Let's start with the tooth of the bandsaw blades.
We know
that tooth load is very important with all bandsaws, and keeping tooth load
down per square inch is the most important thing that a sawyer/sawfiler can
do to preserve his bandsaw.
For an illustration of this let's look at the
teeth of a swage tooth or a welded tooth band. Note that all comparisons will
have to be made of equal width bands for this comparison to be accurate. The
swage tooth and the welded tooth have a full tooth width at the top or the
leading edge. The full tooth has three cutting edges. It has the top or leading
edge and the two sides.
The tooth load is created by all edges that cut wood.
The tooth width on these type blades is usually twice the width of the body.
For instance, if the body is .042 then the tooth is usually created to be
at least .084. So each tooth is cutting a kerf (path) through the log of at
least .084.
The sides of the tooth are also cutting and adding tooth load.
All of the cutting load is distributed into the gullet and most of the stress
is at the narrowest place of the band which is the deepest part of the gullet.
The body is pulling the tooth through the cut and the tooth is under load
and is pryed backward in micro amounts and the steel molecules at the narrowest
part of the gullet are being stretched and being put under great stress.
The
bandsaw steel is, for basic understanding a spring, or spring steel. I say
this because it has the ability to stretch a certain distance and return to
its original shape. Remember, the term 'certain distance' this is important.
Each molocule of steel is round in shape, and can be stretched or enlongated
to a certain distance before it is stressed and begins to remain elongated.
When a molocule remains elongated it has been distorted in the gullet from
pulling too large (more than the body molecules can take) of a tooth load.
The band gullet has what we call a long front. We say this because the front
has stretched but the molecules at the back of the band have remained unstretched.
This condition causes a hump, that rises up in the middle portion of the band,
or in other words the middle of the band will hump toward the slab side and
the mirror image is on the log side being concave. At this point the band
will dive into the log every time!
Note: A good spring has the ability to
elongate or compress a certain or prescribed amount and return to its original
shape millions of times without losing strength. It is only when the prescribed
amount is exceeded that it becomes distorted and loses strength. Think of
valve springs in a gas/diesel engine. They operate within their ability to
extend, compress and return every time and run almost forever under harsh
conditions. They do not exceed their steels ability to stretch and return.
A good understanding of the above two paragraphs is very important to understand
an accurate comparison between the swage tooth band and the set tooth that
we will discuss below.
Now let us look at a set tooth band of .042 thickness.
The Kerf of a set tooth band is also two times the thickness. Usually .084
(sometimes more sometimes less). The difference is the set tooth shares the
tooth load between 3 teeth. Each tooth is only .042 on the top or the leading
edge. The most any tooth can cut is .042 wide. The average that the 3 teeth
cut is only .028. The right and left tooth only cut the set amount beyond
the body width.
Only 2 of the teeth cut on the side and they only have 1 side
to create side load which is very minimum. It is so small that I do not consider
it to have a side load. The reason there is not a side load is the angle of
the side of the tooth. The tooth is bent or set at such a sharp angle on the
side that only the the top corner of about .010 touches the wood fiber. Of
course, if you run your band excessively dull it does take on more side load.
Now compare the gullet stretch per tooth. Remember both band bodies are .042.
The set tooth has the total kerf stress divided between 3 teeth. The swaged
full tooth carries the entire kerf stress per tooth, plus the side load on
2 sides per tooth. It is easy to understand that the full tooth carries twice
the load just because of the top width of .084. Then add the 2 sides of load
and we have at least 3 times more tooth load per square inch than the set
tooth bands have.
What does this mean? From my experience the set tooth pound
for pound will perform 2 to 3 times better than a full tooth of the same band
width and thickness. This is true even if a special steel is welded to from
the full tooth.
Let me talk about the body of the band for a bit here. I have spoken
of the distortion and how it changes the middle portion of the band to dish
on the log side. This causes a band to start diving into the log in an uncontrolled
manner.
Remember that distortion is caused when a band is driven with a tooth
load greater than the body steel is capable of withstanding and still return
back to it's original shape. Any bandsaw that is pushed to this ponit of stretch
has become distorted. This will cause the band to not run good and it will
dive into the log because of the natural change of curvature called anti-clastic
curvature.
Myth of restoring bandsaws debunked
When this happens the band
cannot be restored to new by the Saw Repairman/Sawfiler. However, the sawfiler
can distort the middle of the back of the band to equal the front stretch/distortion.
Of course for those who know the term, this is called benching. This is when
a roller or hammer is used to expand/distort the molecules to match the front
distortion. The roller stretches the molocules by enlogating them as it is
pressed between two specially shaped steel rollers. The hammer expands the
molocules in a starbusrst shape to achieve the distortion of making the middle
of the band back longer.
The hammer needs to be crowned to give a starburst
expansion. If a sawyer runs the band too hard or too dull he will distort
the front. Then, when he sends it out for repair the sawfiler has no choice
but to distort the middle and the back to make the blade equal everywhere.
He can make the blade saw and appear as if it is restored it to original,
but it is not.
Of course the tooth sharpness can be restored, but I am speaking
of the body distortion. Now after 3 or 4 runs the band has been distorted
by the sawyer and then distorted by the sawfiler and the molecules pulled
apart (cracks in the gullets) and the sawyer or owner asks what is the sawfiler
doing wrong. It is not the sawfilers fault. It is a matter of stressing the
blade more than it can stand.
If you want this cracking to stop then you have
to lower tension which in turn makes you slow down and pull the band for sharpening
more often. Or leave the tension the same and just sharpen much more often.
All in all keep the tooth load lower.
It is my opinion that a good set tooth band with correct curvature that is
1-1/2 wide will outperform dollar for yield, a band twice the width at 3 inches
that has a full tooth. It is easier to keep the tooth load down on a set tooth
band and it is easier to maintain a set tooth band.
Full tooth bands require
a lot of attention because of the larger tooth load. They require more professional
attention. This Professional attention is at a large expense.
I like the set-tooth
blades best because of the ease and the lower cost of maintaining the blades.
Be sure to check out our new Cooks Super Sharp bandsaw blade. You can order these blades online through our online store any time you need them. Cooks Super Sharp is taking the sawblade industry by storm!
Until Next Time,
aka... the "Saw Doctor"
Co-Owner CooksSaw.com
Leaders in Bandsaw Technology!
