made through the flitch. When used as above, only one shear blade (not shown) is fitted behind the saw 44, which saw is nearer the crowder roll than the saw '45. The one shear blade serves to prevent a flitch, produced from the larger flitch 40, from rotating'axially or wandering laterally and possible interfering with other flitches produced concurrently. The crowder roll is positioned adjacent to the cutting portion of the saw 44 and rotates to feed the flitch through the saws. A cant can be cut as above, using one shear blade only,and satisfactory results are obtained.

In FIG. 4, the flitch or cant is fed through the saw on a moving slat bed, thus the spiked conveyor chain of FIG. 3 is eliminated and the slat bed is used in lieu. Other means of feeding lumber through the saw are known.

It was stated previously with reference to FIG. 3 that the gauge 33 of the shear blade is less than the gauge 32 of the saw. This is usual but in some circumstances the shear blade gauge can be greater than the saw gauge. Best results are obtained when the shear blade gauge is less than the width 31 of the kerf 30, however in some circumstances satisfactory results can be obtained whenthe shear blade gauge approaches or even exceeds somewhat the width 31 of the kerf. In such circumstances, excessive binding of the kerf on the shear blade may result and adequate lubrication is provided to reduce build-up of resin etc.

FIG. 5 I I The shear blade 16 is a quadrilateral having an upper end 61 having a width 62, a lower end 63 having a width 64, a leadingedge 66 and a trailing edge 67, leading and trailing edges being defined with reference to the direction 19 indicating motion of the lumber past the shear blade.

A mid position 68 of the end 61 and a mid position 69 of the end 63 are joined by a center line 71. The upper end 61 is held in an upper anchor plate 73 and the lower end 63 is held in a lower anchor plate 74, slots in the plates accepting the ends of the shear blades. A plurality of dowels 75 and 76 (shown in end elevation) in the anchor plates 73 and 74 locate the ends of the shear blade and are fitted through location holes in the plate and the shear'blade after fitting as above. The anchor plate 73 is hinged at an upper pivot pin having an axis 79, and the anchor plate 74 is hinged at a lower pivot pin having an axis 81. A line 83 joins the axes 79 and 81, which line represents a mean line of action and direction of tension forces applied on the blade, the line being forward of the centerline 71. A column (not shown) supporting the shear blade has a vertical center line designated 85 which center line, as seen in FIG. 5, is parallel to the leading edge 66. The leading edge 66 is at an angle 87 to an upper edge at the upper end 61 and an angle 88 to a lower edge at the lower end 63, both the angles being 90. Since the direction 19 is at right angles to the leading edge 66, there is no vertical reaction component tending to lift the log off the chain conveyor. The width 64 is greater than the width 62, so the center line 71 of the shear blade is inclined at an angle 91 to the vertical center line 85 of the column and the trailing edge is longer thanthe leading edge. The axis 79 of the upper pivot point is on the center line 85 and the axis 81 of the lower pivot point is displaced behind the center line 85 by a distance 89 so as to incline the line 83 to the center line 85 at an angle 93 as shown.

The pivot pin at the axis 81 is secured to the column of the band mill so that the assembly 74 is free to rotate about the pin in a limited range. Load is applied to the pin at 79 in a direction 95 parallel to the center line 85 to tension the shear blade to increase effective stiffness of the shear blade, to be explained.

A central concept of the invention is to ensure that the leading edge of the shear blade is stiffer than a remaining portion of shear blade. Stifiness above is attained by applying load to the shear blade in such a manner that the leading edge is at a higher tensile stress than the remaining portion of the shear blade without subjecting the shear blade to a material bending moment. Such a distribution of stress in the leading edge of the shear blade is referred to as producing a tight leading edge, three approaches being used to attain this tightness. A tight leading edge has been defined as maintaining the leading edge at a higher tensile stress that the center portion and trailing edge of the shear blade when the blade is under load in the mill and is attained by using at least one of three approaches, namely asymmetrical loading, biased loading, or roll-tensioning. Insufficient tightness of the leading edge can result in the leading edge going slack because of local heating, such slackness reducing effectiveness of the shear blade.

Asymmetrical Loading (FIG. 5)

Asymmetrical loading is attained by positioning the axes 79 and 81 of the pivots forward of the center line 71 of the shear plate, additional stress above symmetrical loading of the shear blade being applied to dowels adjacent the leading edge'of the blade, thus tightening the leading edge of the shear blade. In this instance both axes are displaced forward of the center line 71 to tighten the leading edge, however, displacement of only one forward of the center line may be sufiicient in some cases; typical displacement being in the order of one-quarter of an inch for a blade having a width from about 7 to 10 inches.

Such forward displacement of at least one point of application of load to the anchor plates of the shear blade results in a gradual decrease in stress from the leading edge to the trailing edge across a typical section of the shear blade.

