Grooving core before sinewing?

What is convention among horn bowyers – should the core groove before sinewing? Or is just light roughening with sand paper enough?

@bownarra said in Primitive Archer -forum, that " Collagen glues work best on smooth surfaces. Sand the back flat with 60 grit intially then go down through the grits and use a very sharp scraper as the last prep. This roughening the back is something that has transferred over from hornbows butthey are not ‘roughened up’ they have grooves scored into the back. These grooves aren’t to help adhesion they are to help stop the core breaking - which isn’t an issue with wood bows." (Bold is mine.)

So, I don’t get it. How is it possible that grooves are helping to stop the core breaking? I can not get any kind of physical reason in my mind.

And, what is the “correct” way to prepare the core for sinewing? In view of glue adhesion and/or core strength?

I just scratch the surface mildly. But in general i don’t stress about it too much. Just clean the surface of dust and tiny shaving hanging on. I don’t think I’ve ever had sinew lift off, even though the profiles haven’t been exactly the most mild.
I wouldn’t create too big grooves, because these can easily create air pockets in my opinion.
Its pretty hard to make glue surfaces with maple to break. For example if horn delaminates, its always the horn adhesion that is the weakest link. When gluing clean, fresh maple, its easy life. :smiley:

I think: Grooves made with a scraper with ideally 60 degree angles give a clean surface, which is benefitiall. But the area of the surface increases and by that you get better adhesion between the layers. How that should stop core breaking i am not sure, mayby if you think of a core break due to delamination of the sinew layer.

It is the same principle as modern composite materials. Individual fibers can break but the epoxy stops the fibers next to the broken ones breaking also.
The glue fills the grooves during sizing so no chance of air pockets. The cured glues stiffness is somewhere inbetween the wood stiffness and the cured sinew layer, thus giving a nice ‘transistion’ of material properties.

So that is proven method?

But, the glue in itself should not be a filler. It is with every glue that the glue layer should be as thin as possible. Also, stiffness is not only parameter, cohesive strength is also important.

I think (just think…) that it does not really matters. As @JNystrom said, wood-sinew bond is always strong enough. Also, how many times core has been the problem or broke instead of core-horn delamination, for example?

All glues are not the same.
The stiffness of the cured collagen glue does matter in a composite. How about the traditional method of mismatched grooves between horn and core?
What do you mean by cohesive strength?
The bond strength is only there if the steps are carried out correctly. Sure you can lay sinew down on a non-grooved core and you may be fine. However I prefer to follow the methods of traditional Turkish construction as laid down in Adams book. I feel I don’t know any good enough reasons to deviate away from them. Until someone can prove there is a benefit to any partiicular way of doing things I will stick with the old methods. They were perfected over many years by many people with signifcant consequences if they got things wrong :slight_smile: That is good enough for me!

1 Like

Ok this and the horn grooving get a bit mixed, but only the very best bows were grooved with laser straight grooves. Mongolian bows for example had many times really poor grooving. Also the v-joints have a long tradition of being made poorly. Still these bows function.
Its hard to say what is “traditional”. Also, Adam has some things wrongly in his book. At the time of writing he had not made a concave/convex glueline bows. At least it seemed like it.

In my opinion the true skill is to know where to spend the most attention to. And what really doesn’t matter.

Peter Dekker published pictures of disassembled chinesse bow some time ago and here is the bamboo core from the sinew side ( please, read the comments too ) :

I suppose that clean grooves - with well defined - sharp “peaks” of ridges - on compromised back of the core ( which is not composed from the undamaged summer part of growth ring - like in selfbows ) create something like " wwwwww " - type splice ( instead of plain " ---------- " type ) between damaged growth rings ( or end “grains” of wood fibers ) when filled with glue and effectively stops the propagation of the cracks ( which could start on the tips of ridges ), like bownarra wrote . IMHO It would be better if these grooves could be under 0,3 mm wide ( deeper is better ) for best “glue line” properties.

Tuomo, you have an unique chance now, to make simple test and find an answer personally, if you still have a surplus from your core wood . Maybe 2 pieces of ( 20cm x 1cm x 1 cm ) would be enough - both backs shaped like the wood in the core ( with damaged fibers ) in central part , but one grooved , glue applied to both and then bent to certain radiuses, to achieve required strains. ( But maybe samples of “wood glued on horn” have to be tested ? )

Kind Regards to all (excuse me if my message is not understandable)!

Welcome Jano! Is this your original nickname, or do we maybe know you from past conversations?

Oh now i get it you mean some kind of “backing” for core wood. The glueline itself would act as a backing underneath the sinew, interesting… However i don’t really think its something we should be worried about.
The picture Peter has there is weird, since i have taken apart my otherwise faulty hornbows and none of those show any of such crosswise cracks. If such would happen i think the wood was faulty to begin with. Maybe its more than important to gather your own wood. Buying kiln dried maple might work, but it can end up in situations bownarra earlier in atarn/pa showed. The core was covered in thousands of small cracks. Like totally tore apart. With my own gathered Finnish maple i have never seen any of that happening.

Jere, yes, it is my old nickname ( from previous versions of this forum ).

Maybe you are right about the kind of “backing”, but I will try to explain my thoughts better :slight_smile: .
The glue which is soaked into / between ends of fibers holds them together ( even if the depth of soaking is minimal ) and most critical is the border between two growth rings created by porous spring wood ( which is visible even in maple wood and probably could soak the glue deeper ). After grooving the back of the core, the line of this border will be much longer ( visible as oblique lines on the slopes of the ridges ), creating “V”-s when looking from the top on the grooved surface. Hence the glued “surface” will be much greater than on flat surface. Even the stresses from bending in the “valeys” are smaller than on the peaks of ridges, which is another advantage.

