Douglas fir for OM style guitar top?

[Lillian]

Now, doesn't it depend on the piece of wood itself if it is usiable or not?

[jeffhigh]

At that thickness, it would have to be incredibly stiff to be usable for a standard braced OM, well outside normal douglas fir properties IMO.

[Lillian]

I agree that it is most likely too thin, but that doesn't mean that it is. And bracing plays a big part in it as well.

[jeffhigh]

To me it makes no sense to start with a soundboard that is so thin, just because you have it, and then try to compensate with bracing.
With the hours you put into building a guitar, it is not worth risking the soundboard failing.

[Lillian]

Jeff, I understand what you are saying and I'm not disagreeing. I'm just saying that it shouldn't be dismissed out of hand because of a subjective number. Would I build with it, probably not, but I don't have the wood in hand to make that call. Would I be comfortable building with it if it was stiff enough? Maybe, but honestly probably not. But in the end, it would all come down to the wood.

[jeffhigh]

Hi Lillian,
It's actually because there is only the subjective statement from the original poster
"I left the top at 2 mm cause it felt hard and heavy compared to the usual red cedar or spruce tops"
that I urge caution.
Jay has used Douglas fir at 100 thou quite successfully for an OM
At 78 thou before any finish sanding the moment of inertia of the crossection is reduced to 47% of what Jay has used.
The OP has no objective evidence to show that the modulus of elasticity of the wood he has is outside the norms for Douglas fir and so asking it to cope with this 53% reduction in a normal bracing arrangement is just asking for trouble.
Anyhow Lillian, I am certainly not being dismissive of you point that "It all comes down to the wood". That is certainly true, just that when you look at the numbers for this case, it would be well outside the variation you would get within the species.
Cheers
Jeff

[Bob Connor]

I'd be extremely cautious about making statements about what this piece of wood can or can't do without having my hands on it. You've just got to remember that "oils ain't oils, Sol"

I've got an OM sitting next to me that has an Engelmann top that is a hair over 2mm thick. I built it about 4 years ago. It has a little telegraphing of braces (which I would expect at this thickness) but the overall geometry of the top is fine and the instrument sounds great.

I also have another OM that I built for a bloke in Tassie. Same deal, Engelmann top but this one is sitting right at 2mm. It's around the same vintage and is probably the best sounding instrument I've built. And structurally it's fine.

Conventional wisdom is that Engelmann is not as stiff longitudinally as other spruces so leave it a bit thicker. if I believed everything I read on the internet I'd have left those tops at around 3.2mm and I'd have probably built two average sounding instruments.

If I were you, I'd build with it and see what happens. At worst it'll distort the top and you'll have to replace it, which is not a difficult task.

But you will have learnt a great deal in the process and the experience of having done this will be invaluable in assessing future tops and how you will deal with them.

There is nothing wrong with making a mistake or two by following your intuition. In fact you'll learn faster.

[jeffhigh]

Interesting that you were able to go that thin Bob. Engleman cetainly can be very stiff. Did you use your double cross bracing behind the bridge for these?
How are you determining how thin you can go. serious question. By feel in your hands based on experience or by some form of deflection testing?
Either can be valid.

[tomigv]

thanks jay, for sharing about using 2.5mm d. fir. the piece was thinned as it seemed very heavy to me at the time . I also have some really old d . fir about 40 yr old stock that was scrounged from an old shed for brace stock, . I am originaly from vancouver BC , and have used d. fir on a lot of woodwork projects in the past. I have cut up some sets of bubinga, and osage orange, and will try your combo suggestion. I am more inclined to use spruce and red cedar. I have maybe 1 other d. fir top in my inventory. The next door neighbour had beautiful 1/4 sawn 20 yr old roof red cedar shakes taken down . I managed to get some , but there is a lot of cleanup, sanding and planing as the shakes are tapered. cheers ernie

[Bob Connor]

Jeff

I don't do any deflection testing. I just do it by hand and I'll pull out a pile of tops each time I do it. I've got about a dozen Red Spruce tops that have a lot of those really thick late growth lines so they are incredibly dense and stiff. So they're my benchmarks for what is stiff and what isn't.
I also know where most of my stock sits in the stiffness range (plus I get to fondle my wood often)

I've also got a dozen Engelmann sets that came from Graham Hein that were comparably stiff but not as dense. So that was why I decide to go so thin on them.

