Investigating bandsaw vibrations
After having tested my big bandsaw with big stuff like logs, I switched it back to a 1/4" blade and used it for this video. Cutting small stuff with it, the bandsaw's vibrations became more noticeable. So I figured I should try to improve on that.
Wheels not round or off center?A common source of vibrations on commercial bandsaws is that the wheels are not quite round, or at least slightly off center. This is often caused by tires that weren't evenly stretched when they were put on.
This causes blade tension to vary as the wheels turn, and that in turn causes the frame to slightly flex back and forth, introducing shaking. A good way to check for this is to tighten the blade, then, while slowly turning the wheels, keep plucking the blade to see if the pitch changes from changes in tension.
On my saw, I didn't notice any significant change in pitch, so it wasn't that.
For a test, I put some box board shims behind part of the tire of the saw. The change in pitch was just noticeable. The box board was 0.3 mm (0.0012") thick. Realistically, this amount of variation is high, but might still be within reason. It also depends on the size of the saw, for a 9" bandsaw, 0.3 mm would be far too much.
Motor is shaking?On my first bandsaw (this one, the only one I ever bought), the motor mount was not very rigid. The V-belt caused the motor to shake, and that shook the whole saw. I jammed a piece of wood between the motor and the stand to reduce vibrations on that one.
On this one, the motor is still not very rigidly mounted, because I will probably still change motors on it. I noticed the motor was moving a tiny amount, so jammed some pieces of wood against it to stop it from moving. But that didn't make a noticeable difference, so motor shaking wasn't the cause either.
Wheels unbalanced?I had previously balanced the wheels, but not as precisely as I could have. I balanced the wheels on their own bearings, and they have some resistance to turning.
So I put some roller skate bearings on a shaft and put the wheels onto those. The small roller skate bearings (22 mm diameter) roll inside the holes of the main bearings (25.4 mm diameter), so the wheels could turn more easily.
I found the wheels were a bit unbalanced, and screwed in some screws to add weight on the light side. But overall, it was just a few grams worth of weight that I added. And this again only made a minor difference in vibrations. So whatever amount of unbalance was left, that probably didn't account for the shaking.
Too light and resonant frequency?My next experiment was to put a 10 lb (4.5 kg) barbell on top of the saw. This had quite a noticeable effect on vibrations. It's possible that a saw itself has a resonant frequency near the speed of the wheels, and adding a weight can change that. When I turned the saw off, I noticed the vibrations briefly getting much larger as it slowed down, no doubt hitting some resonant frequency.
Stand is too wobbly?Up to this point, I had deliberately put the bandsaw on an uneven spot on the floor, so it was effectively only on three of four castor wheels. Moving it around to a spot that was flat so it wouldn't rock back and forth reduced vibrations, but not entirely. But then adding some weight to the saw again made it vibrate much less.
I only had the saw on a dolly made with office chair casters (really wimpy, but
those office chair casters are designed to take my weight, and the saw doesn't way much more
than I do). My next experiment was to put pieces of wood under the stand so it rested more rigidly
against the floor. This made a dramatic difference, but for the worse!
With the more rigid support, the saw's resonant frequency of rocking back and forth
became higher, much closer to the speed of the wheels. So that was a bad idea!
So I put this saw back on the wimpy office chair casters. And testing it, I was able to balance a faceted 5 cent coin on the table (though only after starting the saw), so maybe it's good enough.
I may at some point add a hook on the back to securely hold the 10 lb weight. But for now I figured this is good enough.
Testing without a bladeAfter I filmed the video, I had the idea of testing it without any blade on it. The vibrations were the same. So that rules out the top wheel and non-roundness of the wheels as possible sources of vibrations. That leaves the lower wheel unbalanced, motor, or V-belt as possible sources.
But I had already improved the balance of the lower wheel by quite a lot, without making much of a difference, so it probably wasn't that, and reducing the motor shaking didn't help either. But putting my fingers on the V-belt while turning it slowly, I noticed it ran quite unevenly, even though the pulleys didn't have much in the way of "bumps" on them (the large pulley is one I made, so I checked that). At any rate, running the saw without a blade helps narrow down the source of vibrations, and I should have tested that earlier.
I suspect the V-beltV-belts are not ideal, and on my 14" bandsaw, I experimented with using an old timing belt as a flat belt instead, but that was more trouble than it was worth, and I later changed it out for a V-belt. My 16" bandsaw uses a V-belt, and it has very little vibrations. What helps with that one is that it uses a slower motor (1750 RPM), and a larger pulley, so the V-belt doesn't develop as much of a "kink" where it's bent around a small pulley while the saw is not running.
My thinking is, if you have a choice between a 1750 RPM motor or a 3500 RPM motor, pick the slower motor and use a larger pulley. Your saw will be quieter, less vibrations, and the V-belt will be more efficient and last longer from not having to bend as tightly.
Physics of bandsaw re-
sawing (Why blades drift)
More about bandsaws