Sawmilling Concepts
Part III: Blade Theory and Terminology
This article is the third in a series of reports that will appear in the S&W Report focusing on sawmilling concepts. The purpose of the series is twofold. It will provide some useful tips to those who own a portable sawmill. It will also assist landowners, who may want to purchase a mill or hire a sawyer, gain a better understanding of proper sawing techniques.
Topics covered in the series include: sawing lumber, using the quarter scale, saw blade maintenance, introduction to drying lumber, determining moisture content of wood, wood drying defects, methods of drying lumber and dry kilns.
A key component to successful sawmill operations is to perform the necessary maintenance of the sawmill and blades. In part III, we will learn about the proper maintenance of saw blades.
Blade Theory and Terminology
If any area of sawmilling can determine the success or failure of a cutting operation, it is the blade. Because of this, a clear understanding of both definition and theory about bandsaw blades is essential.
Here are some facts about blade construction
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blade thickness from .035 inch to .045 inch;
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180 teeth spaced 7/8 inch apart;
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made for standard cutting or frozen/hard wood;
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special steel alloy used; |
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average blade life between sharpenings is 300 to 800 board feet; and |
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average blade life before it breaks is 2,000 to 3,000 board feet. |
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Tooth shape characteristics
Pitch – is defined as the number of teeth per inch. Purpose: the smaller the pitch, the more the stress load on each tooth. The pitch cannot be altered in the filing process.
Depth of gullet (tooth height) – the gullet is the area between each tooth and is governed by the shape, pitch and height of the saw teeth. Purpose: to carry away all sawdust produced. Filing can alter the height of the gullet.
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Figure 1: Blade design where A - tooth spacing, B - radius, C - tooth height (depth of gullet) and D - hook angle. |
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Hook angle – is the number of degrees that the tooth face leans forward of 90 degrees. It is one of the most critical factors in good blade sharpening. Purpose: allows the teeth to hook themselves into the timber to remove a certain amount of wood so the blade feeds itself into the wood. It is recommended to use a larger hook angle on softwoods (12.5 - 15 degrees) compared to 10 - 12.5 degrees for hardwoods.
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 (kerf) of the blade. Purpose: to allow the body of the blade to pass through the wood. Tooth set will be altered based on the density of the wood or condition (frozen). Tooth set is approximately .020 inch to either side of the body of the blade.
Face angle – is the angle of the tooth in relation to the body of the blade; it should be 90 degrees. Purpose: to pull sawdust forward out of the kerf to produce a good cut.
How is sharpening accomplished?
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Automatic sharpeners are available that will (after proper adjustment and dressing of the stone wheel): file the proper hook angle; dress the gullet; and prepare a 90-degree face angle.
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The teeth are set using a toothsetter device. This device ensures that each tooth is set to the nearest .01 inch.
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Other blade-sharpening options include sending the blades out for professional sharpening (i.e., Re-Sharp Program). The benefit is that the sawyer does not spend evenings filing blades, and the disadvantage is the additional cost to the sawyer
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Typical problems arising from poor sharpening techniques
The majority of problems that arise in sawing lumber are directly related to the quality of filing the blades. Figure 2 illustrates the problem that a poorly filed blade will not perform to the expected standard.
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Figure 2: Poorly filed blades will not perform to the expected standard. |
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In the Next Edition
In Part IV of “Sawmilling Concepts” you will be introduced to the reasons for drying lumber and background information necessary for understanding the process of drying wood.
Acknowledgement
The information (including illustrations) for this article is from the Instructional Guide to Wood Technology and Sawmilling, 2000. A publication of Wood-Mizer Canada Co, 217 Salem Road, Manilla, Ontario, K0M 2J0, 1-877-866-0667, <www.woodmizer.ca>.
This article appeared in the Winter/Spring 2005 (Volume 38) edition of the S&W Report the newsletter of the Ontario Woodlot Association.
© Ontario Woodlot Association
Visit the ‘S&W Library’ section of our website to read Parts I to II of Sawmilling Concepts.
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