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From The Horses Mouth - Getting More From Your Bandmill By Chris Lincoln
S&W Report / Winter/Spring 1998, Vol 10
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Bandsaws have gotten a bad rap over the years, mainly because of their reputation of producing wavy cuts. Contrary to popular belief, these cuts are the result of operator error, not machine error. Unfortunately, few bandsaw mill manufacturers provide training courses. As these machines became more affordable, they also became more popular. Suddenly there were dozens of manufacturers producing a variation on them. This resulted in more untrained people buying mills and calling themselves sawyers. The problem grew, and the reputation of the bandsaw mill was badly tarnished.
A few things to remember when considering the purchase of a mill. First and foremost, they can be loaded with gadgets for lifting, turning, toeing, clamping, which can result in greater efficiency etc. It is easy to be impressed with all the bells and whistles, but the efficiency of the cut portion of the cycle is determined by available horsepower, the sharpness of the blade, the feed rate into the material and the ability to remove the sawdust from the cut. You can increase the efficiency of a bandsaw, even if it does not have all the gadgets, by properly sharpening and setting the blades. The blade is, by necessity, flexible, so a straight cut depends on the teeth (over tightening will break the blade, not straighten the cut). Sharpness, set and feed are all critical factors. Keep in mind that the terms hardwood and softwood refer to density of wood, not necessarily deciduous or coniferous woods.
The best explanation of tooth geometry can be found in Forest Engineering Research Institute of Canada (FERIC) Handbook # 10, reproduced here by permission.
Tooth Spacing: There are several different teeth per inch configurations, but the most common are 1", 7/8", and ¾". The greater the tooth spacing, the more stress or load on each tooth. Having more teeth per inch results in a smoother sawn surface.
Depth of Gullet: The area between teeth carries sawdust out of the cut is called the gullet. The gullet is determined by the shape, pitch and height of the saw teeth, and needs to be large enough to carry away all sawdust produced. If the space of the gullet is too small, the sawdust is forced out at the side of the blade into the kerf. This increases the friction between the blade and the wood, resulting in heat, sap build-up on the blade, and increased power consumption. Whenever the blade is sharpened, the tooth height will decrease somewhat, which can result in a gullet area too small to carry the sawdust. Too short a tooth will also result in inability to 'set' the teeth. To overcome this, the gullet should be ground deeper when tooth height becomes less than 3/16".
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Clearance Angle: The clearance angle is the angle at the top of the tooth. This angle extends far enough down the blade so that it should not need to be ground during the normal life of the blade. This angle should never be less then 5 degrees, and can be as large as 30 degrees. If this back clearance is insufficient, the back of the tooth will press against the wood, causing a rubbing action that will overheat and cause early dulling of the blade. The higher the feed rate, the larger required clearance angle.
Tooth Angle: The tooth angle determines the strength of the tooth. It can range from 45 to 75 degrees.
Hook Angle: The hook angle is the number of degrees that the tooth face leans forward 90 degrees. Hook angle and tooth set are the two most critical factors in the cutting ability of a blade. The type of wood determines the proper hook angle and the intended feed rate. The generally accepted rule is that softwoods require greater hook angle than hardwoods.
Generally, the smaller the hook angle, the slower the cutting speed of the mill will be. The object of the hook angle is for the teeth to 'hook' themselves into the timber to cut a certain amount of wood so that the blade feeds itself into the log. At slow feed rates, if the hook angle is too great in relation to the feed rate, the blade will 'pluck' and attempt to move forward in the cut. The result is chatter, a coarse cut, and poor cut quality. The slow feed speed doesn't allow the teeth to fulfill their function and the tooth points will develop a rubbing action, resulting in premature dullness.
Conversely, at higher feed rates, if the hook angle is too small, the blade must be forced into the log to make the saw cut. This creates side deflection in the blade and will also cause a rubbing action that will wear the teeth faster. Recommended hook angles are 12 ½ to 15 degrees for softwoods and 10 - 12 ½ degrees for hardwoods. In theory, you should use less of a hook angle when cutting wider boards because of the slower feed rate.
Face Angle: The face angle is the angle of the tooth face in relation to the body of the blade. The angle should remain 90 degrees for all teeth to pull sawdust forward out of the kerf. This is critical to getting a good cut and the primary reason for sharpening new blades. Blade manufacturers 'set' the blade after it is sharpened, turning the face angle out as much as 7 degrees. Grinding a blade on a properly aligned sharpener will true all the teeth face angles to 90 degrees.
Tooth Set: The tooth set is measured as the distance that a tooth is bent in relation to the body of the blade. The further a tooth is set the wider the cutting path of the blade. Wider sets require higher horsepower. Total set for 0.042" blades should be 0.084" (0.021" left and right +0.042" material thickness = 0.084" total set). Remember that as you sharpen your teeth, the set will decrease and require a wider set than hardwoods with a total set not to exceed 0.090".
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