Bow Screw Problem

[scordatura]

I have a bow that the screw has come out of the button (screw end). I know that it is customary to put burrs on the tang. Does anybody use some type of glue in addition to burrs when doing this repair? If so what type? I am wary of making it too permanent.

[martin swan]

4 minutes ago, scordatura said:

I have a bow that the screw has come out of the button (screw end). I know that it is customary to put burrs on the tang. Does anybody use some type of glue in addition to burrs when doing this repair? If so what type? I am wary of making it too permanent.

I think you are asking for permission to use CA/superglue?

Is it an important or historic bow?

 

[scordatura]

1 minute ago, martin swan said:

 

Is it an important or historic bow?

Yes

 

 

[martin swan]

3 minutes ago, scordatura said:

 

Then I would take it to a specialist in such bows and ask them to do the work. Personally I can't see any issue with a permanent repair - the more permanent the better - but when it comes to the conservation of historic bows it's up to the person taking on the job to decide what methods are appropriate.

[scordatura]

32 minutes ago, martin swan said:

Then I would take it to a specialist in such bows and ask them to do the work. Personally I can't see any issue with a permanent repair - the more permanent the better - but when it comes to the conservation of historic bows it's up to the person taking on the job to decide what methods are appropriate.

This is my feeling. The repair in itself is not difficult. I have done it before on lesser bows with a little CA for security. It seems like it is a matter of opinion.

[Craigers]

Often the adjuster will come off the screw because the screw is shorter than the hole it is going into. If this is the case then simply gluing the screw back to the adjuster is only a short term fix. It may be that the stick needs a bushing to support the end of the screw and keep it from pulling out of the adjuster again.

[Blank face]

29 minutes ago, Craigers said:

Often the adjuster will come off the screw because the screw is shorter than the hole it is going into. If this is the case then simply gluing the screw back to the adjuster is only a short term fix. It may be that the stick needs a bushing to support the end of the screw and keep it from pulling out of the adjuster again.

I’m not sure if this isn’t a misconception. If it’s true a fraction of a millimeter gap would be enough to pull the screw out of the button. In fact there was no bow I ever found where the screw was of the exact length of the hole, and if so there would be soon a gap by the drilling effect. It mustn’t be too wide.

[Craigers]

Usually it's more than a fraction of a millimeter. Also if the end of the screw is rounded, why would it drill into the wood?  And if the screw was drilling into the wood, the hole would end up being exactly the same length as the screw, not longer

[Craigers]

My feeling is that if the screw is properly supported then it has no reason to pull itself out of the adjuster, but if it is not supported then the likelihood that it will pull out is much greater.

[martin swan]

1 hour ago, Craigers said:

My feeling is that if the screw is properly supported then it has no reason to pull itself out of the adjuster, but if it is not supported then the likelihood that it will pull out is much greater.

I can't understand the physics of this.

What causes the screw and the adjuster to part is that the screw is attached to the brass eye, which is attached to the hair and is therefore being pulled forward by the hair tension, yet the adjuster is prevented from pulling forward by the nipple.

Ergo the adjuster and the screw are always being pulled apart by the very act of tensioning the bow. 

Traditionally the screw is held in the adjuster by gripping force, and ebony expands and contracts, so inevitably over time the screw will work its way out of the adjuster.

A few clever makers have used/do use a threaded screw. They didn't do this because of some issue with the screw mortice, but because screws are inherently liable to work their way out of the adjuster.

[Craigers]

 If the screw is bottoming out at the end of the hole, it can't pull forward and work its way out of the adjuster. The pull of the hair is being supported by the stick and not relying solely on the gripping force of the adjuster. There are many bows where this is not the setup, but I find this arrangement to work best.

[Blank face]

15 minutes ago, Craigers said:

 If the screw is bottoming out at the end of the hole, it can't pull forward and work its way out of the adjuster. The pull of the hair is being supported by the stick and not relying solely on the gripping force of the adjuster. There are many bows where this is not the setup, but I find this arrangement to work best.

If the end of the screw is really in tight contact with and supported by the wood there would be wear, even if the tip is rounded, and this wear  will inevitably loosen the contact over time, and support will be lost either. Therefore the drill hole would need a regular bushing after a certain time of use (and not a very long time IMO). That’s not a very sustainable idea of construction, and as I wrote, I never found it at any bow, at least not that it was noticeable.

[martin swan]

30 minutes ago, Craigers said:

 If the screw is bottoming out at the end of the hole, it can't pull forward and work its way out of the adjuster. The pull of the hair is being supported by the stick and not relying solely on the gripping force of the adjuster. There are many bows where this is not the setup, but I find this arrangement to work best.

Ok now I understand your concept and it makes sense, but on older bows that I've studied I don't think they were made this way as the screw mortise tends to be quite a bit longer than the screw. Often the screws have quite a sharp point too!

So the question would be, on a historic bow should you glue the screw back into the adjuster or bush the screw mortise so that it accommodates the screw exactly ...? For me the former is significantly less invasive.

