Keep That Bandsaw Blade Tight!

[rgsparber]

When a bandsaw blade is not tight, you can get a meandering cut. I knew that. What I had not stopped to consider was exactly what was going on here. The result is this theory which awaits the considered eye of experts. It might be true but it might also be pure fiction.

If you are interested, please see

https://rick.sparber.org/SlackBlade.pdf

Your comments are welcome. All of us are smarter than any one of us.


Thanks,

Rick

[Toolmaker51]

I agree. IIRC, 4 x 6's use 1/2" blades, economical but flexible. Because guides cannot run zero clearance and they are predetermined returning to periphery of the tilted [to cutting orientation] drive and tension wheel. Can't account why certain horizontal machines comply and others defy proper setup. We have 2 Kalamazoo's 40 years old, cut well regardless the variety of material fed.


Vertical bands cut without change being wrapped around their wheels. Discounting worn raker blades that mis-cut because one side dulls, still guides are culprit. I haven't seen any machine company prepare better setup gauges than DoAll, literally put metal cutting bandsaws on the map.
But if you want to hotrod one of many brands, Carter is the go to world wide.
https://www.carterproducts.com/band-saw-products

[tonyfoale]

Rick,

This is something that I know a little about. I have been trying to find the time to reply but I have been a bit busy.

Firstly let me talk about a bandsaw without twisted blades - i.e. a vertical machine.
There is a ton of info on the net about tensioning and tracking blades, most is wrong but a smaller proportion is sound. What follows will upset some people because nobody likes to be told that they have been doing it wrong. I would suggest that anyone with doubts should have a look at videos by Alex Snodgrass from Carter Products.

Specifically I am talking about tracking setup. Most advice is to set the middle of the blade over the largest diameter part of the crowned rim. This works but relies on the guides to keep the blade straight because there is no natural directional stability in the travel of the blade through the workpiece
A better way is to place the front part of the blade (just behind the teeth because the teeth area can support no tension) over the crown.

It is obvious that the part of the blade over the crown will have the greatest tension. So in the case of setting the middle as the tightest we can see that the front and rear sections will be relatively loose because those areas travel over the smaller diameter of the crown wheel.
On the otherhand if the leading edge of the blade is over the crown then all of the trailing part of the blade will be looser.

So what, you might ask. It is a question of directional stability of the blade travel in the workpiece. The setup with the tight section at the front with the rest waving about behind is akin to an aircraft with a tail fin, that keeps it pointing straight. I have prepared a very short video simulating a section of a bandsaw blade with the tight section (represented by a stick in the video) at the blade front, middle and rear which clearly shows the effect that I am talking about. Translate that to a bandsaw and you'll see that if the leading edge is the tightest then the blade will have directional stability and shouldn't need guide for straight cuts. The case with the centre of the blade being tight has no preferred direction of cut and relies on the guides to hold the blade straight. If it is the rear of the blade that is over the crown then the system is directionally unstable, that is the blade will have a strong tendency to veer off course and give the guides a hard job to hold it straight.

The video is in the following post from Jon.

On my own saw I only need the thrust guides to cut a dead straight line, I only have the side guides mounted toward the rear only for doing curves. The first two pix shows the blade position on the crowned wheel and the others show the side guides at the rear.

Click images for full size.



The preceding relies on the assumption that the blade is tensioned enough. If it is loose then, even with the tracking set as suggested, we lose that tight area about which the blade will twist and so we lose the well defined axis of twist. This can be checked simply by twisting the blade with your fingers at various tensions and also at the three tracking positions in the video.

So Rick, getting back to your observations. A horizontal saw has to have wide guides to maintain an accurate and well supported blade twist. However it is the length of the blade between the guides that is relavent to the cutting stability. For best performance you still need the area with greatest tension to be just behind (above) the teeth. It could be possible that the twist in the blade changes the tension distribution across the blade and so in any particular machine you may need to alter the tracking from simply setting the front over the crown. Only testing can determine that. Twisting the blade with fingers is a very good guide as to which part of the blade is tightest.

As mentioned above you lose a lot of the directional stability with a slack blade.

[Jon]

I have uploaded the video here, Bandsaw blade stabilitymaybe Jon can find a permanent home for it on this forum.

Here we go:

[Toolmaker51]

I think Tony is better than well suited, for all kinds of topics. For example, blade tracking.
There must be correlation in dissecting blade travel and as remote as it seems, being able to engineer successful road racing motorcycle frames. Steering head angles. Rake and trail. Wheel sizes. Wheel base. There are others, but those appear related to bandsaw adjustment.
It's impossible to describe how much minds like that contribute to our little community.