I have a problem with saw blades sometimes wobbling and creating a bad cut. I have heard mention of hammering a blade, but I am not sure what that means. Does anyone have any information on this?
(Architectural Woodworking Forum)
From contributor H:
The opposite happens for me; after a few rum and cokes, I'm hammered and start to wobble.
Give the blade to your sharpener and let him fix it.
Unknown to a lot of people is that plate tension (the stiffness of the plate) is more important than the flatness of the plate. This is also done in the hammering process by pounding the plate in certain areas to increase or decrease the stiffness of the plate. Even a perfect flat plate can wobble if improper tension is not met. Hammering is almost an art. Years of training and teaching are required to be good at this. A lot of people can hammer a saw flat but have no clue how to tension. Just make sure whoever you are sending your blades to for service has the capability and experience.
None of the blades you buy, even new, will be perfectly flat. Say the one you buy is not flat by 0.004". Your total runout after mounting on your saw could therefore be anywhere between 0.010 and 0.002 depending on where the saw blade wound up on the arbor.
This is why I use a cheap (under 20 bucks) dial indicator whenever I install a blade. I repeatedly tighten the blade and check the runout, move the blade 90 degrees with respect to the arbor, and do it again. I do this until I have found the spot where the runout is the least. I have found that I can improve the runout on every saw that I use by doing this procedure.
I would definitely heed contributor R's advice regarding hammering. A friend of mine has an ancient sawmill running a large blade. The blade, at rest, has been hammered and tensioned so it is shaped like a satellite dish when not spinning.
Once it comes up to RPM, it flattens out. He claimed there were three reasons for this: the blade was more stable at running RPM's, if the idiot (him) sawing logs was pushing the carriage too fast, the RPM would drop and the blade would come out of the log on the board side. This would prevent severe damage to the plate, the arbor, etc., and would also prevent ruining more than one board at a time. So, like contributor R says, there is more to it than just banging away with a hammer.
Like someone else said, your arbor may be partly to blame also. I put a readout on my Unisaw and was surprised by the amount of runout. I called their tech support and they told me that it was within specs! I did the same on our Schelling - barely detectable runout! (Of course it cost $130,000!) You never get more than what you pay for.
An alternative to increasing diameter as the saw spins is to get wavy at the rim, which is what actually happens. (Note that this effect occurs progressively less as you move away from the rim.) In fact, there is a frequency at which the sawblade will actually create a "standing wave" along the rim. It is similar to what you might observe in a car antenna at certain speeds. To avoid this waviness, we can make the saw blade very stiff, but that means thick, which means lots of sawdust.
Another option is to hammer the saw blade so that when the blade is not spinning, the blade is dished slightly. (Hammering does not make a flat blade!) With a dished blade, as the blade beings to spin, the outside can get a bit longer and this increase in diameter actually causes the blade to come flat while spinning. Hence, when hammering a blade (sometimes hammering is called rolling or tensioning), one needs to know the amount of dish required, which means one needs to know the rpms that the blade will be working at. You cannot hammer without knowing the rpms.
Hammering is sometimes called tensioning the blade and you are putting in this dishing. If one were to check the ring that is heard when a blade is hit lightly, the flat blade (untensioned or unhammered) will be lower in frequency than a hammered (or tensioned or dished) blade. In fact, measuring the frequency is a quality check when rolling a blade.
Incidentally, this wobble is why we have difficulty using thin kerf blades and why as the rpms drop when sawing, the cut can actually get poorer.
Also note that heat will destroy the hammering or tension. So, a blade that is accidentally heated will possibly cut poorly it the future.
There is another option to hammering and that is adding heat to the eye and cooling the rim. This is often done with band saws in sawmills. They have the same centripetal force issues and resonance. Larger band saws are hammered so that they are slightly curved, edge to edge.
Hope this clears up some of the incorrect info posted previously here.
I took the blade back to the grinding shop where I bought it and the owner suggested grinding the rakers down to below the lower intersection of the two beveled teeth. Now when I cut, the bottom of the saw kerf had a center inverted "V" of stock sticking up which came to a sharp point just like a straight ATB without rakers. The top of the "V" was not lopped off like before. This not only eliminated the problem, but that blade cut better than the straight ATB's I had.
1) What exactly is the purpose of the raker? Is it to just clean the kerf of chips?
2) What height should the rakers be ground to relative to the intersection point of the ATB's on a cross cut saw?
3) Do you think the rakers were inadvertently left too high at the manufacturer?
4) Or do you think it was simply that my grinder did a better job grinding the teeth than the factory, and the raker height makes no difference?
Also, with regard to the rim expansion of say 15" blades, assuming there is not any heating problem, is that expansion generally consistent enough throughout the rim that edge grinding the teeth will result in a cleaner (smoother) cut given no change in the top to bottom or front to back angles?
Note that with small diameter blades, and with a large collar, the centripetal force problem is close to zero. The larger the blade, the higher the rpms and the smaller the collar, the more the problem..
That must be an interesting setup for the computer to control the blade wobble with magnets! What kind of saw uses that system? The blades we had the most problem with are only 14" in diameter, big national brand. Haven't bought any in quite awhile; maybe they've figured it out.
Comment from contributor C:
Just to clarify a point about the WZ/FZ group toothing design the finishing tooth is 0.3mm below the highest point of the angled teeth. The purpose of the finishing tooth is two-fold. It gives a flatter end cut and a cleaner cut too. The more aggressive a tooth design is, like the 15-20 degree bevels used in this instance (and usually with a shear angle too), the rougher the finished edge. The finishing (FZ) tooth helps keep the cut clean.