Biased Loading (FIG. 5)

Biased loading of the shear blade is attained when the trailing edge is longer than the leading edge. A blade as above, when subjected to a tensile load applied generally parallel to the leading edge results in the leading edge being at a higher stress than the trailing edge, such stress resulting from biased loading. Upper and lower edges of the blade can be parallel as shown, differences in widths 61 and 63 being about 2 inches for a shear blade of 4 foot length with a mean width of about 10 inches. Upper and lower edges of the quadrilateral need not be parallel as shown, provided that the leading edge is shorter than the trailing edge and is disposed generally parallel to the load applied to the shear blade.

The two approaches above result in the leading edge of the shear blade having a tensile stress higher than the trailing edge or center portion. Material for the shear blade permits a relatively wide variation of stress from the leading edge'to the trailing edge whilst maintaining the stress in the leading edge below the yield point of the material and maintaining a reasonably stiff trailing edge. Common saw steel has been used for the shear blade and is satisfactory.

Roll-tensioning (FIG. 6)

As stated above, the shear blade is made from saw steel and is roll-tensioned similarly to a band saw, such roll-tensioning being achieved by stretching the center portion of the shear blade by plastic deformation, leaving tire portions adjacent outer edges such that, in an unloaded condition when the blade hangs from an upper or lower edge in a free state, the shear blade has .a section as seen in FIG-6. A center portion 98 of the ing and trailing edges and defines, in part, a datum plane of the shear blade. The bow is maximum adjacent the center line 71, which maximum is a dimension 101 being of the order of thirty thousandths of an inch for a shear blade having a mean width of about 10 inches.

One measure of the bow is that the distance 101 would be maintained along the total length of the shear blade if the shear blade were curved to form an arc of a circle having a diameter of about 35 feet, with a center of the circle on a side of the datum plane 99 remote from the center portions 98.

A tire portion 102 having a width 103 is adjacent to the leading edge 66 and a further tire portion 104 having a width 105 is adjacent the trailing edge 67. The widths 103 and 105 are of the order of three-quarters of an inch and are essentially co-planar with the datum plane above.

One result of the roll-tensioning is that, considering the center portion 98 of the shear blade separated from the

tire portion

102 and 104 and placed in a free state on a flat surface, length of the center portion would be marginally greater than length of the tire portions. Rolltensioning can be effected by hammering, or other means known to the trade.

When the shear blade lies in a relaxed condition on a flat surface, the tire portions are effectively under low tension and the center portion is under low compression. When the shear blade hangs freely in a relaxed condition, the tire portion remains relatively flat while the center portion bows away from the datum plane of the blade, the center portion being referred to as the dished portion. When sufficient load is applied to the shear blade, the dished portion tends to become aligned with the datum plane 99, thus the shear blade approaches a flat sheet. The tire portions which were already under some tension before the blade was loaded, are thus more highly stressed than the center portion. Thus roll tensioning' results in tire portions having a higher stress than the center portion and is the third approach that ensures that the edge portions are at a higher stress than the center portion.

All three approaches above ensure that under normal operating conditions, at no time whilst the blade is loaded will the leading or trailing edges go slack due to expansion produced by heat generated by movement of the logs past the shear blade.

When using two or more of these approaches, contributions of stress due to each of the approaches depends on load applied to the shear blade, dimensions of the shear blade, degree of roll tensioning, and displacement and inclination of the line 933 from the center line 71 of the shear blade. The degree of roll tensioning can be defined as difference in stress between the tire portions and the dished potion of the blade when the blade is in the relaxed condition on a flat surface, and in the particular shear blade described, leading edge tightness is attributed mainly to the roll tensioning.

A significant factor in roll tensioning is choice of a suitable steel that responds adequately to roll tension-.

ing. Saw steel is suitable, although other steels can be heat treated and/or cold worked for roll tensioning. A band saw steel having an ultimate tensile strength range from about 200,000 to 210,000 pounds per square inch and a yield strength of l80,000 to 190,000 poundsper square inch gives satisfactory results in practice. Dif ficulty has been experienced in attempting to use mild steel and stainless steel.

In essence, a leading edge having a stress higher than the remainder of the shear blade ensures that, in spite of heating due to friction, the leading edge remains tight so that the log or. flitch is guided by the tight leading edge 66 in the kerf which, in most operating conditions, is essentially coplanar with the saw blade.

