Bill Rouseau, Silvatech Corp. 7/16/01

  A Linear Positioner carriage with a Scanner can have a very beneficial effect on the profits generated by a sawmill.  Working together, they enable a sawyer get more footage of lumber of higher grades from each log he saws – and to do it more quickly than he can with a set shaft carriage.  The functions of the Scanner system cover a range of assistance, from setting tapers and opening faces to determining the complete sawing pattern for a log and automatically positioning the log for each sawline.  It will be used differently depending on the the species and grade of the logs, as well as the products you want to make out of them – whether grade lumber, dimension, or both.  

 The Linear Positioner Carriage           

 The linear positioner controller typically controls a hydraulic cylinder or electric servo motor on each headblock to make sets and position the knees. This type of headblock control system offers very high accuracy, speed, and greatly improved tapering capability.

 The set drive on a set shaft carriage consists of a drive motor, gear reducer, brake, chain drive to the set shaft, and either chains or racks and pinions from the set shaft to the knees.  These mechanical drive components introduce inaccuracy due to inherent slack and backlash.  When running with a constant speed drive they introduce some inherent variation in stopping distance and therefore some degree of inaccuracy.  Knee speed is limited to a level at which stopping distance is reasonably predictable; the higher the speed, the larger the stopping distance and the more variation. Knee alignment must be maintained by mechanical adjustments and taper must be performed one knee at a time, prior to dogging, typically in increments too coarse to consistently put the log face just where it should be. 

 A linear positioner solves each of these problems making the sawing process faster and more precise than previously possible. 

SIMPLICITY 

 On a hydraulic linear carriage each knee is positioned by its own hydraulic cylinder.  The cylinder is fitted with an internal probe to precisely measure the knee's position. The cylinder is mounted via a trunion mount to the back of the bunk with the rod attached to the knee.   The motor, gear box, brake, chain drive to the set shaft, racks/pinions or knee chains and taper cylinders are thus eliminated.  With them go the major causes of inaccuracy and constant carriage adjustment and maintenance. A hydraulic linear carriage is obviously a simple machine to maintain and with which to produce precisely sized lumber. 

 An electric linear carriage has an variable speed electric servo motor with a planetary gear reducer driving each knee.  An encoder on the motor provides position feedback to the controller.  The motor / gearbox drives the knee via either a chain drive or a rack & pinion.  The electric version obviously requires that slack in the chains or rack & pinion be kept to a minimum and that means more frequent maintenance.  Advantages of the electric drive include the absense of hydraulic oil and the elimination of cylinders hanging out the back of the carriage in mills with a wall close behind the carriage. 

SPEED 

 The variable speed of the cylinder with servo valve or the servo motor allows the computer to ramp the speed of knee travel up and down, providing fast travel and maintaining knee alignment  during a move, and slowing it down as the target is approached for extremely precise positioning. 

 Knee speed will vary depending on the length of the desired move. Max knee speed can be set in excess of twenty inches per second but will typically range up to twelve to eighteen inches per second. This means that all sets will be faster than with an ordinary set drive running at four to five inches per second and a 10/4 set will take the same length of time (less than 1/2 second) as required for a 4/4 set, improving sawing rhythm.   

TAPERING 

One of the biggest advantages to be gained with a linear positioner system is the ability to infinitely taper each log extremely quickly and closer to where it needs to be.  Mills operating carriages without taper capability and most mills with tapers sacrifice a great deal of volume and grade recovery by not tapering each log to the best position. 

 Some logs will yield the best dollar recovery by splitting the taper and sawing each face parallel to the log center.  On other logs, sawing the best faces to full taper (keeping the outside face parallel to the sawline) and removing the taper on the poorer faces will significantly increase dollar recovery. Still others yield the most value by sawing the faces with defects (depending on their number and location) to full taper. The extra length and width of boards sawn in this manner can give you the surface measure and hence the extra cuttings you need to improve the grade. 

 A carriage with taper increments of one or two inches makes it very difficult to taper correctly. The log usually ends up tapered either too much or not enough.  Even when you can get it right, you've spent a considerable amount of time since the tapers must be set one at a time before the dogs are closed.  With the Linear Positioner Set Controller tapering is performed with a rocker or toggle switch on the feed or turner handle.  Pressing one side of the switch causes the log to pivot - around the center knee on a three block carriage and around the second or third knee on a four block carriage. Tapering can be performed either before or after dogging and can be accomplished much more quickly and accurately than on a normal carriage.  Your sawyer can split the taper or full taper to saw parallel to the outside of the log  to maximize grade and put the log closer to where it should be. This infinite skew taper also makes it easier to achieve the desired opening face width.  Both tapering and jogging can also be performed with a miniature proportional joystick in one of the handles. 

