United StatesDepartment ofAgriculture Balanced SawForest Service

Forest PerformanceProductsUtilization

TechnicalReportNo. 12

Reference Abstract

Program “SAW” can help sawmillers toselect and operate their saws accordingto sound design principles. This willenable them to (1) maximize productionfrom their saws, (2) reduce saw opera-ting problems, (3) maximize saw life,and (4) increase lumber recovery effi-ciency by producing more accuratelysized lumber

Keywords: Saw, feed speed, bite, toothspeed, depth of cut, power

May 1985

Lunstrum, Stanford J. Balanced SawPerformance. Technical ReportNo. 12. Madison, WI: U.S.Department of Agriculture, ForestService, Forest Products Utiliza-tion; 1985. 17 p.

A limited number of free copies ofthis Technical Report are availablefrom USDA Forest Service, State &Private Forestry, Division of Coopera-tive Forestry, P.O. Box 5130,Madison, WI 53705.

Forest Products Utilization ismaintained in Madison, WI, in coopera-tion with the USDA Forest Service,Forest Products Laboratory, and theUniversity of Wisconsin.

BALANCED SAW PERFORMANCE1

Stanford J. LunstrumLog Breakdown Specialist

USDA Forest ServiceState and Private Forestry

Division of Cooperative ForestryForest Products LaboratoryMadison, Wisconsin 53705

Abstract

Correctly selecting a saw fora job is one of the most criticaldecisions a sawmiller has to make.Even after the selection is made, thesaw must be set up and operated withincertain specifications for balancedperformance. An understanding of theinterrelationship of raw materials,end products, machinery, and thesawing process itself is essential.

Often sawmillers operate theirsaws with less than adequate knowl-edge about correct bite, feed speeds,tooth speeds, side clearances, depthsof cut, and power requirements. Everysaw is limited to a rather narrowoperating range. Experience hastaught most sawmillers which saws tochoose for specific applications.However, for correct saw selection andsaw operation, it is possible to math-ematically calculate the variablesinvolved. Program “SAW” has beendeveloped to integrate these variablesquickly and accurately. Operators whouse Program “SAW” will be able to(1) maximize production from theirsaws, (2) reduce saw operatingproblems, (3) maximize saw life, and(4) increase recovery efficiency byproducing more accurately sizedlumber.

Background

I would like to relate somestatements I have run across in myreading about the operation and careof saws that have appeared over thelast 10 years or so. I quote:

lPresented at conference entitled“Sawing Technology: The Key toImproved Profits,” sponsored by theForest Products Research Society,Jan. 30-Feb. 1, 1984,San Antonio, Texas.

“There is less technical thoughtgiven to the saw than almost any othercomponent part of a sawmill.”

“No engineer can tell you howmuch feed a given saw will take undergiven conditions.”

“It’s standard practice to over-bite the capacity of the gullet tocarry the sawdust. In other areas,the practice is to underbite. "

“I submit on a basis of factsshown that there is now no knownstandard or specification for saws,saw speeds, or rates of feed.” Andfinally,

“Sixty to 65 percent of the sawsare wrong for the job.” Unquote.

These allegations probably havenot been completely laid to rest, andif they hold any truth at all, it’sa wonder we don’t have more problems.Add to this, problems with harmonicsor lack of quality control in blademanufacture --two areas that will beaddressed in other presentations atthis session.

The saw is one of the most abusedpieces of equipment in the sawmill.It’s run too fast; it’s run too slow;it’s overheated; it is overstressed;it’s overfed or underfed; it’s under-powered or, in rare cases, overpowered.Then there are problems with the wooditself. Often, density varies evenwithin the same log or cant. Some-times the wood is frozen or, worse,partially frozen.

The list could go on. You canbegin to see all the combinations ofproblems possible in any given setup.Sooner or later, if saws are improp-erly set up and operated, problemswill occur. Then we either fix theproblems, or we learn to live withthem.

Designing a saw for a specificapplication is a rather controversialand highly complex subject. Opinionsdiffer as to what may constitute thebest parameters for a given applica-tion. In fact, the problem seems toboil down to specifically defininga saw’s task. Many saws are routinelyoperated beyond their design limits.Even if a saw’s design limit is onlyexceeded 10 percent of the time,damage can still occur. If operatorswould consciously avoid operatingtheir saws beyond their design limits,I am convinced there would be fewersaw-operating problems overall.

Once a saw’s task is clearlyestablished, the saw can be properlydesigned. Critical decision makingcan then be made as to tooth geometry,saw or wheel diameter, tooth pitch,gullet size, side clearance, and sawthickness. Once these specificationshave been established, the operatingvariables or limits for that saw havealso been established.

