Why is there bandsaw blade drift?

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I know it occurs, but I realized last week I don't understand why. Assume the weld is straight. Then the circumference of the back of the blade will equal the circumference of the front (toothed) side. If the blade is centered on the tires, why on earth does it curve, causing drift?
    -- Andy Barss
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Wide bandsaws (ie 3in and above) are tensioned to take into accound the difference between the circumference of the back and the toothed edge due to heating as the blade cuts. This seems to be hardly possible for narrower bands.
Stretch a wide elastic band between finger and thumb and apply pressure to an edge. The band will twist sideways, one way or the other.
I suspect that the only means of reliably getting a straight cut is a very sharp blade that applies minimal pressure to the edge of the band.
Jeff G
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Jeff Gorman, West Yorkshire, UK
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:> I know it occurs, but I realized last week I don't :> understand why. Assume the weld is straight. Then the :> circumference of the back of the blade will equal the :> circumference of the front (toothed) side. If the :> blade is centered on the tires, why on earth :> does it curve, causing drift?
: Wide bandsaws (ie 3in and above) are tensioned to take into accound the : difference between the circumference of the back and the toothed edge due to : heating as the blade cuts. This seems to be hardly possible for narrower : bands.
Every discussion I've seen for correcting blade drift starts with a cold blade, and cuts a few inches into a board. Surely that doesn't produce enough heat to greatly expand the leading edge?
: Stretch a wide elastic band between finger and thumb and apply pressure to : an edge. The band will twist sideways, one way or the other.
Sure, but rubber bands stretch when you apply pressure, much more than steel does, I would think.
    -- Andy Barss
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Andrew Barss wrote:

I think that is done with the intention of getting a bead on the drift in the blade for the purpose of adjusting the fence to match it. I could be wrong, but I don't believe it is done to address distortion caused by a heat gradient in the blade.
er
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: Andrew Barss wrote: :> :> Every discussion I've seen for correcting blade drift starts with a cold :> blade, and cuts a few inches into a board. Surely that doesn't :> produce enough heat to greatly expand the leading edge?
: I think that is done with the intention of getting a bead on the drift : in the blade for the purpose of adjusting the fence to match it. I : could be wrong, but I don't believe it is done to address distortion : caused by a heat gradient in the blade.
Right. But it assumes that there is no distortion due to heat, which is what I was trying to say, counter to Jeff's suggestion.
    -- Andy Barss
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Andrew Barss wrote:

It could be a lot of things. Are your guides tight, and were they adjusted after the blade was set to track properly (i.e., withdrawn from the blade while adjusting tension and tracking)?
If the side guides have too much play, the blade will move around in the space between them. If the rear guide is too far forward or back, pressure against the front of the blade as you move the workpiece in will cause it to bow (or against the back of the blade if its riding hard on the rear guide), and the "outside the bow" edge of the blade will twist or wobble.
What you paid for it is what it's worth, and I made it all up. :)
er
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I use a lot of softwood, like pine and fir, and the dust tends to build up between tyre and blade, probably causing most of the drift on my bandsaw.
There is a brush, but the resin from the wood is rather resistent to this, and if I do not scrape it off with a knife once in a while, I get a lot of vibration and noise.
BjarteR
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There was a good program on DIY Woodworking about this subject. David Marks explained that the set on every bandsaw blade has different variations. The saw has to be tested and the fence adjusted to the true line of the blade cut every time a new blade is installed. Once the proper line/angle is established the blade will cut true. Bugs
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Bugs wrote:

It also has to do with the blades position on the tires. If you look at the tire edge on you'll see it's convex with a crown in the center. If the blade is running towards the back of the tire it's going to make the blade point to the right, because its on the back edge of this curve, whereas if it runs on the front of the tire the blade will run to the left. You can leave the fence parallel to the mitre slot if you use this to your advantage and simply use the upper tire adjustement set scew to counter any drift instead of moving the fence.
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Bjarte Runderheim wrote:

Maybe for the resinous softwoods you could swap in a brush with stiffer bristles? I've been toying with the idea of installing a brush in mine. I thought a finger nail cleaning brush was the right size, but too stiff--but it might be just right for green/soft wood. That and a shoe brush cut to the same size for normal stock...
er
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Besides the other answers, a dull blade will cause drift. Apparently one side of the blade cuts more aggressively than the other. Improper tension, guides improperly set, and the blade does stretch some when tensioned. Unless it stretches perfectly across its width the front and back may no longer be coplanar.
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Wrong size (width) of blade for the thickness of material being cut? Too fast of feed?
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This is all horsesit. Use a sharp blade and keep theside irons in LIGT contact, not too much pressure from the back bearing and it will be fine. go slower.
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Well I think the band saw blade and saw manufacturers seem to maybe have a better grasp that a dull blade will cause a blade to drift.
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On Thu, 20 Apr 2006 13:51:06 GMT, "Leon"

I must be missing something. I'll confess that I'm a novice user of bandsaws, so excuse my ignorance.
I have a 16" Jet and have been using POS BC Saw blades. (That could stand for "plain-old-steel" or the more common meaning.)
These blades are welded so poorly that the teeth near the weld are proud of the balance of the teeth on the band, plus they seem to have welded in a twist. So, when cutting I get a thunk thunk when the proud teeth take a bigger bite -and- my side guides are retracted enough that there is just a tick tick when the twist passes through them. The lower guides are backed completely off.
Lately I have been building a playhouse for my granddaughter and since it will have to be moved 800 miles, a lot of it is bolted together. For this and other reasons, I have for the most part completely resized the framing lumber to get it of uniform size and straight. In some cases, I've remachined a bunch of old, dry Douglas fir two-bys left over from the framing of my neighbor's house.
Needless to say, I didn't want to run this stuff directly through my jointer, planer and tablesaw before cleaning off the dirt, so I have taken skin cuts on a lot of this stuff with the bandsaw. This has not been kind to the blades and I've used up a couple of 3/4 inchers.
Despite the crappy, dull blades, that sometimes require considerable force to get a stick through, I can still saw off a nice uniform (albeit, rough) 16th inch slice from the face of a 2x8 without -any- blade drift. I set the fence parallel to the miter-gauge slot when I set up the saw and it's never changed.
What am I doing wrong?
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wrote:>

Snip
Is that a wide blade perhaps? MiniMax claims that you do not need to use guides at all if the blade is properly tensioned and is wide enough.
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On Thu, 20 Apr 2006 22:28:42 GMT, "Leon"

From my comments:
"This has not been kind to the blades and I've used up a couple of 3/4 inchers."
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Even a blind hog ....
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Andrew Barss wrote:

It's usually caused by the way the blade is sharpened and the teeth set. The geometry for the teeth on one side of the blade doesn't exactly match that of the teeth on the other side of the blade.
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Jack Novak
Buffalo, NY - USA
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Ding Ding Ding Finallly the right answer!
Dave
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