Bow repair question


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I have been re-hairing bows for quite a few years now, and consider that my re-hairing skills are at a competitively high level (which took some years to accomplish), but, I have always shied away from re-cambering decent old bows because of the problems that can occur with them.

Well, I have never really been very confident with my ability to do this - though I have done it before with really bad off student bows, and have seen it being done by someone else- who used an alcohol flame - an alcohol lamp.

I send customers who need a re-camber to Albuquerque for this service, because they do that there at one of the Bigger shops - I know the guy there that does it, and am comfortable referring customers to him. I've watched him do it - and he's really good at it...

How many people here who do bow work, also re-camber old bows, and how often do you find yourself having to refinish the bow because of damage the heat can do to the surface finish?

What heat source do you use?

And, if you don't ever have problems re-cambering - do you ever have occasional surface scorching or blistering ?

Do you re-camber "by eye" or do you use a jig?

I have a box of old practice (junker) bows and I will remedy this lack - I hope to get some hints here.

Thanks.

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I have been re-hairing bows for quite a few years now, and consider that my re-hairing skills are at a competitively high level (which took some years to accomplish), but, I have always shied away from re-cambering decent old bows because of the problems that can occur with them.

Well, I have never really been very confident with my ability to do this - though I have done it before with really bad off student bows, and have seen it being done by someone else- who used an alcohol flame - an alcohol lamp.

I send customers who need a re-camber to Albuquerque for this service, because they do that there at one of the Bigger shops - I know the guy there that does it, and am comfortable referring customers to him. I've watched him do it - and he's really good at it...

How many people here who do bow work, also re-camber old bows, and how often do you find yourself having to refinish the bow because of damage the heat can do to the surface finish?

What heat source do you use?

And, if you don't ever have problems re-cambering - do you ever have occasional surface scorching or blistering ?

Do you re-camber "by eye" or do you use a jig?

I have a box of old practice (junker) bows and I will remedy this lack - I hope to get some hints here.

Thanks.

Ive corrected cambering on lots of old bows , i tend to use an electric hotplate with temp control to do it .Ive never felt comfortable with an alcohol lamp. Most good old bows are quite resistant to blistering or discolouring.If they do its generally because the heat is too hot.You only need around 80- 110 celcius to camber most bows.Occasionally the finish if there is one will go slightly rough or dull but its easily brought back with very fine pumice/rottenstone with a litte renaissance wax . Ive even recambered a bow whose head was 45 degrees to the normal position . It held fine for several years before twisting ever so slightly again,but the grain was not too good which is why it twisted in the first place.Ive always cambered by eye,not all bows have the same type of camber .Many old French bows for instance have the low point more towards the middle of the stick.

Forgot to say ,practice on alot of junk bows first to get a feel for the right temperature.

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I do a lot of bows each week (approx 200 per week) yes, 200 per week. Cambering is not difficult. You work slowly and may have to accomplish it in several sessions. Cambering should be done on good quality pernambuco bows. On those low end Chinese bows or overly stiff brazilwood bows it is probably not cost effective.

You need to be very careful about controlling the heating of the stick as it is very easily scorched. I use an alcohol lamp and keep the bow constantly moving with the bow about 1 inch above the flame. The constant motion keeps the heat from building too quickly. The bow is gripped with 2 hands one at the frog end and one two thirds down toward the tip. As I am heating I check the warmth of the stick by touching the stick to my chin. It tells me how hot the stick is without letting go of the grip. When the bow is moderately well heated I then apply pressure with both hands to restore the curvature of the stick whilst observing that the stick is not warping. It takes a little practice but it is a very effective way of doing it.

Many years ago the late Malcolm Sadler told me how William Retford cambered bows. He heated them over an alcohol lamp , then he did a little push and a twist and bam!, he was done. He was an absolute master.