Once a saw is selected fora task, it must be set up and madeready for operation. In setting upthe saw, further decisions must bemade on tooth functioning, whichinvolves the interaction of toothpitch, tooth speed, and feedspeed.Lastly, power needs must be decided.What are the power requirements forthe setup you have selected?

How do you integrate all thevariables for a successful saw setup?This, as I see it, has been the stum-bling block down through the years.A great deal of hand calculating wasrequired to get the needed answers.Program “SAW” was developed to meetthis need, It can be used to analyzeripsawing operations, except for sashgangs, to determine operating vari-ables. It can also be used to re-design a saw setup to bring the sawmore in line with good designprinciples,

Operating Variables

Saw or Wheel Diameter

The diameter of a circular sawshould be the smallest possible thatcan handle the cant or log size to besawn. Using a larger saw than re-quired increases maintenance time andcosts. For maximum efficiency, thesaw diameter should be commensuratewith the task.

The diameter of a circular sawphysically limits the depth of cut itcan effectively handle, While it istechnically possible to saw a depth ofcut equal to the radius of the sawminus the radius of the collar, it isnot the normal practice. Smaller saws,however, can accommodate this practicemore easily than larger ones. Asa general rule of thumb, a circularsaw should saw a depth of cut nogreater than approximately two-thirdsof the saw radius. I label this the“effective depth of cut” (fig. 1).

The effective depth of cut is notsynonymous with the maximum log dia-meter a saw can handle. When breakingdown a log, a sawyer does not normallysaw down the middle on the beginningcuts. Logs are slabbed, a few boardsare sawn, then the log is turned toa new face. Thus a log is reduced toa size where it can be sawn withoutfurther turning (table 1),

For band saws, determining cor-rect wheel diameter for a given taskis more difficult than for circularsaws. The saw gauge, blade width,tooth pitch and height, and the gulletsize must be considered when makingthis decision. Once depth of cut isestablished, it should be strictlyadhered to. If the depth of cut isexceeded, gullets become overloadedwith sawdust, or the sawdust becomesexcessively fine, which pushes the sawoffline, causing variation in thelumber. Once depth of cut for anoperation is established, the smallestwheel diameter should then be used(table 2).

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For wheels of given diameters,there are standard relationships ofband gauge, blade width, and toothspacing and height combinations(table 3).

Saw Thickness

A standard rule of thumb usedover the years in determining sawgauge for band saws was to allow0.001 inch of saw thickness for eachinch of wheel diameter. For example,a 72-inch wheel diameter would requirea saw 0.072 inch thick, which is15 gauge. For larger saws, the gaugeis usually somewhat heavier; and forsmaller saws, the gauge is somewhatlighter than this rule of thumb.

When saw thickness is excessivefor the wheel diameter, high stressesare placed on the band that often leadto fatigue cracks. Cracked saws leadto operational problems and contributeto excessive variation in the lumber.A saw that is too thin for a wheeldoes not necessarily cause fatiguecracks, but often such a saw cannotwithstand the greater stresses imposedby sawing wider cut depths and gen-erally higher load demands.

Determining saw thickness forcircular saw applications is moredifficult. Much depends on the sawingapplication and the type of mainten-ance the saw receives. For primarylog breakdown in the hardwood industry,7- to 8-gauge Inserted Point (I.P.)saws are very common. If the loaddemand is heavy and maximum productionis desired, heavier gauge saws areused. For lighter load demands,smaller gauge saws may be used. Whenselecting a saw for a specific task,determine the severest load conditionsthat will be encountered on a sus-tained basis. Then a saw can bedesigned to handle those loads mostefficiently. If a saw is exposed tocontinually overstressed conditions,the result is lumber variation, anda shortened service life.

For secondary breakdown, a widerange of saw thicknesses is being usedtoday. For edgers and gang sawingapplications in dimension mills, sawsoften run in the 10- to 12-gaugerange. Cutting edge widths range from0.150 to 0.200 inch. However, withtoday’s guidance systems, some opera-tors are successfully running 16- and18-gauge saws with cutting edge widthsaround 0.100 inch. Reduced platethickness and corresponding smallercutting edge widths may not result inincreased lumber recovery. The mainreason being that a smaller platethickness will not necessarily result

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in the least deviation of the saw ifit is not operated and maintainedcorrectly.

It never pays to reduce platethickness and the correspondingcutting edge width at the expense ofincreased sawing variation. A sawimproperly engineered and set up forits task and operated incorrectly willperform less efficiently. Maintenancetime and costs are increased and,worse, the saw will wander in the cut,resulting in increased lumber varia-tion. This results in a higher targetset. The subject of thin saws will beaddressed at greater length in anotherportion of the program.