As stated before, sufficient leading edge tightness may be obtained by using only one or two of the three approaches above, required degree of tightness being dependent on operating conditions and expected'temperature rises due to heat generated by friction. Heat can be removed from the shear blade by providing coolant/lubricant on both surfaces of the blade, rate of cooling of the blade being controlled generally by volume flow. Friction between the shear blade and the log can be decreased by using a low friction coating on surfaces of the shearblade, a suitable coating being Teflon-S, a registered trade mark of Du Pont. FIG. 7 I

In H6. 7, the leading edge 66 as shown has a chamfer 106 on one side 1070f the shear blade which chamfer is at an angle 108 to surfaces of the shear blade, the angle 108 being of the order of about thirty degrees. The side 107 of the shear blade faces the band mill column, disposition of the chamfer 106 being such as to ease access of the shear blade into the kerf. F K]. 8 a

A portion of the column of the band mill is designated 110 in FIG. 8, the column having the center line 85 (shown only in FIG. The section shown in FIG. 7 is from a section plane containing the center line 85. A support bracket 112 is secured at an inner end 113 to the portion of the column 110, the bracket hav ing a vertical bore 114 as shown. Any sufficiently rigid portion of the band mill having abore aligned with the axis 83 will suffice, eliminating the bracket 112. A pivot clevis 116 is secured by a pin 118 to the upper anchor plate 73, the pin being concentric with the axis 79.. The line 83 joining axes of the pivot points passes as a central axis through the shear blade 14 as shown. The shear blade is held in a slot 119 at a lower end of the plate 73 by the dowels 75 (in broken outline in FIG. 8). The plate 73fis freeto pivot in a limited manner on the pin 1 18 so as to ensure that negligible bending moment is applied to the shear blade by loading of the blade.

A stud 121 extends vertically from the pivot clevis 1 l6 co-axially with the axis 83, and extends through the bore 114. An upper end of the stud 121 has a stack of load-indicating washers 124 interposed between a nut 125 and a locking nut 126. The load-indicating washers 124 can be of a dished. type that deflect under load, measurement of deflection giving an indication of load on the washers, which is effectively load on the shear blade. Suitable washers are Solon spring washers or Belleville spring washers obtainable from ordinary trade sources.

With the ends of the shear blade dowelled in the anchor plates and the plates mounted on the pivot pins, the stud 121 is fitted in the bore 114, and the washers 124 and nuts 125, 126 are threaded onto the stud. The

nut 125 is tightened until desired deflection of the stack of washers is attained, thus permitting application and estimation of load on the blade. Typical load applied to an approximately ten inch wide shear blade of 15 gauge saw steel [0.072 inches thick] is about 10,000 pounds.

Other means of applying and measuring load applied to the shear blade can be used, for example hydraulic ram or lever means, thus eliminating the washers 124.

The description of the blade 16 above applies to the shear blade 17 above. The description relating to FIGS. 1 through 4'is use of shear blade with twin band mills. Cutting operations using a single band mill only benefit by using a shear blade, description of FIGS. 5 through 8 applying to both single and twin band mils.

I claim:

l. A bandmill (10) having a band saw (14) adapted to cut a log (22) moving in a direction on a conveyor (23), the saw having a thickness (32) and a cutting portion (24), the cutting portion having a plane parallel with the direction of movement of logs, the saw being adapted to cut a kerf (30) in the log, the kerf having a width (31 ,l, the bandmill including:

a. a shear blade (16) mounted coplanar with and spaced behind the cutting portion of the saw, the shear blade having leading and trailing edges (66, 67) a center portion (98), and a thickness (33) less than the width of the kerf,

b. tensioning means to tension the shear blade so that the leading edge is at a higher tensile stress than the center portion of the shear blade adapted so that movement of the logs through the saw is essentially axial translation with negligible rotation of the log.

- 2. A bandmill as defined in claim 1 in which the tensioning means to tension the shear blade includes:

7 c. tire portions (102, 104) at the leading and trailing edges of the shear blade, the tire portions being produced by roll tensioning, adapted so that, when the shear blade is subjected to tensile stress, the tire portions are at a higher tensile stress than the center portion (98) of the shear blade.

3. A bandmill as defined in claim 1 in which the shear blade has mid-points .(68, 69) at upper and lower ends (61, 63), and the tensioning means to tension the shear blade includes:

(1. pivots at the upper and lower ends of the shear blade the pivots having axes (79, 81), at least one axis of a pivot being displaced forward of a midpoint of one of the ends of the shear blade, the pivot points being adapted so that a tensile load applied between the pivot points is applied to the shear blade, adapted so that, when a tensile load is applied to the pivots, the leading edge of the shear blade is at a higher tensile. stress than the trailing edge and center portion.

. A bandmill as defined in claim 1 in which:

e. the shear blade is a quadrilateral with the trailing edge longer than the leading edge,so that when the tensile load is applied to the shear blade the leading edge is at a higher tensile stress than the trailing edge.

