Crack in top - Tenor Englemann Spruce

[Pat Eaton]

Hey all, just looking opinions on what might have gone wrong here. I built this last year and just the other day I noticed two cracks running from the bridge to the tail. It almost looks like the cracks are on the same growth ring of the wood.

Based on the location I think the rotation of the bridge may have caused the cracks. I may have made the top too thin.

My plan is to either use Titebond or hide glue for the repairs. I may place some small cleats on the underside for strength.

Thanks for your attention.

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[DPO]

Your attachements are invalid.

 

[Pat Eaton]

Here are the pictures.

 

[jhnmdahl]

That sure looks like bridge forces were helping it crack. Is there something we can't see that would prevent the wider base portion of the bridge from distributing some of the tension out past the narrower top portion of the bridge?

 

[Allen]

Looks like a crack right along the grain. A likely problem is that it was always there to some extent when the instrument was built and only showed up after it's been strung up.

You will need to add cleats to the underside for any sort of peace of mind. HHG would be my choice for repair.

 

[sequoia]

Is there something we can't see that would prevent the wider base portion of the bridge from distributing some of the tension out past the narrower top portion of the bridge?

First of all, you have my condolences. Those are nasty cracks alright... As the poster said above, curious that the cracks are under the pedestal of the bridge and not the wings. Maybe the reason is that most people just clamp the pedestal portion and let the wings go unclamped... You said that you thought you over thinned the top. How thick was it at final thickness?. How about a picture from the side of the bridge rotation? Regardless, you are going to have to cleat, sand, fill and re-finish. You could just try cleating and drop filling but...

 

[Pat Eaton]

Thanks for the responses. I believe the final thickness was about 1.7mm.
I agree the cracks look so similar that they may have been there all along.
I am guilty of only clamping the thicker portion of the bridge and not the wings.
Lesson learned.

 

[mikeyb2]

I'm wondering whether humidity has played a part here. Either humidity was very high when the top was glued on, or the current humidity is very low. Or a combination of all the above.

 

[bazuku]

If the wings are not separating from the top, I think that these cracks would most probably have been present prior to book-matching and glue up. If the grain line count is the same from the centre line to the crack on each side, I would suspect a natural flaw and certainly no build error.

 

[PetalumaRescuke]

Is the sound board arched? If so is the bridge flat? I saw similar cracks when replacing a bridge on a VitaUke with one with a flat base.

 

[Pat Eaton]

Hey PetalumaResuke the sound board is flat. I also saw no signs of bridge rotation.

The top was installed during the winter, however our winters are not cold or dry. Nine degrees (Celsius)and raining is our common forecast from December to March. Humidity (or lack of) may have played a part.

I am an inexperienced builder so this type of problem will provide me with the good experience.

Thanks all for your thoughts. I will see about posting pictures of the repairs as the proceed.

 

[Pete Howlett]

Nothing to do with soundboard thickness, bridge rotation or any of the other suggestions except from Allen who diagnosed correctly. It was always there - you can tell because of the crack symmetry, either side of the centre line and the secondary crack beginning to show beside the main one. This indicates a set taken from the end of a billet...

And that folks is the difference between those who do this for a living and have the 10,000 hours and those who aren't quite there yet.

I don't do it but you may wish to look up 'candling'. I simple flex the boards because this is not an uncommon problem when working with thin stock, quarter sawn and figured timber and this simple check reveals cracks which can be glued ant the time the centre seam issuing done as long as they have not been 'open' for some time.... and I'd defy you to find the join .

 

[Pat Eaton]

Thanks Pete, I found this description of candling testing.

"There is an old method of dating wood called the Candle Test, used to date violins. Holding a light behind wood recently harvested will show a bright round spot of glowing orangeish-yellow. Older wood, of say 100 years in age, allows much less light to show through, while wood that is 200 or 300 years old will show little or no light at all.

Why this happens remains a mystery. But it makes sense to me that the matter inside the cells or between the cell walls (cellulose and sugars, etc.) has become crystallized to the point that it refracts the light, or otherwise somehow absorbs it."

Is this what you are referring to?

 

[Allen]

Candeling......or a more modern method would be a strong even light source (light table) will show you pitch pockets, and other flaws such as cracks, and some builders use them to help determine if their bookmatch joint is perfect.

 

[sequoia]

Here is a picture I took using a 300 watt halogen shop lamp to candle some jointed plates sanded to 125. Note that while the seam was completely invisible under normal light, the glue shows up when candeling. No weird inclusions or grain lines apparent. Actually I've never come across anything deeply flawed in my spruce plates. Probably because I'm not as experienced as some luthiers here on the forum. Or maybe I just have access to higher quality wood. These spruce plates were from Alaskawood: https://alaskawoods.com/

 

[Pat Eaton]

Thanks again, I found this reference;

https://forum.ukuleleunderground.com/showthread.php?101974-Candling-a-Top

 

[jhnmdahl]

In the violin world, a bright light source can also help provide a quick visual indication of how thickness varies across the carved top. A handy way of keeping tabs on things while carving contours and testing tap tones (if that's your thing).

John

 

[Pat Eaton]

Update, I used hide glue, magnets and a home made clamp to fix the cracks. I used magnets to glue the cracks shut. I placed two cleats on the inside with the aid of of a home built clamp. The home built clamp has magnets installed to aid in aligning the cleats. The final finish looks better than i thought it would.

 

[Pete Howlett]

Next time use 'gummed tape' as a cleat. You wet it and apply it and as it shrinks, it pull the crack together. The adhesive is fish glue so it fully integrates with whatever it is sticking to and will only come off when sacked with water. It is an easier and neater solution than cleating with wood.

 

[John Colter]

Nice job, Pat.