Setting up for Double Cut Bandsaws in Saw Mills
A guest article by dick Bernier
In the last year I have had 2 filers contact me on setting up the filing room and bandmill for switching from a single cut to a double cut. One from BC and the other from Alabama. Rather than giving them a few tips over the phone I wrote this. I may have missed something but I don’t think so. It made me realize and appreciate how much is really involved in making this changeover.
In the filing room:
To maintain a tight cutting edge on both front and back we must run a “zero” back measured with a back gauge. The tension and tire lines must be even and equal on both front and back quarters. If the back is uneven the cutting edge will be tighter on the “short” side and the cutting edge will be “looser” on the edge with the long back. Benching requirements are more stringent with less margin for error.
Every effort must be made to keep tooth width (kerf) the same on both sides. We tip both front and back at the same time – every time.
Your bandsaw bench anvil will have to be narrow enough to keep the saw teeth off of the anvil to properly level the saw and keep from damaging the teeth. Most anvils for double cuts have the edges relieved on both front and back for this reason. You may want to put a new rub block on the bench to keep from damaging teeth during the benching process.
Care (or modification) must be taken to prevent tooth tip damage on all of the bench rub blocks or rolls. I set up the stretcher rolls to have the back of the saw (teeth) just lightly rub the rub block or not touch at all as the saw goes around to minimize tooth tip damage. Do not bench a sharp saw for this reason as well.
The bandsaw grinder will have to be modified to minimize damage (dullness) to the sharpened tips of the first side you sharpened. The post brackets will require leather or plastic on them to keep teeth off metal. Your grinder saw carriage will have to be the longer double cut style for the same reason. These parts are listed in the Armstrong parts book. Your Armstrong distributer should be able to help you with these modifications. You will require more grinder floor space and post brackets to grind the “other side.” This will require a feed unit on the back side as well. This requires much more floor space. For your money a Wright Machine air back is best for cost, simplicity, effectiveness and maintenance. If you run Stellite you will have to modify your tipper and side grinder as well.
It is preferable to sharpen the back side of the saw first as this side rides on the saw carriage when you grind the front (primary cutting side). Depending on your manner of breaking down the log, the front edge will be used approximately 10%-30% more than the back edge. The front side always makes the first cut………
The saw width and gullet overhang are two very important considerations. The new saw width must be such that the gullet overhang is kept to a minimum. This keeps the cutting edge of the saw supported for optimum sawing performance. Overhang must not exceed ½” per side as saw performance will be adversely affected due to lack of support and you may get gullet cracks as well. ¼” overhang per side would be considered optimum. The saw must be discarded when the gullets get on the wheel edge. If your mill isn’t very forgiving with gullet cracks, you may have to reduce this number to ¼” maximum per side. The guides may have to be slightly modified to keep the gullets/tips from sawing the guides out.
Tilt must be adjusted so that front and back gullets are equal distance from edge of wheel. For these reasons, saw life is much shorter than with a single cut and saw costs will go up proportionately. I get only ½” life out of our saws for this reason and this is considered above average. When I came to Columbia Vista we got only ¼” life out of saws. Every sawmill & every application will vary and have it’s own requirements.
The shearboard must be modified for a double cut. Rather than a full length angle towards the gullet as on a single cut, the shearboard must be a upside down V to keep sawdust and knots from going around the lower bandwheel. This is a necessary requirement.
If you run a crown on your bandwheels it would be advisable to regrind with the crown in the middle rather than biased to the front quarter. If your wheels haven’t been ground recently you may consider grinding to eliminate the hollow where the back of the single cut used to ride. You will want to chamfer the back edge as well. Bandwheel circumference front edge to back edge is critical for a double cut.
These measurements must be equal. Use a Pi tape for optimum circumference measuring accuracy. This cannot be over emphasized. With the narrower tolerances from gullet to wheel edge, having saw tracking problems is not an option. If the teeth ride on the tooth edge, premature saw tip dulling resulting is poorly sawed lumber
Bandwheel to track alignment is more critical as well as even minor mis-alignment will result in one of the cutting edges to be looser than the other. Guide alignment and crossline adjustment are the same for a double cut. I recommend zero lead as lead is typically put in a mill to hide an alignment problem.
Your rollcase will have to have a slat bed behind the bottom saw guide and and rolls in front of the log loading area to move the cants down the rolls when making a back cut. I’m certain your people knew this but I had to say it…..
You may also need to modify your log loading mechanisms to allow you to drop a fresh log onto the carriage while a cant (backstand) is directly underneath the loading area. You lose valuable seconds when you have to wait for the backstand to clear the loading area.
The sawyers job becomes more challenging due to the faster paced decision making requirements. The time from finishing the front cut, setting the knees out to next thickness and then starting the back cut is typically two seconds or less. Mis-manufactured lumber usually referred to as “snipe” can and will occur when the timing is off. The sawyer will also be required to dump the cants at both ends of the track requiring more skill in timing of cant’s traveling down the rollcase.
With teeth on both edges of the blade, handling of the saw is much more of a safety concern for the filers. When putting the saw on the bandmill, care must be taken to prevent damage to the teeth as they slide across the wheel face.
Columbia Vista Corporation
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