Current Build - Falcate Braced Multiscale OM cutaway

[johnparchem]

I have been finding the spruce is pretty easy to bend. I do bend all of the laminates on a hot pipe before laminating them, even for the main falcate brace.

[Bruce McC]

I have been following this post with particular interest as I have just finished my first falcate braced classical.
Thank you Martin for sharing your experience and others for your constructive input. The T(1.1)2 on my finished
guitar is less than ideal, 176 to be precise which is too low and close to F. T(1.1)1 is 93.5 and T(1.1)3 is 259.5
the back is not a live back. The braces are of laminated Sitka spruce, 2 mm laminates, I chose not to use CF because
I have never used it and don't want to. I started with the braces at 12mm H x 6mm W and tapered and reduced the
height until the stiffness of the soundboard felt "right" to me. Regrettably I am lacking in experience when it
comes to testing stiffness of soundboards. The finished height of the primary braces at the centre line of the bridge
was 6mm and the secondary braces was 4mm. There is no bridge plate. I am now contemplating a retop with taller
braces.

[kiwigeo]

New swiss spruce top glued up today. Luckily I stocked up on this stuff while I was building the lute so Ive got leeway for about 40 more stuff ups

[kiwigeo]

I have been following this post with particular interest as I have just finished my first falcate braced classical.
Thank you Martin for sharing your experience and others for your constructive input. The T(1.1)2 on my finished
guitar is less than ideal, 176 to be precise which is too low and close to F. T(1.1)1 is 93.5 and T(1.1)3 is 259.5
the back is not a live back. The braces are of laminated Sitka spruce, 2 mm laminates, I chose not to use CF because
I have never used it and don't want to. I started with the braces at 12mm H x 6mm W and tapered and reduced the
height until the stiffness of the soundboard felt "right" to me. Regrettably I am lacking in experience when it
comes to testing stiffness of soundboards. The finished height of the primary braces at the centre line of the bridge
was 6mm and the secondary braces was 4mm. There is no bridge plate. I am now contemplating a retop with taller
braces.

Thanks for the input Bruce. Ive actually got a falcate braced classical in the process of being french polished. Once its finished Ill post up some pics and tap test data.

[blackalex1952]

Great comments! Great thread...I agree with all Craig's comments re the service that your posts and others have done to everyone's collective learning experience. I would add that redoing any aspect of a mistake also improves our repair and restoration skills. Also great thanks to Craig for his Selmer Maccaferri posts! And, of course, thanks to Trevor for his generosity in sharing his experience regularly on this forum. In fact, thanks to everyone who shares and is not secretive with their ideas here!
Given that the top braces are not let in to the sides in the lower bout, would it have been possible to remove the back instead (there are a few good posts on the internet of back removal including getting the braces that are let in to the sides sorted)? Even if there was a disaster with the let in braces on the back, rebracing the back is far less hassle than re building a soundboard.It might be possible to unglue the back then cut the brace ends or sculpt them down with a sharp tool thru the soundhole (or a combination of both).If so, couldn't the top braces be stiffened without the risk of destroying the whole soundboard? ie, remove the CF from the top of the braces in question, top them with more wood then replace the CF.That would allow the top deflection to be worked with as per Craig's idea of building putting the back on last. If this was a feasible approach,would there be a problem using spruce to top the King Billy laminations, and would there be a problem heat bending spruce then shaving the sides of the spruce topping flush with the KB to make up for not so perfect bending pre retopping with CF?

[kiwigeo]

Great comments! Great thread...I agree with all Craig's comments re the service that your posts and others have done to everyone's collective learning experience. I would add that redoing any aspect of a mistake also improves our repair and restoration skills. Also great thanks to Craig for his Selmer Maccaferri posts! And, of course, thanks to Trevor for his generosity in sharing his experience regularly on this forum. In fact, thanks to everyone who shares and is not secretive with their ideas here!
Given that the top braces are not let in to the sides in the lower bout, would it have been possible to remove the back instead (there are a few good posts on the internet of back removal including getting the braces that are let in to the sides sorted)? Even if there was a disaster with the let in braces on the back, rebracing the back is far less hassle than re building a soundboard.It might be possible to unglue the back then cut the brace ends or sculpt them down with a sharp tool thru the soundhole (or a combination of both).If so, couldn't the top braces be stiffened without the risk of destroying the whole soundboard? ie, remove the CF from the top of the braces in question, top them with more wood then replace the CF.That would allow the top deflection to be worked with as per Craig's idea of building putting the back on last. If this was a feasible approach,would there be a problem using spruce to top the King Billy laminations, and would there be a problem heat bending spruce then shaving the sides of the spruce topping flush with the KB to make up for not so perfect bending pre retopping with CF?

I chose to remove the top for following reasons:

1. top was fixed with hide glue while back was done with AR. I tried to apply some heat to the back but the IRW was starting to darken from the heat so I abandoned the exercise. Finding a new back to match the sides is a pain compared to finding a top that matches the sides I have a large stash of Swiss spruce).
2. Only one brace (UTB) to deal with on the top as opposed to three on the back.
3. Working on the top laid on the bench as going to be easier than working on braces with the sides restricting activities.

