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| Do reverse kerf linings create stress risers? http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10101&t=53569 |
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| Author: | Clay S. [ Mon Oct 05, 2020 12:50 pm ] |
| Post subject: | Do reverse kerf linings create stress risers? |
One thing I have noticed while regluing the old guitars - the better ones all seem to use the classic triangular shaped linings, either continuous or as tentalones. The linings taper down to nothing where they meet the sides. It is more work to make them this way. Why do you think the better makers bothered to do this? Reverse kerf linings have become popular with some makers (myself included) because of the additional stiffness they impart to the sides. But could this be a mistake, and could we be building in a stress riser at a place where an impact will create a crack in a spot that is difficult to repair? What are your opinions on this? |
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| Author: | jshelton [ Mon Oct 05, 2020 1:27 pm ] |
| Post subject: | Re: Do reverse kerf linings create stress risers? |
I have never used kerfed linings. Tentellones on the top and solid linings on the back. I just don't like the look of kerfed linings. Is there some reason you can't taper the kerfed linings down to nothing (triangular)? |
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| Author: | Barry Daniels [ Mon Oct 05, 2020 2:20 pm ] |
| Post subject: | Re: Do reverse kerf linings create stress risers? |
I have seen cracked sides on guitars with traditional triangular linings. I don't think reverse kerfing is any worse than the triangular linings in creating a stress riser. They both create one. Triangular linings would have to be feathered out a lot more to be a smooth stress profile. |
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| Author: | Bryan Bear [ Mon Oct 05, 2020 2:58 pm ] |
| Post subject: | Re: Do reverse kerf linings create stress risers? |
What Barry says above feels right to this non-engineer. At first thought a tall (roughly) right angle transition form lining to side is an obvious stress riser and one that is much larger than a tapered lining profile. But. . . the narrow sides of the typical triangular linings are never really truly feathered so the ends are still roughly 90 degrees (albeit much shorter). So I think there is always going to be this stress riser that unfortunately runs along the grain. I use solid crossgrain reinforcements (instead of tape) and run them under the linings hoping to limit the spread of any cracks that might form along the linings. I'm not a repair guy at all, so I'd be interested to know how often side cracks form near the linings as opposed to in the middle area of a side and if there is any knowledge about the different locations of cracks from impact as opposed to drying out. |
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| Author: | Barry Daniels [ Tue Oct 06, 2020 10:18 am ] |
| Post subject: | Re: Do reverse kerf linings create stress risers? |
Side cracks obviously follow the side grain so the crack may not proceed along the lining unless the grain is aligned with it. I have seen a number of side cracks occurring at the edge of the lining and this makes them more difficult to repair. I think most of the cracks I have dealt with were impact related. Drying out seems to be less of a problem on the sides probably because the side piece is narrow and there is often no cross grain bracing causing differential expansion, like you see on guitar tops. |
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| Author: | Dave m2 [ Tue Oct 06, 2020 11:14 am ] |
| Post subject: | Re: Do reverse kerf linings create stress risers? |
It seems sensible that using plenty of side splints and ensuring they are notched into the reversed kerf linings will get over this stress problem. Dave M |
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| Author: | Alan Carruth [ Tue Oct 06, 2020 12:48 pm ] |
| Post subject: | Re: Do reverse kerf linings create stress risers? |
I use reverse kerfed liners, and side tapes which run under the liners. The only side cracks I've seen in my guitars have been from impacts, and were not near the liners. The tapes stopped them from spreading. Non-inlet wood fillets on the sides will cause cracks at the edge of the liners, according to a museum restoration expert who used to give us occasional lessons when I was learning violin making. I have had two guitars (not mine) with side fillets come in for side crack repairs, back when I did repairs. The fillets were on the 'straight' section of the side below the sound hole. The sides were a soft acacia wood of some sort, and the bodies were quite deep. The guitars had been made overseas, in higher humidity that we see around here in the winter. Both of them cracked in the center of the side, right at the location of the fillet, which were inlet under the liners. I suspect that the sides had tried to shrink in width, but the fillets did not shrink in length, which is what caused the cracks. Fillets are certainly stronger than side tapes, but maybe not an improvement. |
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| Author: | oval soundhole [ Tue Oct 06, 2020 1:37 pm ] |
| Post subject: | Re: Do reverse kerf linings create stress risers? |
Not sure if they're any more stress causing than triangular lining but they seem to add more stiffness to the rims in my experience. I use wooden reinforcements like below to reinforce the sides; the one pictured is influenced by Dominique Field's side bracing ideas, I don't normally use so many 
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| Author: | Alan Carruth [ Tue Oct 06, 2020 6:38 pm ] |
| Post subject: | Re: Do reverse kerf linings create stress risers? |
Reverse kerf liners do add some stiffness to the 'free' rim, but I suspect that advantage goes away when you glue the plates on. |
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| Author: | Clay S. [ Thu Oct 08, 2020 11:55 am ] |
| Post subject: | Re: Do reverse kerf linings create stress risers? |
Thanks Y'all for your thoughts. The consensus seems to be that reverse kerfed liners don't create a stress riser. |
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| Author: | J De Rocher [ Thu Oct 08, 2020 2:35 pm ] |
| Post subject: | Re: Do reverse kerf linings create stress risers? |
Clay S. wrote: Thanks Y'all for your thoughts. The consensus seems to be that reverse kerfed liners don't create a stress riser. And even if they do, the question would then be does it actually matter for the vast majority of guitars in the real world. |
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| Author: | Bryan Bear [ Thu Oct 08, 2020 6:58 pm ] |
| Post subject: | Re: Do reverse kerf linings create stress risers? |
I feel like the consensus is that they do cause a stress riser but in practical application it is not much more dangerous that that of more traditional linings. |
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| Author: | wbergman [ Fri Oct 09, 2020 7:10 am ] |
| Post subject: | Re: Do reverse kerf linings create stress risers? |
Just musing. Suppose you are using laminated linings and scalloped the edges not in contact with the top or back? Would that be beneficial to reduce possible fracture lines? |
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| Author: | Barry Daniels [ Fri Oct 09, 2020 11:38 am ] |
| Post subject: | Re: Do reverse kerf linings create stress risers? |
I was going to suggest the same thing. Scolloped edges would definitely spread out the stress over the width of the scollop and should lessen the likelihood of a crack running along the lining. At least in theory. |
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| Author: | Alan Carruth [ Fri Oct 09, 2020 4:34 pm ] |
| Post subject: | Re: Do reverse kerf linings create stress risers? |
I think that tapering out laminated liners would work better than scalloping. The solid liners that are used on violins are carved to a taper after they're put on, so that the edges are flexible. There's no reason you could not do that with reverse kerf liners except for the time it takes. |
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