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Home . About Us . Subscribe . Advertise . Editorial Outline . Contact Us . Current Issue . Back Issues . Jon Eakes

© Copyright 2006 Work-4 Projects Ltd.

By Jon Eakes

Why get complicated with Saw Blades?
...because efficiency leads to profit

Most of us probably have a carbide-tipped combination blade in our circular saw, a fairly fine-toothed carbide blade in our mitre saw and, perhaps, a large combination blade in our table saw. Pushed further, you might not know much about the blades you use. When sharp, they cut pretty well.
It is kind of funny to think that we work seriously at efficiencies in our furnaces and in our vehicles, but never think how we could double or triple the power of our saws or spend less time with chipping and splintering by looking at the efficiencies of specialty saw blades. In reality, taking the time to switch a saw blade can often save you much more time in getting the job done, not to mention less wear on your saw.
I love the definition of a combination blade as "one that cuts equally poorly in both directions." If you look at the teeth of a combination blade, either they are some compromise between rip and cross-cut teeth, or you will find four-pointed cross cut teeth and one rip tooth with a large gully before it to clear the shavings. In either case, these blades are useful but never work to maximum efficiency.
Blade choices for shop uses get very specific, but even on construction sites we need to be aware of some important differences because we should cut different materials with different tools. Although most saw blade companies make a variety of products, I am using illustrations here from the Freud catalogue; they have really taken all elements of saw blades to perfection.


Carbide
It is important to realize that the carbide itself varies tremendously. On a microscopic level, carbide is an accumulation of little grains of metal. You can't make a tooth sharper than its smallest grain. Freud likes to brag that it makes its own "micro grain carbide" that gets sharper and stays sharper longer than standard carbide teeth. Teeth get dull by pieces of the grain popping off, so if the grain is larger, the blade gets dull faster. Sharper teeth require less power to make a cut and finer grains mean more cutting between sharpening.
Along with grain size is the fact that if carbide is formulated for impact resistance, it will not stay as sharp; if formulated as extra hard carbide for sharpness, it will not withstand impact as well. You should match the teeth to the working stress it will encounter. Renovation blades designed to easily cut through that occasional nail are not finishing blades designed for a splinter free edge.
Also, when looking at blades at the store, you need to realize that the ability to re-sharpen a blade is related to the thickness of the carbide. More expensive blades can be sharpened more times than a less expensive one, and often the lifespan cost of the blade is less.

Tooth shapes and angles
Saw blade teeth have different shapes for different purposes. The Alternate Top Bevel Grind tooth represent the standard cross-cutting or even multiple-purpose tooth. It scores the fibres on each side of the kerf and the wood in the middle just kind of pops out. The High ATB grind is less durable but sharper, creating even fewer splinters, which is particularly useful for clean cuts in pine and cedar.

The Flat Top Grind tooth is the basic rip tooth. It is like a chisel that literally pulls out a shaving, the reason for a rather large gully in front of the tooth. Use it in a cross cut application and it makes splinters like crazy because there are no points slicing across the fibres. The very common multiple tooth combination blade uses both the ATB and the Flat rip with the pointed teeth just a bit further out than the flat top tooth.

The Triple Chip Grind is often used in combination with a Flat Top tooth. It is specifically for melamine and other composite materials that would destroy the narrow points of the ATB teeth through heat build-up, but where you are not working with wood grain.
The hook angle of the tooth (H in the drawing above), the angle between the radius of the blade and the face of the tooth, is important as well. It can vary from an aggressive 25° for serious ripping, to a negative 7° for cutting nails or for use on sliding mitre saws where you do not want the blade to grab and throw the wood. Negative angles don't cut, but rather scrape their way through the material.
Years ago you may have learned as I did to take a fine-toothed blade and install it backward for cutting thin sheet material - both plastic and metal. Now they make negative hook angle blades to accomplish the same clatter-free cut on both vinyl and aluminum siding as well as aluminum extrusions and plastic pipe. That funny reversed bump on many renovation blades is an anti-kickback device that allows for a very aggressive tooth angle for fast cutting while the heel prevents the blade from biting too much at once. It does cause rub and limits speed but adds safety and prevents motor binding.

Choosing the right blade
How many teeth should your saw blade have? That really depends on what you are cutting and why you are cutting it. Fewer teeth will cut faster, more teeth will cut cleaner. But too many teeth going through a thick material will clog up and burn, so it is also a function of the thickness of the material you are cutting. The chart above will help you to find the right blade. Start with the thickness of material you are cutting on the bottom scale and go up. Choose the cross cut or ripping zone. From the centre of the zone go left to the number of teeth. The higher end of the zone will cut cleaner, the lower will cut faster and the centre is a good average.
How about coatings on blades? It is simple if you have a microscope. Coatings are in fact very useful where there is a lot of glue or sap because even polished metal has a rough surface that can collect wood residue. The coatings fill in those gaps, giving the residue no valleys to collect in. This in turn causes the blade to run cooler, giving it even less of a tendency to collect junk and less drag on your motor.
The strange cut-outs you see in many blades serve two purposes. The relief from the outer edge, a question mark shape on Freud blades, allows the outer rim to expand with heat without warping the disk. This cuts down on vibration because the blade remains flatter, reducing chipping. But cut-outs can create noise and some companies have tried to "plug the expansion hole" with an insert, which doesn't always stay put. That is why Freud created the question mark shape: give the blade the relief it needs but leave the plug there as part of the blade.
The snake-shaped laser cuts in the body of larger table saw blades gives the blade a "dead" sound when you tap it. These "dead" blades eliminate the need for stabilizing hubs and simply do not vibrate, a real advantage when cutting melamine or fine moulding joints.
What is that really weird blade with gaping slots cut right through it? It is essentially a ventilated blade specifically designed for working with wet decking lumber, preventing heat build-up and binding. Try the coated one and you will be amazed, but don't expect a furniture grade cut.
With the advent of battery operated saws we have seen the demand for blades that require less power to cut the same wood - hence the advent of the "thin" blades, 1/3 to 1/2 the thickness of regular blades. They simply remove less wood, using less power, giving us longer battery life. Why wouldn't we use thin blades for everything? Because thin blades will vibrate more and don't give us as clean a cut as a thicker and more stable blade. Also, the carbide tooth is smaller, heats more and doesn't hold its cutting edge as long. But for general carpentry, they are a good choice and a necessity for cordless tools.
Of course many an "improper" blade will get the job at hand done, but the right blade will do it better, safer and with less wear on your saw. Learning to change blades quickly is time better spent than learning how to "get by" with the wrong blade. HB


Montreal-based TV broadcaster, author, home renovation and tool expert Jon Eakes provides a tool feature in each edition of Home BUILDER.
www.JonEakes.com

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