Current Build - Falcate Braced Multiscale OM cutaway

[kiwigeo]

I have a suggestion, it might not be so important, but why don't we all use some kind of standard for the x and y axis with our frequency graphs when comparing the basic resonances? For example, I suggest 50Hz to 1000Hz for the x axis, and the y could also be set up in a standard way so that the peaks are quite clearly defined. That would make visual comparison easier, perhaps? I wouldn't get into a debate around the subject, only to say that we could establish a "convention"...Perhaps Trevor Gore could chime in here?
I have been copying my frequency graphs from Visual Analyser using "snipping tool" (free) and I can size the images so that I can see two of them on my desktop simultaneously for comparison...eg with and without side mass or with and without bridge mass. The reason I use snipping tool is because, on my computer, the screenshot function in VA simply won't work...neither will the presets. Must be the version of windows that I have. I've tried different versions of VA.

The best way to output the VA data for comparison is to load a text file into a spreadsheet. I normally average data from 10 runs and then load into a spreadsheet but at the moment I'm only interested in rough results to decide how the main top resonance is sitting.

Standardising the X scale is easy enough but the Y scale will be an issue as SPL's are highly variable...even on seperate runs on the same guitar.

Screenshot works on my version of VA but loading presets crashes the program.....youll find alot of little bugs like this and they seem to be version and OS dependant.

[blackalex1952]

"Modifying braces is not an option as top braces have CF on top of them. Adding side weights will drop main top resonance....I want to go the other way."
-I was referring to your post where you said you were thinking of stiffening the back. Those braces are not CF...How about some brain surgery where another couple of CF strands are laid on the braces to stiffen them? Beyond my experience, just a thought.There may be an innovative way no one has thought of yet, rather than removing the back, at the same time I have no idea how much of a change one could expect from that. Removing the back might be an option before the binding goes on.The other thought I have is that these resonances will probably move after the box is bound. (in the case of the uncoupled top measurement, bound and gagged!)

[blackalex1952]

I have no idea how to do the spreadsheet dance! It would be lovely to see a tutorial re the subject on this forum, if someone has the inclination. Including any ways to load the data from VA. One that covers open office spreadsheet would be handy too, not just Excel. I have open office, but couldn't get formulas to work properly on it, I wanted to get a column that automatically calculated the Specific Mobility from K an m.

[blackalex1952]

"Standardising the X scale is easy enough but the Y scale will be an issue as SPL's are highly variable...even on seperate runs on the same guitar"
Maybe we need to standardise a hammer for the tapping!!!

[kiwigeo]

-I was referring to your post where you said you were thinking of stiffening the back. Those braces are not CF...How about some brain surgery where another couple of CF strands are laid on the braces to stiffen them? Beyond my experience, just a thought.There may be an innovative way no one has thought of yet, rather than removing the back, at the same time I have no idea how much of a change one could expect from that. Removing the back might be an option before the binding goes on.The other thought I have is that these resonances will probably move after the box is bound. (in the case of the uncoupled top measurement, bound and gagged!)

Alex, the main back brace has a small central section shaved out prior to closing up the box. This reduces stiffness of the back but there is also the option of increasing the back frequency by simply replacing the wood (done through soundhole) that has been removed and increasing the brace stiffness.

The back frequency is the least of my concerns at this stage. The two main top resonances should always be the starting point.

[kiwigeo]

I have no idea how to do the spreadsheet dance! It would be lovely to see a tutorial re the subject on this forum, if someone has the inclination. Including any ways to load the data from VA. One that covers open office spreadsheet would be handy too, not just Excel. I have open office, but couldn't get formulas to work properly on it, I wanted to get a column that automatically calculated the Specific Mobility from K an m.

VA allows export of spectrum data as a text file. A text file is then simply loaded into a spreadsheet and you can then display the data using Excels charting features.

[blackalex1952]

I agree, I have the books...don't fully understand them yet as I haven't spent lots of time applying the ideas yet, but getting right into it, time permitting. I'll have to change my username,Martin, it's actually Ross, Alexander is my middle name....Don't mind either really.

[kiwigeo]

Stuck a dummy bridge blank on the top and tap tested. T(1,1)1 dropped to 92Hz and T(1,1)2 dropped to 160Hz. Bridge blank weighs 46gm but will weight less once its shaped. 27grams of bluetack stuck over the bridge area gave similar results.

[kiwigeo]

That T(1,1)2 seems low already, as the others have mentioned. Do you still have that bridge caul attached inside?

Bridge plate is around 2.5mm thick as advised in the book but is it possible there's a bit too much mass in the bridge plate? If this is not the reason for the low T(1,1)2 frequency then brace stiffness would be the next culprit? Braces are 9mm high over bridge area.

[Trevor Gore]

Bridge caul as per this photo:
download/file.php?id=13579&mode=view

[kiwigeo]

Bridge caul as per this photo:
download/file.php?id=13579&mode=view

The caul went into the "OM #15" shoe box with all the other bits and pieces prior to the top glue up.

[kiwigeo]

Situation overview:

Target resonances are 95/180 and 226Hz for T(1,1)1 2 and 3 respectively.
Actual resonances are 95/175 and 210Hz. T(1,1)2 is way too low.
Back resonance is easily raised by increasing stiffness of main back brace. Raising T(1,1)2 is more difficult.

My options appear to be as follows:

1. Reduce mass of top - difficult/impossible.
2. Increase stiffness of top or bracing - difficult without removing top and either replacing primary bracing or making an entire new top.

At the moment tentative plan is to remove top (it's fixed on with hide glue) and attempt to remove primary bracing and replace with higher bracing (11mm??). If I can't get the braces off then I'll make up a new top.