 Individual knee tapers can also be provided so that the knee alignment can be made to conform to an odd shaped log.  After individual tapering, the relationship of the knees will be maintained through sets and further tapering until the tapers are cancelled. 

 Individual knees can also be "Locked Out" when not needed. A locked out knee will move to the back of the carriage and remain there until the lock out is cancelled. 

INSTALLATION 

 The Silvatech Linear Positioner Set Controller for a three or four block hydraulic linear carriage includes a computer, switch panel, monitor, a DC power unit to operate the valves, a probe for each knee, two probe boxes and cables.  For an electric drive a motor control box and encoders take the place of the DC power unit and probes. 

SCAN LINE CONFIGURATION

Typically, the scanning line covers 24 feet opposite the log deck such that logs loaded onto the carriage pass through it. Photo cells are typically spaced at six inch intervals. The overhead cells are mounted on a bracket attached to the mill ceiling. The lower cells are in an enclosure running along the track. The scanner computer monitors a light beam passing between each pair of photo cells. It also keeps track of the position of the carriage along the track so the scan can be accomplished while the carriage is moving toward the saw. 

SCANNING FUNCTIONS

The purpose of the scanner is to develop a profile of the log as viewed from above denoting the length of the log and its diameter at points along its length. The scanner gets an initial measurement of log length by noting which light beams are interrupted when the log is loaded onto the carriage. After the log is dogged, the sawyer presses the "Scan" button and the system moves the front side of the log through the scan line. 

As each light beam is interrupted (forward scan) or re-established (reverse scan) the computer notes the position of the knees at that moment. The information obtained through the scanning process enables the computer to derive the profile of the log as illustrated below.

Log profile for 12 foot log generated by knee positions and scanner points


The scan generates a more detailed picture of the front of the log than the back side since it has more points on the front. The front side detail, however, is used to help supply the outline of the back side. This profile is then used by the computer in performing the Minimum Opening Face and Cant /Fill scanning functions. 

Each of these functions results in the system positioning the knees at specific points. 

Minimum Opening Face:
The Minimum Opening Face function will position the log so that the saw opens a face of a selected size in terms of face width and length. This function can be used on as many faces of the log as desired. Whether sawing hardwood or softwood, a mill will want to open faces of different sizes depending on the characteristics of the face and of the whole log. 

The size wanted can depend on the thickness of the first board to be sawn. The minimum marketable size for a 4/4 board, for example, is usually different from the minimum for a 6/4 or 8/4. The grade of the face can also determine the opening face wanted as well as the first board thickness. It is certainly not uncommon for a grade sawyer to open four different sized faces on the same log. 

The scanning system enables you to define four opening faces which the sawyer can choose as he moves from one face to the next. Each of the opening face choices is defined in terms of: face width, face length, desired taper, and the thickness of the first board to be sawn on that face. 

An opening face can be defined as a width and length (6 inches wide by 8 feet long for a potential FAS or One Face, for example), or as a width (6 inch) by full log length. 
The taper to be sawed for a given individual face, log, or run of logs can be chosen from fully variable selections. The amount of taper for each selection can be defined by the mill as a percentage with 100% representing full taper (the outside of the log parallel to the sawline), 50% as splitting the taper ( the centerline of the log parallel to the sawline) and 0% as no taper (the back side of the log parallel to the sawline). An experienced grade sawyer may set his "full taper" opening faces at 85% or 90% since he knows that the more taper he puts into a cant sawing around the outside of the log, the more taper must be removed to "square up" the cant. 

In accomplishing the MOF function the computer looks at the diameter at points along the log and at the the log's straightness. It then computes how much wood must be moved past the sawline at the designated length point by calculating a flat side (chord) of the required width on a circle of the log's diameter at that point. It takes into account the straightness of the log and the specified taper, and moves the log to the correct position. A typical set of MOF settings for hardwood are shown below: 

Face Width Face Length 1st Set Taper
1. 4" 4' 4/4 50%
2. 4" 6' 4/4 50%
3. 6" 8' 5/4 50%
4. 6" Full Log Len. 5/4 90%

Log positioned for MOF 6 inches wide by 8 feet long with split (50%) taper. 

