TP–05–02WDecember 2005

TechnologyProfile

Circular Saw Selection for Secondary Manufacturing

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C ircular saws are the most commonly used tool in the woodworking industry. Despite their simplicity, circular saw design is a refined

technology. Attention to detail and quality construction have significant effects on performance. This applies especially as the plate thickness and kerf are reduced. For achieving optimum performance, circular saws should be considered as engineered tooling, not a commodity supply.

The first step to selecting or optimizing a circular saw is to understand that one saw will not do all cutting tasks well. The more specific the process, the more refined the design of the saw needs to be. In extreme cases, saws are designed for one depth of cut, one feed speed, and perhaps only one wood species.

Production GoalsThe process for selecting or designing a saw depends on the product to be produced. The task is to balance the needs of production, recovery, accuracy and surface finish. Usually, three of the four needs can be met with some sacrifices on the fourth. For example, when cutting window blind slats, thin saws with kerf as small as 0.040 inch (1 mm) with many teeth are used, but the feed speed must be slow.

Stock saws are available for most circular saw machines. However, there are many saw makers, each with its own designs and custom designs are also available. There are costs involved in custom orders, but the benefits can be significant. As in all manufacturing processes, continuous

improvement and “stretching the envelope” pay dividends. This Technology Profile presents the factors that control saw performance so you can better understand your current sawing system and plan improvements.

Machine IssuesBefore discussing saw design, the capability of the whole machine needs to be considered. Having a solid, well-maintained machine is critical for consistent production. A machine that does not control the wood will force the use of thicker saws or slower feed speeds.

Heat, resulting in warped saws or in loss of stiffness, is the most common side effect of machine problems such as: bent arbors, worn or nicked collars, worn pressure plates, track or fence straightness and parallelism, and failing arbor or feed bearings. The consequences are scarred or croocked wood surfaces and reduced runtimes. First class maintenance of these issues is critical when running thin kerf saws because they cannot withstand the abuse of wood movement or inconsistent feeding.

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Saw Dimensions

Cross-Cut Saw DesignFor cross-cutting, surface finish and minimal edge tear-out are the main concerns. These goals are achieved by having a small tooth bite (a few thousandths of an inch) and by using a tooth shape that scores the fibres cleanly from the surface. The small bite occurs when the saw has a high rim speed, many teeth and is fed slowly into the wood. The severing action is best done with an Alternate Top Bevel (ATB) tooth shape. The ATB can be combined with an Alternate Face Bevel (AFB) to increase the sharpness of the tip corners. If the tips become too weak, the hook angle can be reduced or even reversed (negative).

Kerf and plate thickness are not usually an issue, but reducing the kerf does reduce the cutting pressure that causes fibre “blow-out” on the edge where the saw exits. Noise – screaming saws – can be reduced with changes in tooth shape (shallower gullets, shorter tip length), adding holes or slots in the blade, and by changing the shape of the saw enclosure.

Plate Thickness controls the bending stiffness of the blade. The effect can be major. For instance, a 10% decrease in thickness results in a 30% reduction in stiffness. This also means the saw is 30% easier to damage.

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Tip Wear is caused by abrasion and corrosion• Using harder materials

reduces abrasion. The downside is that these materials are more brittle and chip more easily.

• Chemicals in the wood and glues can selectively attack elements of the tip material, especially the binder between the hard particles.

Tooth Design Strength and gullet area requirements determine tooth shape. A deep narrow tooth will bend easily, but if the gullet cannot hold the sawdust produced then the sawdust will spill and heat the blade, resulting in a weaker saw and a wavy cut. Tooth shape is also affected by plate thickness – the gullet depth must be reduced for a thinner plate to maintain an acceptable tooth stiffness and strength. The hook angle can be reduced to increase tooth strength. For ripping, the hook is typically 30° for green softwoods, 20° for dry softwoods and hardwoods. For particleboard, a hook angle of 12° to 15° is sometimes used.

Side clearance increases when the wood is poorly controlled or has high pitch content. Under the best conditions, side clearance can be as small as 0.005 inch, but is typically in the 0.010 to 0.025 inch range.

Tooth Shapes & Uses

Common Tip MaterialsCarbides consist of hard carbide particles in a softer metal binder. They are hard and wear resistant, but chipping occurs more often in the harder grades. Many grades and types are available, and new grades use different binder metals to reduce chemical wear. Tips are brazed to the blade.

