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Cane Prep - Pith

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I have just confirmed a long held theory I have held for years to my satisfaction.  There is a question as to whether it will satisfy others, but I really don't care much.  As some of you have heard, I have been dinking around with a beveler.  I still am, but I have had some trouble with the holddowns.  Yesterday I ran a strip through checking it out, and the holddowns slipped of the strip and I routed most of the pith away for about 12" to 14".  Ruined strip.  As I took it in hand to break  it up so throw away in my trash barrel, I noticed that the portion of the strip with no pith (only power fibers about 1/8') broke cleanly with no effort.  The portion of the strip with the pith still present broke with difficulty and presented that sprayed brush effect of power fibers, and did not break in half at all, but was held together by the pith. I have long suspected that the integrity of hollow built and double-built rods was suspect.  To my mind this little experiment proved that the strength of the rod without the pith is materially reduced.  God put that pith there for a purpose, to support and act as a binder for the power fibers.  Removing it is in my mind a NO NO.  I also begin to think that the selection of cane with a proper proportion of power fibers to pith might be much more important than we have hitherto suspected.  In cane selection I have heard and read again and again, get an adequate depth of power fibers.  Should we not be thinking get a proper proportion????  (Ralph Moon)

You're sorta mixing apples and oranges.  You're talking about a single strip, with power fibers on one side, and pith on the other.  In that case, the single strip may be stronger with the pith than without, because because the pith is contributing to the overall area of the cross section of the strip.  A tube of a given material will always be stiffer than a solid rod of the same material.  When you bent the strip where the pith was, the pith offered resistance to the bending, whereas the area of the strip where the pith had been milled away didn't have the pith, so was a thinner cross section, easier to bend past the breaking point, and easier to break.  When you look at beam strength, the opposing sides of the beam, aka power fibers, reinforce the other side, and offer resistance to bending, and don't allow the "flange" to bend as far, so there is less chance that you will exceed the elastic point of the web.  When you make the composite structure of a hollow rod, you basically have a tube of bamboo, with no web necessary to keep the flanges of the beam apart, depending on how you hollow build the rod section.  The web of a beam contributes no real strength to the beam, but it does contribute to the distance the flanges are held apart.  As an experiment, if you feel the pith imparts a bunch of strength to a rod, build a section of rod that is nothing but pith.  Do a stress test on the pith section, then compare it to a comparable section in area of bamboo that is power fiber only in a hollow built configuration.  Which section do you think will  be able to survive the test better?  The pith on bamboo is really nothing much more than filler, with not much apparent strength.  (Mark Wendt)

We're talking theory, only, at this point, but I would echo Mark's observations.  The principles and instances he cites from other engineering applications seem very compelling, indeed.  But someone far more knowledgeable that I would need to run tests to see whether or not Mark's case apply to a bamboo rod.  I think when a hollow-built rod fails, it's because of inappropriate specs. and not because there's no pith.  Meantime, I am entirely satisfied that a properly constructed, hollow-built rod is absolutely the cat's meow (a scientific term).  (Bill Harms)

First I reject your assertion that I don't know my fruit.  I just didn't hold it up for you to see.

There is no question that if we are looking at 1/8" strips of power fibers Vs 1/8" of pith which is the stronger.  I don't have to run a stress test.  But, what I did not make clear is the manner in which the unsupported power fibers broke as opposed to the way that pith supported power fibers broke.  My fault in not being more clear.

I am not going to ask that you do any type of scientific analysis in comparing the bending of a tube and the bending of a similar solid structure.  You know and I know that the tube collapses it approaches the limit of the bend.  When it collapses, it (as you will find in composite rods) virtually explodes.  Rephrase that.  loses all integrity.  Not so with Bamboo.  a break in a bent bamboo rod is rarely if ever a clean break.  Instead it is the rupturing of the outer fibers from expansion.

Finally I disagree most emphatically  with your last statement.  "The pith on bamboo is really nothing much more than filler, with not much apparent strength. "

How could you?  Take a look at the end of a bamboo culm .  Do you really see just a filler?  The pith is the medium in which and on which the power fibers depend.  Can you imagine a culm without pith?  I think that Don Philips  in his book The Technology of Flyrods males a lot of these points with more authority than I.  Vince Marinaro has something to say as well  "The weak pith prevents rupture. All wood products begin to break on the concave or compression side of a bent piece. The collapse of fibers forms a hard wedge that pushes up against the convex side until the limits of tension are reached and the fibers in tension are rent asunder. The soft pith on the compression side cannot form a hard wedge to cause such a rupture. That is how the pith contributes, in a negative way, to bamboo's enormous resistance to fracture."

Still think I have a point, but willing to listen to you et. al.  (Ralph Moon)

All I was saying, in testing with a single strip of bamboo, you are not comparing the stresses that a composite section of a bamboo rod to the stresses that a single strip sees.  It is easier to bend a single strip of bamboo, with no pith on it, past the breaking point, since the cross sectional area of that strip is far less than a strip with the pith still attached.  But, and this is a big but, the strip has not been assembled into the final product, whether its a hollow built rod section, or a rod section with the pith remaining.

Not all tubes explode.  Bend a piece of copper tubing past it's elasticity point and it will just crumple.  It all depends on the material used to make up the tube.

You have to do some type of scientific analysis to determine how much, if any, strength pith adds to the structure of a bamboo fly rod.  How else can you determine whether or not it does.  Just looking at it doing something, without repetitive testing, and some form of true objective analysis is not going to decisively determine that pith adds strength to the structure.  Would you depend on the next bridge designer to come up with a bridge span with no scientific testing as to the statics and strengths of the materials and their construction to build that bridge span?  What I talked about in my original post is just plain old statics and strengths of materials.  Would I ever build a rod made completely out of pith?  Not hardly.  Would I ever build a rod that can't  support the beam structure.  No.  I would however, and have, built a rod that does make use of the power fibers that aren't as compressed as the ones on or near the surface to form the webs necessary for beam support.  As Bill mentioned, the failures of hollow built rods were more than likely because the specs they were built to weren't enough to handle the stresses of what they were doing when they broke, IE: they exceeded the design limitations. because the wall of the tube was too thin.