Log positioned for MOF 6 inches wide by log length with 90% taper.

     The two examples above illustrate the need for flexibility in MOF selection. In the upper illustration a MOF 6" by 8' is obviously preferable since opening a face the full length of the log would send too much grade lumber to the chipper and severely limit the number of boards which could be sawed before reaching heart defects. On the opposite face of the same log, a 6" by 8' face would result in a slab with very heavy ends so an MOF of 4" by 4' or 6' would probably be chosen with potential for an FAS on the second board.

In the lower illustration, assuming a large defective heart atn the butt end, the full length opening face selected will provide the best grade and yield recovery. When MOF is used on a face where the opposite face has already been sawed, the system remembers how much was removed on that previous face to maintain the proper working diameters. Up to nine different sets of MOF specs. can be stored in the computer and called up whenever needed. The system can also be set up to saw MOF's on two opposite faces and in Cant / Fill on the other two faces. Another feature is the ability to designate MOF's by log diameter. 

Cant / Fill Mode: 
When this function is selected, the scanner computer goes to a table filled in by mill personnel. For each diameter range and length range, a cant (or dogboard) size for each axis and prioritized sets for filling out the remainder of the log are entered. Each cant as well as fill set thickness is also defined for minimum face required. 

Upon your pressing the "Scan" button, the computer will find the measured log in the table, center the specified cant, and fill the remaining area of the log to the outside faces with sets based on your priorities and the room available. When the Cant/Fill function is activated, the computer looks at log diameter and configuration and fills the available space on either side of the cant to maximize yield. The board thickness with the highest priority is used as long as space allows. If the computer determines that it can gain an extra board on one side by moving the cant slightly off center it will do so as long as the required face sizes for all pieces will still be met. 

For the softwood log in the example below, an 6" cant was called for and board thicknesses of 5/4, and 4/4 were prioritized in descending order. The computer's solution is two 5/4 sets on each side, then a full length 4/4 and finally a 4/4 which will have to be trimmed. The face required for a thicker set is typically larger than that for a thin set. Once the sawyer has begun sawing the sequence he may rearrange the order of pieces at any time. 

Mill personnel can enter information into the tables regarding desired sawing patterns (cant or dogboard and fill sets) for the two remaining faces also. The computer can then choose, indicate to the sawyer, and make all positioning moves for the appropriate pattern. Using the Cant / Fill Mode on all four faces results in the scanner determining the sawing pattern for the entire log based on instructions specified by mill management for each diameter and length to be sawed. This function is obviously very useful in sawing softwood dimension. It is also commonly used in hardwood on low grade logs and those with a predictable sawing pattern such as when sawing only 4/4 or 5/4 to standard sized pallet cants. 

In this example the scanner measured a 14 inch log 12 feet long. For a log of these dimensions, mill management had previously specified instructions leading to one of the sawing pattern shown. 

The sawing instructions are entered into Set Up Tables in the computer. Up to nine different tables can be set up for different species or different grades of logs. 
This capability ensures that each log can be sawed in the way mill management has determined is the most profitable. 

The Silvatech scaner allows you to enter up to three different sawing solutions for the same size log. If you prefer different cant sizes or sawing patterns depending on the grade of the log, the sawyer can choose from cant / fill #1, #2, or #3. This feature might be used where one pine log has a lot of black knots and another of the same size has red knots showing. He can choose a solution with a smaller cant for the black knotted log so he doesn't saw wide boards with black knots on the edges. In Hardwood, a sound low grade log ofa given size might be best sawn down to a tie while another of the same size has defects which make its best use two 4" X 6" pallet cants. 

In sawing with the scanner, the computer must know when you turn the log and which face you are on. Some sawyers normally turn a log 180 degrees in turning from first to second face, while others prefer to turn the first face down 90 degrees. Most good sawyers use both methods to maximize grade recovery. The system has an "auto turn" feature which lets you designate which way you normally turn. If set for a 180( turn, the system will assume you're turning this way unless you press the turn rocker to tell it otherwise. A graphic display on the operator screen shows the selected face and available turning options. A rocker switch on the scanner switch panel allows you to quickly indicate which way you are turning if not the way designated for "auto turn".


OVERRIDING THE SCANNER

In any of the scanning modes the sawyer can quickly override the scanner and saw the log as he sees fit. With any scanning system, the computer is working with a limited amount of information about the log particularly with regard to grade. A sawyer, particularly one with years of experience, will know some things about the log that the computer doesn't. This is especially true with regard to grade considerations. The sawyer should be prepared and encouraged to override the scanner whenever he feels it is appropriate. 

