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Repairing a split head
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Mat Roop
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Joined: 24 Mar 2007
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Location: Wyoming Ontario

PostPosted: Sat Aug 10, 2013 8:13 pm    Post subject: Repairing a split head Reply with quote

In the January 2010 Strings magazine (just getting caught up on some reading!) with reference to repairing a bow head... here is what was written...
" First, the head is glued back into place, then the break is reinforced by one of several procedures. In the old days a slice was cut out of the center of the repaired head and filled with new wood, called a spline. These days, common fixes include screwing, pinning or tying the glued pieces together in a repair that can be barely visible form the outside."
Really?? never heard of screwing a violin bow tip to repair it... somehow I can't imagine that would work for very long due to the very great differential in expansion rates both from heat and humidity. Likewise, I gave up pinning, as others have, as the pins eventually release, again due to differential expansion.
... and how would one tie the pieces together?

I thought the modern and successful method was to insert a spline... at least that's what works for me.
Maybe I'm just old and behind the times .... what are your thoughts... and how do you do it?
Cheers, Mat
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whatwasithinking
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Joined: 26 Jan 2013
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PostPosted: Tue Aug 13, 2013 10:59 am    Post subject: Reply with quote

Hi Mat,

Have you seen this thread?

http://www.maestronet.com/forum/index.php?/topic/327788-snapped-bow

Or this one?

http://www.maestronet.com/forum/index.php?/topic/317304-bow-repair

Some folks are evidently using CF rods. I have some rods, but haven't tried this yet. Well, I haven't splined a head yet, either, although I did repair a broken stick. I do have a bow with a splined head, though. In Friedrich Wunderlich's book, he says a good head repair, using a spline, can be nearly invisible.

What kind of glue do you like to use on Pernambuco? And what have you tried? (I've tried G2 epoxy and various incarnations of CA, with and without mixing Pernambuco in the solution).
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Mat Roop
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Location: Wyoming Ontario

PostPosted: Tue Aug 13, 2013 11:26 pm    Post subject: Reply with quote

thanks wwit...
I have tried a couple of repairs using hardwood pins from under the tip plate, using 5 min epoxy and they lasted about 1 year...
about 5 years ago, I upgraded to G2 epoxy using pernambuco splines in the nose of the bow....the kerf is cut with a 2"diam fine toothed hollow ground blade of .41 mm thick. the splines are about .4 to .5mm thick and the grain is oriented to about 30 degrees from the head grain. I chose to not go perpendicular for the same reason cleats that reinforce plates are not at 90 degrees. So far, having done perhaps 4 or 5 repairs this way, none have come back.
I seem to have pathetic success with CA glues... so generally I do not use them except for setting splinters and hairline cracks in frogs.... anywhere where there is no stress... not even on tip plates and liners.
I had ordered some CF rods a few years ago, but never had the nerve to try them... maybe for a test, I'll break the head off one of my own student bows and repair it with a CF pin from under the tip plate.... when I have time!!
Thanks for the threads... I had seen them at the time, but this was a good refresher.
Cheers, Mat
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whatwasithinking
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PostPosted: Wed Aug 14, 2013 12:30 pm    Post subject: Reply with quote

Interesting thought on the angle of the spline. I wonder if that also makes the repair a bit less obvious.

I've been using G-2 more and more, but I know that not everyone likes it. It does have one advantage--if you don't like the way the alignment turns out, you can heat it up and move things around, and then let it reset. But I think once you've used epoxy, it's difficult to go back to CA on the same repair--they're not compatible. System Three, the makers of G-2, told me this: "Make sure that both surfaces are well coated, and then apply only enough clamp pressure to close the joint. Over clamping will starve the joint which could lead to future joint failure." I heard about one failure where G-2 was used, and the repairman couldn't tell me why he thought it failed. But perhaps it was clamped too tightly.

Bob
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Mat Roop
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PostPosted: Thu Aug 15, 2013 9:10 am    Post subject: Reply with quote

Yes, it does get closer to the graining of the nose and so is less noticeable, but my practice is to do a good clean and not obvious finish, but still evident on inspection, so that any future buyer can be aware of the history.
Clamping too tightly creates a problem for most glues except maybe CA glue.
I did not know you could heat the joint and it would reset... How hot does it need to get to become soft? Epoxy is a chemical reaction when it sets... I wonder what heating would do to the strength?
cheers, Mat
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whatwasithinking
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PostPosted: Thu Aug 15, 2013 10:25 am    Post subject: Reply with quote

I wonder if this could even be a problem with CA. I've read that increasing the clamping force doesn't improve the bond for CA glue.

As far as epoxy goes, any epoxy has a heat deflection (or distortion) temperature (HDT), at which point it starts to become a liquid. Since G-2's maximum working temperature is 160F, its HDT must be just above that. Once it cools back down below this point, the strength is just the same as before.

I had thought that nearly all epoxies could be strengthened with post curing, but System Three advised me that G-2 won't benefit from post curing, and that they don't advertise a Tg (glass transition temperature). To post cure, you elevate the temperature of the part until you reach a point below the spec'd Tg, but gradually heat it further to raise it to a new, higher Tg, and leave it there for several hours. The epoxy will now possess a higher Tg and HDT (which is a little below the Tg). This works with West Systems epoxy, which some folks use successfully on Pernambuco. I've used post curing on aircraft parts with good success, but not using G-2 epoxy.