This was a few years ago so the bracing was fairly standard Martin bracing with no forward shifting of the X-brace.

The new double X lattice that I use should support the lower bout in a much more even manner which should allow me to lighten things up even more. I'm only on guitar 2 with the new bracing pattern so I'll be able to give more informed comment as to whether it has been successful in about another 10-15 instruments.

If you've had a look at that new bracing you'll notice that the bridge plate is fairly large. That's to spread the load the the strings trying to pull the bridge off the top. (again so I can lighten up elsewhere)

I am going to set up a deflection testing jig just to check that my hands aren't telling me lies. Very Happy . I laso want to test some of that piano bracewood I got and deflection testing would be much more accurate for braces.

I still reckon everyone should get their hands on their luthier lumber at every available opportunity.

[jeffhigh]

Thanks Bob,
I am about to set up for deflection testing myself so I can have an objective measurement. I don't trust my memory of what a top felt like, and it will enable me to compare properties regardless of their current thickness.

Having a benchmark sample like you do certainly makes sense.

What you are doing with your new bracing arrangement certainly should reduce bellying.

My last 2 (flamenco and classical) were done with engleman and I really liked it

This is the final re-constructed message of this topic posted by the ANZLF help team.

[pmatolcsy]

sorry to bring this up so much later. Great info. Believed so myself for years .In our building code(canada) can span greater distances than red,white or blue spruce. Have intuitively made very thin tops from it that seem to sound great. Works as braces for same reason.
Thanks for the confirmation. Much cheaper here than "real" guitar spruces here(even old growth recycled at lumberyard prices)

On the downside, tends to be splintery and splits very easily.

Is this attribute OK for a brace wood(lack of shock resistance ability)?

Thanks in advance. I love this site. I've learned so much

Phil

[charangohabsburg]

I think that critical splintery-ness of wood (as far as it is interesting for instrument building, bows and arrows for example are a different story) is more a function of improper drying than of the wood species. During the drying process wood can develop cracks which, when drying is completed are perfectly closed again, but of course mean a weakening of this part of the wood. Maybe this happens more often to Douglas Fir because it is more difficult to dry it as it should be dried, or maybe it does not really matter for its intended use if the wood is structurally perfectly OK or not.

To answer the question if you can use a given piece of Douglas Fir (or any other "non-luthierie grade" wood) for bracing I would cut a few test brace blanks and bend them until they start to crack. Would that kind of force and deflection applied probably happen during normal use and maybe even slight abuse of the guitar? If yes, take that batch of wood to your fireplace. Then, when actually building, test-bend your brace blanks to what you believe they should be able to withstand, just to be sure that you don't use a piece with a hidden (closed) drying crack that could present a problem with the finished guitar.

In any case though, the main key to non-breaking braces (for any wood species) is using runout free wood. Split the blank in order to be able to cut it up into braces as close as possible parallel to the splitting direction, or alternatively (only when not scalloping, and not tapering the hight the braces, for several reasons) use it with the growth rings "flat" (parallel to the top) which even will make a slightly stiffer brace.

[Philstix]

My personal guitar is a dreadnaught I built over 10 years ago with a Douglas fir top, Martin style x bracing. It is loud and has a very full sound. I thinned the top by feel, that is I didn't measure thickness as I worked. I just took it out and measured the thickness of the top. It is 0.08 inches thick, i.e. 2 mm. It has held up very well. I would use it for a top if it feels stiff enough.

[pmatolcsy]

thanks so much for the thoughtful replies. I too have built tops and braces with perfect runnout free ,old growth ,very dense ,high ring count true douglas fir. Unfortunately its as brittle a glass of the same thickness. No doubt it is strong in compression, flexion tension. It does not appear to be "tough" i.e. there is so little flexibility that it exceeds both its plastic and elastic limit easily and will catastrophically fail at a very percentage over its
intended use. Like a thin wine glass, elegant ,light and functions perfectly until the day you ding it in just the wrong way and it breaks easily.

my 2c worth
phil