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Ripping Planks: The movie

6/4/17       
cabmaker

I typed this response to Ian's post below about 'sheetgoods bending when ripping'.
I certainly would not ever want to hijack anybody else's thread so I thought I would proffer this in it's own post.

It is based on purely anecdotal science so could also just be the movie in my own head. Maybe Gene Wengert could weigh in this if he should so desire.

In the beginning.................Wood is actually a series of straws. The straws constitute arteries that connect each leaf with nutrients in the soil, These straws are arranged in a circle that emanates from the center of the tree. The tree undulates as it grows and each plank represents a different slice of that circular undulation of straws. (rift cut, plain slice, quartersawn etc)

During the lifecycle of the tree it sometimes has prosperous seasons and sometimes has to struggle for nutrients. Trees that grow in a young forest grow quicker because there is less competition for sunlight and moisture. Trees that grow after a forest fire get different juju out of the soil than trees that grow on managed plantation forests. As the forest canopy increases the growth rings get tighter (straws get smaller. (Imagine this as a symphony).

I am not sure about why this banana cut happens to sheets cut with a saw but not with a router. I suspect, however, that it might have something to do with heat.

Watch what happens when you hand feed a piece of lumber through a table saw. The lumber itself stalls ever so slightly while your hands are changing position during the rip. Sometimes the fibers of the board collapse in a way that binds the lumber onto the blade. When this happens you can sometimes get a burned edge on the side of the board. In extreme cases you can actually see smoke come off the board.

What is happening here is that the lumber is actually catching on fire. That is a LOT of heat!

Sometimes the tension release manifests in the opposite direction. Rather than pinching onto the blade the saw kerf gets grotesquely wider at the cut end as you are pushing through the blade.

The tension that causes boards to twist while they are being cut has sometimes to do with species or how the plank was cut. Sometimes it has to do with moisture intrinsic in the lumber. Sometimes it is the sharpness, toothset or cleanliness of the tablesaw blade.

About 20 years ago someone told us to try ripping lumber on a bandsaw rather than a tablesaw. They said we would get straighter boards out of the saw than if we ripped them on a table saw. This turned out to be true.

It used to be when we were building cabinet doors we would always allocate for the tall doors first. If we needed four 60 inch stiles we would start with at least eight pieces then pick the straightest ones and use the extras for smaller needs. As soon as we started doing initial ripping with a bandsaw, however, the need for extra planks was significantly reduced.

As it was explained to me, the cutting action of a bandsaw approaches the lumber fibers 90º opposite from that of a table saw. In this respect the cutting is more like how it is when you crosscut fibers with a miter gauge vs ripping with a fence. The cutting influence is not parallel with the fibers but opposite them There is no opportunity here for the wood fibers to seize on the blade and each tooth of the blade has a chance to cool down slightly as it rotates through the spin cycle.

The heat is easy to observe. What is less obvious is the influence of this heat as it is imparted to the woodfibers just at the location and point in time as the intrinsic tension of the plank is being modified by the blade.

Think for a second about what happens to lumber as it is being milled. If you take a perfectly straight & flat board and rip it down the middle one or both resulting planks is no longer straight. This has to do with the intrinsic tension in the board.

Try it for yourself: Rip 10 boards with a tablesaw and then rip 10 similar boards with a bandsaw and see how generally straight the sticks are.

6/4/17       #2: Ripping Planks: The movie ...
Adam

Thanks Cabmaker. This will be an interesting thread. I will definitely do a taste test.

There are a couple of major differences between a band saw and a circular saw. Band saw blade is narrow in width and very thin. The teeth have a large amount of set. That keeps the back of the blade from binding. Ripping band saw blades have few teeth and large gullets. You can stuff a piece of wood thru a good bandsaw very fast.

Circular saw blades have a wide profile when ripping(gullet visible above the piece of wood). The plate set ratio is close. Not much set. These factors contribute to binding. Requires a reeving knife to prevent kick back.

The kerf cut is wider with a circular saw which releases more tension instantly. Try using two 8 x 24 dado blades in a saw and rip a board. Wow it will move.

I'll try it.

