Bandsaw formulas

[nikolas britton]

Hi,

I made this for my benefit and thought it would be useful to others... 236906

236907

Does anyone else have formulas or rules of thumb that would help take out the trial and error from creating these machines? speeds and feeds for example? torque or power sizing? blade selection?

[Erik Loza]

Out of curiosity, what do you hope to accomplish with a formula like this? Are you intending to manufacture a bandsaw or are you shopping for one and want to know what size or specs you would need?

Thanks,

Erik Loza
Minimax USA

[Jerry Thompson]

Anything other than adding, subtracting, dividing and multipling is past me.

[Van Huskey]

I have a host of mental rules of thumb for bandsaws but they all center around picking and evaluating commercially built bandsaws. If you are going to build a saw then Woodengears would be a place to start but I don't think Mathias worked from formulas more of a trial and error.

For HP/Torque I don't think much about torque with a bandsaw, since most of the time you are dealing with a 4 pole motor with a synchronus 1800 rpm speed torque will follow HP (or more correctly visa-versa) so you will have equal torque in motors with the same NEMA HP. If you are building a large BS with very heavy wheels a motor with increased starting torque might be useful but it will still have the same torque at synchronus speed. My rule of thumb for HP is 1hp per 4" of resaw gives a BS all the power it needs for resawing dense woods at hand fed speeds, if you want faster power feeder speeds a lot more thought has to go into blade/hp and speed selction.

Speed of the band is often limited by wheel size but I like 5000-6000 SFPM for a woodcutting saw though if you are building a saw for a single purpose this could vary by a good bit. In a perfect world if I was building a BS I would like very large wheels and variable speed from about 4,000 SFPM to 8,000 or so. But for a hand fed machine 5,500 SFPM would be a good compromise.

I can give more of my gut level opinions if I know more about a specific build project but they are just anecdotal based on my experience with NO science from me, just gut.

[Myk Rian]

My rule of thumb is using a string, measure the distance around the wheels with the tension slack, and at high point.
Someplace in the middle is what you want.

Another way is measure the diameter of ONE wheel, and add 2xB.
There is no need to deviate from the KISS principle.

[Van Huskey]

Myk, I think he is interested in a lot more than the length of the band.

[Myk Rian]

Well then, I certainly don't know what he's after.
Tape measure and yard stick give me everything I need to know.

[Erik Loza]

I am likewise puzzled. Not a criticism at all to the TS, but I can 100% guarantee you that no bandsaw manufacturer uses a formula to design their machines. The design is very arbitrary, in fact. I'm just interested to hear whether the TS is using this as an exercise in math (and there is nothing wrong with that..) or if the TS has a specific goal in mind for an actual machinery operation. I guess we will have to wait to hear....

Erik Loza
Minimax USA

[Phil Thien]

How about the fundamental frequency of oscillation of a misaligned saw?

Let me start with this: When wheels are not aligned in the same plane, the wheels impart a twist to the blade. Hopefully it is obvious that when you apply uneven force to a band (towards the front of the band on the top wheel and towards the back of the band on the bottom wheel, for instance), a twist will be imparted to the blade as it revolves around the two wheels.

So as the blade leaves the top wheel and travels to the bottom wheel, it twists slightly. Similarly, when it rounds the bottom wheel and travels back to the top wheel, it twists back.

The amount of twist will be dictated by the amount of offset between the the two wheels, and where the blade rides on those wheels.

The typical 14" saw seems to have a 93-1/2" blade (105" with riser), moving at approx. 3200-FPM.

Given the length and speed of the blade, we can determine that a 93-1/2" blade is making 6.84 revolutions around the wheels each second. On a 105" blade, that would come to about 6.10 revolutions per second.

That is, ((3200 * 12) / 60) / 93.5 (in the instance of a 93.5" blade).

Another way to look at this is 6.84-CPS (Cycles Per Second) and 6.10-CPS. Or even better, 6.84-Hz and 6.10-Hz, respectively.

So I'd say the fundamental frequency of the oscillation imparted by wheels that are not coplanar is controlled by the length and speed of the blade, and the amplitude would be controlled by the offset between the upper and lower wheels (the greater the offset, the greater the twist imparted to the blade).

So it would seem to me that the bandsaw can be treated as a sort of inefficient infrasonic transducer.

Now, you'd think the twisting, happening on both sides (spine and guide) of the wheels would cancel each other out. That is where the fun begins, because the bandsaw frame is asymmetric. The blade may contact wood, guide blocks (or bearings) on one side. And the guide side is substantially more flexible than the spine side. So whatever is not cancelled, is likely amplified due to the arrangement of the frame design.

WHY IN THE WORLD am I pondering this stuff? Some guy at woodnet (another know-it-all engineer) called me out when I used the term "basic physics" when arguing that bandsaw wheels benefit from being sort of closely aligned. I never said they should be perfect. "Close enough is good enough," I said. But this engineer claimed I was using scare tactics by using the term "basic physics."

So I explained the importance of coplanar wheels was first mentioned to me by a retired (but now gone) engineer that holds patents in flywheel power storage systems. I intended to ponder the actual physics and post back. But when I went back to post, the thread was gone.