Gullet Size

One of the main functions of thetooth gullet is to chamber and removesawdust particles from the saw cut.Anything that hinders the tooth fromperforming this task should be averted.Hindering action might include feedingtoo fast, feeding too slow, cuttingtoo deep, running the teeth too fast;or underpowering the saw. The endresult might include excessive amountsof sawdust, sawdust particles exces-sively fine, or sawdust particles thatare so big they clog the gullets,possibly stopping the saw dead cold inthe cut.

How much sawdust can a gullethold? That is indeed the $64,000question! Gullet-holding capabilitynever has been resolved satisfactorily.The answer is not easily obtainedbecause it depends on many factors,such as wood density, moisture content,percent of the gullet that can beeffectively utilized, and the amountof sawdust spillage incurred.

Freshly cut sawdust occupies from3 to 6 times the space in the freestate--that is, before any compactionoccurs. Green, soft, low-density woodin sawdust form expands the least,while dry, hard, high-density woodexpands the most.

Sawdust packs in the gulletcavity because of the pressure thatis exerted from sawdust particlestraveling at high velocity slamminginto the sawdust that has already cometo rest in the gullet. The fuller thegullet becomes, the greater the pres-sure. Laboratory tests have shownthat pressure in the gullet can buildup to 2,000 pounds per square inch in

the compaction process. We also knowthat about 2,000 pounds of pressureper square inch are required to com-press sawdust into an equal volume ofsolid wood. Conditions that wouldallow a buildup of 2,000 pounds persquare inch in the gullet probably arenot achieved regularly during normalsawing. Experts generally agree thatsawdust will normally pack in thegullets to about 50 percent of itsvolume in the free state for band sawsand larger circular saws. If this istrue, then gullet size needs to befrom one and one-half to three timesthe volume of the solid wood fromwhich the sawdust is generated.

gullet Holding Index (or GHI) canbe used to denote the sawdust-handlingcapability of gullets. The GHI isdefined as a ratio of the volume ofsolid wood content, after it is con-verted into sawdust and compacted intothe gullet, that the gullet can handleto the total gullet volume. For ex-ample, suppose a gullet with a volumeof 0.160 cubic inch can handle0.112 cubic inch of solid wood afterit is converted into sawdust and com-pacted into the gullet; we would saythe GHI is 0.7 (0.112/0.160). Or,for ease of understanding the mathinvolved, the cross-sectional area canbe used as shown in the illustration(fig. 2).

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Many sawmill experts agree thatfor good saw performance, sawdustshould be chambered in the gulletcavity and carried along and dis-charged as the tooth emerges from thecut. Overloading gullets results inexcessive sawdust spillage. As saw-dust particles are squeezed betweenthe plate or blade and the wood, itcauses friction, heating, and drag onthe saw. In severe cases, overloadingthe gullets can stop a saw dead coldin the cut. Power demand dramaticallyincreases at the point of overloading.Sawdust spillage usually occursunevenly, thus forcing the saw offlineand causing variation in the resultantlumber.

The volume of sawdust produced bya tooth depends on the cutting edgewidth, bite, and depth of cut. In nocase should these factors combine toproduce more sawdust than the gulletcavity can handle. Forcing more saw-dust into the gullet cavity beyond itscapacity puts severe strain on thetooth assembly. To help insureagainst overloading the gullets,a tradeoff can be made between bite,which is actually controlled by feed-speed, and depth of cut. For example,when increasing feedspeed, and thusthe bite, the depth of cut can bedecreased to make the same volume ofsawdust. This tradeoff can be regu-lated within established limits.

Excessively large gullets fora task can also lead to sawdustcarrying and discharge problems. Anoverly large perimeter in a gulletthat is never filled may allow sawdustto escape more easily. Sawdust thatescapes from the gullet may cling tothe sides of the cut in an unevenfashion. This often happens in wintersawing and can result in excessivetooth vibration, which in turn cancause plate or blade cracks.

In general, when sawing smallerlogs or cants and thus narrower depthsof cut, tooth bite can be increased tothe maximum. Maximum tooth biteshould be translated into the corre-sponding feedspeed and care exercisednot to exceed it. Overbiting on nar-rower depths of cut can damage thetooth assembly as well as the saw. Onnarrow depths of cut, the tendency isto overfeed because power is usuallymore than adequate. Overfeeding manytimes results in tearing the wood,particularly around larger knots,making a rough board surface.

When sawing smaller logs, it isusually best to use the maximum numberof teeth to allow for maximum feed-speed, productivity, and smoothestboard surface. Gullet size should becommensurate with the task. Usingoverly large gullets for sawing smalllogs or cants reduces the number ofteeth that could otherwise be put intothe saw. Thus the feedspeed is slowedand production is hindered. Forsawing larger logs or cants, uselarger gullets to handle the largervolume of sawdust. Sawing larger logswith inadequate gullet capacity gener-ally results in gullet overloading andits attendant problems.