6/4/17       #3: Ripping Planks: The movie ...
Larry

Virtually all of our ripping is done on a SL ripsaw at 99'/min. So unless the blade is terribly dull there is likely very little heating from the blade. We still get the wild curves on occasion. I therefore think it is something in the board. A leaning tree, uneven moisture content, poor kiln drying, the god of the forest?

6/4/17       #4: Ripping Planks: The movie ...
cabmaker

Larry,

You bring up a point about timing that might be germane.

I have often wondered if some of the wood movement had to do with introducing heat to the wood fibers right at the time the relative density is being modified.

Since your boards are moving so fast there is no time for heat to impart from blade to wood fiber. Are the boards coming off your rip saw as intrinsically straight as a similar board might be with a hand fed saw?

I know that I get intrinsically straighter sticks from our bandsaw than our table saw. Am just trying to figure out why.

6/4/17       #5: Ripping Planks: The movie ...
Joe Calhoon

Tim, I tried this a few years ago when you first mentioned it and believe it is true if you are hand ripping on a table saw. I think the issue that can cause tension in the material is the blade pinching and burning.

Using a riving knife on the table saw will help. And using the fence in the European method where the fence adjusts back to be at a 45 degree angle to blade front based on material width. The Unifence is the only US fence that I know of that adjusts like this.

We have a SLR and do not experience much burning with it. Two reasons - the fence is behind the blade and the material held firmly with the chain and rollers.

We do some ripping by hand on our sliding saw using a riving knife and the fence in a rearward position. This produces burn free cuts (mostly) and makes the blade life longer.

Ripping with a bandsaw is a safer method than table saw for sure. Most smaller - mid size shops in Europe rip with powerfed bandsaws. Especially in the UK.
The bigger shops usually have a Weinig type multirip. For some reason you don't see the single SLR saws so much over there.

6/5/17       #6: Ripping Planks: The movie ...
Larry

Cabmaker
"Since your boards are moving so fast there is no time for heat to impart from blade to wood fiber. Are the boards coming off your rip saw as intrinsically straight as a similar board might be with a hand fed saw?"
I have no way of telling. The only things ripped on the table saw or slider are short parts.

6/5/17       #7: Ripping Planks: The movie ...
David R Sochar Member

I have read in Hoadley and from Dr Gene that lumber shows internal stress - movement at the ripsaw - from only two causes.
One is that there is internal stress in the tree/log/lumber. There is nothing the woodworker can do but make shorts or kindling. But this is somewhat rare.
Two is internal stress induced by the drying process. This is far more common. Drying takes natural gas/money as well as some skills different from woodworking, so is vulnerable to cost cutting, etc.

I learned to do a case hardening test on lumber before unloading any semi-load when my employer bought cheaper loads. Several boards were tested. If they showed signs of case hardening, the load was sent back.

cabmaker - Have you tried a case hardening test on several boards?

I link a Woodweb discussion, but Purdue has a great explanation, in a pdf.

https://www.extension.purdue.edu/extmedia/FNR/FNR-132.pdf


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Woodweb Discussion

6/5/17       #8: Ripping Planks: The movie ...
cabmaker

David,

We are not having a problem with the lumber we are ripping. I was prompted to create this thread in response to a previous one about banana cut in sheetgoods. The general topic was wood movement concurrent with cutting operations so it seemed timely.

We don't have a straightline rip saw so break all of our lumber down with a bandsaw. Anecdotally it seems to result in intrinsically straighter planks so I offered some conjecture about why this seemed so. The movie in my head explained everything real simply but now I am not so sure.

6/6/17       #9: Ripping Planks: The movie ...
Kevin Jenness

I use a bandsaw for rough ripping lumber with a short fence. The narrow blade width relative to a circular saw reduces friction (heat) and binding, but I don't find any reduction in the incidence of crooking after ripping. Maybe there is some minor influence on bending from heating the kerf but I believe most comes from inherent tension in the wood due to growth factors and improper drying.

With sheet goods, I generally rough rip and dimension after straightening one edge, always with mdf and pb. Plywood is usually less of a problem, but when I take the shortcut of ripping direct to dimension the occasional piece goes bananas and the time saved goes out the window. Maybe if I got material direct from the factory this issue would not exist (?).


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