On Fri, 23 Jan 2004 09:10:52 -0500, "Stephen M"
wrote:

|
|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.