Cleats: How thin is too thin?

[MachTwang]

I am currently working on an old Martin ukulele with two cracks in the back and as I was crafting some cleats out of Mahogany I got to wondering how thin is too thin for a cleat?

I have some mahogany veneer that is .065mm thick and as you can see in the photo I have cut it 20mm long with the grain and 5mm wide, my thought was to lay them across the crack at the beginning, middle and end almost like sutures.

How this in too thin for a cleat? What are your thoughts?

[Bizman62]

Hi and welcome to the forum!

Cleats usually are a bit thicker than that, something about 1.5 mm. And also a bit larger, almost square. I've seen both squares and diamonds, the main thing is that the grain direction is perpendicular to that of the instrument. And the cleats should be quarter sawn for maximum strength.

Mahogany is quite soft despite being a "hardwood" and it's also quite brittle. However, if the bottom is mahogany, using the same species should be OK. Also, there's pieces and pieces, some are tougher than others.

For a small instrument like a ukulele the 0.65 mm might suffice. - I believe there's a typo in your thickness! .065 mm is only .0026", the thickness of a hair or paper. Then again, it depends on the nature of the cracks. If the cracks stay closed without pressing then wood glue like Titebond should be enough. A non-stressed crack won't break again at the glue joint. But if there's stress at the crack, meaning you need force to push the surfaces together, some cleats are required. In that case instead of sutures I'd use a bit longer strips just for looks. Either long enough to cover the space between braces or as long as you can get with the grain being perpendicular.

If the bottom is of some other wood than mahogany I'd use either that wood or maple for cleats.

 

 

[MachTwang]

2 hours ago, Bizman62 said:

I believe there's a typo in your thickness! .065 mm is only .0026",

You are correct! It should have been 0.65mm.

This is a Martin uke that is about 100 years old, the cracks run from the upper bout to the first brace and they do not require any force to get them to meet and I can see no light through them. So you are saying that for these kinds of cracks all I would need to to blow some glue in?

[Bizman62]

8 hours ago, MachTwang said:

they do not require any force to get them to meet and I can see no light through them. So you are saying that for these kinds of cracks all I would need to to blow some glue in?

Exactly that. Modern glues like Titebond Original are so strong that the wood will break in another place rather than in the glue joint. Some folks may say that it's not "original" meaning it was not used 100 years back. Fact is that Martin and other quality builders have always used the best glues available at each time.

Cleats are needed just for pulling an open crack shut or keep the sides of the crack level. Think about it this way: The top and and bottom of the ukulele are glued to the sides. Do you see any cleats in the seam? No! The surfaces have been carved to match perfectly without any stress. Should you leave the uke in the rain so it warps you'd get stress issues in the glue joints. The logic is that simple!

When gluing those cracks, do some dry fitting first. Try to get the seam level and clean - any tiny shards sticking sideways should be either removed (x-acto knife and tweezers) or straightened. If the sides of the crack aren't level you can carefully try bending or pulling the whole instrument to open the crack so you can nudge any hidden ridges into their place. Hope that made sense...

Before applying glue you can also drive some warm water with a small (artist's) brush to the cracks. The water is thinner than glue so it will wick in easier. Titebond can be thinned with water so the moisture won't do any harm. Wipe the excess off.  As it gets sucked in to the pores of the wood it creates some suction to the crack so the glue will follow more easily. A suction cup can be used to drive the glue deeper. Compressed air can also help as well as wiggling the crack for a pumping action. Try to look inside the uke to see any squeezeout, both for water and glue. If you see driplets of glue inside the joint has been saturated and you can clamp the uke, Again, wipe any excess glue off with a damp rag. Let dry overnight.

[MachTwang]

Excellent, Thank you so very much for the advice.

[Bizman62]

For the record, most of that information is based on the Rosa StringWorks videos. Knowing that Jerry Rosa gives a lifetime guarantee to his repairs they can't be too badly off.

[TheCaffeinatedOne]

Somewhere I picked up the trick of using a shop vac to pull glue through an irregular crack. In this case you'd flood the crack with titebond and apply the hose to the sound hole. Magic!

And I agree that Jerry Rosa is a master!

Edited January 20, 2023 by TheCaffeinatedOne
A bow to Jerry Rosa