If you are working on oil rubbed finish bows you need not worry about damaging the finish as that type of finish is quite durable. If the bow is finished with a spirit varnsih which is mostly sandarach, you shouldn't have too much trouble with carefully controlled heat. If it is a bow of varnished white wood,do what my wife does, use it to help guide seedling tomato plants.

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Part of my business to service bows from music firms (rental providers, music stores, school districts) throughout the US. It generates a lot of volume, thats why I have to work 7 days a week.

++++++++++++++++

May I ask you a question? (sorry off topic)

When I try to tighten my bow all the way, the bow is only half way extend (in short,

the hair of the bow has been lengthened) so I try to shorten it by doing something on the tip where

there is a wood plug. Take the plug out to look at. But it is glued in. Which it seems strange to me. Is it normal?

i.e. the way it is supposed to be ? )

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ctviolin said:
How many people here who do bow work, also re-camber old bows, and how often do you find yourself having to refinish the bow because of damage the heat can do to the surface finish?

What heat source do you use?
And, if you don't ever have problems re-cambering - do you ever have occasional surface scorching or blistering ?
Do you re-camber "by eye" or do you use a jig?

 


I've been re-cambering and straightening bows for almost 20 years. I use an alcohol lamp now, but I've used an electric hot plate in the past. I think that the advantage of a hot plate is that the heating area is bigger, so you can heat a large portion of a bow faster. The advantage of the alcohol lamp is that you can keep the heat focused on a small section of the stick if you want.

I only re-camber/straighten bows that are fully haired. Before starting, I mark the stick with chalk to indicate where I want to change the shape and in what direction. With the frog and hair dangling from the head to my left, I put the stick right in the flame, constantly moving it from side to side and rotating it to avoid overheating any spot. I check the heat with my hand occasionally. When I think it might be ready, I take it out of the flame and bend it with my hands as indicated by the chalk mark. I only apply the bending force for several seconds. Then I quickly mount the frog on the stick with the screw, tighten the hair very slightly and check to see how it came out.

There are several different ways of sighting the bow to check for straightness, camber and twist. I never use a jig or pattern. To check for straightness, I generally sight with one eye down the length of the bow from the frog to the tip, holding the frog just below the direct line of sight between my eye and the head. I observe the straightness of the stick. I check to see if both sides of the head are equally visible on either side of the stick. I know that the hair is running in a straight line, so I compare the stick against the hair for straightness, which is why I straighten with the bow haired. I also hold the bow at arm's length across my field of vision with the hair away from me to compare the straightness of the stick against the hair which I know is straight.

To check for camber, I lay the bow on my workbench, hair side down, with the hair just tight enough to be pulled into a neat ribbon. I want the stick to just touch, or not quite touch, the bench at the mid point of the hair and curve gracefully and gradually toward both ends. I also tighten the hair a bit and sight down the length of the bow, looking for a smooth even curve to the camber as the hair is tensioned.

To check for twist, I hold the bow at arm's length across my field of vision with the hair away from me. I rotate the stick so that equal amounts of the head are visible on either side of the stick. Without moving the stick, I shift my gaze to the butt and see if equal amounts of the frog are also visible on either side of the stick. If they aren't, it means the stick is twisted.

Some bows bend easier and at a lower heat than others, and I cannot tell before I start heating any particular stick how readily it will respond. Often I get it just right on the first heat, but if I find that that I haven't changed anything, I re-heat the stick a bit hotter, bend the stick again with a little more force, hold it bent for a little longer and check it again. And sometimes I find that I've bent the stick farther than I wanted, so without dismounting the frog I quickly try to bend it the other way before it cools off to remove some of the bend that I've just put in.

I keep re-heating and re-checking the stick until I've got it where I want it. Rarely I've had very difficult bows where I'd get the camber right only to find that the bow was crooked, then I'd straigten it and find that I'd lost some camber. Sometimes I bend the sick "on the diagonal" so that I'm attempting to correct both camber and straightness at the same time. I get easy bows where I want them in one or two heats. A moderately difficult one might take five or six, and I've probably gone as many as about a dozen heats on one or two extremely difficult ones.