At the moment it would appear that the stiffness of the King Billy Pine is the root cause of my issues. In hindsight I should have done some testing on the KPB blank but didnt as Ive used the same wood for bracing on a classical and parlour and T(1,1)2 resonances were within the ball park.

[blackalex1952]

Does AR refer to aliphatic resin glue? Great that the thread has helped with the KB stiffness issue via Trevor's generous input as usual. A lesson there for all of us re testing. Did you use the methods in the book to test your soundboard first in order to find the optimum thickness? What soundboard thickness did you end up with? Why AR glue rather than HHG?

[kiwigeo]

Does AR refer to aliphatic resin glue? Great that the thread has helped with the KB stiffness issue via Trevor's generous input as usual. A lesson there for all of us re testing. Did you use the methods in the book to test your soundboard first in order to find the optimum thickness? What soundboard thickness did you end up with? Why AR glue rather than HHG?

1. Yes Aliphatic Resin glue..Titebond I.
2. Yes soundboard and back were tap tested and target thicknesses calculated as per the Books. Target thickness for the top was 2.7mm and thickness after closing up the box was around 2.8mm.
3. Why AR on the back and hide on the top? I was actually going to use AR all around as Trevor generally does but for some reason decided to have one of the panels easily removable.....I opted for the top.
4. Yes I should have actually tested the KPB but as already mentioned Ive used the stuff on two guitars with no major issues.

[johnparchem]

On my current build I also had to remove my first top. I ended up with a lower top resonance even though I increased the height of the braces relative to my first falcate classical guitar. I've been thinking about how to achieve more consistency for stiffness of the falcate braces. One way as mentioned in this thread is to get an idea of the stiffness of the brace wood and choice consistent braces. My other thought is to do deflection tests while shaping the falcate braces to normalize the top for stiffness before adding the top CF. Has anyone tried this process?

[kiwigeo]

Assuming the top thickness is OK (I couldn't find anywhere you said what it is) my guess is that you must have some really soft KBP for the bracing. I've always used the brace sections given in the plans or smaller. When using KBP for falcates, the Elong for the brace wood I have is close to 6GPa (see Table 4.4-1). It's quite possible that you have wood as low as 3.0GPa.

Trevor,

I tested a piece of the KBP I used for bracing on the OM. Results as follows:

L = 690mm
W = 53mm
d = 27mm
Wt = 375g
F = 207.7Hz
E = 4.8GPa

[Trevor Gore]

Well, that's a lot lower than spruce but not desperately low for KPB. However, in the small brace sections (the size I measured at) and with a bit of build variability you could easily be 25%-30% lower on EI than where I ended up and that's enough to account for a lot of the difference.

I've noted before (and in the book) that mine seem to come out stiffer than most people's, though it is very difficult to identify what is being done differently. I can't think of one where I've had to come up in frequency to a T(1,1)2 target. Mostly I'm struggling to get down. Those who have done a few generally seem to be getting them close to target. I guess there's not much room for build variation when we're aiming for pretty precise targets.

If I were doing your multiscale, I'd have gone in with the assumption that the bracing asymmetry wouldn't have made much difference, but that is not something I've tested. There are one or two people who read these pages who are very adept at FEA. Maybe they can model the asymmetry and see what happens?

If you have some bendable spruce, I think I'd be going with that. Much easier to take a T(1,1)2 down than up.

Let's hope the re-top works out.

[kiwigeo]

Well, that's a lot lower than spruce but not desperately low for KPB. However, in the small brace sections (the size I measured at) and with a bit of build variability you could easily be 25%-30% lower on EI than where I ended up and that's enough to account for a lot of the difference.

I've noted before (and in the book) that mine seem to come out stiffer than most people's, though it is very difficult to identify what is being done differently. I can't think of one where I've had to come up in frequency to a T(1,1)2 target. Mostly I'm struggling to get down. Those who have done a few generally seem to be getting them close to target. I guess there's not much room for build variation when we're aiming for pretty precise targets.

If I were doing your multiscale, I'd have gone in with the assumption that the bracing asymmetry wouldn't have made much difference, but that is not something I've tested. There are one or two people who read these pages who are very adept at FEA. Maybe they can model the asymmetry and see what happens?

If you have some bendable spruce, I think I'd be going with that. Much easier to take a T(1,1)2 down than up.

Let's hope the re-top works out.

Thanks Trevor,

No the KBP E value isn't outrageously low.

I'm pretty well resigned to using spruce bracing on the replacement top. Ive done some tap testing on some red and Lutz spruce billets.....E values averaging around 10GPa but a few pieces of Red Spruce ranging up to as high as 16GPa.

I'm also looking very hard at the secondary and tertiary brace layout and may tweak same.

