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Can this broken bow be repaired?


IVSymphony
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I used to think that a broken bow was just destined for the trash pile; however, a number of years ago a student borrowed the symphony's very old Otto Durrschmidt cello bow, jammed it in her cloth bag case, THEN shoved her cello in the bag. The bow stick snapped at the frog area into three sections, plus a little sliver. I though it was hopeless, but I took it to the late Raymond Wise of San Diego. I thought about cutting it off at the end of the eye-screw channel, re-drilling and re-channeling it to turn it into a 3/4 bow, but Raymond said he would try to fix it for just about $250. Unfortunately, the budget just didn't have $250 at the time, so I just gave it to him. HE FIXED It! (perhaps using an epoxy glue). One of the orchestra members paid him for the repair and took the bow home. All three pieces, plus the sliver, were glued so well, that you cannot even see that there was ever a break (unless you look at it when the light is shining on it just in a certain way). The bow is as strong as it ever was. So I know broken bows can be fixed.

I just don't know if THIS bow (below) can be fixed.

I received this bow from China last week. Actually a beautiful looking bow; however, upon tightening it and starting to apply the rosin, it immdiately snapped at the tip, making me think there had been a crack or some flaw in the wood. If they don't want me to ship it back to China, might there be a way to fix this so it still might be somewhat useful as a bow? I don't know if it can be drilled, pinned and glued; drilled, pegged and glued; glued; crazy glued; gorilla glued; resin-epoxy glued; or just forget about it and salvage the gorgeous frog. After checking out the photo, let me know what you think.

BrokenBow.jpg

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This is a place bows occasionally break, and repairing it is a pretty standard proceedure. I glue the pieces back together, saw a slot across the glue joint through the center of the head of the bow and glue a re-inforcing spline in the slot. The grain of the spline runs at 90 degrees to the grain of the bow, which essentially turns the head of the bow into plywood making it stronger than it was when it was new. The weight, balance and playing characteristics of the bow are unchanged. I've fixed several dozen bows broken like this.

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If you are not as dexterously skilled as Brad, you can glue the head to the stick with epoxy (2 component) and after that is secure a layer or two of transparent (white) fiberglass tape over the stick covering the head and another layer or two on the sides of the head and around the front of the tip.

This is then coated with epoxy which secures the bow quite well and adds almost nothing to the weight.

The original color of the wood will show through, although the grain will appear distorted.

The right kind of epoxy and the fiberglass tape are sold by may hobby/model shops.

I've done it with a good Pfretzchner bow, but would not on anything better - therefore, it's what I would do with a cheap Chinese bow if they would not replace it. The repair will cost less than one-way shipping.

Andy

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As the others have said - no problem.

My approach is...

i) clean away all the little whiskers from the broken faces

ii) glue the head back to the stick with 2 part epoxy (instant glue will also do)

iii) drill a 1.5 mm hole parallel to the leading edge of the head and about 3mm back from it

iv) take a 2mm carbon fibre rod and sand it down to a slip-fit

v) sand a flat along the length of the rod (allows the air/epoxy to escape from the hole)

vi) fill the hole with epoxy (coat the drill with epoxy and "undrill" the hole)

vii) spin the carbon fibre dowel into the hole until it bottoms

viii) after the epoxy has cured trim the "outstand" and polish everything.

In this particular case - because the break is so high I might be tempted to drill all the way through.

I have two "beheaded" bows coming in some time next week for exactly this repair - I'll record the procedure and post pics.

Never cared for the spline approach - traditional - yes, but smacks of poor understanding of where the stresses that cause the failure are lurking.

The fibreglas that Andy refers to would be called "surfacing tissue" - I've used it to repair a just-behind-the head splitting stick. That imminent failure was due to poor grain direction.

cheers edi

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I use Brad's method with good success. I use super glue for my initial gluing of the head and gorilla glue for gluing the spline in. If you use gorilla glue make sure you keep and eye on it and wipe off the excess because it expands 3x and you don't want it to harden on the outside of the joint or you will have a hard time cleaning it off.

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I had a broken bow once and used West Systems Epoxy, which is used

to hold boats together in salt water.  It is a two part epoxy

that is totally waterproof.  I am only mentioning that it is

waterproof as I understand that the epoxy actually penetrates into

the wood pores, thus creating an extremely strong bond.  I use

this glue to hold hammered dulcimer frames together and I have

never had an instrument fail once.

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Hello Oded - epoxy is pretty tolerant about the "exactitude" of the mix.

When repairing sailplanes I use a beam balance to make sure that I have decanted the correct proportions of the components, but for a small amount I just eyeball it - or count droplets.

Any excess left over (and I'm feel tempted to challenge anybody to actually succeed in mixing less than required) I use to glue two pieces of scrap together. An informal test piece.

cheers edi

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"Never cared for the spline approach - traditional - yes, but smacks of poor understanding of where the stresses that cause the failure are lurking."

Edi,

When I try to analyze these stresses mentally, I come up with two models: 1. There is a neutral axis where there is no stress at, or somewhere near, the center of the glue joint, similar to when a beam is subjected to a bending load. On the frog side of the neutral axis the joint is under compression, and on the far side it is under tension. Or 2. The glue joint acts as if there is a hinge at the frog end of the joint, putting the entire joint under tension.

I'm not sure which of these models is correct. What do you think?

Whichever of my models is correct, at least half of the joint is under tension. Considering the tensile strength of pernambuco along the grain, I believe that the traditional spline re-inforcement is adequate, assuming that the glue holding the spline in place doesn't fail.

Now that I think about it a little more, maybe neither of my models is entirely correct. The joint might also be subject to considerable shear, because the direction of the hair tension is parallel to the joint, and the point of hair attachment is pretty close to the joint. Shear at the joint would mean that the spline is subject to both shear and tension.

I'm sure your carbon fiber rod method would do the job also. I recall you describing it previously on this forum. It sounds interesting, and I would like to see pictures of it.

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Good structural analysis, Brad. You're correct,a shear force, equal to hair tension, is there.

For preventing failure in the repaired bow, the spline method is a better fix IMO. Ignoring the strength of either,the support that a spline or rod contributes to this end depends on the shear strength of the original wood and the glueing area. The rod method, while apparently sufficient, is 3mm distant from the point of maximum stress, and affords 19 sq mm of glueing area above the break, plus a small possible mechanical advantage. The spline method allows nearly 3 times more area and extends to the point of maximum stress. I do not understand the purpose of Edi's "drilling all the way thouugh" suggestion.

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"How do you measure out the resin and hardener, do you have a metered pump, or do you measure by volume or weight??? "

Oded,

Volume to volume is the usual way that is understood from the directions, but it is not always stated so. You can use weight to weight but you have to know the density of the material to use the correct weight. Most manufacturers will give you this information. Some epoxy glues are tolerant of mixing errors but others are very critical in the proportions mixed. Too much or too little of one component or the other can lead to improper curing and weaker bonds. Temperature is important too and with some glues the reaction needs higher temperature to effect a good cure and bond. If the glue cures at too low a temperature then a weaker bond, more susceptible to failure and creep will result.

Terry

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