So this has been rattling around in my head since then. Glad to get it down in writing. :)

[Erik Loza]

Phil, I would just make sure to clarify that you are discussing bandsaws with crowned tires, not ones with flat tires, which negates all of that.

Best,

Erik Loza
Minimax USA

[Phil Thien]

Phil, I would just make sure to clarify that you are discussing bandsaws with crowned tires, not ones with flat tires, which negates all of that.

Best,

Erik Loza
Minimax USA

It shouldn't actually negate it. In both cases (crowned and flat), the wheels are typically close enough that the amplitude is negligible. It is only in cases where the misalignment becomes significant that the vibration becomes problematic.

[Van Huskey]

Just so we don't completly obliterate the OPs question, I assume he is considering building a bandsaw. Something I have thought about many times (particularly a horizontal resaw) and a number of people have built them some pretty good, but I have yet to see one that can handle "real" resawing.

To expand on my thoughts the best thing to do is crib from the established designs. The basic designs of BS has been around for over 100 years and the engineers today probably spend 90% of their time reducing materials and streamlining production over worrying about optimum blade speed, bandsaws are built to a price point with a specific level of performance in mind, at least for bandsaws you would find in our shops, gang rip/resaws in the flooring industry are likely a whole different story.

[Stephen Cherry]

Out of curiosity, what do you hope to accomplish with a formula like this? Are you intending to manufacture a bandsaw or are you shopping for one and want to know what size or specs you would need?

Thanks,

Erik Loza
Minimax USA

Minimax

If you're promoting a commercial enterprise, aren't you supposed to pay the few extra bucks before you comment on peoples posts?

In any case, this looks like a nice way to come up with blade lengths if you don't have good math skills to figure this out for yourself, that is unless you have a flat tape measure around.

[John Coloccia]

re: coplanar

If you have flat wheels, it's not critical that they are coplanar. They should form two planes that are parallel to each other. That does make it somewhat easier to get the two wheels aligned properly, and unless you're way off the tracking on the top wheel is sufficient to do that.

[Rick Fisher]

Its really cool having people who work for big tool companies on the site.

When I restored my old SCM bandsaw .. I slowed the feet per minute down to about 4300 by changing the motor sheave. It doesn't have co-planar wheels.

[Phil Thien]

re: coplanar

If you have flat wheels, it's not critical that they are coplanar. They should form two planes that are parallel to each other. That does make it somewhat easier to get the two wheels aligned properly, and unless you're way off the tracking on the top wheel is sufficient to do that.

Again, I'll reiterate that close enough is good enough. You quickly reach the point of diminishing returns.

But if alignment was altogether unimportant, then manufacturers would spend absolutely no time getting the wheels in the same neighborhood. You could shift one wheel two to three inches from the other, and expect no consequences.

But the reality is, trying that would result in a saw that would not keep a blade mounted.

I performed an experiment on my Inca (with flat wheels) once. I had to glue the tires down, which required removing the wheels. The top wheel was fixed, by design. The bottom wheel was on a keywayed shaft and could be located anywhere on that shaft by virtue of a set screw.

When I replaced the bottom wheel, I mounted it such that the blade's back was parallel to the table. But the saw ran awfully rough. I figured I had gotten the tires out of round with my glue. So I trued the tires.

Still pretty rough. So I moved the wheel back on the shaft a bit at a time until the saw ran smooth. With each shift, I knew the blade was more and more out of parallel to the table. But a smooth running saw was important.

When I was done, I discovered that the table was out of square to the frame, and I needed to add shims (duh!). Once I did this, I had a bandsaw with somewhat aligned wheels, and a blade back that was square to the table.

If one had a hypothetical bandsaw where the bottom wheel could be shifted in/out by 2" or 3" in relation to the top wheel, we could perform an experiment thusly:

Shift the bottom wheel out by 2". Now try to mount and run a blade. The saw will likely not keep the blade mounted.

Now shifting the wheel back towards alignment with the top wheel by maybe .25" at a time, we'd find at some point that we'd have some success being able to keep a blade on the saw, under power. But at this point, the saw would run very rough.

As we continued in our test, shifting the bottom wheel another .25" at a time, we'd find the saw would begin to run more smoothly. We'd reach a point (diminishing returns) where adjustments probably would have little impact on how the saw ran. But we might find other benefits. For example, we might find that a saw aligned to tension/track a larger blade needs little, if any, adjustment when you mount a narrower blade.

This is an exaggerated example of what I went through with my Inca. The Inca was the perfect test bed, as those 8" wheels are very sensitive to misalignment.

But there is a payoff with other, larger saws with flat tires. Here is a page from someone w/ a Minimax that also found he was able to adjust the saw so minimal adjustment was needed when changing blades:

http://home.comcast.net/~jaswensen/machines/bandsaw/bandsaw.html

I will finally add that again, "close enough is good enough." My current saw has crowned wheels, and I have no problems changing blades w/o having to adjust tracking. Just set the tension, and go. I've never checked the wheels for how close they are.

But I'm betting they are real close.