I am not worrrying about it, because I would like to use undamaged / ungrooved growth ring for the core-s back, when I shall begin horn bow manufacture. Were your failed hornbows drawn to the max (strained to what degree ), before disassembling ?

Some rules for harvesting suitable wood personally ( what species of maple is your Finnish maple )?

Thanks for your contributions!

Ah ok, a veteran!

I really don’t see a reason for creating more glue surface for maple. We make grooving for horn, but for wood its pretty useless. Or have you had a situation the glue up fails from the wood side?
My wood cores are not on one growth ring either.

My failed bows were in the very beginning because of twist. Broke during tillering and such. Then when switching to non matching grooves, i had a lot of horn glueline failures before i understood. Then i had one core i broke because of reeeeally small amount of sinew. Like 1,5mm covering and big reflex.
Since those lessons, i’ve made about 10 functioning bows drawn to really proper drawlengths, like 112cm bow to 30" at 60#. Mongol 105# @ 28" and only 115cm. Tatar 110# 32" (130cm), turkish bows mostly 50-80# @ 28", but one is 110-115# @ 28". Turkish bows are 108-115cm long. This heavy one is 108cm.
When you learn the proper way, its all clear afterwards. :smiley:

Jere, I only tried to reply on original Tuomo-s question and presented my unverified thoughts ( because I am veteran in theory only ). I hope this winter I will do some tests.

If your mentioned bows are functional, then your wood is good and could cope with the applied stresses and do not need grooving ( althought I would not be surprised if the maple backs would be full of visible “microcracks” after their disassembling :slight_smile: - which probably do not influence the function leastly ).

But how we can be sure beforehand, that next used wood will be good enough again ? I simulated some turkish type heavy warbows in VirtualBow program and the relative strains of core-s backs were up to 2,3 %, when selfbows are designed to not overcome around 1 % strain and breaks quite often :slight_smile: . I also tested several samples of maple wood, which had very different propperties, from very flexible to quite brittle ( both harvested personally ), hence I hesitate to give any general advices.

I am not sure what you are asking by " Or have you had a situation the glue up fails from the wood side? ", but in general, good ( applied ) hide glue is stronger in shear than wood, hence splices fails in wood when tortured to break.

Tuomo, thank you for repairing my contributions. I hope I will learn " the rules" soon.

Great! Someone other is also using VirtualBow!! I have used it and made a lot of development ideas for further versions. I have made a few simulation models for composite bows and the most recent model gives only about 1,8 % strain for 150# bow. Of course, there are a lot of variables and designs are different. If possible, I would like to see your model…

But, the main question is that how core can withstand such a stress? Maybe it has something to do with wood-glue transition layer? On the other hand, as Jere said, wood-sinew joint is almost never a problem. So, we shouldn’t worry about it.

Tuomo, I have fought with it as it was and hoped for more agreable version :slight_smile: . I liked to simulate most strained/thickest ones, mentioned probably in “? Turk Okculugu ?” ( but I have only the picture with the thicknesses of layers ). I did not take into account the prestressing of wood and horn layers by reflexing the limbs throughout the sinewing operation, nor changes in the Young moduli at higher levels of strains. Here is the link :

There is also one .xls file with the ( thickness ) summary tables and maximal relative strains in this folder ( if you will be interrested I will translate the headings into english language ).

It seems that cores are still stressed under their breaking strength somehow ?
Yes, I suppose too, that wood-sinew joint is not a problem, because the weakest member is the ( cohesion of ) sinew layer, when overcoming the reflex at first time bending the bow to the tepeliks.

You have nice, very precisely created models @Jano! Although VirtualBow is not perfect, it is nice tools to see “what if”, thus the effect of different parameters. For example, the thicker sinew layer is or the more stiffer the sinew/glue matrix is, the less strained is core’s back side.

By the way, what is your source for elastic modulus for dry sinew? For example Kooi (1994, 16) gives very low value, only 9e8 Pa. You have used value 2,41e9 Pa. There are a lot of variation, depending on the source.

Yes, I am interested in your Excel-table, so please translate it.

Thank you for appreciation. I am planning to compare practical results with this software simulations and SuperTiller values too ( one day :slight_smile: ), because actual ash war/longbow simulation ended with too thick end parts of bending arms against real bow ( maybe only my mistake ). I tried to keep ( until possible ) the sinew layer thickness under 3 mm as in historical bows.

I took an average values of elastic moduli for the layers from the different sources including my own tests ( made back in 2011 ). For sinew : I tested thin deer lateral extensor tendon in elongation and also gelatine/sinew laminate ( around 5 mm thick ) in three point bending test to determine modulus of elasticity. In pure elongation I did two sequences of loads - first with “fresh” dry tendon, then second after long lasted loading which caused permanent elongation / set in tendon, because I read in S. Selby-s book : Chinesse Archery, that the sinews were stretched to the max before application ( if I remember correctly ). Also the sinew bowstrings have to be stretched before use. I was also surprised that sinew laminate could withstand compression in bending strains of almost 6 % ( without complaints ) and maybe more, but I did not continue to higher values. Of course, I am afraiding, that these tests were not made on the " scientific level of precision " :slight_smile: .

I already loaded translated .xls file.

1 Like