Gullet size should be measuredaccurately. One method that can beused LO determine gullet cross-sectional area is to trace the gulletoutline on graph paper that has100 squares of equal size per squareinch. After the gullet outline hasbeen traced carefully, count thenumber of squares enclosed within thegullet boundary. Divide the number ofsquares by 100 by simply moving thedecimal point two places to the left.You now have the gullet cross-sectional area in square inches.

Side Clearance

The subject of side clearancemust necessarily be discussed alongwith cutting edge width and saw thick-ness. Once side clearance is estab-lished for a task and the saw thick-ness or gauge is known, the cuttingedge width is automaticallyestablished.

Side clearance should be matchedto the task. Side clearance isaffected by the saw and tooth type;species of wood sawn; wood moisturecontent; condition maintained on thecutting edge; alinement of equipment;type of guidance system; and toothgeometry.

The cutting edge must cleara pathway in the wood wide enough forthe saw blade to pass through withoutexcessive friction (fig. 3). Friction,of course, results in heat and this,in turn, may cause a heat gradient todevelop in the saw. This, then,affects a saw’s tension forces.Excessive side clearance may causeoverstressing of the tooth assemblyand thus cause saw instability. Italso increases power demand.

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As the cutting edge penetratesthe wood, the fibers are compressedslightly until they are actuallysheared. After shearing, the com-pressed fibers adjacent to the shearpoint spring back nearly to theiroriginal position. The side clearancemust be sufficient to keep the fibersin the sprung-back condition fromcontacting the saw.

Softwoods in general tend to bemore stringy grained and therefore donot cut as cleanly and smoothly as domost hardwoods. There are exceptionsof course. Softwoods in general, then,require somewhat more side clearancethan do hardwoods. Typically, soft-wood sawing requires side clearancefrom 40 to 50 percent greater than thesaw plate thickness, while hardwoodsawing requires side clearance about25 percent greater than the saw platethickness.

Less side clearance is requiredwhen sawing frozen wood because thefibers generally cut cleaner than whensawing unfrozen wood. Therefore,a narrower path for the saw plate topass through is possible. Reducingside clearance also reduces powerrequirements, which is an importantfactor in sawing frozen wood. To helpoffset the tendency to underfeed inwinter sawing because of increasedpower demands, use the smallest sideclearance possible.

Tables 4 and 5 show standard sideclearances used on headrigs for bothband and circular saws.

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Tooth Geometry

Saw tooth geometry includessetting up the clearance angle, thetooth angle, and the hook angle(figs. 4 and 5). These angles shouldcarefully be matched to a saw’s task.In general, circular saws use largerhook angles than do band saws; fasterfeedspeeds require larger hook anglesthan do slower feedspeeds; conven-tional sawing requires larger hookangles than does climb sawing; andsoftwood sawing requires larger hookangles than does hardwood sawing(table 6).

A proper hook angle insures thatthe tooth will literally “hook” itselfinto the wood to consummate the sawingaction without adverse effects. Whenthe hook angle is excessive for thetask, the saw may self-feed and, inthe case of band saws, may pull itselfoff the wheels. If the feedspeed isexcessively slow for the hook angle,

the teeth, instead of hooking into thewood, will be forced. This producesexcessive drag and increases powerconsumption. The cutting edge of theteeth dulls more rapidly, thus in-creasing the sharpening frequency. Asa rule of thumb, use the greatest hookangle a task will allow.

The tooth angle, or sharpnessangle, determines tooth strength andstiffness. In general, for band

sawing, faster feedspeeds used onsofter woods require smaller toothangles and slower feedspeeds used onharder woods require larger toothangles (fig. 6). Feedspeeds should bematched with the hardness of the woodsawn. If tooth angles are excessivelysmall for the task, the cutting edgebecomes weak, and the tooth is moreinclined to crumble under load. Withan excessive tooth angle, feedspeed isrestrained and power demand isincreased.

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The clearance angle must be suf-ficient for the tooth back to clearthe wood as it progresses through thecut. If the tooth back drags againstthe wood during cutting, friction andheating of the saw result. Forcircular I.P. saws, the clearanceangle should be from 9 to 11 degrees.For band saws feeding above 150 feetper minute, the clearance angle shouldbe from 12 to 16 degrees. Slowerfeedspeed should be coupled withsmaller clearance angles. Largeclearance angles have a tendency todecrease the feed force but also causethe saw to dull faster because ofcausing a smaller tooth angle. Con-versely, a smaller clearance angleresults in a stronger tooth and slowerdulling of the cutting edge. Balancedsaw performance again demands thattooth geometry be matched carefullywith the saw’s task and the way thesaw will be operated.

Tooth Functioning

In discussing the subject oftooth functioning, three componentsmust be considered do,i;ysmrpi;duz:tooth speed, tooth pitch, andfeedspeed.