I find correcting twist to be the most difficult and nerve-wracking stick adjustment. I heat almost the entire length of the stick and apply torque gently by holding the head in one hand and the frog in the other. This always feels very unsafe. The straightness and camber will often require adjustment after the twist is removed.

My experience is that good bows always have a clear oil or shellac finish which is unaffected by the heat that I use, or no finish at all. Cheap bows often have some sort of colored varnish which gets damaged by the heat. Since they're cheap bows, and usually mine, I don't worry about it too much. I usually sand the areas where the finish is messed up the most and French polish the stick with orange shellac.

The ever-present worry when straightening or recambering is that a stick will break, but I have not broken one in the hundreds that I've worked on. If someone asked me to do a very good bow, like a Hill or an old French, I would refuse the job.

Practicing on cheap or throw-away bows is the only way to get experience in and a feel for this work.

 

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++++++++++++++++

May I ask you a question? (sorry off topic)

When I try to tighten my bow all the way, the bow is only half way extend (in short,

the hair of the bow has been lengthened) so I try to shorten it by doing something on the tip where

there is a wood plug. Take the plug out to look at. But it is glued in. Which it seems strange to me. Is it normal?

i.e. the way it is supposed to be ? )

The plug should be held in place by the accuracy of the fit. On student bows they are usually glued in place. In order to remove it you will need to very carefully chisel it out. The best thing to do is to try to cut it out from the middle of the plug and not by the side where it contacts the tip, otherwise you might end up splitting bone, ivory or plastic tip. What I usually do is to carefully drill a small hole in the center of the plug and insert a dental tool into the hole and wiggle the plug out gently.

If the hair is stretched and the hair is old, you might want to just rehair the bow. Old hair can be very brittle and will end up splitting if you try to shorten it.

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Brad--thanks for a great job in describing the methods and processes that you use. The only thing that I would add to what you've written is that I sight the bow from both ends. Most often, I sight the straightness as you do (holding the frog, sighting towards the tip), but I often will reverse the bow to sight towards the frog. Sighting this way, I often find small variances (warps, twists) that I didn't see the other way.

In general, I would recommend that anyone that is straightening/cambering bows be mentally fresh and focused. If I find that I have a difficult stick that I am having trouble straightening, I work on it until I lose focus :). I then set it aside and return to it the following day. I find that with fresh eyes (and better mental focus), I can much more accurately see slight warps in the stick.

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Brad, thank you for that information. I am going to try and fix a twisted bow I got recently as a gift from a supplier. It is a beautiful pernambuco bow with gold fittings. But this summer has been terrible for woods here in Spain and last weekend I noticed it is twisted, not too much but twisted. As it is a gift I don't care too much if I screw it up, I can always save the gold fittings for spare parts.

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The only thing that I would add to what you've written is that I sight the bow from both ends. Most often, I sight the straightness as you do (holding the frog, sighting towards the tip), but I often will reverse the bow to sight towards the frog. Sighting this way, I often find small variances (warps, twists) that I didn't see the other way.

Sighting from both ends is such an obvious thing to do that I am embarassed to admit that it never occurred to me. I will have to try it.

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I don't hesitate to recamber... its not that tough. I do it freehand and heat the bow with a hot air gun(paint stripper) I am too chicken to use alcohol and I don't like the hazard.

I have a wood "V trough with a couple of cross ribs, that I lay the bow in and the ribs hold it up off the bottom. With one hand move the air gun along the shaft holding the gun about 8-12" away from the bow, and with the other I rotate the shaft. The V trough ensures the heat gets to all sides around the shaft. I feel with the hand for the right temperature... as hot as the hand can stand. The biggest hazard is being impatient... go slow as it takes several minutes for the center of the shaft to heat up.

Cheers, Mat

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I'm sorry it took me so long to get back to this.