[rudall]

I vaguely remember a bow maker/repairer saying here some years ago that they inserted a stone of some kind into the screw hole for the end of the screw to bear on. 

[nathan slobodkin]

3 hours ago, rudall said:

I vaguely remember a bow maker/repairer saying here some years ago that they inserted a stone of some kind into the screw hole for the end of the screw to bear on. 

Yes, I know of at least one maker who installs a wear resistant bearing for the end of the screw.

On bows without such a bearing I think the pull of the hair is not as much of a problem as having the thread of the screw misaligned with the length of the mortise which allows the screw to pull the eyelet up against the butt end of the mortise acting as a jack which pulls the screw out of the button.

[Blank face]

I can see a lot of complications using such a stone or bearing. It’s nearly impossible to remove such a device if it’s necessary for some reason, the wear would happen at the screw, and any hard object in the hole could cause cracking when the wood of the stick shrinks.

This all sounds to me like a lot of unnecessary sophistication to avoid a simple and easy repair like fixing the screw again into the button.

[martin swan]

6 hours ago, nathan slobodkin said:

Yes, I know of at least one maker who installs a wear resistant bearing for the end of the screw.

On bows without such a bearing I think the pull of the hair is not as much of a problem as having the thread of the screw misaligned with the length of the mortise which allows the screw to pull the eyelet up against the butt end of the mortise acting as a jack which pulls the screw out of the button.

Surely the best protection against this problem is to use the right length of hair?

[SCorrea]

This has all the appearance of a thread class fit problem.  If the threads on the shaft have worn in side the fitting that pulls the hair,or the threaded shaft itself, it will take more force to move the fitting on the threaded shaft as this wear progresses.  Im thinking that both threads on the button shaft started with the same thread class fit as they were probably threaded identically.  As the bow wears, the stationary threads will start to unscrew because they take a very little bit less torque to move than the threads that are wearing.  I believe this to be the root cause of the issue.  It seems to be an unavoidable byproduct of a lack of lubrication and just plain old wear over time.  A suggestion to get the button threads to not unscrew would be to ever to slightly bend the shaft in a very gentle curve in that part of the shaft inserted into the button and to install it frozen to shrink fit it together.  The shaft can be super cooled in Methylene Chloride.  Fill a small metal cup with Methylene Chloride and place it on a chunk of dry ice.  That should get you a -150F differential to ambient and allow the gently curved shaft to go in as a much looser thread fit, and as it warms up, it will normalize and the curve will add greatly to the anti-rotation need.

Edited April 27 by SCorrea
spelling

[fiddlecollector]

I dont see a problem putting a little CA on the tang (as long as the fits ok) before putting the button back together . The heat from a soldering iron on the screw near the button is enough to break the bond if it needed removed again.  Id be more concerned with any rust on the tang  than putting a drop of CA to secure.

[nathan slobodkin]

I certainly don't see a problem with using adhesives to reattach a bow screw to the adjuster. It's probably better than raising burrs on the shaft which becomes a broach which will enlarge the hole. However neither will last long if the eyelet travel doesn't stop before hitting the back of the mortise. 

[fiddlecollector]

1 hour ago, nathan slobodkin said:

I certainly don't see a problem with using adhesives to reattach a bow screw to the adjuster. It's probably better than raising burrs on the shaft which becomes a broach which will enlarge the hole. However neither will last long if the eyelet travel doesn't stop before hitting the back of the mortise. 

I have read this thread twice and cant see the OP mentioning any problem other than the button is detached from the screw. Lots of waffle about the stick hole, mortice  etc,but mainly  irrelevant to the OP`s simple question.

[David Burgess]

3 hours ago, fiddlecollector said:

I have read this thread twice and cant see the OP mentioning any problem other than the button is detached from the screw. Lots of waffle about the stick hole, mortice  etc,but mainly  irrelevant to the OP`s simple question.

Would it not be pertinent to figure out why the button has become detached from the screw, or is the objective just to send stuff out the door, charge for the "repair", with no thought to what happens after that?

[fiddlecollector]

9 minutes ago, David Burgess said:

Would it not be pertinent to figure out why the button has become detached from the screw, or is the objective just to send stuff out the door, charge for the "repair", with no thought to what happens after that?

I suppose so

[Blank face]

1 hour ago, David Burgess said:

Would it not be pertinent to figure out why the button has become detached from the screw, or is the objective just to send stuff out the door, charge for the "repair", with no thought to what happens after that?

 

On 4/26/2023 at 8:53 PM, martin swan said:

Traditionally the screw is held in the adjuster by gripping force, and ebony expands and contracts, so inevitably over time the screw will work its way out of the adjuster.

 

[Blank face]

6 hours ago, nathan slobodkin said:

However neither will last long if the eyelet travel doesn't stop before hitting the back of the mortise. 

The only way against this might be to inform players that they should stop to turn the screw if it's becoming unusual difficult. There are many ways to damage a bow or an instrument by careless ignorance.