The irony of all this......this is the first guitar Ive made where the back appears to be coupling with the top

[Craig Bumgarner]

I hate when this happens. I really don't know the solution when the top goes below the target, seems much easier to take it down than punch it up. Minimize the mass in the bridge of course, maybe go radical on that somehow. Add more carbon to some of the bigger braces with the intention of stiffening the top to raise the T2? I've done that, but my purpose was to stiffen up deflection and my notes indicate it did not change the T2 much at all. Mostly, I just wanted to thank you for posting this build thread, great pictures. Good luck with the T2, very interested to hear the solution.

[Trevor Gore]

The caul went into the "OM #15" shoe box with all the other bits and pieces prior to the top glue up.

Good to hear!

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. The last all KPB braced medium box I did came out at 96,190, 217 just boxed and finished on 90, 170, 214 after edge thinning, binding, finishing etc., no side mass (Engelmann, Aus blackwood). I wouldn't expect the asymmetry of your bracing to have much impact on the resonances, but I've never done one like that. If I use all spruce falcates I have to reduce the brace sizes to be sure of getting down to a T(1,1)2 of 180Hz when finished.

As an aside, when Strato, at the Australian School of Guitar Making, takes students through a falcate build he uses 9*7 braces to accommodate the variability in building talent/wood properties and ends up (finished) at much the same resonances as I do.

[Craig Bumgarner]

If you go the way of taking the top off, you could just trim the tops off the braces you want to change, plane the edge and glue thicker pieces back on. Reshape them and add the carbon back. That would save the trouble of trying to get the entire brace off the back of the top, with the carbon stuck between, sounds like an unlikely proposition.

I notice you have mass bolting blocks. You might consider removing these to boost T2 a little.

[Craig Bumgarner]

As an aside, this makes me wonder why do most builders put the back on before the top? I started putting the top on first to get the geometry of three dimensional arched tops and the match to the sides right, but the added benefit is being able measure top deflection and resonances once attached to the sides. The back, bridge, etc., seem to have little effect on deflection. They lower the T2 resonance, but this is largely predictable based on experience. Before gluing the top to the sides however, it is difficult to know what you have, other than just hoping based on previous builds. If the back is glued first, then the top, the options for adjustment are significantly reduced. I'm probably missing something here as back first, then the top seems very much the norm.

[kiwigeo]

Thanks for the input Trevor, Craig and others.

Alot going on in and outside the workshop so today I'm going to let the guitar lie and have a chew over the comments and a long think about my options.

Cheers Martin

[kiwigeo]

As an aside, this makes me wonder why do most builders put the back on before the top?

You make alot of good points Craig. Why do I put the back on first? The main driver is being able to clean up the glue squeeze out from the back linings....theyre visible through the soundhole.

Im definitely going to have a re-think on this one along with a few other matters.

[kiwigeo]

Made a decision to lift the top today. In the process of undoing the hide glue joint the centre seam split at the tail. The top will go on the wall of shame and Ill make up a new top with appropriate modifications to ensure T(1,1)2 doesnt come in low like it did with the first top. First job will be to beef up the lower main back brace to help with the T(1,1)3 peak.....will add some wood to the brace and make it a bit higher.

[Mark McLean]

Thanks for sharing all the pain with us Martin! I am sure it is disappointing to take a few backward steps in a build that has been looking so great. But we are all on the learning journey with you! Can you rescue the rosette? It took quite a bit of effort!
Mark

[kiwigeo]

Thanks for sharing all the pain with us Martin! I am sure it is disappointing to take a few backward steps in a build that has been looking so great. But we are all on the learning journey with you! Can you rescue the rosette? It took quite a bit of effort!
Mark

Thanks Mark...if my publicizing my gaffs and problems can help someone else then I guess that's a good thing.

The rosette....actually since I managed to split the centre seam while removing the top it's a write off so rescuing the rosette is an option. I plan to cut it out and then run it through the drum sander to remove spruce from the back of same. I can then mount the rosette on an MDF base plate and rout inside and outside of the rosette.

Ive bulked up the lower back brace and trimmed it down to around 20mm (original brace was around 18mm). Ill probably leave the brace as is and remove any material once the new top is on.

[kiwigeo]

Pics of work on the lower back brace. I shaved the brace down as far as possible to give a good gluing surface. I then glued on some more spruce and am in the process of shaping same after trimming height to 20mm.

[Mark McLean]

That will be something else interesting to look at through the soundhole. It does look very clean and tidy in there!

[Dave M]

I was going to say something a bit facetious like 'that's a bummer' but honestly this has been a roller coaster of emotion, no doubt for other forum members too.
Following the wonderful workmanship, the back and sides coming together so well, getting that damned cutaway side to bend, the excitement of the box being closed, then the horrible let down when you realise it hasn't worked...

It's been great to share the ride but more importantly:

1 We appreciate the effort that goes into sharing your experiences.

2 The information is really useful. We who are trying to learn how to make a good guitar, or even a great one, need this everyday practical knowledge to improve our own work.

3 It quickly becomes clear that instrument building imposes a realistic attitude to one's skills. We do get better but things still go wrong. The trick is to get over it, as you have done, with a careful assessment of where to go next and get on with it.


So after that bit of guff.

Practically, do you have enough of that King Billy Pine to do a test for Young's Modulus? This could prove, or not Trevor's hypothesis. Otherwise what was going on?

All The best Dave

[kiwigeo]

Practically, do you have enough of that King Billy Pine to do a test for Young's Modulus? This could prove, or not Trevor's hypothesis. Otherwise what was going on?

All The best Dave

Thanks for the positive words Dave.

I have plans to measure Youngs Modulus of the Chunk of KBP Im cutting my bracing from but judging by the way it bends Id say it's quite low. With this in mind Im leaning towards using spruce for the bracing.