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The Dynamics of Power Saws and Blades

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Blog entry by MrRon posted 08-03-2012 09:25 PM 1635 reads 7 times favorited 6 comments Add to Favorites Watch

Concerns regarding saw cutting accuracy usually puts the blame on the blade. Blades can cost anywhere from less than $5 to well past $100. One might think that the more expensive blade will cut better than a cheap blade. This is true up to a point. The quality of any circular saw blade will vary by manufacturer. Some manufacturers make blades with other company’s names on them. Others make them exclusively under their own name. These latter manufacturers usually are in the upper range of quality saw blades and consequently cost more. What makes a high quality saw blade compared to blades in the medium and low price range? The plate is the starting point for a quality saw blade. The plate must be high quality tempered steel. The thickness of the steel plate must be sufficient to prevent the plate from developing harmonic vibrations while cutting. The vibrations will cause a blade to “flutter” from side to side, degrading the quality of the cut. The side to side flutter is similar to the action of a “wobble” dado blade. Unless the blade runs true, it will not follow the line accurately. The thicker the plate is, the less chance it will wobble. Saw blade manufacturers have developed “thin kerf” blades in order to save material and reach a larger population of under-powered saw users. The normal saw kerf is .125” ± .002”. Thin kerf blades are around 3/32” kerf (.093”). Because they are thinner, their tendency to vibrate is greater. Saw manufacturers have come up with innovative ways to reduce vibration. This is done by cutting slits or scrolls in the plate with a laser. These slits break up the harmonics. It is effective, but if pushed too aggressively, the quality of the cut can be compromised. The next factor in a good saw blade is the cutting teeth.
The teeth are made from carbide. Tungsten carbide otherwise known as just carbide is the material used for the teeth. Carbide comes in many different grades, formulated for different cutting situations. Carbide is very brittle and the right combination of the carbide formulation must suit the application. Wood requires a grade that is very sharp and can sever wood fibers easily and cleanly. Carbide used to cut metals is a whole different grade of carbide. It must withstand the impact of hard metals. A sharp edge would immediately chip if trying to cut metal. Even the difference in density between hard and soft woods would require a different carbide grade; therefore saw blade manufacturers take into account the materials to be cut and formulate their carbide grades to suit. The number of teeth, size of tooth and tooth configuration is the next factor. Basically, there are two types of cut, rip and crosscut. Blades intended to rip, that is, parallel to the wood grain, will have fewer teeth and will cut with a “chisel” action. Each tooth resembles a chisel and takes a cut that is the width of the chisel. This chisel action creates a lot of small chips as opposed to “sawdust” and requires fewer teeth. The space between teeth is used to carry away the chips. A blade with many fine teeth, would clog up quickly. The number of teeth on a rip blade is between 12 and 30 depending on the diameter of the blade. In general, this discussion pertains to 10” diameter blades, the most common size among home use saws. Blades intended for crosscut service, that is, across the wood grain have many more teeth, the more teeth, the cleaner the cut. The number of teeth varies on a crosscut blade between 40 and 160 teeth. One might assume a blade with 160 teeth will cut the smoothest of any blade, but this would not be true. A blade with 160 teeth is great for cutting very thin materials, like ¼” plywood, but if you were cutting 1-2” wood, the kerf would be filled with sawdust and would cause excessive friction and burning. The number of teeth on a blade will be to suit the thickness of the wood. A third type of blade is called the combination or “novelty” blade. It is usually 40 to 50 teeth comprising 4 or 5 teeth with an alternating top bevel and a raker tooth which has no bevel or set. Its purpose is to clean out from the kerf the debris left by the cutting teeth. Combination blades serve the purpose of a general all around blade, good for ripping and crosscutting. The teeth on a blade have a bevel and a rake. An alternate top bevel has a bevel that varies between 0 and -30° from the horizontal. The more the bevel, the cleaner is the shearing action, but the quicker it will dull. A less beveled tooth will stay sharper longer, but will not give as clean a cut. Rack is the angle made between the face of the tooth and a line drawn from the center of the blade to the tip of the tooth. This angle can be positive or negative. The higher the positive rake is, the more aggressively the saw will bite into the wood. Blades with positive rake are always used on table saws where the blade is below the table. Compound sliding miter saws and radial arm saws need blades with a negative rake because the cutting action of a blade that is above the wood has a tendency to lift the work from the table. Although positive rake blades may be used on these types of saws, they pose more danger to the operator as the blade wants to pull itself into the wood. Simple miter saws that don’t have the sliding feature can use either type of blade. Going back to carbide teeth, the smoother the carbide edge is, the smoother will be the cut without splintering. As the cutting edge of each tooth makes contact with the wood, it severs the fibers on each side of the kerf. If the tooth is not finely ground, instead of severing the fiber cleanly, it will mash the fiber and the trailing edge of the tooth will lift the un-severed fibers above the wood’s surface. This is usually not a problem with rip blades. Assuming we have a good sharp blade with the correct number of teeth and the correct rake, we still get splintering, burning and a just plain poor cut, where do we go from there? The only other place to visit is the saw itself. Too many woodworkers, especially those new to the hobby, will assume because the saw is brand new and cost $3000 that the problem is in the blade. If you have a high quality saw blade, (Forrest, Tenyu, Everlast, Freud, etc.) the fault may well lie with the saw itself. A clean cut requires a blade to rotate with zero deviation (no wobble) from vertical. The arbor of the saw must turn in its bearings with no axial or radial play. Play is measured with a dial indicator set to rub on the large washer on the arbor and is read as so many thousands of an inch per revolution or total run-out. The best spindles can maintain a run-out of .00001” due to the high precision bearings used. They are costly, so you won’t find them in your $3000 machine. Most machines in the middle price range will have run-out readings in the ±.002” range. How does this affect the cut? If the blade cut in a perfectly vertical plane (zero run-out), the cut would be perfectly clean with no splintering, tears or irregularities. But with an arbor that has run-out of say ±.001”, a 10” blade will deviate ±.005” at its outer edge. This is a very small amount, but it can kick up a splinter or two. Although there is not much one can do to improve on run-out, one can check all other saw functions to make sure they are all within operating tolerances. A blade must be exactly 90° to the table for square edge cuts. The fence must be exactly parallel to the blade for accurate width rips. A ripped board that measures 3” at the start of the run through and 2.995” at the end of the cut, can add up to poor fitting joints. The same goes for miter gauges. There are after-market miter gauges and fences that can improve the accuracy of you saw. One other factor that affects cutting quality is the power of the saw. Most “contractor” type saws have 1-1/2HP motors which are adequate for a contractors purpose, but in a home woodworking environment, it can prove to be inadequate when cutting hardwoods. This also goes for the “table top” saws that use universal motors, the same type as used in portable hand held saws and miter saws. They can claim all sorts of HP, but bottom line is; they slow down under pressure. Clean cuts in hard woods and even soft woods, depends on a saw blade maintaining its RPM and that means HP. Not HP from a universal motor, but steady HP from a synchronous motor of at least 3 HP. If a saw doesn’t have the power to push through the wood, it slows down; the kerf closes down around the blade and the friction causes the wood to burn and burning the teeth. A 3+ HP motor maintains speed and powers the blade through the wood. I hope this will increase your understanding of saws and blades a bit more. There is a lot more to know, but this is only a brief introduction. I tried to hit upon the most basic.