In general, tooth speed is exces-sive in American sawmills. Whilethere may be some advantages inrunning saws fast, the disadvantagesare of greater consequence. Factorsto consider when selecting tooth speedfor a given setup include saw type,tooth type, species, production needs,and maintenance practices.

In general, saws with carbideteeth are run at higher tooth speedsthan other similar saws. Circularsaws are usually run at higher toothspeeds than band saws. Unfrozen woodis normally sawn at higher toothspeeds than frozen wood. Forobtaining maximum production, use thefastest tooth speeds allowable fora Particular saw design. Faster toothspeeds do not necessarily induce sawvibration, but any tendency a saw hasto vibrate will be aggravated toa greater degree by higher toothspeeds. Saw vibration contributes topoor performance and results in exces-sive lumber variation. Saws run athigher tooth speeds demand topnotchmaintenance. The slower a saw runs,the easier it is to maintain (table 7).

Tooth pitch determines primarilyhow much work a saw can accomplish.If the maximum amount of work isdesired, the saw must contain themaximum number of teeth commensuratewith the proper gullet size. Gulletsize, in turn, determines to a largeextent the maximum number of teetha saw can effectively carry. Toothpitch should allow for sufficientstrength in the shoulder to supportthe tooth during sawing.

If small logs are cut with sawsthat have large gullets, and thus con-tain fewer teeth, feedspeed must nec-essarily be slowed to insure againstoverbiting. This restricts produc-tivity. Because the tendency is tofeed fast on small logs, overbitingthus becomes a problem, and damage tothe saw is likely. Therefore, whensawing small logs, use the maximumnumber of teeth with gullet size

pitch. When pitch becomes greaterthan 2 inches on smaller saws, toothheight should be approximatelyone-fourth the pitch for best results.

To help insure saw stability,tooth pitch should always be less thanhalf the smallest cutting depth sothat at least two teeth will be inaction during cutting.

The interaction of tooth speed,tooth pitch, and feedspeed combine toproduce a finite tooth bite duringsawing (fig. 7). When any of thesethree components change, tooth bitealso changes. Obtaining the properbite is essential for good saw per-formance. Overbiting can cause sawdamage. Underbiting can cause sawundulation which results in lumbervariation.

matched to the task.

If large logs are cut with sawsthat have small gullets, the feedspeedwill possibly be slowed because of thehigher power demand and the need toavoid overloading the gullets withsawdust. Again, productivity suffers.Underfeeding results in a bite lessthan the side clearance, thus pro-ducing sawdust particles that moreeasily spill from the gullet cavity.This forces the saw offline, thuscreating friction and heating of thesaw. All of this contributes tolumber variation. Sawing large logsrequires increased gullet capacity toavoid overloading the gullets. Largergullet capacity means fewer teeth withwider shoulders, thus providing morestrength for the tooth to do its work.This allows proper feedspeed withoutadverse effects on the operation.

For band saws, tooth heightshould be in proper proportion to thepitch. If tooth height is excessive,the saw tends to flutter, thusresulting in vibration and deviationin the cut. For softwood operations,gullet depth should be approximately43 percent of tooth pitch. Forheavier gauge saws, higher toothspeeds, and greater depths of cut,tooth depth can be up to 50 percent oftooth pitch without adverse effects.Heavier gauge saws provide sufficienttooth stiffness for accurate sawingwithout excessive tooth flutter. Forsmaller gauge saws with a tooth pitchof 2 inches or less, tooth heightshould be approximately one-third the

Tooth bite can be defined as thedistance a log or cant advancesforward when the tooth has traveledthrough the wood a distance equal tothe tooth pitch. Tooth bite deter-mines sawdust particle size, whichis important in determining itscambering characteristics.

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As a log or cant advances intoa saw, the sawdust particles producedmay vary in size or characteristics.In band sawing, the wood moves ata right angle in relation to thedirection of tooth travel. The actualpath of the tooth through the wood,however, is slightly inclined from theperpendicular. As long as saw speedand saw feed are constant, the result-ant sawdust particles will be sizedfairly uniformly.

With circular saws, sawdustparticles vary in size from the timea tooth enters the wood until itexits. In conventional sawing theteeth travel in cycloidal curves suchthat the paths of two successiveteeth diverge as they pass through thewood. Therefore, the sawdust par-ticles formed on the side where theteeth exit are somewhat larger thanthe sawdust particles formed on theside where the teeth enter. Eventhough the horizontal distance betweensuccessive teeth is constant, thusproducing a constant bite, the physicsof the formation and breakup of thewood into sawdust also causes a dif-ference in sawdust particle size.Therefore, with circular saws whenspeaking of sawdust particle sizerelated to a given bite, we are reallyspeaking of an average particle size.

