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| Kaboom Forensic Analysis http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10101&t=55833 |
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| Author: | Ken Nagy [ Mon Aug 14, 2023 4:52 pm ] |
| Post subject: | Kaboom Forensic Analysis |
I've had strings break. My tiny baroque bridge wouldn't stay on. I never had a tailpiece break in pieces! I thought it was pretty chunky. The one on the other archtop is much smaller, and thinner. Maybe 4-5 mm thick, and 7 mm or so left above the strings. No problem with that one. The 1/6" x 3/4" piece of brass holding it was not a problem. It is still solid. Attachment: IMG_1389.jpg Attachment: IMG_1390.jpg Any ideas? Padauk just isn't the right choice? |
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| Author: | Barry Daniels [ Mon Aug 14, 2023 6:42 pm ] |
| Post subject: | Re: Kaboom Forensic Analysis |
All the breaks are in areas where the grain is very short. Compare it to a violin tailpiece where there is no short grain. Also, Padauk can be very prone to cracks. |
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| Author: | rbuddy [ Mon Aug 14, 2023 6:55 pm ] |
| Post subject: | Re: Kaboom Forensic Analysis |
Sorry Ken, I have to agree with Barry. All the short grain in the tailpiece design was destined for this end if not sooner then later. At least in my opinion. Brian |
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| Author: | Ken Nagy [ Mon Aug 14, 2023 6:59 pm ] |
| Post subject: | Re: Kaboom Forensic Analysis |
Barry, I don''t understand what it is you mean by short grain. The north/south distance to the end is about the same. My violin tailpieces are less than half that thick. They even taper at the end. They are all quartered stock. So this one seems at least 4 times stronger. (edit) No, 2 cubed; 8 times stiffer. What am I missing? I am very surprised how it broke on both sides at the bottom end. It is quite a chunk of wood there. Broke at the top first, and pulled to the sides at the same time? It has to be the Padauk. I do know that it can split. I've heard that Beech is not normally used for necks for the same reason. But I don't know. I only saw that one time. |
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| Author: | bcombs510 [ Mon Aug 14, 2023 7:07 pm ] |
| Post subject: | Kaboom Forensic Analysis |
No answers, but sorry that happened, Ken! Sent from my iPhone using Tapatalk Pro |
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| Author: | Ken Nagy [ Tue Aug 15, 2023 6:41 am ] |
| Post subject: | Re: Kaboom Forensic Analysis |
I'm not upset about it. I almost expect stuff to happen! If it didn't I'd be surprised. I just want to know what I did wrong, so it doesn't happen again. I'm guessing it is just the wrong choice of wood for a tailpiece. |
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| Author: | Clay S. [ Tue Aug 15, 2023 7:13 am ] |
| Post subject: | Re: Kaboom Forensic Analysis |
Hi Ken, I agree with Barry and Brian - it broke where the grain is relatively short. If you had cut that piece down the middle and laminated a piece between, with the grain running in the opposite direction (like plywood) you may have had better luck. |
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| Author: | rbuddy [ Tue Aug 15, 2023 7:19 am ] |
| Post subject: | Re: Kaboom Forensic Analysis |
Hey Ken It would help to have a little more info. A picture of the whole tailpiece and the application. Was it a guitar? What kind of strings and tension tuned to pitch? Your tailpiece looks like it had a large hollow in the middle between where it attaches to the tail block and where the ball end strings attach. That design creates a strip of cross grain wood (the "short grain" area) where the strings attach that isn't very strong. Then holes and slots cut thru it weaken it further. Leading to the unfortunate Kaboom. To demo the weakness you could cut a strip of padauk or wood of choice with the same grain orientation and mock up just the section section where the strings attach, holes and all, and just bend it with your fingers to do a rough stress test. More accurately, you could figure out the string tension, suspend your test strip of wood between 2 blocks and add a weight to the test strip similar to the string tension and see what happens. An alternative of course would be to make a tailpiece similar to a violin or cello that is solid wood from the string attachment to the "gut" attachment to the tail pin or body attachment. That would be a lot stronger. I think a stronger wood with tighter grain would help but even ebony would be too weak for your tailpiece design. A narrow piece of cross grain wood is just weak and prone to splitting under stress. Hope that helps. |
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| Author: | Ken Nagy [ Tue Aug 15, 2023 9:15 am ] |
| Post subject: | Re: Kaboom Forensic Analysis |
