Mandolin Top design keeps cracking; repost

[George Semenov]

I hope I'm not breaking any rules by reposting. I accidently posted in the bowed instruments forum and I wasn't able to edit it.

I have never built a mandolin. All my work is with guitars. My daughter sketched up a design of a mandolin that she felt would be easier to hold on her lap. She also wanted it to have a pickup.
The top was made from a red wood post. It was not from a tone wood supplier but from a lumber yard. I chose it because of my ignorance. It's grain was 40 rings per inch in the center and it was light. When it was finished my daughter said that it had as good volume and tone as a quality mandolin. Unfortunately in one month it developed cracks in the top (see .jpg First top cracked). The top was not made to traditional thickness, again my lack of experience with mandolins. I was thinking guitar and had no appreciation of the force a mandolin top has. The center was 0.19" and it tapered to 0.10".
So I removed the top and made another top with the redwood that I purchased from Alaskan Specialty Woods for the back. It is beautiful wood. I don't have a picture of the finished front, the back is an equivalent look. The jpg titled Second top shows the bracing further apart on the second top as I thought the cause was that I didn't brace directly under the bridge. I made the top thicker, center 0.20" tapered to 0.110. This top didn't have a such a good tone or volume. Well it cracked anyway almost in the same spot three months later. I didn't take a picture where it cracked.
This time I purchased Adirondack spruce. I am carving top number three. I read in Roger Siminoff's "Constructing a Bluegrass Mandolin", he recommends that the rim not be any thinner than 0.10" and it looks like the center is around 0.23".
I am seeking recommendations of building this top for the best tone considering I still am going to put a pickup next to the neck and not crack. I am so snake bit after the second top. I want this one to work out. Was using Red Wood a mistake? Will using spruce be the answer? should I use X bracing?
I thank you all ahead of time!

[Mario Proulx]

It's not the tops that are the problem, but the bridge.

Make a new one with a full-contact base.

And lose the massive soundhole reinforcements; they only add to the problem by creating stress-risers.

[Michael Lewis]

The blocks at the ends of the pickup hole are part of the stress riser issue. This is an architectural problem, with the stress going through the neck end of the soundboard, and the hole is right in the path of the stress. The top needs a smoother line of support around the edges, unlike the blocks which jut out into the air chamber and add stiffness to small parts of the top right next to flexible parts of the top.

[Mark Swanson]

You have a radically different design here, and that is adding to your problems as already noted. On top of that you are mixing acoustic and electric elements. It would probably be better to pick one or the other, acoustic or electric mandolin. If it was meant to be electric, then you could brace all you wanted and not worry about killing the acoustic tone, and if it were to be an acoustic mandolin then there are a lot of things you could do to focus on that direction and get rid of that pickup, which is causing some of your trouble and taking up a lot of important real estate on the top.
One other thing you could do right away is to get rid of the type of pickup, it's not the right type. Use one that does not need a hole in the top, like a Fishman bridge pickup, or a K & K pickup that mounts under the top.

[George Semenov]

The two blocks for the pickup are not touching the top sound board. They are attached only to the face of the neck heel and are floating away from the top. Either way, the pickup hole will not be in my next top and I will not have the pickup blocks. The piezo pickups are a good alternative.

Michael,
Could you clarify by what you ment by "The top needs a smoother line of support around the edges, unlike the blocks which jut out into the air chamber and add stiffness to small parts of the top right next to flexible parts of the top."

A few more questions;
1. Is there any problem with having an f-hole with a shape that has a sharp point in it?
2. Are the f-holes too close to the center?
3. What about the minimum thickness of the top? From center to graduated outside

Thank you all for your input. It is much appreciated.

[George Semenov]

What specifically is a stress-riser? This is the first time I heard the term used.

[Mario Proulx]

It's best for the soundholes to not have any sharp corners as yours do, and yes, they should be farther away from the center. A good alternative with this design would be a single, oval(or some funky shape) soundhole.

