Here is an excerpt from Feb 2014. You can order the issue for the complete article and pictures:
TOOTH ANGLE
There are three angles of a saw tooth; these three total 90 degrees. The first angle is the clearance angle, which is an angle required so that the back of the tooth (the metal immediately behind the tooth) does not rub on the wood as the blade advances into the cut. This clearance angle, if not present, would result in rubbing, heating and extreme difficulty in feeding the saw into the wood. This clearance angle is typically 10 to 12 degrees.
The next angle is the hook angle. The greater the hook angle, the more the saw is going to grab the wood and dive into the cut. [Have you ever tried to cut lumber with a radial arm saw and the saw actually wanted to come flying through the piece of wood so much that you had to hold the saw back? This is because there was too much hook in the blade. A radial arm saw blade usually has zero hook or even slightly negative; use the wrong saw and the radial arm saw will be difficult to control. On the other hand, if you used a zero or negative hook angle saw blade on a table saw, the end result would be that the wood piece being sawn would require a lot of force to feed into the saw and usually the piece will want to come flying back at the operator.]
The hook angle for bandsaws for sawmills typically is between 4 degrees to 13 degrees. With less hook than 4 degrees, the effort to feed the saw into the wood becomes greater; with more hook than 13 degrees, the saw can come off the wheels or can break.
A second effect is that with less hook, the horsepower required for the saw is reduced.
With more hook, especially for dense or frozen wood, the blade will have to be fed very slowly; if fed too fast, the blade will likely stall. With slow feed, we get fine sawdust and this leads to heating (see previous discussion), rapid dulling, poor performance, poor thickness uniformity for the lumber, etc. [I have heard from many mill operators about the limited amount of footage they saw until they have to change blades...my initial guess, assuming they did not have dirty logs, is that they have too much hook on the blades, so they fed slowly and got heating.]
Here are general recommendations:
4 degree hook Use for well frozen (under 20 F) wood and/or dense wood
7 degree hook Use with higher horsepower mills for frozen and/or dense wood, including many dense tropical hardwoods; also use for larger diameter logs and medium or lower power on medium or low density wood
9 degree hook Use with lower horse power mills cutting smaller logs; also with other powered mills cutting denser hardwoods
10 degree hook Use for most hardwoods (except the densest like hickory) and most softwoods except for a lower powered mill; this is a general purpose, all around hook
13 degree hook Use for most softwoods on most mills, although a smaller hook might work better for southern pine, as SYP is very dense.
IMPROVING TOOTH TIPS
The tip of the saw tooth can be just ordinary metal, identical to the metal of the saw body. Typically, such metal is fairly soft and will wear quickly; as a result, the tooth will dull quickly.
One, somewhat inexpensive, alternative that makes the tooth harder is to heat the tooth and then quench it in a cold liquid; the resulting tooth is much more durable, but is slightly more brittle. Usually the hardening process uses induction heating, thereby heating only the tooth and making the tooth harder, but not affecting the properties of the body of the blade.
Another alternative is to put a more durable material on the tip of the tooth. Durable materials include
Carbide (a combination of tungsten, carbon and cobalt in various proportions to increase times between sharpening by ten, although the material is prone to chipping when it hits hard items like rocks, tools, etc.)
Stellite (R) (Stellite, invented in the early 1900s, is a proprietary carbide material with cobalt, chromium, and a variety of other elements such as tungsten, nickel, molybdenum and carbon. It is less brittle than carbide and lasts longer but does cost more.)
Chrome (chromium carbide, very durable and expensive)
There is no question that logs with dirt and other hard debris on the outside will quickly short useful tooth life, especially when using these special teeth that tend to be more brittle. To lengthen tooth life, several options are to clean logs with a power washer, remove bark (as the crevasses in bark often hold logs of grit), and store cleaned logs up off the ground.
SET OR SWAGE
Now that we have the tooth ready to go, we need to figure out a way to make the slot that the tooth cuts, the kerf, just a bit wider than the body of the blade. Without this extra space, the blade will rub, heat, etc. Actually, when we cut a slot in the wood, the wood cells will spring back slightly, closing the slot just a bit after the cells are cut, so the kerf must be just a bit wider than we might calculate initially.
Basically, there are two choices for cutting a a wide kerf: Make the tip of the tooth a bit wider, called swaging and done by hitting the tooth tip with a hammer and a special tool called a swage; or by bend the teeth, one to one side and the next one to the other side of the blade body, called setting the teeth. With small band saws, setting is the most common technique. Setting must be done carefully so that the protrusion in each side of the blade is identical. If uneven, then the blade will wander and can even take off cutting on its own, undesired path. Special, simple equipment can be purchased to set the teeth yourself. Setting is usually done after sharpening.
The required set is less for denser woods than for softer, less dense species.
In some cases, after two teeth are set, one to each side, the third tooth, called the raker tooth, is left straight. Then the fourth and fifth teeth are set and the sixth is left straight, and so on. This is called a triple chip blade and would be used for crosscutting, but is not widely used in most small band sawmills, but such blades are available.
TOOTH SPACING OR PITCH
The closer the teeth are spaced, the less power per tooth (but overall the power is the same if the feed rate is the same at any tooth spacing) and finer the sawdust. With wider spacing, however, the amount of sawdust that the gullet of each tooth increases. With large logs, the gullets may fill and then the excess sawdust will spill out and cause heating.
In my experience, 7/8" is the most common spacing, but closer and further spacing is available. My favorite is 1-1/8" especially for most softwoods and low density hardwoods. For well frozen logs, 3/4" is probably more reasonable. In general, it is common to find that the spacing is too close for many mills. One of the best ways to judge the result of spacing and feed rate is to look at the sawdust; very fine sawdust is often an indicator of slow feed and close spacing. The larger spacing does require a wider blade, however, to handle the extra stress per tooth.
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