I guess you are all saying that it was too short, north/south? The ends of my violin tailpieces are only 7 mm to the tip, and 3.5 mm thick. The bottom where the tailgut goes is thicker, but that is mainly for balance. So I figured this one being 9mm by 9 mm at the end, it was HUGE. The sides were it broke off is 9mm thick by 15mm long. I don't have a photo of it. I just finished it Friday. The 13" acoustic archtop. An ElectroLess Paul. It's been strung up with light Phosphor Bronze strings. 160 pounds. The body is very light, with HD cedar, and it moved some, but not as much as I was expecting; I could push the bridge down fairly easily. The pull of the strings counteracted the push on the bridge. It did rise some. The guitar IS in the video of the latest OLF roundtable. You probably can't tell, but it sounds pretty good for just set up. I just tuned it back to pitch minutes before it broke. Stress already building up as creep? It rings like crazy. The string length is 625mm. I think it is the wood. Padauk is very resonant, but I don't know about its structural strength. The design might have had a "wishbone" effect to it. But that make the fact that it broke on both sides even more amazing. I might go for something fancier, the guitar is pretty plain. My wife says she likes one of my violins, but she has to look through them to find it. I think it is the one that kept self destructing. The belly kept cracking, red spruce. Then the neck just broke off. The block split. I kept the neck and the tailpiece, and threw it away. I'll make another one for her, and put the tailpiece on it. Maybe I'll make a tailpiece with some inlay. The one that my wife likes. Done years ago, before I heard about fill. Attachment: IMG_1394.jpg Attachment: IMG_1395.jpg
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| Author: | bobgramann [ Tue Aug 15, 2023 10:12 am ] |
| Post subject: | Re: Kaboom Forensic Analysis |
I built a padauk guitar many years ago. I remember that splinters tangent to the curve kept popping out when I was hand bending it. That leads me to believe that it doesn’t stick to itself well across the grain which is where yours failed. I haven’t made any instruments with tailpieces other than a mandolin where I bought a simple tailpiece. But, I play upright bass. Many years ago, following the guidance in Chuck Traeger’s book, I reworked my bass making sure I had no extra weight at the tailpiece and modifying the tailgut length until tailpiece vibration was not interfering with noter production. That, and a couple of other modifications, made my plywood bass into a monster. I pulled all of the tailpiece tuners off of my fiddle. Removing that weight changed my fiddle from sounding pretty dull into a very nice sounding instrument. My conclusion is that tailpiece weight matters and ought to be considered in tailpiece design. |
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| Author: | Ken Nagy [ Tue Aug 15, 2023 10:31 am ] |
| Post subject: | Re: Kaboom Forensic Analysis |
Bob. Thanks for that! I make everything as light as I can. I thought the tailpiece was chunky! I think it is the wood. Are we making Toyotas, or Ferrari's? I like Ferrari's. This tailpiece came out at 26g, the same as the ugly maple one I had on my first archtop. I switched it out for a nicer one I made. I have no idea how heavy that is. It looks lighter. The violin one is 14g with the adjuster. I like to make the length between the bridge and the string, 1/2 the length from there to the saddle. Why? It just seems like you'd get a 2:1 rocking ratio. Maybe it keeps the strings "lively"? "They" say the after length should be 1/6 of the string length. But that only works for certain length violins that conform to modern standards. Some models don't, but they put the bridge where they want it, and make the necks 130 mm long.They want them a certain way, and will beat them into submission! But violins and cellos are different from basses. For my cello I bought an adjustable tailpiece, it was cheap and easy, and fairly light. Maybe I'll try something different on it. I never put tuners on. Isn't that why they have pegs? The cello has FOUR tuners! |
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| Author: | Barry Daniels [ Tue Aug 15, 2023 12:36 pm ] |
| Post subject: | Re: Kaboom Forensic Analysis |
No Ken. It's not the wood but your design. The large hole in the middle of the tailpiece seriously weakened the structure. This is wood, not metal. You created places where there was short grain which is evidenced by the places that broke. They broke along the grain and the those spots are actually small in surface area. A re-design would be in order. |
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| Author: | Ken Nagy [ Tue Aug 15, 2023 6:06 pm ] |
| Post subject: | Re: Kaboom Forensic Analysis |
Well, I had no idea. No one ever told me about wood! I tried, and the end broke where the string hole was fairly easily, but not on the long straight piece. Who knew? Not me. I suppose that is why sometimes you cut a belly out, and the wood splits easily in your hand. Is there some sort of chart for that kind of strength? Is it something like cross grain strength? So generally, soft woods are worse in this respect? I did plan on making a different design. I cut the center out because it looked so huge. |
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| Author: | Barry Daniels [ Wed Aug 16, 2023 4:42 pm ] |
| Post subject: | Re: Kaboom Forensic Analysis |
OK, I advise an experiment. Take a board and rip off a piece along the edge, parallel to the grain, maybe a half inch wide. Cut another piece off the end, perpendicular to the grain, a half inch wide. Now these pieces have the same cross sectional area and if it was a uniform material, like metal, they would have the same strength. Now grab each end of the wood pieces and try to snap them in half. Report your results. Although a simple test, I do this to offcuts all the time just to let me know the strength or weakness of the wood I'm building with. |
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| Author: | Clay S. [ Thu Aug 17, 2023 10:25 am ] |
| Post subject: | Re: Kaboom Forensic Analysis |
It may be a combination of the two - the wood, and the design. A tough wood with interlocked grain like lignum vitae or elm may have held up better, but an improvement to the design (laminated/ply construction?) might also help to make it stronger and lighter. |
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