A stress-riser is wherever a weak, flexible area is suddenly stiffened. What that does is cause all of the stress to be concentrated in a very small area, usually leading to a failure. What you want is to transition smoothly from stiff to supple in a stressed area.

[George Semenov]

Mario,
With the single sound hole, do mean off on the side or in the center? If in the center than I am assuming I need to use an X brace.

[Mario Proulx]

I'm thinking that as unique a design as this is, you sdhould be able to find a way to make a single soundhole fit somewhere abouts where the current pickup hole is. Might have to make it elongated..?

An X brace system would not be a bad idea.

[Michael Lewis]

George, I was assuming the blocks were attached to the top, so discount that remark. It looked to me as though the blocks were attached to the top and caused an irregular and abrupt line of attachment.

Consider the shape of the heel of the neck, where it flows from the shaft of the neck into the body. It curves rather than being cut square for a reason. If it were left as a sharp angle the intersection of the surfaces would create a stress riser, and this is where cracks would most likely begin. Sharp edges and angles are often smoothed to avoid causing a stress riser, or rather to prevent failure from stress. In dealing with sound holes in instrument tops we must consider the strengths and weaknesses of the materials we use, and since wood is much more likely to crack or fail along the grain than across it, we try to smooth the edges and lines of the holes so they are less likely to crack from stress. This is probably why we seldom see square or triangular sound holes.

[George Semenov]

Here is my third top made of Adirondack spruce. I got rid of the top mounted pickup and cutout needed for it. I will work on installing piezo pickups from the inside later since I already have a hole in the side for a jack. I just finished the bridge and gave it to back to my daughter. She likes the tone and likes it better than the first top. I will post some completed photos in a few days. The attached picture is of the extended allen wrench and the bolt used to set the neck from the inside. The first top with the hole for the pickup allowed easy access to the neck mounting area. But since I got rid of the the hole for the pickup, the only way I can attach the bolt on neck was through the jack hole. So I had to make a custom made extended allen wrench to reach to there.

[Michael Lewis]

George, you are on the right track. I think this one much less likely to crack.

[George Semenov]

I have a better feeling with this top. Thank you Michael.

[Michael Lewis]

George, how about a close up of your tail piece? What can you tell us about it?

[George Semenov]

The concept design of the mandolin was my daughter's. She wanted a mandolin that was comfortable when used without a strap. I made the tail piece to fit the shape of the mandolin. This one is a second revision. I had to put a bar for the strings to go under to help keep the strings from pulling the end into the top. Without it the bar the strings twisted the tailpiece. I am not totally happy with it, as it still wants to pull down. It is floating off the top, but by only a 1/16". I am thinking of making a third version that is shorter, closer to the end, to keep the tension in a straight line

[Michael Lewis]

Interesting approach to the metal work.

What is the measurement between the top and strings at the bridge ? String break angle at the bridge?

[George Semenov]

The average height would be 11mm, top to strings on bridge. The break angle is 17 degrees.

[Michael Lewis]

That seems like a low measurement for a bridge. That being dependent upon the neck angle, so can you change the neck angle? I see it is a bolt on neck. A taller bridge (15 - 20mm) will raise your string after length and the tail piece angle, but will also increase the string break angle over the bridge, which is already more than enough. I guess I don't know what to suggest at this point. If the instruments works well then that is all we really need to know, but if you were going to do this again I would suggest you make a full size drawing of the side view and put all the parts in the drawing with a string break angle over the bridge of about 12 degrees and bridge height of 15 - 20mm. Most of the rest of the parts will "fall" into place around those parameters as you make the drawing.

[George Semenov]

Michael, you are right. I will plan better on my next mandolin. The first top was shallower and I had a higher bridge. I wanted a higher crown on this last top to make it structurally stronger. Only after carving the top did I realize I over did it, but decided to stay with it.

[Dick Hutchings]

I'm a little late to this and I just want to say how much I like this design. That's a great looking mandolin. I guess any time you stray from proven designs you need to expect to go through a few iterations. So how is the top holding up and has the tailpiece given any problems?