Researchers at the University ofMaine recently determined that forcircular saws the minimum bite shouldbe approximately 32 percent largerthan the side clearance for the saw.This helps to insure that the smallersized sawdust particles will not beless than the side clearance. If biteis less than side clearance, sawdustparticles will more easily escapebetween the kerf wall and the saw.Escaping sawdust results in excessivefriction and thus a heat gradientbuilds up in the saw. Sawdust thatescapes from the gullet rarely spillsevenly on both sides. Uneven sawdustspillage inevitably pushes the sawoffline, thus causing variation in thelumber. While some sawdust spillageis inevitable, it can be kept toa minimum by making proper-sized saw-dust particles.

The upper bite limit is morearbitrary than the lower bite limit.For band saws, the upper bite limit isgenerally set at not more than thegauge of the saw. For other saws, itis set as follows: large circularheadsaws--0.l25 inch; smaller circular

saws with other than carbide teeth--0.063 inch; and circular saws withcarbide teeth--0.040 to 0.050 inch.The main reason for establishing anupper bite limit is to prevent over-stressing the tooth assembly. Over-biting imposes heavy strain on thetooth shoulders since they bear thebrunt of the sawing stresses. Over-biting is also likely to cause dis-tortion of the shoulders and thuseventual fatigue and failure. Tearoutcaused by overbiting can also becomea problem, especially around largeknots.

Implementing correct tooth func-tioning is critical in the setup andoperation of saws. For balanced sawperformance there must be properinteraction between tooth speed, toothpitch, and feedspeed to produce a bitethat makes sawdust particles that canbe correctly handled by the gullet.If tooth functioning is left tochance, saw operating problems andexcessive lumber variation willinevitably result.

Power

It seems that many formulas forcalculating horsepower do not takeinto account all the variables thataffect power needs. It’s difficult todetermine all variables involved.It’s even more difficult to determinehow these variables interact and tobring them together into a singleformula that accurately accounts fortheir behavior in the sawing process.

For a saw to work properly, theteeth should travel at constant speedand bite into the wood within pre-scribed limits. To do this, adequatepower must be supplied to the saw.This includes a power source designedfor the task and a proper belt/pulleyhookup system correctly installed.

Power needs constantly changeduring the sawing process, Widerdepths of cut require more power thando narrower depths of cut; harderwoods require more power than dosofter woods; faster tooth speeds, andthus faster carriage speeds, requiremore power than do slower tooth speedsand thus slower carriage speeds; widercutting edge widths require more powerthan do narrower cutting edge widths.However, increasing the bite does notnecessarily mean increasing powerneeds. Let me illustrate threesituations.

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In the first situation (fig. 8),assume the number of teeth and carri-age speed are constant and the toothspeed is decreased--bite will increase,power needs will decrease. This isdue to fewer across-the-grain sever-ances per inch of feed.

In the second situation, assumethe tooth speed and carriage speed areconstant and the number of teeth aredecreased--bite will increase, andpower needs will decrease. This isalso due to fewer across-the-grainseverances per inch of feed.

In the third situation, assumethe number of teeth and tooth speedare constant and carriage speed isincreased--bite will increase, andpower needs will increase. This isdue to more work being done per unitof time as compared to situations 1and 2. However, the across-the-grainseverances are the same as insituations 1 and 2.

The force used at the cuttingedge is the same for producing a thinchip as for a thick one. It normallyexceeds that used along the sides andunder the cutting point. By doublingthe chip thickness, the minor forcesused for side-shear and chip-crumblemay be double, but the major forceused at the cutting edge to severacross the grain remains the same.Therefore, a chip that is twice as

thick can be produced using less thantwice the original force.

The formula used to calculatepower needs in Program SAW was devel-oped by Phil Quelch in his well-knownbook “Sawmill Feeds and Speeds,” pub-lished by the Armstrong ManufacturingCompany in the 1960’s. In hisformula, Quelch used a constant0.003 horsepower for each square inchof gullet area per minute, witha baseline derf of 0.250 inch forband saws and 0.344 inch for circularsaws. In each case, when using a kerfother than the baseline value, anadjustment for horsepower is made.The formula has been further refinedto account for additional factors thataffect power needs. These refinementswere largely the work of Hiram Hallock,a Forest Products Technologist retiredfrom the Forest Products Laboratoryin Madison, WI. These factors arebite, wood hardness, and depth of cut.

For the bite factor, 0.125-inchbite represents a baseline value of 1.When bite is less than 0.125 inch, anadjustment is made to power require-ments that is less than 1.

For the wood hardness factor,0.46 specific gravity representsa baseline value of 1. When wood hasa specific gravity greater than 0.46,an adjustment is made to powerrequirements that is greater than 1;and when specific gravity is less than0.46, an adjustment is made to powerrequirements that is less than 1.