Other Features:

In either MOF or Cant / Fill modes, the scanner system can automatically advance the log to put the first board in the slab to be recovered by a resaw. Sawing patterns can also include sizing cants to be further processed at a resaw. 

The system can also advance the log and position a chipping slabber so that the first piece will be sawn by the headsaw while the slab is removed by the slabber. 

REPORTS

Two types of reports are generated by the system. These reports can be displayed on the screen or printed out on the printer if a hard copy is desired. 

Log Tally:

The computer will keep a record (backed up by battery through power loss) of the dimensions of each log. Tally reports can be generated upon command and include log count, line count, production rates, board footage by designated log scale and breakdown of logs by diameter and length. 

Downtime report:

The system will also keep track of downtime by duration and reason. Whenever a pre-selected number of minutes or seconds have passed without a line being sawn, the system will automatically enter downtime mode. Once in downtime mode, A list of mill defined reasons (each with a numnber) is displayed and the sawyer must enter the the number of the reason for the delay before resuming normal function. Again, the reason and the duration (including the pre-selected number of minutes or seconds) are recorded. 

The report indicates each category with the number of instances, total downtime for the category, percentage of shift, and total for all categories with percentage of shift.

TAPER - Hardwood 

One of the most important factors in keeping a mill profitable these days is to maximize the grade recovery you get from your logs. If you look at the difference in the price you get for FAS versus 1 common or the difference between 1 common and 2 common you don't have to be the proverbial rocket scientist to see how important this is. One of the most critical issues you have to address in accomplishing maximum grade recovery is setting the tapers properly. To maximize grade, there are two things, directly related to setting tapers, we need to accomplish. 

First, we need to put the highest possible percentage of the log volume, particularly the higher quality outer portion of the log, into boards which meet the size requirements of the valuable grades, whether Select & btr. or Face & Btr. Second, we need to isolate the low grade portion at the center of each log in the smallest possible pallet cant or the least number of boards of the smallest size. Lets take a look at how proper taper is essential to meeting both these goals. 

By the time your sawyer has made a slab cut on each of two adjacent faces he has typically determined the alignment of all the sawlines in the log, whether it will be completely sawn on the headrig or sent to a linebar or gang. For a large majority of logs we want to position the tapers such that the sawlines are parallel to the heart center or to "split" the taper. A good sawyer, if he's paying attention, will probably average getting the log positioned to within a couple of inches of having the taper evenly split. When I was sawing, at the end of the day I'd load one more log after sawing the last of the day and set the tapers. Then I'd go out and measure how well I'd alligned the centerline of the log to the sawline. Sometime when you think you're getting too cheerful and you want to add a little depression to your life, try this for a week or two. 

Evenly split taper on opposite faces of a log will look something like this.

What's more likely, though, is that our sawyer will go closer to full taper (sawing more parallel to the outside of the log) on the first face because it makes a nice looking face, and then end up taking most or all the taper off the opposite face. This results in faces on opposite sides which look more like this. 





This makes for some nice looking boards as you saw into the first face.






When you get to the opposite side, however, you're sawing boards which require heavy edging and trimming .




This can obviously interfere with the goal of putting the highest possible percentage of the log volume into boards which meet the size requirements of the valuable grades. It also puts more potential high grade from the outside of the log into the chipper in the form of heavy edgings and cut-offs. 



As you saw into the log on this face, you'll hit heart defects on the butt end long before you do on the small end. Now we're putting low grade into one end of boards (possibly even before we're through trimming wane off the other end). 
This means we're either going to drop the grade of these boards or cut them back or both. This hurts both volume yield and grade yield. The reverse side of the third board may look something like this. 

Boards like the one above become more and more common as we saw smaller second growth timber.

Now lets take a look at the effects of proper taper on the lumber sawn near the heart. As I said earlier, one of our primary goals is to isolate the low grade near the heart into the smallest possible piece or pieces. This next drawing pretty well illustrates the proper isolation of the heart and that the sawlines are parallel to the heart center. 



The alignment of these sawlines (and all the others) were obviously established at the slab cuts. Lets see what would happen if our sawyer had missed the proper taper position by a couple of inches.




We've now brought some of the heart outside the pallet block and into the opposite ends of two boards. These two boards could have easily dropped a couple of grades from what they would have been with the tapers properly set
(or the cant is left larger so it still contains the heart defects).