6 comments so far

View Alexandre's profile

Alexandre

1417 posts in 847 days


#1 posted 08-03-2012 09:41 PM

Thanks for the info!
That was a HUGE chunk of info!

-- My terrible signature...

View davidroberts's profile

davidroberts

1002 posts in 2142 days


#2 posted 08-03-2012 11:51 PM

I read about half and will save the rest for this evening, but wanted to thank you now for posting this. I noticed a 10” thin kerf sawblade I have (not a primo blade, but not a $10 Skil either) will ring on occasion, and occasionally will sound like it is scraping a zci on my tablesaw. Now I know it is probably vibrating a bit.

-- God is great, wood is good. Let us thank Him for wood......and old hand tools.

View MrRon's profile

MrRon

2834 posts in 1899 days


#3 posted 08-04-2012 05:09 PM

a 10” thin kerf sawblade I have (not a primo blade, but not a $10 Skil either) will ring on occasion, and occasionally will sound like it is scraping a zci on my tablesaw. Now I know it is probably vibrating a bit.

It doesn’t take much for a thin kerf blade to vibrate. That little run out is enough to set the blade into harmonic distortion, hence flutter. You know it is so if you examine the zero clearance insert (ZCI). A brand new ZCI will be larger than the width of the saw blade tooth right after the initial plunge cut and will get wider with use. These are things I have noticed over the years and I’m trying to understand the dynamics of why it happens and what I can do about it. There are so many variables involved. Run out being the one thing we have to regard as a constant. We can improve on the grind of the carbide tips. We can go to thicker plates and we can tweak the fence and miter gauge, but the run out will always be there. We may have to just learn to live with it. An interesting experiment would be to focus a high speed camera on a running saw blade. Maybe I will contact Myth Busters.

View davidroberts's profile

davidroberts

1002 posts in 2142 days


#4 posted 08-19-2012 06:14 PM

I’ve check my arbor runout and it’s low, very low. Don’t remember the exact number but the needle on the dial caliper did deflect a smiggen, Certainly under 0.001. However, there is measurable runout or at least a limiting value. Actually it could have been variation in thickness on the end of my arbor, probably a combo of both. I recently installed a vetrified Norton white grinding wheel on my slow speed grinder and noticed, even after a lot of dressing, it wobbles upon startup and just before spinning down when I power off. I think unless the wheel is precision made, verses just pressed, imbalance is inherit, especially given the cheap plastic bushings it comes with. Same for saw blades. Perfect balance cost money. There s a diminishing rate of return on value. Also, my full kerf WW II blade rarely rings.

-- God is great, wood is good. Let us thank Him for wood......and old hand tools.

View MrRon's profile

MrRon

2834 posts in 1899 days


#5 posted 08-21-2012 12:02 AM

Davidroberts,
Don’t forget, that “smiggen” at the arbor will be magnified at the tip of the blade as much as 5X. 0005” at the arbor will read .0025” at the blades circumference.

View MrRon's profile

MrRon

2834 posts in 1899 days


#6 posted 10-06-2012 07:24 PM

Davidroberts;
As mentioned in my previous post, .0025” of TIR is a very small amount not worth getting upset over. It just means the edge of the wood may have some fine splinters that can be removed with sandpaper. It would be nice to have zero TIR, but this is not possible with todays saws. A blade with a sharp bevel is best for that very clean cut

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