The depth-of-cut factor uses anadjustment to power requirements thatrecognizes the fact that cuttinga wider depth of cut requires morepower than cutting a narrower depthof cut, with all else being equal.This adjustment factor is based onearlier work done in the late 1940’sand early 1950’s by C. J. Telford ofthe Forest Products Laboratory andG. W. Andrews of the Forest ProductsLaboratory of Canada.

In Program SAW, power needs arebased on filling the gullets tocapacity at the stated Gullet Holdin~Index value. Thus,using the saw underthen the horsepowerwill be misleading.values from Programpreted correctly to

if you are notthose conditions,values calculatedThe horsepowerSAW must be inter-be meaningful.

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My experience has been that thehorsepower formula in Program SAWgives reliable results. As with mostthings, however, it can probably beimproved upon.

Balancing Saw Performance

Achieving balanced saw perform-ance cannot be left to chance if thegreatest efficiency of operation isdesired. It can only come about first,by determining what task is to beaccomplished; second, by designinga saw to do that task; and third, bysetting up the saw properly and oper-ating it within its design limits.Of course, maintenance must be givena high priority.

Saw Design Using Program SAW

I have already mentioned thatProgram SAW was developed to helpassess a saw’s operating limitations.It can also be used to design a sawto best fit a particular task.

Program SAW requires seven piecesof input information: saw or wheeldiameter; arbor or tooth speed; sawplate thickness; cutting edge width;tooth pitch or number of teeth(circular saws only); gullet area ofone tooth or gullet depth (band sawsonly); and the specific gravity valueof the heaviest wood sawn. To prop-erly design a saw for a task, thetypical depth of cut range and thetypical feedspeed must also be known.Program SAW can currently be run onan HP-41 CV handheld calculator or anAPPLE II Plus desktop computer. Itcan be run in either English or metricunits.

Output from Program SAW includesthe range of feedspeed, depth of cut,and bite values. However, justbecause these values are calculatedfor a saw does not mean that they arecorrectly established for that par-ticular situation. This is where theredesign capability of Program SAWcomes in handy. A saw can be properlydesigned and the operating variablescorrectly established for a particularsetup. Program SAW also shows toothspeed and the power required to fillthe gullets to capacity with sawdust.

To make the output from ProgramSAW easier to understand and apply, itcan be plotted on a special graphentitled “Operating Range and Perform-ance Limitations.” This graph consists

of a vertical axis labeled feedspeedand a horizontal axis labeled depth ofcut.

After obtaining output data, themaximum feedspeed line is plotted.This line terminates at the depth-of-cut value where the gullet is filledto capacity. Similarly, the minimumfeedspeed line is plotted. Next, thetermination points of the intermediatefeedspeed/depth-of-cut values areplotted. A curved line can be estab-lished through these terminationpoints. You now have the boundarylimits within which the saw should beoperated provided it is properlydesigned for its task (fig. 9).

When a log or cant is fed intoa saw, the observed feedspeeds andcorresponding depths of cut shouldfall within the established boundarylimits for a saw that is properlydesigned for the job. If observa-tions fall outside the boundarylimits, the saw is not operatingefficiently.

There are five possibilities whenobserving feedspeed and depth-of-cutcombinations. First, observationswill fall within the boundary limitsas established by Program SAW(fig. 10, No. 1). Second, observa-tions will fall below the minimumfeedspeed line, thus indicating thatsawdust particles are smaller in sizethan the side clearance (fig. 10,No. 2). Third, observations will fall

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within the maximum and minimum feed-speed lines but also fall to the rightof the curved line, thus indicatingthat gullets are overloaded with saw-dust (fig. 10, No. 3). Fourth, obser-vations will fall above the maximumfeedspeed line but also to the left ofthe projected curved line, thus indi-cating that overbiting has occurred(fig. 10, No. 4). And fifth, observa-tions will fall above the maximumfeedspeed line but also to the rightof the projected curved line, thusindicating that both overloading thegullets with sawdust and overbitinghave occurred (fig. 10, No. 5).

In figure 11, the minimum feed-speed is 135 feet per minute and themaximum feedspeed is 293 feet perminute. The curved line delineatesthe points at which feedspeed anddepth of cut are in balance with thegullets being filled to capacity withsawdust at the assumed GHI value. Forexample, at the maximum feedspeed of

293 feet per minute, the correspondingdepth of cut is 15 inches, at whichpoint the gullets are filled to theircapacity.

Figure 12 shows how saws aredesigned to perform different tasks.Saw A is designed with a maximum feed-speed of about 415 feet per minute ata depth of cut of about 8 inches.Saw B is designed for a maximum feed-speed of about 280 feet per minute ata depth of cut of about 15 inches.This vividly illustrates the differentcapabilities of two different saws.