Stellite™ and similar chromium-cobalt alloys are better for woods with corrosive chemicals, such as red cedar. It can be welded or brazed to the blade.

Poly-crystalline Diamond is very expensive and easily chipped, but lasts a long time. It is best for abrasive, clean materials such as MDF and coated panels. The diamond is coated onto a carbide tip and then brazed to the blade. Tips can be sharpened by electro-discharge machining (EDM).

Cermets are hybrid metal-ceramic materials that are long-wearing and very resistant to corrosion. Brittleness is an issue. With special preparation, the tips can be brazed.

A Flat Top is the most common, inexpensive and easiest to maintain. Used mainly for ripping, but does not produce the best surface.

B Triple Chip improves the surface finish for man-made materials and also works well for solid wood.C Alternate Top Bevel (ATB) is sometimes used for ripping, but the scoring action of the tooth corners makes it ideal for

cross-cutting. The teeth may also have Alternate Face Bevel (AFB) to increase the severing ability. The corners are fragile, but the bevel angle can be reduced from the usual 15°, to 10° to increase corner strength.

D Chamfered Corner is used for ripping harder materials where the corners of the teeth may be broken easily.E Hollow Ground provides the best surface finish for cross-cutting, but requires special and careful grinding procedures.F Combination pattern used on table saws for mixed ripping and cross-cut operations. Four ATB tips followed by a flat raker

tooth. G Triple Chip with Raker Flat Top is used for products with laminated surfaces. The triple-chip tooth is slightly higher than

the raker and pre-cuts the center of the kerf. The raker removes only a small amount on the edge, which reduces the cutting pressure, resulting in less chipping of the laminate.

H Scoring tip for a pre-cutting saw.

info@valuetowood.ca

Forintek Canada Corp.Value to Wood Co-ordinator (East)

319, rue Franquet, Quebec, QC Canada G1P 4R4

Tel: (418) 659-2647Fax: (418) 659-2922

Forintek Canada Corp.Value to Wood Co-ordinator (West)2665 East Mall, Vancouver, BC Canada V6T 1W5

Tel: (604) 224-3221Fax: (604) 222-5690

• Send a request via valuetowood.ca (Help Desk).• Contact a Value to Wood co-ordinator at one of the following locations:

As part of the Value to Wood program, funded by Natural Resources Canada, Forintek’s Industry Advisors are providing technical services to value-added wood product manufacturers in all regions of Canada. If you need information on any technical issue related to wood product manufacturing, you can:

Ce Profil technologique est également disponible en français.

This Technology Profile has been edited by Bruce Lehmann, Research Scientist, Lumber Manufacturing, Western Division, Forintek Canada Corp.

For more information on the 2005-2006 Value to Wood research program, visit www.valuetowood.ca (Research and Development). The partners involved are:

Surface FinishSurface finish is primarily determined by the alignment of the teeth and the bite per tooth (chip thickness). Grain direction is also involved, but is not generally a factor that can be controlled.

If the teeth are not in the same plane, the high teeth will leave scratches on the wood surface. Improper grinding can cause this, but the more common causes are that the plate is not flat or that a damaged collar is pushing the saw off square from the arbor. Sawdust trapped between the collar and the saw will also cause the saw to wobble. Much of the cost of high performance blades is in achieving an initially flat plate.

The smaller the bite per tooth, the smoother the finish. Typically, bites are in the range of 0.002 to 0.010 inch. This is achieved using saws with many teeth fed at slow speeds. Sometimes the arbor speed can be increased to improve production. Another option is to reduce the distance the saw protrudes out of the wood. This reduces the instantaneous chip thickness and aligns the tooth motion parallel to the grain.

Also, an online course, "Feed and Speeds Course" is available in the Members section of the Forintek website, www.forintek.ca.

Feed SpeedsOne or more of the following factors limits the feed speed:• The bite is as large as it can be for an acceptable surface finish.• The gullets are overloading with sawdust, resulting in heat in the saw and a

wandering cut. Deeper depths of cut require slower feed speeds because more sawdust is produced.

• The cutting forces on the teeth are bending the saw, producing a wandering cut and perhaps saw damage. As plate thickness is reduced, so are feed speeds to reduce the cutting forces.

On the other hand, feeding too slowly results in rapid dulling of the teeth and higher power draw on the motor.