5. A bandmill as defined in claim 3 in which:

f. the upper end (61) of the shear blade is dowelled in an upper anchor plate (73) by dowels (75), the anchor plate being mounted on a first pivot having an axis (79),

g. the lower end (63) of the shear blade is dowelled in a lower anchor plate (74) by dowels (76), the lower anchor plate being mounted on a second pivot having an axis (81), one pivot being moveable relative to the other pivot for applying a tensile load to the shear blade.

6. A bandmill according to claim including:

h. a pivot clevis (1 16), the clevis having a stud (121) extending co-axially with an axis (83) adjoining the pivot axes,

i. a pin (118) concentric with the first pivot axis and secured to the clevis,

j. a portion (112) of the bandmill having a bore (1 14), adapted to support the stud,

k. a nut (25) being threaded on the stud,

l. measuring means cooperating with the nut, stud and the portion of the bandmill to measure load applied to the shear blade, constructed and arranged so that tightening of the nut applies a load to the shear blade, indication of load being read on the measuring means.

7. Structure according to claim 6 in which the measuring means to measure load applied to the shear blade includes:

m. a stack of load indicating washers (124) interposed between the nut (125) and the portion (112) of the bandmill, so that tightening of the nut on the stud applies a load to the shear blade and deflects the stack of washers, deflection being an indication of load applied to the shear blade.

8. Structure as defined in claim 1 wherein the shear blade is a saw steel having an ultimate tensile strength range from about 200,000 to 210,000 pounds per square inch and a yield strength of 180,000 to 190,000 pounds per square inch.

9. Structure as defined in claim 2 wherein the shear blade is a saw steel having an ultimate tensile strength range from about 200,000 to 210,000 pounds per square inch and a yield strength of 180,000 to 190,000 pounds per square inch.

Claims

1. A bandmill (10) having a band saw (14) adapted to cut a log (22) moving in a direction on a conveyor (23), the saw having a thickness (32) and a cutting portion (24), the cutting portion having a plane parallel with the direction of movement of logs, the saw being adapted to cut a kerf (30) in the log, the kerf having a width (31), the bandmill including: a. a shear blade (16) mounted coplanar with and spaced behind the cutting portion of the saw, the shear blade having leading and trailing edges (66, 67) a center portion (98), and a thickness (33) less than the width of the kerf, b. tensioning means to tension the shear blade so that the leading edge is at a higher tensile stress than the center portion of the shear blade adapted so that movement of the logs through the saw is essentially axial translation with negligible rotation of the log.

2. A bandmill as defined in claim 1 in which the tensioning means to tension the shear blade includes: c. tire portions (102, 104) at the leading and trailing edges of the shear blade, the tire portions being produced by roll tensioning, adapted so that, when the shear blade is subjected to tensile stress, the tire portions are at a higher tensile stress than the center portion (98) of the shear blade.

3. A bandmill as defined in claim 1 in which the shear blade has mid-points (68, 69) at upper and lower ends (61, 63), and the tensioning means to tension the shear blade includes: d. pivots at the upper and lower ends of the shear blade the pivots having axes (79, 81), at least one axis of a pivot being displaced forward of a mid-point of one of the ends of the shear blade, the pivot points being adapted so that a tensile load applied between the pivot points is applied to the shear blade, adapted so that, when a tensile load is applied to the pivots, the leading edge of the shear blade is at a higher tensile stress than the trailing edge and center portion.

4. A bandmill as defined in claim 1 in which: e. the shear blade is a quadrilateral with the trailing edge longer than the leading edge, so that when the tensile load is applied to the shear blade the leading edge is at a higher tensile stress than the trailing edge.

5. A bandmill as defined in claim 3 in which: f. the upper end (61) of the shear blade is dowelled in an upper anchor plate (73) by dowels (75), the anchor plate being mounted on a first pivot having an axis (79), g. the lower end (63) of the shear blade is dowelled in a lower anchor plate (74) by doweLs (76), the lower anchor plate being mounted on a second pivot having an axis (81), one pivot being moveable relative to the other pivot for applying a tensile load to the shear blade.

6. A bandmill according to claim 5 including: h. a pivot clevis (116), the clevis having a stud (121) extending co-axially with an axis (83) adjoining the pivot axes, i. a pin (118) concentric with the first pivot axis and secured to the clevis, j. a portion (112) of the bandmill having a bore (114), adapted to support the stud, k. a nut (25) being threaded on the stud, l. measuring means co-operating with the nut, stud and the portion of the bandmill to measure load applied to the shear blade, constructed and arranged so that tightening of the nut applies a load to the shear blade, indication of load being read on the measuring means.

7. Structure according to claim 6 in which the measuring means to measure load applied to the shear blade includes: m. a stack of load indicating washers (124) interposed between the nut (125) and the portion (112) of the bandmill, so that tightening of the nut on the stud applies a load to the shear blade and deflects the stack of washers, deflection being an indication of load applied to the shear blade.