I'm not sure if this is a good thing or a bad thing, but I have had a glut of recent repairs which I just finished.

My thanks to all of you who answered. I have read through these replies, and find I have to re-read through them, slowly this time, and I will probably have some questions about some of this.

Special thanks to Brad who went way beyond what I expected - in such clear terms.

ct

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No one has mentioned letting wood 'rest' between heatings. Do those of you who have posted find this to be a non-issue, and re-heat and work the stick to your heart's content in a single session?

Good point.

I have been told to do this also. "The wood must cool off completely between bends", according to the only competent bow man I personally have talked with about the matter. Of course, now I will have to talk to some other bow people I know and trust, and see if I can arrive at a consensus about this...

It seems a very pertinent point in recambering.

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No one has mentioned letting wood 'rest' between heatings. Do those of you who have posted find this to be a non-issue, and re-heat and work the stick to your heart's content in a single session?

It seems to me that there are two things required to change the shape of a stick: 1. Temperature above the point at which the wood becomes plastic. (The exact temperature could vary from stick to stick.) 2. A bending force applied to the heated portion of the stick.

If I want to do some further bending of an area I have previously heated and bent, I see no reason to let this area cool down before re-heating it. But when I have heated an area and bent it to where I want it, I either let it cool down below the plastic temperature or I am careful not apply any bending force to that area if I want to work on other areas of the stick.

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I don't know what the rationale for the 'rest' idea is. I speculate that it may be an assumption about allowing moisture content to reach equilibrium. When the idea comes up it's generally an 'overnight' thing. I've also seen this mentioned with regard to bending wood for ribs or guitar sides.

I'm pleased to see that it doesn't seem to be universal, as I'd much rather recamber a bow in one day than three. :)

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There is a reason for letting things cool completely. Let's assume you are working on a given problem spot. You heat it, bend it, and decide it needs more tweaking.

There are two possible procedures, the first is that you let it cool completely, and start working on it again. The second is that you start heating immediately from the warm stage.

In the first and second procedures, a certain amount of heat will be conducted from the point of correction, to adjacent regions. In the first procedure, the heat will likely be insufficient to cause problems in the form of relaxing wood above or below the point of correction, however, in the second procedure, you begin heating before the wood has thoroughly cooled, which will result in a greater quantity of heat being transmitted through conduction to adjacent regions, and this may result in partially relaxing the camber in an area that was perfectly adequate.

The risk of accidental wood relaxation in regions above and below the zone of correction is greater if the wood is not allowed to cool thoroughly. It can be argued that bending stress applied by the hands mitigates this phenomenon, however in practice, the wood can be somewhat ornery and unpredictable when low to moderate amounts of heat are found in areas not being considered during the bending process.

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I would say if it's the same temperature to the touch on the area just corrected as it is in an area far from the zone of correction, it is sufficiently cool. This is approximately fifteen minutes on the safe side (probably more like ten), and will vary depending on the thickness. If you want to accelerate things, you could apply a flexible cold pack (well wrapped in paper towel to prevent condensation from touching the stick). Even a vigorous fan or stream of compressed air will help remove heat much faster.

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in the second procedure, you begin heating before the wood has thoroughly cooled, which will result in a greater quantity of heat being transmitted through conduction to adjacent regions, and this may result in partially relaxing the camber in an area that was perfectly adequate.

In theory this line of reasoning is correct, but in practice I don't think it makes much difference, for two reasons: 1. The fairly low thermal conductivity of wood. 2. When I bend the stick where I have just re-heated it, I will also be bending the ajacent portions that the heat may have spread into.

But I suppose that if one wanted to be very careful, it wouldn't hurt to follow your suggestion.

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  • 2 years later...

After reading and studying this post, I'm looking for an alcohol lamp. I would assume it's the temperature of the flame that matters. Will any alcohol lamp do, or are there models better suited for bending bows? Anyone (Brad?) care to show a picture? Thanks all.

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