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Broken Bow Repair


troutabout
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Black and Decker #19104 1/8". Just a standard high speed bit. 2 speed Dremel on low speed. On the angled surface I did use a diamond ball to start the centers so the drill for the pilot hole had a direct surface to drill into.

thanks trout, next time use triaxable veranium, it will be available as soon as i invent it....its kinda a ceramic

nano polymer wood fiber :)

i hear what others are saying about carbon snap back, but really i think the glue joint/wood would give way long before we would find out if it would matter

a lot of this is based on the strength factor based on the length of the pin, a shorter aluminium pin such as your would require two sets of pliers to bend into an L, at that point one end of the pin or another would blow through the wood imo

the only advantage i can see to using carbon fiber is that it may snap and do less damage...

here again we are assuming we are breaking an already repaired bow in the same spot

and at that point i would assume the bow possessed by demons and would burn it at the stake anyway

thanks for sharing

you big carbon based silly you :)

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Yeh Jezz - like I said, it's splitting hairs between carbon being stronger/more resilient than aluminum. The weight factor is worth the difference as far as carbon rod is concerned but will this increase my carbon footprint or do I have to purchase carbon credits from Gore to feel green ? I'm feeling green enough after eating a whole bag of salted roasted peanuts watching NASCAR last night as it is. :)

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I am a bit worried that this repair creates a very weak points at the ends of the aluminium shaft. I believe that if it is going to break again in the same place it will happen in the end of the shaft and it doesn't meter if you use carbon, alluminium or... titaniumissium... Even the wood will still create a stronger area in the middle than in the area between the end of the shaft and wood of the bow.

I wonder if there exist an easy way to make the hole and the shaft that tapers towards the sides. I mean so that the thickest part of the shaft will be in the break area and will gradually zero at the ends.

Sorry for bad explanation, hope you understand what I mean...

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Roman,

I think that you're probably right about the weak points at the ends of the splice shaft, although the tube of wood left around the splice would be quite strong by itself. After all, look at arrow shafts. Most are very thin aluminum or carbon.

Also, look at the other end of the bow. Granted, it's thicker, but it's got a big long hole drilled right down the center, with nothing glued in it.

I think the key here is that we're getting a bow back into a player's hands (at a reasonable cost), that might otherwise be trash, or require a very expensive shaft/head splice.

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Well Zuppe, your logic is as flawed as another fellow's.

yes fellow, if we took the lead out of two pecils and in one we put carbfib' and the other an aluminium rod...the dovetailed the ends and glued them together......then at that junctionproceeded to bend the pencil into an L shape...i would not matter what the insert was, the wood would break anyway...

which also goes to prove, that it would not matter what the material is as long as it is light and strong...

This proves nothing. What is light? What is strong? Proof of what? In order for your statement to have any meaning, you need context. Helium is very light, especially compared to argon, but it is very heavy compared to hydrogen. This tells us nothing of substance, in much the same way.

Structual simpson aluminium nails intended for 835 clip applications{and others} have a tensil shear strenght of 10,000 lb's

This too tells us nothing. There is a universe of materials that have shear strength higher than wood. Does that mean all of these materials are suited as reinforcement for the original poster's repair? Glass has a similar shear strength to your hardened aluminium nails. Perhaps that would also make a good choice?

thanks trout, next time use triaxable veranium, it will be available as soon as i invent it....its kinda a ceramic nano polymer wood fiber

If you invent Triaxial Veranium, and it has the same or greater Modulus of Elasticity as carbon fiber, but with lower density (thus more closely approximating wood) then it will be superior to carbon fiber the same way a hardened aluminium nail might have been superior to silly putty before the advent of carbon fiber.

i don't think we need to concern ourselves with the strenght of the dowel, when it is clear that the hollowed out wood section would give way long before that kind of force was exerted...and the same would go for carbon fiber...

the only advantage i can see to using carbon fiber is that it may snap and do less damage...

Of course we have to concern ourselves with strength, and density too.

There are several advantages to carbon fiber, none of which has anything to do with how the dowel might fail. It cannot be overstated, how carbon fiber fails is irrelevant in this instance.

There is an obvious advantage to leaving the hollow supporting walls sufficiently thick, to prevent thier failure, and this can be best accomplished by using the material that can occupy the smaller space for approximately the same value of strengthening. Using carbon fiber will allow one to use dowels of smaller diameters, thus allowing the supporting walls to be thicker, thus reducing the likelihood of surrounding wood breakage, the same surrounding wood breakage that you have repeatedly drawn attention to.