Program SAW Results

The best way to illustrate thecapabilities of Program SAW is to showan example of how a specific saw setupcan be improved.

Tables 8 through 11 show howchanges in saw design change operatingparameters so the saw is better fittedfor its task. Situation 1 (table 8shows the specification for theoriginal saw. The operator normallyfed this saw at about 280 feet perminute. This being true, the bitewas always smaller than the sideclearance. The saw constantly pro-duced fine sawdust particles thatwould not readily remain in the gullet.Notice that the saw as originallydesigned had a maximum feedspeed ofabout 500 feet per minute, which

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represents capacity that was neverused. It costs to maintain capacitythat is not used. If the stated feed-speed of 280 feet per minute is thedesired upper limit, then the saw isnot properly designed for its task andwill not achieve the greatest effi-ciency of operation. Changes in sawdesign will make this saw more effi-cient and will likely alleviate opera-tional problems.

Situation 2 (table 8) shows theside clearance reduced from 0.055 to0.048 inch-- a reduction of 0.007 inch.For a 13-gauge saw, 0.048 inch isa normally acceptable side clearance.It can possibly be reduced furtherdepending on the situation. The newside clearance is achieved by reducingthe cutting edge width from 0.205 to0.191 inch. Lowering the side clear-ance increases the feedspeed range byextending the lower end. Notice alsothat this extends the depth of cutcapability from 16 to 18.3 inches.Reducing the cutting edge width alsoreduces power requirements.

Situation 3 (table 9) showstooth speed reduced from 10,560 to8,560 feet per minute--a reduction of2,000 feet per minute, This isachieved by reducing the arbor speed

from 373 to 303 RPM. Reducing toothspeed shifts the feedspeed rangedownward. Maximum feedspeed has nowbeen reduced from 502 to 407 feet perminute and the minimum feedspeed hasbeen reduced from 290 to 203 feet perminute.

Situation 4 (table 10) showstooth pitch increased from 2 to2.75 inches --an increase of 3/4 inch.Again, the feedspeed range is shifteddownward. The maximum feedspeed hasnow been lowered from 502 to 296 feetper minute, and the minimum feedspeedhas been lowered from 290 to 148 feetper minute. Notice also the corre-sponding change in depth of cut.Fewer teeth are now available toaccomplish the work. The remainingteeth will perform their task moreefficiently by taking a larger bite.Less power will be consumed with fewerteeth in the cut at any given time.

Situation 5 (table 11) shows thegullet cross-sectional area increasedfrom 1.14 to 2.16 square inches. Theaccumulated changes to this point nowaccommodate an 18-inch depth of cut ata feedspeed of 280 feet per minute.Larger depths of cut may also beaccommodated although at realisticdecreased feedspeeds.

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This saw is now better designedto do its task albeit more efficientlythan the original saw. With the newsaw, the mill should experience feweroperational problems and lower main-tenance costs. There is less oppor-tunity for gullet overloading sincethe saw is designed to handle largerlogs that may occasionally beencountered.

Figure 13 is another example ofsome changes that were instituted inone band saw setup. First, notice thebite of 0.019 inch, which is somewhatless than the side clearance of0.030 inch. The saw originally hada hook angle of 30 degrees. Themaximum feedspeed was 349 feet perminute. Tooth pitch was 1.75 inches.The mill operator normally fed the sawat about 100 to 125 feet per minutefor hardwoods.

In redesigning this saw, toothpitch was increased from 1.75 to2.25 inches-- an increase of 1/2 inch.Hook angle was reduced 3 degrees.With the mill’s feedspeed of about100 feet per minute, bite increased

from 0.019 to 0.026 inch with the newsaw-- an increase of 0.007 inch. Maxi-mum feedspeed dropped from 349 to271 feet per minute, while minimum

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feedspeed dropped from 145 to 113 feetper minute. Notice what this accom-plished (fig. 14). Number of teethdecreased 23 percent; gullet size in-creased 52 percent; and bite increased37 percent. Power consumption dropped3 percent according to actual meterreadings, Maintenance time forbenching the saw dropped 28 percentaccording to the head filer. Runningtime for the saw increased 1.5 hours.

Summary

Some general observations forProgram SAW over the last few yearsare that saws are routinely pushedbeyond their design limits; gulletsare frequently overloaded with saw-dust; teeth are run too fast for theconditions; and saws often contain toomany teeth, thus producing excessivelyfine sawdust. I feel that many opera-tional problems can be traced directlyto these shortcomings. When there isa better way of doing things and youdo not take advantage of it, thenthere is a price to pay for not makinga change.

To obtain balanced performancefrom your saws, they must be designedproperly for the task, then operatedwithin their design limits, Know whatthose limits are and insure that theyare not exceeded.

It does not cost to upgrade yoursaw’s efficiency of operation butrather it pays you in the long run.

Bibliography