| |Charlie, | |Nice reponse. Thanks for taking the time.
I second that. | |> DO NOT remove the wheels. That's asking for a ticket to Set Up Hell. |>
|> While on the subject - you want dynamically balanced wheels - and cast |> iron at that. Wheel weight = inertia = more continuous cutting power |> when the teeth hit harder areas. Dynamically balanced wheels mean |> smoother cutting and that's a really good thing. | |What does dynamically balanced mean? Is it like balancing a a car tire? |(spin it and add/remove material until it's even?)
In my youth I ran an automotive machine shop where we did engine balancing so maybe I can explain. There are two types of out of balance forces that can be generated in rotating objects:
Stewart-Warner, the maker of my balancing equipment called these "force" and "couple", although they are commonly called "static" and "dynamic" respectively. Usually the term "dynamic balancing" is used to indicate that the balancing was done while the object was rotating, but depending on the object, this may or may not be anything more than hype.
To explain this, I will use a pair of wheels from a bandsaw. Let's say that our wheels are cast iron and 1" thick at the rim and hub. When the manufacturer machined the castings, he bored the hole in the hub slightly off-center and then machined the rim concentric with the hole. If we measure the runout at the rim with a dial indicator, everything looks fine; perfectly round and concentric. (From what I've seen of woodworking machinery, this is not a hypothetical)
First let's use just one wheel and assume that it has a set screw for locking it on the shaft We place the wheel on the middle of a perfectly ground shaft of say 2 feet long and lock it down. We then suspend this shaft horizontally on a set of totally frictionless bearings located at the ends of the shaft. Since the "meat" of the wheel is off-center, there is a spot on the wheel that is "heavier" than anywhere else and that spot causes the shaft to rotate until the heavy spot rests at the location closest to the center of the Earth. There is a "force" proportional to the mass and its distance from the center of the shaft that causes this rotation.
This is pretty intuitive and should be clear to all. We all should have a feel for what happens when we try to spin this shaft up. At low enough speed nothing much happens but as the rpm increases, this weight flying around starts trying to turn our perfect bearings into junk.
If we go back to our "static" case where the only rotation is due to the off-center mass we can, by trial and error, drill holes in the spokes or along the rim of the wheel until we remove the heavy spot so that when turned to any position and released, the wheel remains motionless. We have removed the force and the wheel is statically balanced. Alternatively, we could add an equal weight opposite the heavy spot and accomplish the same thing. (I used to use modeling clay to achieve balance and then weigh the clay and knowing the density of the metal, know how much to drill out.)
If we now bring this shaft/wheel assembly up to operating speed, it should run very smoothly, thus it is also "dynamically" balanced, although we didn't spin it up to achieve this. So what's the big deal about dynamically balanced bandsaw wheels you ask. In a word (or two), not much, other than it indicates that they *were* balanced.
Where is does matter can be explained by another example: Let's mount two wheels on our shaft and space them 12" apart. Let's assume that the manufacturer has implement process controls that have reduced variability to zero (six sigma). (We won't ask about the off-center hole bore) So, both wheels are identically flawed. We also assume that the wheels can be indexed with respect to each other anywhere we want them.
Unless we routinely win the Powerball, there will be some angular separation between the heavy spots other than 180 degrees. In any other case the shaft will rotate so that it stops with the heavy spots equally spaced about a downward pointing line bisecting the smaller included angle between them. We now have too little information to know exactly where the heavy spots are. All we know is that they are equally spaced with respect to the virtual "heavy spot" and they aren't 180 degrees apart. By trial and error, we can rotate one wheel with respect to the other until we position the two heavy spots 180 degrees apart, where they exactly counteract each other. Our assembly is now statically balanced. Are we done? No, let's see what happens when we spin it up.
Because the two heavy spots are separated 12" from each other along the length of the shaft, they try to "do their own thing." At any instant in time one mass is trying to move the end of the shaft in one direction while the other mass is trying to move the other end of the shaft in the opposite direction. Unrestrained, the shaft would wobble around the point midway between the wheels. So when our shaft is at rest, i.e. static, it is in balance but when it is rotating, the two forces "couple" to each other and the assembly is "dynamically" out of balance.
The only way to correct this is to spin it up and measure, and correct, the forces independently. Note that with a given amount of off center mass, the effect is worse the farther apart the two wheels are along the shaft. Conversely, if we slide the two wheels together, since they are relatively thin, the effect is negligible and our static balancing method is probably good enough.
Lest anyone think that the static method I describe isn't used, we had a couple of industrial strength crankshaft grinders that used grinding wheels 36" in diameter and two or more inches wide. The wheels had a center hole about eight inches in diameter and were mounted on a hub that captured the wheel between two flanges. Since the wheels were molded, the holes weren't terribly accurate and the wheel was never concentric when mounted. The hub contained a set of sliding weights and we did mount it to a shaft and put it on a set of bearings and tweaked the weights just as I described earlier.
When we figured it was close enough to not self-destruct (it happened once...you think a table saw kick back is something....) we would diamond dress it round and rebalance.
Since tire balancing was mentioned, if you're old enough to remember the old skinny tires, you might remember "bubble balancers." These balanced the tire/wheel assembly statically by suspending the assembly horizontally on a point and using a bubble level to see which direction the tire moved. Weights were added on the high side until the tire was level.
With today's wider tires (the wheels on my Camaro SS are 9" wide) it matters on which side of the wheel the balance weights are fixed, especially at 130 mph.
I know this doesn't have anything to do with woodworking but I don't know much about woodworking so I've gotta write about something else :-)
Whew.