The other main advantage of carbon fiber (yet again) is that it more closely approximates the density of wood, since the idea is too render the repair as close to the original state as possible, so we anxiously await the invention of Veranium. Until then, carbon fiber is better than aluminium, in all categories. The numbers cannot lie, or be disregarded.

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i think my understanding of the inherent issues are quite clear...

really i think when we are talking about properties that are "wood like" that the importance of them being wood like is that they may effect the sonic qualities...in a bow repair i'm not thinking that is all too important...

related to dowel thickness...i would assert that the smaller the shaft, the less likely a blow out would occur by forces being exerted on the wood by the dowel itself...however, based on my experience....a dowel sized the same thickness in carbon fiber as trouts pin would be too flexible and not stiff enough...much of this to me would have to do with where on the bow the break is...carbon fiber would be good for close to the frog breaks where a thicker dowel could be used, or perhaps at the very tip...

and here again like so many things in the violin world...its all speculation...

IF we had 50 repaired bows, half with carbon, and half with aluminium...and then proceeded to then re-break them...and then documented our observations in a controlled scientific manner...at that point only after conclusive documentation would i be willing to go out on a limb and definitively say one is better than another...

and no i don't buy into the "well, the hundreds of years that people have been doing this, they have gone before you and figured all these things out, so don't bother, just do what we say and what traditionaly is done"...

no i'd rather see the 50 bow's repaired and broken...not just their performance after being repaired....re-broken

carbon fiber's availability and use in violin repair does not have a long history...

and while i would not debate its viability or even superiority, i would debate that metal rods are not viable material for repair...

i would say there are and have been WAY more bows that have been repaired with metal dowels than carbon...

time is on my side related to that....

i would not say that future repairs are not going to be better if everyone uses carbon...

but it does not negate all the successful repairs that have been done in the past with metal pins

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Just as an update (and I promise we can let this thread die....) The bow performed flawlessly during two and a half hours of practice with my church band this afternoon. Not once did I remember or think about it's being broken.

FiddleDoug has his thumb on the pulse of the procedure. It's more about getting on with getting on....

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Roman,

I think that you're probably right about the weak points at the ends of the splice shaft, although the tube of wood left around the splice would be quite strong by itself. After all, look at arrow shafts. Most are very thin aluminum or carbon.

Also, look at the other end of the bow. Granted, it's thicker, but it's got a big long hole drilled right down the center, with nothing glued in it.

I think the key here is that we're getting a bow back into a player's hands (at a reasonable cost), that might otherwise be trash, or require a very expensive shaft/head splice.

Doug,

Yes, you right that tube can be a very strong construction. And I am not trying to say that this repair will not hold. However I can say that I seen similar kind of repair that failed in the end of the tenon.

As far as i understand, usually breaking point is where there is a sudden change in the thickness. All the stress in this case goes to one thin point and that is why it breaks there. The hollowness part in the frog side of the bow is weaker part too (and it often breaks there too). It is quite thicker though (as you said) and also stress in the frog area distributed on a much longer area.

The other thing is the direction of the stress (when bow in tension). At the frog most of the stress is in the direction of the head and very little sideways (or downwards if you can say so...). In the middle of the stick almost all the stress goes sideways. Tubes are very weak when the stress goes sideways.

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While one could respond technically to your latest observations Zuppe, there is no overly compelling reason to do so, and thus one can further along the OP’s desire for a peaceful thread death.

However, I offer one last observation about the potential for breaking the fragile tube structure that results from drilling holes in the bow (regardless of dowel type).

A tight wrap of silk thread around the point of break, solidified with CA, will go a long way in reinforcement. It might not look great, but it will greatly reduce the chance of new cracks propagating in this region.

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<<<<< "Tubes are very weak when the stress goes sideways" >>>>>>

What tube ? THERE IS NO TUBE. You're completely ignoring the fact that the hollow 'tube' is now filled with aluminum/carbon shaft and epoxy ! I also challenge the idea that there is a weak point at the end of the aluminum/carbon shaft. Weaker than what ? The day it was made ? With the reinforcement locked in behind it - that area will be stronger if anything.

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The other thing is the direction of the stress (when bow in tension). At the frog most of the stress is in the direction of the head and very little sideways (or downwards if you can say so...). In the middle of the stick almost all the stress goes sideways. Tubes are very weak when the stress goes sideways.