If we're two inches off in tapering the other pair of faces we've now made a couple of more boards drop a grade or two.



You don't want to think about what just happened to the profit you could have made from sawing this log. Again, with smaller second growth timber these mistakes have more impact on the percentage of your lumber which will make 1C. & Btr. 

You don't have to visit very many veneer slicer mills to find some who are paying someone to mark the ends of their logs for the heart center so the sawyer who's making the flitches can get his sawlines parallel to the heart. All they are trying to do is isolate the heart defects in as small a portion of the flitch as possible and keep the heart out of the maximum number of veneer sheets. Now I know that they're paying more for their logs than you are, but my guess is that they were doing this when they were paying what you're now paying for your good sawlogs. 

Though it's probably not practical for the average sawmill to pay someone to mark the ends of logs, the obvious importance of proper taper settings certainly helps explain the number of mills who are installing scanners. Setting tapers accurately is very difficult for a sawyer to do particularly from where he typically sits. A scanner positions the tapers as the result of mathematical calculations based on the measured log outline and will consistently set them more precisely. Most mill managers think about a scanner in terms of what they see happening on the outside faces of the log (particularly if they're running a linebar). The biggest impact on your pocketbook, however, can be the impact the scanner has on what happens around the heart. 

TAPER - Softwood

In the sawing of pine or other softwood logs, particularly those where the lumber will be sold in width increments of 2 inches, there are two critical goals which must be accomplished to maximize yield. The first is to center the cant in the log so that the minimum number of boards need to be edged. The second, and this bears directly on the first, is to set the tapers accurately so that the cant is centered on both ends of the log. This must be done in both axis of the log as well. The next drawing shows a log with the tapers set accurately to split the taper evenly on both sides of the log. 







Now we see sawlines on a log where the taper was set too close to full taper on the first face and most of the taper removed on the second. This makes good boards boards on the first face but means wasteful and costly edging and trimming on the second face. 





   The most costly effects of inaccurate taper on the first two faces, however, show upm when you get to faces 3 and 4. Here we see the sawing pattern for a log which was properly tapered. We have maximized the number of boards sawn on faces 3 and 4 which are full width and minimized the number which must be edged narrower (in increments of two inches). On boards which do need to be edged, we get wane distributed to two corners where it hurts the grade a minimum amount. 

If the tapers were set inaccurately on faces 1 and 2, the cant is placed on the respective ends more like this. 



We obviously have to edge more boards (two inches at a time) and throw away more lumber now. If we try to edge a given board to the same width we did with the tapers properly set, we get all the wane on one edge. This board will typically grade lower because the wane will extend further down the side of the board. If there isn't enough taper in the log to cover the inaccurate taper, we'll get some boards with wane on the opposite ends of both edges. Now we have to edge the board diagonally and waste even more. 

If we also set the tapers inaccurately on the last two faces, the pith and any red rot streaks running along the heart will be distributed into multible boards rather than being isolated in the minimum possible. 

Setting the tapers is the function the scanner does very well which is the most difficult for the sawyer to do (particularly from where he sits). When I was sawing, after I'd sawn the last log of the day, I'd load the next log onto the carriage, set the tapers and jog the log up to the first sawline. Then I'd shut the mill down and go measure how accurately I had set the tapers. If you think you're just getting too cheerful and you need a little depression in your life, have your sawyer do this for a couple of weeks and watch the results - particularly with an eye to capturing the heart defects in the smallest possible piece. 

We have installed over 120 scanners in both hardwood and softwood mills. These mills include companies who have installed up to five scanners. One company who has four just ordered two more. These guys are installing scanners for one simple reason. More profit. Mills who have accurate yield numbers report volume yield increases of up to 12% by installing both a linear carriage and a scanner. More typical numbers are 3% for an L.P. carriage and 5% from a scanner. Your grade yield should improve as well. This means that the volume yield increase will show up in the upper grades as higher priced lumber. Higher production rates show up as reduced sawing cost per thousand.

If it cost you $120,000. to convert and you depreciate that over 10 years that's 12,000 per year. If you borrow the money at 10% for ten years your payments total about 19,000. per year. If you sawed 5,000,000 ft per year and get an 8% yield increase that's 400,000 ft of lumber that you didn't have to buy any more logs to get. If you get an average of $500. /M (keeping in mind that a improvement in grade recovery will increase your average selling price) that's $200,000 from spending $19,000. You can also look at it as less than a year payback and you get the extra $200,000 in your pocket the second year.