If G-2 doesn't post cure, then perhaps it's even possible to repeatedly make position adjustments by heating to 170F or so--might be an interesting experiment.

A maker told me an interesting story about a French maker and inventor of a composite bow. Someone complained that his bow lacked sufficient camber. He said, "Give it to me." He then wrapped a cloth rag around the stick, and rubbed it rapidly to raise the temperature of one area. He then bent it, increasing its camber. He must have exceeded the HDT of the epoxy to accomplish this. BTW, he never indicated that he agreed there was a problem with the initial camber.

Disclaimer: I'm not a chemist or composites engineer. Just a user. Maybe there's someone here who can enlighten us more, and who's actually made adjustments to epoxied parts using heat.
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whatwasithinking
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PostPosted: Mon Aug 19, 2013 11:24 am    Post subject: Reply with quote

Update: Following a bit of research, and reading the applicable MSDSs, I learned that G-2 uses a polyamide hardener, while West uses polyamines exclusively. Otherwise, the epoxies aren't a lot different. Comparing amines with amides, one trade site notes that "Amides...are more surface tolerant and less troubled by moisture." So G-2 is evidently suited to Pernambuco mostly because of the hardener they use. It may not be the strongest possible epoxy, but it's quite sticky.

Bob
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Mat Roop
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PostPosted: Mon Aug 19, 2013 5:04 pm    Post subject: Reply with quote

Thanks wwit... good info to have... guess I will continue with G2!
Cheers, Mat
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PostPosted: Tue Aug 20, 2013 1:52 pm    Post subject: Reply with quote

I just keep learning.

It might be fun to start a "glue" thread, assuming some in the community would be willing to contribute from their experiences. There are so many different adhesives in use on bows--CA, various epoxies, hide glue, Ponal Wasserfest, etc. And some folks seem to use combinations of adhesives on a single repair.

A friend just asked me to repair the neck on a guitar. I'm leaning toward hide glue, but I haven't seen it yet. I fear that a previous repairman used CA. I've worked on bows that looked like CA was used, which makes me stop and consider what I'm about to do.
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Chet Bishop
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PostPosted: Wed Aug 21, 2013 8:58 am    Post subject: Reply with quote

Isn't CA fairly dependably soluble in pure acetone? I realize that would almost certainly take off any finish involved as well, but if the repair of the break is the primary consideration, can you simply saturate the joint in acetone and thereby remove the CA? Or is it not that simple? (Things seldom are, of course....)
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whatwasithinking
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PostPosted: Wed Aug 21, 2013 9:52 am    Post subject: Reply with quote

Hi Chet,

Yes, that's what I've been doing, and I follow it up with 99.5% isopropyl alcohol. I'm sometimes not sure I'm getting it all off, though, especially when the surface is rough. I know that some have suggested soaking the wood in acetone, as opposed to simply brushing it on. That would certainly take all of the finish off!

Do you prefer CA for Pernambuco repairs?

Bob
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Chet Bishop
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PostPosted: Wed Aug 21, 2013 4:23 pm    Post subject: Reply with quote

Sorry-- I have never performed enough serious bow repairs to speak from experience. So far, I am pretty much limited to the instruments.
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Dave Chandler
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PostPosted: Tue Oct 27, 2015 10:44 am    Post subject: Reply with quote

Just completed a broken bow tip repair, thought I'd share some photos. The spline insert is nearly invisible.

http://www.chandlerviolins.com/repair-examples.html Click on the photos to enlarge, and there is some captions under each.

My only caveat -- I just finished this today, am delivering back to owner tonight with caution to give it a couple days for a complete cure both the epoxy and the finish. We'll see how it holds up over time.

I used a mix of burnt umber and indian red oil paints to get a color that exactly matched the finish to cover the spline and fill the end grain that was showing. It also filled the fine line showing at the original break. Then a coat of clear lacquer sprayed on.

Its undetectable without bright lighting and some method of magnification.
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Mat Roop
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PostPosted: Wed Oct 28, 2015 12:40 am    Post subject: Reply with quote

Nice job, Dave....

Somewhere I had read that Michael Vann had commented that he repairs broken heads just by gluing using modern materials.... I looked for the reference, but can't find it... so I can't be sure what materials he was referring to.

I did see that he (Michael Vann) wrote a chapter on restoration of a fractured bow head in the IPCI books ( Vol 3 PtII, pg 148) , and there he uses only G2 epoxy but also that he lets it sit for at least a week before rehairing.... does this suggest that inserting a spline is not really necessary?

I have used wood pins inserted inside the nose and hidden by the faceplate.... but that does not work ( for me anyway!.
Cheers, Mat
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Dave Chandler
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PostPosted: Wed Oct 28, 2015 3:47 pm    Post subject: Reply with quote

Mat, I tried to just glue the piece back on -- used fresh high strength hide glue, let is set ovrnight, and was unable to bring the hair up to full tension before it gave way. I suppose I could have used super glue. By the way, the secret to super glue, is to wet the surfaces, just a bit of moisture, will activate the super glue.

The "Henry Wake" solution is the spline method. This is from his book on bow repair and rehair updated 2003. Certainly if using the spline method the bow can be in use in a couple days. 24 hours after I put this together, I tightened the bow well past full tension and saw no movement in the splice.
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