Odd resaw experience with jatoba

[Bob Jones 5443]

I'm new to bandsawing.

Last week I ran a carefully foursquare-milled jatoba board, 7/8" thick by 2.5" wide, a foot long, through my little 10" band saw that could (Rikon 10-306). I wanted to resaw the stick into two parts about 3/8" thick by 2.5" wide.

I put on my nearly unused 1/2" 3 tpi Timber Wolf blade, tensioned it, made the wheels coplanar, and centered it on the wheel tires with the teeth very slightly back from each tire's front edge. That's as far back as a 1/2" blade will go on this machine. With narrower blades I center the teeth on the hump of the tire. (You guys with 18" and 24" saws may now take a moment to sigh.)

I set my fence and, after trying a test board about the same dimensions, I pushed the jatoba gently, steadily, and slowly through the cut. Then something unexpected (for me) happened. Three times during the cut the timber lurched ahead, cutting between a half-inch and a full inch of the resaw in one fast, slightly jagged chomp. As the board moved forward it also wavered slightly, giving a slightly wavy cut line in those places. It was as if there was a void in the wood.

Jatoba is supposed to be about the same hardness as teak. There are certainly no voids in the piece I was working on.

After the first little jump forward, I stopped in my tracks and tried to make sense of what had just happened. Then I resumed the cut. In the end I was able to guide maybe 90% of the 12-inch cut nice and straight with no turbo jumps. The three little surface anomalies were minor enough that they planed out fully and still yielded full 3/8" thickness.

But what happened? I can understand how a blade can drift, and that could be what happened, I suppose. But why would the wood jerk forward and cut an inch quickly? Is there a large amount of differential tension in this wood or something?

[Robert Engel]

I’ve had this happen too. The only thing I can think of is the gullets clearing themselves.

[Brad Shipton]

Jatoba is much harder than teak. The janka # for teak is around 1070 and it is 2700 for jatoba. I am not sure of the cause. I have resawn a bunch of cumaru, and it is difficult.

[Lee Schierer]

I've had this type of thing happen when stringey parts of the grain hang up on the throat plate for a while and then let go. I feels like there are hard and soft places in the piece being cut.

[Andrew Hughes]

It’s the rear bearings or what ever your saw has not contacting the back of the blade evenly.
I aslo get long fibers stacking up behind the blade sometimes. But I think that’s mostly due to the blade not being vertical when it’s cutting.

[Mike Henderson]

I've noticed that happen occasionally (jumps in the cut). I never could figure out why.

Mike

[Dick Strauss]

I think you are seeing the blade flex slightly and roughly align with the softer wood between the yearly growth rings. It will follow this path until there is enough grain misalignment combined with the blade tension to help it cut straight and vertical again.

I see this happen when resawing turning blanks where the wood cuts quickly if it is aligned vertically with the blade.

[Bob Jones 5443]

Dick, that seems plausible. I'll hold with that theory for now, and see if pushing much more gently and slowly will remove the problem. I have a 3" x 3" bloodwood blank to resaw soon. Maybe not soon, but soon enough.

To everyone, once again I find myself in the comforting glow of those who care about this hobby/craft of ours. Thank you all for your ideas.

Off-topic and by the way, I love how this wood steps up to a hand plane. Replacing jointer and planer marks with mirror finish was effortless. Then there's the dense, crisp surface of end grain after a crosscut on the table saw. I wasn't even tempted to put it on the shooting board. For someone who has largely worked with poplar in my formative years, and cherry more recently, jatoba is a revelation. Now I think I can appreciate Derek's jarrah.

[Erik Loza]

I had this happen numerous times during my bandsaw days. Almost always tropical hardwoods. These would be carbide blades on a big Italian saw, so it was definitely not a machine or blade-thing. It felt like you hit a void in the wood, even though there wasn’t one. I always referred to it as “the blade taking a dive”, for lack of a better description. What I learned was, watch the feed rate and don’t be pressing the stick so hard you lurch forward when it happens. Strange phenomenon.

Erik

[Andrew Hughes]

I don’t think it’s that strange at all. I can get my saw to make that wavy self feeding cut by simply backing off the rear thrust bearing the upper one seems to make the most difference. If the blade touches the bottom one first and the blade isn’t getting any support from the top one. The blade gets forced into a shape that I’m guessing changes it pitch.
I don’t have a scientific proof or videos back up my experience.
I guess I stand alone on this one.

[Bob Jones 5443]

Food for thought, Andrew.

[Mick Simon]

I don’t think it’s that strange at all. I can get my saw to make that wavy self feeding cut by simply backing off the rear thrust bearing the upper one seems to make the most difference. If the blade touches the bottom one first and the blade isn’t getting any support from the top one. The blade gets forced into a shape that I’m guessing changes it pitch.
I don’t have a scientific proof or videos back up my experience.
I guess I stand alone on this one.

That sounds very likely to me. I'll take that into account next time I'm setting up for resawing. It's pretty common, in my experience. Almost like a barking sound on some bandsaws.

[peter gagliardi]

Under the right circumstances, with the right blade, right feed speed, and enough positive hook in the teeth, the blade will "self-feed". This is a fairly common phenomenon. Keep your hands in a safe position for when this happens, and take advantage of the situation.

[Steve Rozmiarek]

I don’t think it’s that strange at all. I can get my saw to make that wavy self feeding cut by simply backing off the rear thrust bearing the upper one seems to make the most difference. If the blade touches the bottom one first and the blade isn’t getting any support from the top one. The blade gets forced into a shape that I’m guessing changes it pitch.
I don’t have a scientific proof or videos back up my experience.
I guess I stand alone on this one.

Yes, sounds right to me!

[Tom Bender]

This is good to hear, sort of. I get this ripping Oak with a 3/16" blade. Don't know why it doesn't snap.

Self feeding is a good term if that's what's happening but if so, why does it stop and not eat the whole piece?

Maybe the piece is getting airborne with vibration and the lack of friction with the table is letting it feed fast.

Great post, and not done yet.