There are huge side-loads on the bow screw. I've broken several custom screws with material properties calculated to take this load, on strong cello bows. On the stick, a high percentage of the wood may be in compression (reducing when approaching the frog and tip).

Tubes can be very strong in side-load. Material properties and wall thickness come into play. Theoretically, the material at the very center of a cylinder contributes almost nothing to bending stiffness.

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Regarding setting time: Epoxy sets by chemical reaction, and all chemical reactions procede faster at higher temperatures. So you can make it set faster by warming the parts under a light bulb. Or you can slow it down to gain more working time by refrigerating the epoxy and the parts.

Brad, while the first part of your post is correct, the second part could lead to some problems. It is true that the setting of epoxy can be slowed down by cooling, however the strength of the bond can be greatly diminished by letting the temperature drop too low during the curing process. Once the temperature drops the bonding of the epoxy is reduced and will never achieve full strength. The process is an exothermic one and the generation and maintaining of the heat is necessary to get the strongest bond.

Terry

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Brad, while the first part of your post is correct, the second part could lead to some problems. It is true that the setting of epoxy can be slowed down by cooling, however the strength of the bond can be greatly diminished by letting the temperature drop too low during the curing process. Once the temperature drops the bonding of the epoxy is reduced and will never achieve full strength. The process is an exothermic one and the generation and maintaining of the heat is necessary to get the strongest bond.

Terry

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Brad, while the first part of your post is correct, the second part could lead to some problems. It is true that the setting of epoxy can be slowed down by cooling, however the strength of the bond can be greatly diminished by letting the temperature drop too low during the curing process. Once the temperature drops the bonding of the epoxy is reduced and will never achieve full strength. The process is an exothermic one and the generation and maintaining of the heat is necessary to get the strongest bond.

Terry

I agree and its particularly important with some of the difficult to use restoration epoxies like Hxtal,which are excellent for making almost invisible crack repairs in bows.

Also if you want to drill holes which dont `wander` ,i made a few of these which were basically designed for boring muskets .Silver steel is available in any size and the drills are relatively easy to make.bore drill or D drill

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What tube ? THERE IS NO TUBE. You're completely ignoring the fact that the hollow 'tube' is now filled with aluminum/carbon shaft and epoxy !

I was replying to Doug on his argument about strength of "the tube" of the original wood left at the ends of the shaft...

I also challenge the idea that there is a weak point at the end of the aluminum/carbon shaft. Weaker than what ? The day it was made ? With the reinforcement locked in behind it - that area will be stronger if anything.

I am trying to say that if we could find an easy method of creating a conical shaft that will be zeroed at the ends it will give a stronger repair that will avoid a weak point. I think epoxy joint will not be as strong at the connection with cross grain part at the end of the shaft which can create a breaking point. By weaker I mean weaker for sideway bending compared to areas around this place.

There are huge side-loads on the bow screw. I've broken several custom screws with material properties calculated to take this load, on strong cello bows. On the stick, a high percentage of the wood may be in compression (reducing when approaching the frog and tip).

David, you right, I was not expressing myself properly. There are quite loads of side-loads on the bow screw. On the stick, the bottom part of the stick is in compression as you said, but as far as I understand, the top part of the wood has quite lot of sideway load.

Tubes can be very strong in side-load. Material properties and wall thickness come into play. Theoretically, the material at the very center of a cylinder contributes almost nothing to bending stiffness.

That is absolutely true and I have to admit that I was not explaining myself clearly here as well. I agree that when the material is intact - tube is as strong (or almost as strong) as the same thickness hollowless stick. However when shooting archery I seen that if carbon arrow gets any smallest crack - than you should expect that it will be broken very fast. You can even still shoot it a few times, but it breaks very easily if you try to bend it sideways. Personally I wouldn't really trust on wood consistency in the stick when doing such a repair and my gut feeling says that we can expect microcracks going a bit further in the stick from the place were it was broken.

Anyways, I just wanted to suggest a way to avoid a possible weakness...

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Brad, while the first part of your post is correct, the second part could lead to some problems. It is true that the setting of epoxy can be slowed down by cooling, however the strength of the bond can be greatly diminished by letting the temperature drop too low during the curing process. Once the temperature drops the bonding of the epoxy is reduced and will never achieve full strength. The process is an exothermic one and the generation and maintaining of the heat is necessary to get the strongest bond.

Terry

Thanks for the correction. Arnold Bone used 12-hour epoxy for bow repairs, but he got it to set in about half an hour by heating the glued joint. He said that speeding up the reaction increased the epoxy's strength.

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