[Dave M]

The spruce I am using - given KBP doesn't seem to be available in the UK - is pretty reluctant to bend.
I have to use 4 thin laminates for the secondaries. Even then there are quite a few breakages. It does mean there is little springback.

Presumably reckoning out the Youngs modulus of the composite would be quite hard. The vertical sheets of epoxy, though thin, must contribute to the stiffness...?
It would be interesting to measure it and compare to a solid brace of the same timber.

[Mark McLean]

I am building a falcate neo-classical at the moment. It is my first try at this bracing style so I don't have a lot of expertise to contribute. However, I did them with Sitka, just because it was what I has to hand and I was only viewing it as an experiment. I didn't really expect that I would get a usable set of braces at my first attempt. However, at about 1.8mm it bend very easily on a hot pipe, even for the tighter bends of the secondary braces. There was little spring back, and I found it fairly easy to get them lined up and glued in a shaping jig. So, they got used and I have just closed the box. Have not got around to doing any tap testing yet.

Perhaps I was lucky with this spruce, as others have obviously found it difficult to bend. But I certainly suggest that you give it a go. If the first lot you try won't co-operate, try a different spruce variety. Martin, it is not like you are short of wood to experiment with!

[kiwigeo]

I am building a falcate neo-classical at the moment. It is my first try at this bracing style so I don't have a lot of expertise to contribute. However, I did them with Sitka, just because it was what I has to hand and I was only viewing it as an experiment. I didn't really expect that I would get a usable set of braces at my first attempt. However, at about 1.8mm it bend very easily on a hot pipe, even for the tighter bends of the secondary braces. There was little spring back, and I found it fairly easy to get them lined up and glued in a shaping jig. So, they got used and I have just closed the box. Have not got around to doing any tap testing yet.

Perhaps I was lucky with this spruce, as others have obviously found it difficult to bend. But I certainly suggest that you give it a go. If the first lot you try won't co-operate, try a different spruce variety. Martin, it is not like you are short of wood to experiment with!

Thanks for the input Mark. Did you do a 3 or 4 ply laminate with the braces?

Ive got some red spruce ready to run through the drum sander but today attentions are focused on a moderate sized piss up here.

[Mark McLean]

I did three laminates, each about 1.7mm. I ended up with final brace dimensions of 5.4mm by 20mm, and they got glued with CF above and below.

[Bruce McC]

Hi Mark
When you say your final brace dimensions were 5.4 x 20 am I correct in assuming that was
under the bridge area and from there you tapered the braces towards the ends?

[kiwigeo]

I did three laminates, each about 1.7mm. I ended up with final brace dimensions of 5.4mm by 20mm, and they got glued with CF above and below.

Thanks Bruce.

The width of the finished bracing is narrow side but this is more than compensated for by the extra height. Ive just glued up a secondary brace with 3 x 1.7mm lams of red spruce. The stuff doesn't bend like KBO does but I managed to get it into shape with softener and a bit of cursing.

[kiwigeo]

Was looking at the old top last night and noticed that the parted centre seam has closed up

[Mark McLean]

I think cursing definitely helps in the bending process.

Yes, Bruce that is the maximum height and they taper towards the tail. After sanding they probably ended up a bit less than that maximum dimension anyway. I really should get more careful at recording exact measurements as I go along, and before closing the box!
Mark

[Trevor Gore]

I did three laminates, each about 1.7mm. I ended up with final brace dimensions of 5.4mm by 20mm, and they got glued with CF above and below.

That is massively tall bracing. Was it the dimension before the brace was sawn in two?

[kiwigeo]

I have a tentative plan to make up the new top with over height braces to the stage where top layer of CF would normally get laid in. At this stage Im going to temporarily glue on the top (hide glue) and tap test. If the T(1,1)2 or 3 resonances are way out I'll pull the top and trim the braces. Of course there will have to be some allowance made for the stiffness added to the brace by the top layer of CF....

[Mark McLean]

I did three laminates, each about 1.7mm. I ended up with final brace dimensions of 5.4mm by 20mm, and they got glued with CF above and below.

That is massively tall bracing. Was it the dimension before the brace was sawn in two?

Whoops, yes, you are quite right Trevor! I did seem to forget to mention that the 20mm height was the initial lay-up, and then of course it got cut down the middle to give the paired final braces, each about 9.5mm (and closer to 9 by the time it was all tidied up). That small detail could make a difference

[kiwigeo]

Whoops, yes, you are quite right Trevor! I did seem to forget to mention that the 20mm height was the initial lay-up, and then of course it got cut down the middle to give the paired final braces, each about 9.5mm (and closer to 9 by the time it was all tidied up). That small detail could make a difference

20mm high braces........just thinking about them is giving me a stiffy

[kiwigeo]

Got a secondary brace done today....seemed to go ok with very little spring back. Second secondary brace on the jig with the glue setting and tomorrow I'll get the primary braces done.

The new top has been jointed and thinned to around 2.9mm...final thickness will be around 2.6mm.