If you look at a typical beam, they are designed to have the minimal amount of web necessary to support the flanges, which flanges do all the work in compression and tension.  Take a peek at the way they are designing floor joists today.  They use a piece of particle board/MDF as the web structure with 2 by X's taking the load and stress.  The web really does nothing more  than that,  in fact  the web  at times,  if it's over-designed can interfere with the beam doing it's job.  In most cases, less is better.  I'm not saying the pith is worthless, but just as in beam construction, the pith doesn't do quite as much as you think it does.  The majority of the work that a beam does is towards the outer parts of the beam; they are the parts of the beam that see the loads of compression and tension.  The closer you get to the center of the beam, the more the web just becomes dead weight.

The soft pith is not going to stop a hard wedge of power fibers from developing when the limits of tension are reached.  The pith material is much softer than the power fibers, and in this case, may cause the rod section to rupture out to the sides of the bend, due to the compressive forces acting on the material, and the material having no where to go.  (Mark Wendt)

Newbee here,  but I'll to disagree on the pith as filler statement.  I find it hard to believe that any part of any plant does not serve a very specific and needed function.  In the case of bamboo pith, doesn't it serve as "sap wood", allowing moisture to be taken up by the cane?  However, if that is the case, it would have to be more porous.  (Lee Orr)

But then again, you are talking about the function of a portion of the plant whilst said plant is still alive.

Going back to the pith in the rod section, if you look at most rod sections, how much pith is really left on the individual spline after the final plane.  To tell the truth, I don't see a whole lot there.  I do see "power" fibers, for the lack of a better term, packed together much more loosely in the lignin than I do closer to the surface.  On our tip sections, it's all power fiber.  So the pith doesn't contribute a thing there  (Mark Wendt)

I am still working on your last statement re: beams.  I will respond later.  However in your reply to Lee you point out that tips are  almost all power fibers.  I don't disagree, but ask what usually breaks first on a fly rod?  (Ralph Moon)

Quite possibly the tips are the most broken, but how are they broken?  Most times, slammed in a car door or trunk, or a screen door, or as some enterprising individuals have done, in ceiling fans.  I think it's possible that more flexure breaks happen on rods in and around the ferrules.  Usually, the tips don't see as much of the flexure that approaches limits of the material as much as the more middle sections of the rod.  And even here, on most trout rods, I don't really see much, if any pith in the composite.  I've haven't broken a rod in over thirty years of fishing, whether they be 'boo, glass, or graphite, so I'm not truly qualified to say where most rods break...   ;-)   Course, now that I've said that, I've probably jinxed myself.   (Mark Wendt)

I had a 1920 3 piece Edwards break just above the mid male ferrule, while fishing. Exactly the same thing with a Leonard 7" 3 piece rod. In each case there was a node right at the ferrule.  (Sean McSharry)

I agree with everything you said except about a tube Vs solid rod of same material.  If the tube and the solid rod of same materials are also the same diameter, then the solid rod has to be stiffer by virtue of the greater Section Modulus.  It doesn't take much of an increase in diameter, though, to give the tube an equal section modulus since the Section modulus is proportional to the diameter to the 3rd power.

For example:

Consider a tube 0.375 OD x 0.187 ID and a solid rod
         0.375 OD
The section modulus for the tube is 0.00485
The section modulus for the solid rod is 0.00538

A solid rod with the same section modulus as the tube would have an OD of 0.367  Only 0.008 difference  (Al Baldauski)

You are correct.  I should have said a rod of a given cross sectional area, and a tube of the same cross sectional area, not a tube of the same diameter as the rod.  My fingers were working faster than my brain at that point...  (Mark Wendt)

If you examine how the rod actually bends during a cast, then take a freeze frame at the point the rod is at its maximum bend, you can then determine at which location the stress is highest.  The area of the rod that bends the most, and is subject to the highest stress, is the midpoint of the rod (give or take 6 inches).  (Kyle Druey)

Not to get totally off the topic at hand, but wouldn't this imply that a 3 piece rod would be "better", since you've moved any ferrules/discontinuities away from the point of maximum dynamic stress?  Would it not also pay to stagger nodes such that they are as far as practical from the midpoint of the rod for the same reason?  Seems logical, but what do I know?  (Todd Enders)

I have split all strips out, and planed the pith side where the nodes are. Should I continue planing off all the pith from the strip until I am close to the power fibers or will this cause problems down the road. Somewhere I thought I read to do this and also square up the edges of the strips so they fit better in the roughing form.  (Dennis VanHoose)

I plane the pith side until it is flat, removing the dam, at the node.  After that I begin planing to get the 60 degree angles followed by running them through my beveler.  I do not plane to the fibers as the pith will be removed as a result of the beveler and final planing.  (David Gerich)

Your good advice reminds me of an general 'rule of thumb' for efficiency I got from Tony (S.), "Don't waste time and energy working on something that's going to wind up on the floor."   As long as I keep remembering this it keeps saving me time and Bengay.  :^)  (Darrol Groth)

I thickness my strips to about .020 over the size of the bigger end of each section with a drum sander and a pin fence. This way I follow the node bump  so the strip is the same thickness from end to end, then I heat and press in the vise, following the node bump allows it to straighten and the nodes heat faster because I've gotten rid of the excess material.  (John Channer)